CHAPTER 3

Microphones

 

The Moving-coil or Dynamic Microphone

The Ribbon Microphone

The Capacitor or Condenser Microphone

Basic capacitor microphone

Electret designs

RF capacitor microphone

Directional Responses and Polar Diagrams

Omnidirectional pattern

Figure-eight or bidirectional pattern

Cardioid or unidirectional pattern

Hypercardioid pattern

Specialized Microphone Types

Rifle microphone

Parabolic microphone

Boundary or ‘pressure-zone’ microphone

Switchable Polar Patterns

Stereo Microphones

Microphone Performance

Microphone sensitivity in practice

Microphone noise in practice

Microphone Powering Options

Phantom power

A–B powering

Connectors

Radio Microphones

Principles

Facilities

Licenses

Digital radio microphones

Aerials

Aerial siting and connection

Diversity reception

 

A microphone is a transducer that converts acoustical sound energy into electrical energy, based on the principle described in Fact File 3.1. It performs the opposite function to a loudspeaker, which converts electrical energy into acoustical energy. The three most common principles of operation are the moving coil or ‘dynamic’, the ribbon, and the capacitor or condenser. The principles of these are described in Fact Files 3.2–3.4

 

 

 

THE MOVING-COIL OR DYNAMIC MICROPHONE

The moving-coil microphone is widely used in the sound reinforcement industry, its robustness making it particularly suitable for hand-held vocal use. Wire-mesh bulbous wind shields are usually fitted to such models, and contain foam material which attenuates wind noise and ‘p-blasting’ from the vocalist’s mouth. Built-in bass attenuation is also often provided to compensate for the effect known as bass tip-up or proximity effect, a phenomenon whereby sound sources at a distance of less than 50 cm or so are reproduced with accentuated bass if the microphone has a directional response (see Fact File 3.5). The frequency response of the moving-coil mic tends to show a resonant peak of several decibels in the upper-mid frequency or ‘presence’ range, at around 5 kHz or so, accompanied by a fairly rapid fall-off in response above 8 or 10 kHz. This is due to the fact that the moving mass of the coil-diaphragm structure is sufficient to impede the diaphragm’s rapid movement necessary at high frequencies. The shortcomings have actually made the moving coil a good choice for vocalists since the presence peak helps to lift the voice and improve intelligibility. Its robustness has also meant that it is almost exclusively used as a bass drum mic in the rock industry. Its sound quality is restricted by its slightly uneven and limited frequency response, but it is extremely useful in applications such as vocals, drums and the micing-up of guitar amplifiers.

One or two high-quality moving-coil mics have appeared with an extended and somewhat smoother frequency response, and one way of achieving this has been to use what are effectively two mic capsules in one housing, one covering mid and high frequencies, one covering the bass.

THE RIBBON MICROPHONE

The ribbon microphone at its best is capable of very high-quality results. The comparatively ‘floppy’ suspension of the ribbon gives it a low-frequency resonance at around 40 Hz, below which its frequency response fairly quickly falls away. At the high-frequency end the frequency response remains smooth. However, the moving mass of the ribbon itself means that it has difficulty in responding to very high frequencies, and there is generally a roll-off above 14kHz or so. Reducing the size (therefore the mass) of the ribbon reduces the area for the sound waves to work upon and its electrical output becomes unacceptably low. One manufacturer has adopted a ‘double-ribbon’ principle which goes some way towards removing this dilemma. Two ribbons, each half the length of a conventional ribbon are mounted one above the other and are connected in series. They are thus analogous to a conventional ribbon that has been ‘clamped’ in the center. Each ribbon now has half the moving mass and thus a better top-end response. Both of them working together still maintain the necessary output.

The ribbon mic is rather more delicate than the moving coil, and it is better suited to applications where its smooth frequency response comes into its own, such as the micing of acoustic instruments and classical ensembles. There are, however, some robust models which look like moving-coil vocal mics and can be interchanged with them. Micing a rock bass drum with one is still probably not a good idea, due to the very high transient sound pressure levels involved.

THE CAPACITOR OR CONDENSER MICROPHONE

Basic capacitor microphone

The great advantage of the capacitor mic’s diaphragm over moving-coil and ribbon types is that it is not attached to a coil and former, and it does not need to be of a shape and size which makes it suitable for positioning along the length of a magnetic field. It therefore consists of an extremely light disc, typically 12–25 mm in diameter, frequently made from polyester coated with an extremely thin vapor-deposited metal layer so as to render it conductive. Sometimes the diaphragm itself is made of a metal such as titanium. The resonant frequency of the diaphragm is typically in the 12–20 kHz range, but the increased output here is rather less prominent than with moving coils due to the diaphragm’s very light weight.

Occasionally capacitor microphones are capable of being switched to give a line level output, this being simple to arrange since an amplifier is built into the mic anyway. The high-level output gives the signal rather more immunity to interference when very long cables are employed, and it also removes the need for microphone amplifiers at the mixer or tape recorder. Phantom power does, however, still need to be provided (see ‘Phantom power’, p. 64).

Electret designs

A much later development was the so-called ‘electret’ or ‘electret condenser’ principle. The need to polarize the diaphragm with 48 volts is dispensed with by introducing a permanent electrostatic charge into it during manufacture. In order to achieve this the diaphragm has to be of a more substantial mass, and its audio performance is therefore closer to a moving-coil than to a true capacitor type. The power for the head amplifier is supplied either by a small dry-cell battery in the stem of the mic or by phantom power. The electret principle is particularly suited to applications where compact size and light weight are important, such as in small portable cassette machines (all built-in mics are now electrets) and tie-clip microphones which are ubiquitous in television work. They are also made in vast quantities very cheaply.

Later on, the so-called ‘back electret’ technique was developed. Here, the diaphragm is the same as that of a true capacitor type, the electrostatic charge being induced into the rigid back plate instead. Top-quality examples of back electrets are therefore just as good as conventional capacitor mics with their 48 volts of polarizing voltage.

RF capacitor microphone

Still another variation on the theme is the RF (radio frequency) capacitor mic, in which the capacitor formed by the diaphragm and back plate forms part of a tuned circuit to generate a steady carrier frequency which is much higher than the highest audio frequency. The sound waves move the diaphragm as before, and this now causes modulation of the tuned frequency. This is then demodulated by a process similar to the process of FM radio reception, and the resulting output is the required audio signal. (It must be understood that the complete process is carried out within the housing of the microphone and it does not in itself have anything to do with radio microphone systems, as discussed in ‘Radio microphones’, p. 68.)

