32

CHAPTER

Acoustics and Audio

IN SOUND RECORDING AND REPRODUCTION, ESPECIALLY WITH MUSIC, FIDELITY SUPERSEDES ALL other considerations. Audio enthusiasts consider low distortion and esthetics—and in some cases massive output power—more important than sheer efficiency.

Acoustics

Acoustics is the science of sound waves. Sound consists of molecular vibrations at audio frequencies (AF), ranging from about 20 Hz to 20 kHz. Young people can hear the full range of AF sound. As they age, people lose hearing sensitivity at the upper and lower frequency extremes.

Audio Frequencies

Musicians divide the AF range into three broad, vaguely defined parts, called bass (pronounced “base”), midrange, and treble. The bass frequencies start at 20 Hz and extend to 150 or 200 Hz. Midrange begins at this point, and extends up to 2 or 3 kHz. Treble consists of the audio frequencies higher than midrange. As the frequency increases, the wavelength, in any particular medium, grows shorter.

In air, sound travels at about 1100 feet per second (ft/s) or 335 meters per second (m/s). The relationship between the frequency f of a sound wave in hertz and the wavelength λ_ft in feet is therefore

λ_ft = 1100/f

The relationship between fin hertz and λ_m in meters is

λ_m = 335/f

These formulas also hold for frequencies in kilohertz and wavelengths in millimeters.

A sound disturbance traveling in sea-level air at 20 Hz has a wavelength of 55 ft (17 m). A sound of 1.0 kHz produces a wave 1.1 ft (34 cm) long. At 20 kHz, a sound wave in the air measures only 0.055 ft (17 mm) long. In other media such as air at extreme altitudes, fresh water, salt water, or metals, the foregoing formulas do not apply.

Waveforms

The frequency, or pitch, of a sound disturbance represents one of several variables that acoustic waves can possess. Another important factor is the shape of the wave. The acoustic waveform determines the timbre (sometimes erroneously called “tone”). The simplest acoustic waveform comprises a sine wave (or sinusoid), in which all of the energy exists at a single frequency. Sinusoidal sound waves rarely occur in nature, but we can synthesize them with specialized audio oscillators. A good artificial example of an audio sinusoid is the beat note or heterodyne produced by a steady, unmodulated carrier in a communications receiver using a product detector.

In music, most of the notes produced by “real-world” instruments have complex waveforms containing energy at a specific fundamental frequency and its harmonics. The simplest examples include sawtooth, square, and triangular waves. The shape of the waveform depends on the distribution of energy among the fundamental frequency and the harmonics. In theory, a sound wave can have infinitely many different shapes at a single frequency such as 1 kHz. As a result, a note at any particular frequency can exhibit unlimited variations in timbre.

Path Effects

A flute, clarinet, guitar, and piano can each produce a sound wave at 1 kHz, but the timbre differs for each instrument. The waveform affects the way that acoustic disturbances reflect from physical objects. Acoustics engineers must consider this variability when designing sound systems and concert halls to ensure that all of the instruments sound realistic everywhere in the room.

Suppose that you have a sound system set up in your living room, and that, for the particular placement of speakers with respect to your ears, sound waves propagate well at 1, 3, and 5 kHz, but poorly at 2, 4, and 6 kHz. This performance-versus-frequency variability will affect how musical instruments sound to you, distorting the sounds from some instruments more than the sounds from other instruments. Unless all sounds, at all frequencies, reach your ears in the same proportions as they come from the speakers, you will not hear the music as it originally came from the instruments.

Figure 32-1 shows a listener, a speaker, and three sound reflectors called baffles. The waves following paths X, Y, and Z, when reflected by the baffles along with the wave following the direct path D, add up to something different, at the listener’s ears, for each frequency of sound. This phenomenon