DIRECTIONAL RESPONSES AND POLAR DIAGRAMS

Microphones are designed to have a specific directional response pattern, described by a so-called ‘polar diagram’. The polar diagram is a form of two-dimensional contour map, showing the magnitude of the microphone’s output at different angles of incidence of a sound wave. The distance of the polar plot from the center of the graph (considered as the position of the microphone diaphragm) is usually calibrated in decibels, with a nominal 0dB being marked for the response at zero degrees at 1 kHz. The further the plot is from the center, the greater the output of the microphone at that angle.

Omnidirectional pattern

Ideally, an omnidirectional or ‘omni’ microphone picks up sound equally from all directions. The omni polar response is shown in Figure 3.1, and is achieved by leaving the microphone diaphragm open at the front, but completely enclosing it at the rear, so that it becomes a simple pressure transducer, responding only to the change of air pressure caused by the sound waves. This works extremely well at low and mid frequencies, but at high frequencies the dimensions of the microphone capsule itself begin to be comparable with the wavelength of the sound waves, and a shadowing effect causes high frequencies to be picked up rather less well to the rear and sides of the mic. A pressure increase also results for high-frequency sounds from the front. Coupled with this is the possibility for cancelations to arise when a high-frequency wave, whose wavelength is comparable with the diaphragm diameter, is incident from the side of the diaphragm. In such a case positive and negative peaks of the wave may result in opposing forces on the diaphragm.

Figure 3.2 shows the polar response plot which can be expected from a real omnidirectional microphone with a capsule half an inch (13 mm) in diameter. It is perfectly omnidirectional up to around 2 kHz, but then it begins to lose sensitivity at the rear; at 3 kHz its sensitivity at 180° will typically be 6 dB down compared with lower frequencies. Above 8 kHz, the 180° response could be as much as 15 dB down, and the response at 90° and 270° could show perhaps a 10dB loss. As a consequence, sounds which are being picked up significantly off axis from the microphone will be reproduced with considerable treble loss, and will sound dull. It is at its best on axis and up to 45° either side of the front of the microphone.

High-quality omnidirectional microphones are characterized by their wide, smooth frequency response extending both to the lowest bass frequencies and the high treble with minimum resonances or coloration. This is due to the fact that they are basically very simple in design, being just a capsule which is open at the front and completely enclosed at the rear. (In fact a very small opening is provided to the rear of the diaphragm in order to compensate for overall changes in atmospheric pressure which would otherwise distort the diaphragm.) The small tie-clip microphones which one sees in television work are usually omnidirectional electret types which are capable of very good performance. The smaller the dimensions of the mic, the better the polar response at high frequencies, and mics such as these have quarter-inch diaphragms which maintain a very good omnidirectional response right up to 10 kHz.

Omni microphones are usually the most immune to handling and wind noise of all the polar patterns, since they are only sensitive to absolute sound pressure. Patterns such as figure-eight (especially ribbons) and car-dioid, described below, are much more susceptible to handling and wind noise than omnis because they are sensitive to the large pressure difference created across the capsule by low-frequency movements such as those caused by wind or unwanted diaphragm motion. A pressure-gradient microphone’s mechanical impedance (the diaphragm’s resistance to motion) is always lower at LF than that of a pressure (omni) microphone, and thus it is more susceptible to unwanted LF disturbances.

Figure-eight or bidirectional pattern

The figure-eight or bidirectional polar response is shown in Figure 3.3. Such a microphone has an output proportional to the mathematical cosine of the angle of incidence. One can quickly draw a figure-eight plot on a piece of graph paper, using a protractor and a set of cosine tables or pocket calculator. Cos 0° = 1, showing a maximum response on the forward axis (this will be termed the 0dB reference point). Cos 90° = 0, so at 90° off axis no sound is picked up. Cos 180° is −1, so the output produced by a sound which is picked up by the rear lobe of the microphone will be 180° out of phase compared with an identical sound picked up by the front lobe. The phase is indicated by the + and − signs on the polar diagram. At 45° off axis, the output of the microphone is 3 dB down (cos 45° represents 0.707 or 1/√2 times the maximum output) compared with the on-axis output.

Traditionally the ribbon microphone has sported a figure-eight polar response, and the ribbon has been left completely open both to the front and to the rear. Such a diaphragm operates on the pressure-gradient principle, responding to the difference in pressure between the front and the rear of the microphone. Consider a sound reaching the mic from a direction 90° off axis to it. The sound pressure will be of equal magnitude on both sides of the diaphragm and so no movement will take place, giving no output. When a sound arrives from the 0° direction a phase difference arises between the front and rear of the ribbon, due to the small additional distance traveled by the wave. The resulting difference in pressure produces movement of the diaphragm and an output results.

At very low frequencies, wavelengths are very long and therefore the phase difference between front and rear of the mic is very small, causing a gradual reduction in output as the frequency gets lower. In ribbon microphones this is compensated for by putting the low-frequency resonance of the ribbon to good use, using it to prop up the bass response. Single-diaphragm capacitor mic designs which have a figure-eight polar response do not have this option, since the diaphragm resonance is at a very high frequency, and a gradual roll-off in the bass can be expected unless other means such as electronic frequency correction in the microphone design have been employed. Double-diaphragm switchable types which have a figure-eight capability achieve this by combining a pair of back-to-back cardioids (see next section) that are mutually out of phase.

Like the omni, the figure-eight can give very clear uncolored reproduction. The polar response tends to be very uniform at all frequencies, except for a slight narrowing above 10 kHz or so, but it is worth noting that a ribbon mic has a rather better polar response at high frequencies in the horizontal plane than in the vertical plane, due to the fact that the ribbon is long and thin. A high-frequency sound coming from a direction somewhat above the plane of the microphone will suffer partial cancelation, since at frequencies where the wavelength begins to be comparable with the length of the ribbon the wave arrives partially out of phase at the lower portion compared with the upper portion, therefore reducing the effective acoustical drive of the ribbon compared with mid frequencies. Ribbon figure-eight microphones should therefore be orientated either upright or upside-down with their stems vertical so as to obtain the best polar response in the horizontal plane, vertical polar response usually being less important.

Although the figure-eight picks up sound equally to the front and to the rear, it must be remembered that the rear pickup is out of phase with the front, and so correct orientation of the mic is required.

Cardioid or unidirectional pattern

The cardioid pattern is described mathematically as 1 + cos θ, where θ is the angle of incidence of the sound. Since the omni has a response of 1 (equal all round) and the figure-eight has a response represented by cos θ, the cardioid may be considered theoretically as a product of these two responses. Figure 3.4a illustrates its shape. Figure 3.4b shows an omni and a figure-eight superimposed, and one can see that adding the two produces the cardioid shape: at 0°, both polar responses are of equal amplitude and phase, and so they reinforce each other, giving a total output which is actually twice that of either separately. At 180°, however, the two are of equal amplitude but opposite phase, and so complete cancelation occurs and there is no output. At 90° there is no output from the figure-eight, but just the contribution from the omni, so the cardioid response is 6 dB down at 90°. It is 3 dB down at 65° off axis.

One or two early microphone designs actually housed a figure-eight and an omni together in the same casing, electrically combining their outputs to give a resulting cardioid response. This gave a rather bulky mic, and also the two diaphragms could not be placed close enough together to produce a good cardioid response at higher frequencies due to the fact that at these frequencies the wavelength of sound became comparable with the distance between the diaphragms. The designs did, however, obtain a cardioid from first principles. The BBC type 4033 was one such example.

The cardioid response is now obtained by leaving the diaphragm open at the front, but introducing various acoustic labyrinths at the rear which cause sound to reach the back of the diaphragm in various combinations of phase and amplitude to produce a resultant cardioid response. This is difficult to achieve at all frequencies simultaneously, and Figure 3.5 illustrates the polar pattern of a typical cardioid mic with a three-quarter-inch diaphragm. As can be seen, at mid frequencies the polar response is very good. At low frequencies it tends to degenerate towards omni, and at very high frequencies it becomes rather more directional than is desirable. Sound arriving from, say, 45° off axis will be reproduced with treble loss, and sounds arriving from the rear will not be completely attenuated, the low frequencies being picked up quite uniformly.

The above example is very typical of moving-coil cardioids, and they are in fact very useful for vocalists due to the narrow pickup at high frequencies helping to exclude off-axis sounds, and also the relative lack of pressure-gradient component at the bass end helping to combat bass tip-up. High-quality capacitor cardioids with half-inch diaphragms achieve a rather more ideal cardioid response. Owing to the presence of acoustic labyrinths, coloration of the sound is rather more likely, and it is not unusual to find that a relatively cheap electret omni will sound better than a fairly expensive cardioid.

Hypercardioid pattern

The hypercardioid, sometimes called ‘cottage loaf’ because of its shape, is shown in Figure 3.6. It is described mathematically by the formula 0.5 + cos θ, i.e. it is a combination of an omni attenuated by 6 dB, and a figure-eight. Its response is in between the cardioid and figure-eight patterns, having a relatively small rear lobe which is out of phase with the front lobe. Its sensitivity is 3 dB down at 55° off axis. Like the cardioid, the polar response is obtained by introducing acoustic labyrinths to the rear of the diaphragm. Because of the large pressure-gradient component it too is fairly susceptible to bass tip-up. Practical examples of hypercardioid microphones tend to have polar responses which are tolerably close to the ideal. The hypercardioid has the highest direct-to-reverberant ratio of the patterns described, which means that the ratio between the level of on-axis sound and the level of reflected sounds picked up from other angles is very high, and so it is good for excluding unwanted sounds such as excessive room ambience or unwanted noise.

SPECIALIZED MICROPHONE TYPES

Rifle microphone

The rifle microphone is so called because it consists of a long tube of around three-quarters of an inch (1.9 cm) in diameter and perhaps 2 feet (61cm) in length, and looks rather like a rifle barrel. The design is effectively an ordinary cardioid microphone to which has been attached a long barrel along which slots are cut in such a way that a sound arriving off axis enters the slots along the length of the tube and thus various versions of the sound arrive at the diaphragm at the bottom of the tube in relative phases which tend to result in cancelation. In this way, sounds arriving off axis are greatly attenuated compared with sounds arriving on axis. Figure 3.7 illustrates the characteristic club-shaped polar response. It is an extremely directional device, and is much used by news sound crews where it can be pointed directly at a speaking subject, excluding crowd noise. It is also used for wildlife recording, sports broadcasts, along the front of theater stages in multiples, and in audience participation discussions where a particular speaker can be picked out. For outside use it is normally completely enclosed in a long, fat wind shield, looking like a very big cigar. Half-length versions are also available which have a polar response midway between a club shape and a hypercardioid. All versions, however, tend to have a rather wider pickup at low frequencies.

Parabolic microphone

An alternative method of achieving high directionality is to use a parabolic dish, as shown in Figure 3.8. The dish has a diameter usually of between 0.5 and 1 meter, and a directional microphone is positioned at its focal point. A large ‘catchment area’ is therefore created in which the sound is concentrated at the head of the mic. An overall gain of around 15 dB is typical, but at the lower frequencies where the wavelength of sound becomes comparable with the diameter of the dish the response falls away. Because this device actually concentrates the sound rather than merely rejecting off-axis sounds, comparatively high outputs are achieved from distant sound sources. They are very useful for capturing bird song, and they are also sometimes employed around the boundaries of cricket pitches. They are, however, rather cumbersome in a crowd, and can also produce a rather colored sound.

Boundary or ‘pressure-zone’ microphone

The so-called boundary or pressure-zone microphone (PZM) consists basically of an omnidirectional microphone capsule mounted on a plate usually of around 6 inches (15cm) square or 6 inches in diameter such that the capsule points directly at the plate and is around 2 or 3 millimeters away from it. The plate is intended to be placed on a large flat surface such as a wall or floor, and it can also be placed on the underside of a piano lid, for instance. Its polar response is hemispherical. Because the mic capsule is a simple omni, quite good-sounding versions are available with electret capsules fairly cheaply, and so if one wishes to experiment with this unusual type of microphone one can do so without parting with a great deal of money. It is important to remember though that despite its looks it is not a contact mic — the plate itself does not transduce surface vibrations — and it should be used with the awareness that it is equivalent to an ordinary omnidirectional microphone pointing at a flat surface, very close to it. The frequency response of such a microphone is rarely as flat as that of an ordinary omni, but it can be unobtrusive in use.

SWITCHABLE POLAR PATTERNS

The double-diaphragm capacitor microphone, such as the commercial example shown in Figure 3.9, is a microphone in which two identical diaphragms are employed, placed each side of a central rigid plate in the manner of a sandwich. Perforations in the central plate give both diaphragms an essentially cardioid response. When the polarizing voltage on both diaphragms is the same, the electrically combined output gives an omnidirectional response due to the combination of the back-to-back car-dioids in phase. When the polarizing voltage of one diaphragm is opposite to that of the other, and the potential of the rigid central plate is midway between the two, the combined output gives a figure-eight response (back-to-back cardioids mutually out of phase). Intermediate combinations give cardioid and hypercardioid polar responses. In this way the microphone is given a switchable polar response which can be adjusted either by switches on the microphone itself or via a remote control box. Some microphones with switchable polar patterns achieve this by employing a conventional single diaphragm around which are placed appropriate mechanical labyrinths which can be switched to give the various patterns.

Another method manufacturers have used is to make the capsule housing on the end of the microphone detachable, so that a cardioid capsule, say, can be unscrewed and removed to be replaced with, say, an omni. This also facilitates the use of extension tubes whereby a long thin pipe of around a meter or so in length with suitably threaded terminations is inserted between the main microphone body and the capsule. The body of the microphone is mounted on a short floor stand and the thin tube now brings the capsule up to the required height, giving a visually unobtrusive form of microphone stand.

STEREO MICROPHONES

Stereo microphones, such as the example shown in Figure 3.10, are available in which two microphones are built into a single casing, one capsule being rotatable with respect to the other so that the angle between the two can be adjusted. Also, each capsule can be switched to give any desired polar response. One can therefore adjust the mic to give a pair of figure-eight microphones angled at, say, 90°, or a pair of cardioids at 120°, and so on. Some stereo mics, such as that pictured in Figure 3.11, are configured in a sum-and-difference arrangement, instead of as a left-right pair, with a ‘sum’ capsule pointing forwards and a figure-eight ‘difference’ capsule facing sideways. The sum-and-difference or ‘middle and side’ (M and S) signals are combined in a matrix box to produce a left-right stereo signal by adding M and S to give the left channel and subtracting M and S to give the right channel. This is discussed in more detail in Fact File 3.6.

A sophisticated stereo microphone is the Soundfield Research microphone. In this design, four ‘subcardioid’ capsules (i.e. between omni and cardioid) are arranged in a tetrahedral array such that their outputs can be combined in various ways to give four outputs, termed ‘B format’. The raw output from the four capsules is termed ‘A format’. The four B-format signals consist of a forward-facing figure-eight (‘X’), a sideways-facing figure-eight (‘Y’), an up-and-down-facing figure-eight (‘Z’), and an omnidirectional output (‘W’). These are then appropriately combined to produce any configuration of stereo microphone output, each channel being fully adjustable from omni through cardioid to figure-eight, the angles between the capsules also being fully adjustable. The tilt angle of the microphone, and also the ‘dominance’ (the front-to-back pickup ratio) can also be controlled. All of this is achieved electronically by a remotely sited control unit. Additionally, the raw B-format signals can be recorded on a four-channel recorder, later to be replayed through the control unit where all of the above parameters can be chosen after the recording session.

The ST250 is a second generation stereo microphone based on soundfield principles, designed to be smaller and to be usable either ‘end-fire’ or ‘side-fire’ (see Figure 3.12). It can be electronically inverted and polar patterns and capsule angles are variable remotely.

MICROPHONE PERFORMANCE

Professional microphones have a balanced low-impedance output usually via a three-pin XLR-type plug in their base. The impedance, which is usually around 200 ohms but sometimes rather lower, enables long microphone leads to be used. Also, the balanced configuration, discussed in ‘Balanced lines‘, Chapter 12, gives considerable immunity from interference. Other parameters which must be considered are sensitivity (see Fact File 3.7) and noise (see Fact File 3.8).

 

Microphone sensitivity in practice

The consequence of mics having different sensitivity values is that rather more amplification is needed to bring ribbons and moving coils up to line level than is the case with capacitors. For example, speech may yield, say, 0.15mV from a ribbon. To amplify this up to line level (775mV) requires a gain of around x5160 or 74 dB. This is a lot, and it taxes the noise performance of the equipment and will also cause considerable amplification of any interference that manages to get into the microphone cables.

Consider now the same speech recording, made using a capacitor microphone of 1mVμB− ¹ sensitivity. Now only x775 or 57 dB of gain is needed to bring this up to line level, which means that any interference will have a rather better chance of being unnoticed, and also the noise performance of the mixer will not be so severely taxed. This does not mean that high-output capacitor microphones should always be used, but it illustrates that high-quality mixers and microphone cabling are required to get the best out of low-output mics.

Microphone noise in practice

The noise coming from a capacitor microphone is mainly caused by the head amplifier. Since ribbons and moving coils are purely passive devices one might think that they would therefore be noiseless. This is not the case, since a 200 ohm passive resistance at room temperature generates a noise output between 20Hz and 20kHz of 0.26μV (μV = microvolts). Noise in passive microphones is thus due to thermal excitation of the charge carriers in the microphone ribbon or voice coil, and the output transformer windings. To see what this means in equivalent self-noise terms so that ribbons and moving coils can be compared with capacitors, one must relate this to sensitivity.

Take a moving coil with a sensitivity of 0.2mVμB− ¹, which is 2mV for 94 dB SPL. The noise is 0.26 μV or 0.00026 mV. The signal-to-noise ratio is given by dividing the sensitivity by the noise:

and then expressing this in decibels:

This is an unweighted figure, and ‘A’ weighting will usually improve it by a couple of decibels. However, the microphone amplifier into which the mic needs to be plugged will add a bit of noise, so it is a good idea to leave this figure as it is to give a fairly good comparison with the capacitor example. (Because the output level of capacitor mics is so much higher than that of moving coils, the noise of a mixer’s microphone amplifier does not figure in the noise discussion as far as these are concerned. The noise generated by a capacitor mic is far higher than noise generated by good microphone amplifiers and other types of microphone.)

A 200 ohm moving-coil mic with a sensitivity of 0.2mVμB− ¹ thus has a signal-to-noise ratio of about 77 dB, and therefore an equivalent self-noise of 94 − 77 = 17dB which is comparable with high-quality capacitor types, providing that high-quality microphone amplifiers are also used. A low-output 200 ohm ribbon microphone could have a sensitivity of 0.1mVμB− ¹, i.e. 6 dB less than the above moving-coil example. Because its 200 ohm thermal noise is roughly the same, its equivalent self-noise is therefore 6dB worse, i.e. 23dB. This would probably be just acceptable for recording speech and classical music if an ultra-low-noise microphone amplifier were to be used which did not add significantly to this figure.

The discussion of a few decibels here and there may seem a bit pedantic, but in fact self-noises in the low twenties are just on the borderline of being acceptable if one wishes to record speech or the quieter types of classical music. Loud music, and mic positions close to the sound sources such as is the practice with rock music, generate rather higher outputs from the microphones and here noise is rarely a problem. But the high output levels generated by close micing of drums, guitar amps and the like can lead to overload in the microphone amplifiers. For example, if a high-output capacitor microphone is used to pick up a guitarist’s amplifier, outputs as high as 150mV or more can be generated. This would overload some fixedgain microphone input stages, and an in-line attenuator which reduces the level by an appropriate amount such as 10–20 dB would have to be inserted at the mixer or recorder end of the microphone line. Attenuators are available built into a short cylindrical tube which carries an XLR-type plug at one end and a socket at the other end. It is simply inserted between the mixer or recorder input and the mic lead connector. It should not be connected at the microphone end because it is best to leave the level of signal along the length of the mic lead high to give it greater immunity from interference.

MICROPHONE POWERING OPTIONS

Phantom power

Consideration of capacitor microphones reveals the need for supplying power to the electronics which are built into the casing, and also the need for a polarizing voltage across the diaphragm of many capacitor types. It would obviously be inconvenient and potentially troublesome to incorporate extra wires in the microphone cable to supply this power, and so an ingenious method was devised whereby the existing wires in the cable which carry the audio signal could also be used to carry the DC voltage necessary for the operation of capacitor mics — hence the term ‘phantom power’, since it is invisibly carried over the audio wires. Furthermore, this system does not preclude the connection of a microphone not requiring power to a powered circuit. The principle is outlined in Fact File 3.9.

It will be appreciated that if, for instance, a ribbon microphone is connected to the line in place of a capacitor mic, no current will flow into the microphone because there will be no center tap provided on the microphone’s output transformer. Therefore, it is perfectly safe to connect other types of balanced microphone to this line. The two 6k8 resistors are necessary for the system because if they were replaced simply by two wires directly connected to the audio lines, these wires would short-circuit the lines together and so no audio signal would be able to pass. The phantom power could be applied to a center tap of the input transformer, but if a short circuit were to develop along the cabling between one of the audio wires and the screen, potentially large currents could be drawn through the transformer windings and the phantom power supply, blowing fuses or burning out components. Two 6k8 resistors limit the current to around 14 mA, which should not cause serious problems. The 6k8 value was chosen so as to be high enough not to load the microphone unduly, but low enough for there to be only a small DC voltage drop across them so that the microphone still receives nearly the full 48 volts. This is known as the P48 standard. Two real-life examples will be chosen to investigate exactly how much voltage drop occurs due to the resistors.

First, the current flows through both resistors equally and so the resistors are effectively ‘in parallel’. Two equal-value resistors in parallel behave like a single resistor of half the value, so the two 6k8 resistors can be regarded as a single 3k4 resistor as far as the 48 V phantom power is concerned. Ohm’s law (see Fact File 1.1) states that the voltage drop across a resistor is equal to its resistance multiplied by the current passing through it. Now a Calrec 1050C microphone draws 0.5 milliamps (=0.0005 amps) through the resistors, so the voltage drop is 3400 × 0.0005 = 1.7 volts. Therefore the microphone receives 48 − 1.7 volts, i.e. 46.3 volts. The Schoeps CMC-5 microphone draws 4mA so the voltage drop is 3400 − 0.004 = 13.6 volts. Therefore the microphone receives 48−13.6 volts, i.e. 34.4 volts. The manufacturer normally takes this voltage drop into account in the design of the microphone, although examples exist of mics which draw so much current that they load down the phantom voltage of a mixer to a point where it is no longer adequate to power the mics. In such a case some mics become very noisy, some will not work at all, and yet others may produce unusual noises or oscillation. A stand-alone dedicated power supply or internal battery supply may be the solution in difficult cases.

The universal standard is 48 volts, but some capacitor microphones are designed to operate on a range of voltages down to 9 volts, and this can be advantageous, for instance, when using battery-powered equipment on location, or out of doors away from a convenient source of mains power.

Figure 3.13 illustrates the situation with phantom powering when electronically balanced circuits are used, as opposed to transformers. Capacitors are used to block the DC voltage from the power supply, but they present a very low impedance to the audio signal.

A–B powering

Another form of powering for capacitor microphones which is sometimes encountered is A–B powering. Figure 3.14 illustrates this system schematically. Here, the power is applied to one of the audio lines via a resistor and is taken to the microphone electronics via another resistor at the microphone end. The return path is provided by the other audio line as the arrows show. The screen is not used for carrying any current. There is a capacitor at the center of the winding of each transformer. A capacitor does not allow DC to pass, and so these capacitors prevent the current from short-circuiting via the transformer windings. The capacitors have a very low impedance at audio frequencies, so as far as the audio signal is concerned they are not there. The usual voltage used in this system is 12 volts.

Although, like phantom power, the existing microphone lines are used to carry the current, it is dangerous to connect another type of microphone in place of the one illustrated. If, say, a ribbon microphone were to be connected, its output transformer would short-circuit the applied current. Therefore 12 volt A–B powering should be switched off before connecting any other type of microphone, and this is clearly a disadvantage compared with the phantom powering approach. It is encountered most commonly in location film sound recording equipment.

CONNECTORS

The 3-pin XLR connector (see Fact File 12.3) is the industry standard for balanced microphones. Increasing activity in the surround sound field has given rise to an AES standard for a 5.1 microphone multiway output socket with pins as outlined in AES65-2012. This consists of a single connector to replace the six XLR outputs of the control box, simplifying connection and greatly reducing panel space needed. Table 3.1 outlines the pin designations. A standard circular 19-pin socket, M16 screw-locking, is employed, one pin being left unused.

RADIO MICROPHONES

Radio microphones are widely used in film, broadcasting, theater and other industries, and it is not difficult to think of circumstances in which freedom from trailing microphone cables can be a considerable advantage in all of the above.

Principles

The radio microphone system consists of a microphone front end (which is no different from an ordinary microphone); an FM (frequency modulation) transmitter, either built into the housing of the mic or housed in a separate case into which the mic plugs; a short aerial via which the signal is transmitted; and a receiver which is designed to receive the signal from a particular transmitter. Only one specified transmission frequency is picked up by a given receiver. The audio output of the receiver then feeds a mixer or tape machine in the same manner as any orthodox microphone or line level source would. The principle is illustrated in Figure 3.15.

Table 3.1 AES Standard for Multipin Surround Sound Microphone Connector

The transmitter can be built into the stem of the microphone, or it can be housed in a separate case, typically the size of a packet of cigarettes, into which the microphone or other signal source is plugged. A small battery which fits inside the casing of the transmitter provides the power, and this can also supply power to those capacitor mics which are designed to operate at the typical 9 volts of the transmitter battery. The transmitter is of the FM type (see Fact File 3.10), as this offers high-quality audio performance.

Frequently, two or more radio microphones need to be used. Each transmitter must transmit at a different frequency, and the spacing between each adjacent frequency must not be too close otherwise they will interfere with each other. In practice, channels with a minimum spacing of 0.2 MHz are used. Although only one transmitter can be used at a given frequency, any number of receivers can of course be used, as is the case with ordinary radio reception.

Facilities

Transmitters are often fitted with facilities which enable the operator to set the equipment up for optimum performance. A 1 kHz line-up tone is sometimes encountered which sends a continuous tone to the receiver to check continuity. Input gain controls are useful, with an indication of peak input level, so that the transmitter can be used with mics and line level sources of widely different output levels. It is important that the optimum setting is found, as too great an input level may cause a limiter (see ‘The compres-sor/limiter’, Chapter 13) to come into action much of the time, which can cause compression and ‘pumping’ noises as the limiter operates. Too weak a signal gives insufficient drive, and poor signal-to-noise ratios can result.

The receiver will have a signal strength indicator. This can be very useful for locating ‘dead spots’; transmitter positions which cause unacceptably low meter readings should be avoided, or the receiving aerial should be moved to a position which gives better results. Another useful facility is an indicator which tells the condition of the battery in the transmitter. When the battery voltage falls below a certain level, the transmitter sends out an inaudible warning signal to the receiver which will then indicate this condition. The operator then has a warning that the battery will soon fail, which is often within 15 minutes of the indication.

The ‘wireless guide’ download available at www.lectrosonics.com provides a huge amount of information about radio microphones.

Licenses

Transmitting equipment usually requires a license for its operation, and governments normally rigidly control the frequency bands over which a given user can operate. This ensures that local and network radio transmitters do not interfere with police, ambulance and fire brigade equipment, etc. In the UK the frequency band for which radio mics do not have to be licensed is between 173.8 MHz and 175 MHz. Each radio mic transmitter needs to be spaced at least 0.2 MHz apart, and commonly used frequencies are 173.8, 174.1, 174.5, 174.8 and 175.0MHz. An additional requirement is that the frequencies must be crystal controlled, which ensures that they cannot drift outside tightly specified limits. Maximum transmitter power is limited to 10 milliwatts, which gives an effective radiated power (ERP) at the aerial of 2 milliwatts which is very low, but adequate for the short ranges over which radio mics are operated.

In recent years the VHF frequencies have been largely replaced by those in the UHF band, and these have proved to be notably free from interference problems. Aerials are correspondingly shorter, and thus more convenient to wear. Frequencies are mainly in the 800 MHz band, and frequencies for which a UK license is not required are between 863.1 and 864.9 MHz. The Joint Frequency Management Group Ltd is the body which administers licenses in the UK, and their website www.jfmg.co.uk provides much useful information on the subject. The situation in the USA is considerably more complicated, with both licensed and widespread unlicensed use of wireless microphones, often in the ‘white space’ between other services such as local television channels. There is increased competition for the bandwidth previously used by wireless microphones, from services such as commercial data networking and homeland security. The digital television transition in 2009 also forced licensed wireless microphones onto frequencies below 698 MHz. However, this is accompanied by an FCC ruling that allows continued use of wireless microphones on unused television band channels provided that they meet very strict technical criteria that will limit their potential for interference.

Digital radio microphones

Virtually everything in the audio chain is now digital, but radio mics have been slow to adopt this technology. It has been partly due to difficulties in design, ensuring that bandwidth for a given frequency is sufficiently small to enable a large number of channels to be accommodated across a given frequency range, with any data reduction techniques being used maintaining high sound quality and low latency. Also, because high quality UHF analog systems have performed excellently, the need to go digital has been rather less compelling than within other applications. Nonetheless, digital radio microphone systems are now increasingly prevalent.

Analog FM transmission involves the frequency modulation of a carrier wave, the modulation index (how far above and below the carrier wave frequency the signal can deviate) defining the minimum possible frequency spacing of adjacent channels, and hence the maximum number of channels that can be fitted into an allocated frequency band. Other issues such as inter-channel interference further limit the practical number of usable channels. Digital transmission can be accomplished in a variety of ways. The carrier wave can be modulated by the digital code using simple amplitude shift keying (ASK) where a 1 modulates the carrier wave up in level, and a 0 modulates it down in level. This is equivalent to analog AM radio. With frequency shift keying (FSK) the 1s and 0s modulate the frequency of the carrier wave up or down. This is equivalent to the analog FM system. Phase shift keying (PSK) modulates the phase of the carrier wave. Quadrature phase shift keying (QPSK) is a more complex technique which allows a high bit rate to be accommodated in a manageably small bandwidth, and this is the technique normally used for digital radio microphones. Required bandwidth for a particular channel is further kept to a minimum by using data reduction techniques similar to MP3 and other broadcasting systems so that a usefully large number of channels can be fitted into the allowable frequency band. As with analog systems, certain combinations of frequency are best avoided. Bandwidth and data reduction techniques cause latency (delay) of up to 4 or 5 milliseconds in commercial systems. This in itself is insufficient to cause noticeable loss of synchronization between sound and vision, but it can become significant if the signal passes through other equipment such as digital mixers and processors which themselves have latency, and the cumulative effect can become noticeable in some situations.

Other approaches have been marketed. One manufacturer uses uncompressed coding in its SpectraPulse system (Audio-Technica). It operates in a band above 6 GHz using UWB (Ultra Wide Band) transmission. 24 kHz sampling with 16-bit resolution gives 14 channels of uncompressed audio with a data rate of 8Mbit/s− ¹. The penalty is an audio bandwidth of 12 kHz. Lectrosonics’ ‘digital hybrid’ system encodes the audio digitally but transmits it as an analog data signal via conventional FM transmission. A compressed error signal is sent to the receiver using FM, representing amplitude-compressed deviations between the input signal and a computed extrapolation of that signal. In the receiver a similar predictor reconstructs the signal by using the transmitted error signal. Conventional digital transmission mutes abruptly when signal strength falls below a certain threshold, but such a hybrid system tends to fade into noise in a similar manner to an analog FM system.

The Joint Frequency Management Group (JFMG) (www.jfmg.co.uk) regulates permitted frequencies and issues appropriate licenses; currently, the frequency allocation for digital radio microphones across Europe is 470–865 MHz, enabling up to at least 32 digital radio mic channels to be accommodated, the exact number depending on how the manufacturer has specified the system with regard to required bandwidth for each channel and the combinations of frequencies that will work well together. The user is however advised to consult the JFMG’s website for the latest situation as frequency allocations can be altered as situations develop. In the USA, permitted frequencies occupy a similar range, but again the user is advised to enquire which band of frequencies is appropriate for a particular region or city; the Shure website (www.shure.com) for instance, gives helpful information.

For licensing, it does not matter in principle whether the radio mics are analog or digital, it is the frequencies over which they operate that is the issue. The following information on aerials, diversity and the like applies equally to both types.

Aerials

The dimensions of the transmitting aerial are related to the wavelength of the transmitted frequency. The wavelength (λ) in an electrical conductor at a frequency of 174.5 MHz is approximately 64 inches (160 cm). To translate this into a suitable aerial length, it is necessary to discuss the way in which a signal resonates in a conductor. It is convenient to consider a simple dipole aerial, as shown in Figure 3.16. This consists of two conducting rods, each a quarter of a wavelength long, fed by the transmitting signal as shown. The center of the pair is the nodal point and exhibits a characteristic impedance of about 70 ohms. For a radio mic, we need a total length of λ/2, i.e. 64/2 = 32 inches (80 cm).

A 32-inch dipole will therefore allow the standard range of radio mic frequencies to resonate along its length to give efficient radiation, the precise length not being too critical. Consideration also has to be given to the radiated polar response (this is not the same as the microphone’s polar response). Figure 3.17 shows the polar response for a dipole. As can be seen, it is a figure-eight with no radiation in the directions in which the two halves are pointing. Another factor is polarization of the signal. Electromagnetic waves consist of an electric wave plus a magnetic wave radiating at right angles to each other, and so if a transmitting aerial is orientated vertically, the receiving aerial should also be orientated vertically. This is termed vertical polarization.

The radio mic transmitter therefore has a transmitting aerial of about 16 inches long: half of a dipole. The other half is provided by the earth screen of the audio input lead, and will be in practice rather longer than 16 inches. The first-mentioned half is therefore looked upon as being the aerial proper, and it typically hangs vertically downwards. The screened signal input cable will generally be led upwards, but other practical requirements tend to override its function as part of the aerial system.

Another type which is often used for hand-held radio mics is the helical aerial. This is typically rather less than half the length of the 16 inch aerial, and has a diameter of a centimeter or so. It protrudes from the base of the microphone. It consists of a tight coil of springy wire housed in a plastic insulator, and has the advantage of being both smaller and reasonably tolerant of physical abuse. Its radiating efficiency is, however, less good than the 16-inch length of wire. At the receiver, a similar aerial is required. The helical aerial is very common here, and its short stubby form is very convenient for outside broadcast and film crews. A 16-inch length of metal tubing, rather like a short car aerial, can be a bit unwieldy although it is a more efficient receiver.

Other aerial configurations exist, offering higher gain and directionality. In the two-element aerial shown in Figure 3.18 the reflector is slightly larger than the dipole, and is spaced behind it at a distance which causes reflection of signal back on to it. It increases the gain, or strength of signal output, by 3 dB. It also attenuates signals approaching from the rear and sides. The three-element ‘Yagi’, named after its Japanese inventor and shown in Figure 3.19, uses the presence of a director and reflector to increase the gain of a conventional dipole, and a greatly elongated rectangle called a folded dipole is used, which itself has a characteristic impedance of about 300 ohms. The other elements are positioned such that the final impedance is reduced to the standard 50 ohms. The three-element Yagi is even more directional than the dipole, and has increased gain. It can be useful in very difficult reception conditions, or where longer distances are involved such as receiving the signal from a transmitter carried by a rock climber for running commentary! The multi-element, high-gain, highly directional UHF television aerial is of course a familiar sight on our roof-tops.

These aerials can also be used for transmitting, the principles being exactly the same. Their increased directionality also helps to combat multipath problems. The elements should be vertically orientated, because the transmitting aerial will normally be vertical, and the ‘direction of maximum sensitivity’ arrows on the figures show the direction in which the aerials should be pointed.

Another aerial type which has proved very useful where a large number of radio mics are being used, for example in big musical productions, is the log-periodic type. It covers a considerably wider bandwidth than the aerial types previously discussed. This type is rather like a dipole but with an array of graduated dipole elements along a boom, the longer ones to the rear, progressing down to shorter ones at the front. The appropriate pair then resonate according to the frequency of the transmission, and by these means a single aerial (or, more usually, a pair in a diversity system) can cover the wide band of frequencies over which the numerous transmitters operate. The presence of other elements behind and/or in front of a particular resonating pair gives the aerial a cardioid-like polar response, which is useful for pointing the aerial to the area of desired coverage. Superficially it resembles a Yagi but is fundamentally different in two respects. First, the pairs of elements are somewhat different in length from their neighbors; and second, each pair is an active dipole in its own right, the two halves being insulated from each other. Usually such aerials are fabricated within a solid flat plate rather like the paddle on the end of a rowing oar.

Another technique for improving the signal-to-noise ratio under difficult reception conditions is noise reduction, which operates as follows. Inside the transmitter there is an additional circuit which compresses the incoming audio signal, thus reducing its overall dynamic range. At the receiver, a reciprocal circuit expands the audio signal, after reception and demodulation, and as it pushes the lower-level audio signals back down to their correct level it also therefore pushes the residual noise level down. Previously unacceptable reception conditions will often yield usable results when such transmitters and receivers are employed. It should be noted though that the system does not increase signal strength, and all the problems of transmission and reception still apply. (Noise reduction systems are covered further in Chapter 7.)

Aerial siting and connection

It is frequently desirable to place the receiving aerial itself closer to the transmitter than the receiver, in order to pick up a strong signal. To do this an aerial is rigged at a convenient position close to the transmitter, for example in the wings of a theater stage, or on the front of a balcony, and then an aerial lead is run back to the receiver. A helical dipole aerial is frequently employed. In such a situation, characteristic impedance must be considered. As discussed in ‘Principles’, Chapter 12, when the wavelength of the electrical signal in a conductor is similar to the length of the conductor, reflections can be set up at the receiving end unless the cable is properly terminated. Therefore, impedance matching must be employed between the aerial and the transmitter or receiver, and additionally the connecting lead needs to have the correct characteristic impedance.

The standard value for radio microphone equipment is 50 ohms, and so the aerial, the transmitter, the receiver, the aerial lead and the connectors must all be rated at this value. This cannot be measured using a simple test meter, but an aerial and cable can be tuned using an SWR (standing wave ratio) meter to detect the level of the reflected signal. The aerial lead should be a good-quality, low-loss type, otherwise the advantage of siting the aerial closer to the transmitter will be wasted by signal loss along the cable. Poor signal reception causes noisy performance, because the receiver has a built-in automatic gain control (AGC), which sets the amplification of the carrier frequency to an appropriate value. Weak signals simply require higher amplification and therefore higher noise levels result.

The use of several radio microphones calls for a complementary number of receivers which all need an aerial feed. It is common practice to use just one aerial which is plugged into the input of an aerial distribution amplifier. This distribution unit has several outputs which can be fed into each receiver. It is not possible simply to connect an aerial to all the inputs in parallel due to the impedance mismatch that this would cause.

Apart from obvious difficulties such as metallic structures between transmitter and receiver, there are two phenomena which cause the reception of the radio signal to be less than perfect. The first phenomenon is known as multipath (see Figure 3.20). When the aerial transmits, the signal reaches the receiving aerial by a number of routes. First, there is the direct path from aerial to aerial. Additionally, signals bounce off the walls of the building and reach the receiving aerial via a longer route. So the receiving aerial is faced with a number of signals of more or less random phase and strength, and these will sometimes combine to cause severe signal cancelation and consequently very poor reception. The movement of the transmitter along with the person wearing it will alter the relationship between these multipath signals, and so ‘dead spots’ are sometimes encountered where particular combinations of multipath signals cause signal ‘drop-out’. The solution is to find out where these dead spots are by trial and error, and re-siting the receiving aerial until they are minimized or eliminated. It is generally good practice to site the aerial close to the transmitter so that the direct signal will be correspondingly stronger than many of the signals arriving from the walls. Metal structures should be kept clear of wherever possible due to their ability to reflect and screen RF signals. Aerials can be rigged on metal bars, but at right angles to them, not parallel.

The other phenomenon is signal cancelation from other transmitters when a number of channels are in use simultaneously. Because the transmitting frequencies of the radio mics will be quite close together, partial cancelation of all the signals takes place. The received signals are therefore weaker than for a single transmitter on its own. Again, siting the receiving aerial close to the transmitters is a good idea. The ‘sharpness’ or ‘Q’ of the frequency tuning of the receivers plays a considerable part in obtaining good reception in the presence of a number of signals. A receiver may give a good performance when only one transmitter is in use, but a poor Q will vastly reduce the reception quality when several are used. This should be checked for when systems are being evaluated, and the testing of one channel on its own will not of course show up these kinds of problems.

Diversity reception

A technique known as ‘spaced diversity’ goes a good way towards combatting the above problems. In this system, two aerials feed two identical receivers for each radio channel. A circuit continuously monitors the signal strength being received by each receiver and automatically selects the one which is receiving the best signal (see Figure 3.21). When they are both receiving a good signal, the outputs of the two are mixed together. A crossfade is performed between the two as one RF signal fades and the other becomes strong.

The two aerials are placed some distance apart, in practice several meters gives good results, so that the multipath relationships between a given transmitter position and each aerial will be somewhat different. A dead spot for one aerial is therefore unlikely to coincide with a dead spot for the other one. A good diversity system overcomes many reception problems, and the considerable increase in reliability of performance is well worth the extra cost. The point at which diversity becomes desirable is when more than two radio microphones are to be used, although good performance from four channels in a non-diversity installation is by no means out of the question. Good radio microphones are very expensive, a single channel of a quality example costing over a thousand pounds today. Cheaper ones exist, but experience suggests that no radio microphone at all is vastly preferable to a cheap one.

RECOMMENDED FURTHER READING

AES, 1979. Microphones: An Anthology. Audio Engineering Society.

Rayburn, R., 2011. Eargle’s Microphone Book: From Mono to Stereo to Surround — A Guide to Microphone Design and Application, third edition. Focal Press.

See also ‘General further reading’ at the end of this book.

USEFUL WEBSITES

www.lectrosonics.com: The ‘wireless guide’ download provides a huge amount of information about radio microphones.

www.jfmg.co.uk: The UK’s radio microphone licensing authority; contains useful information about frequencies and other issues.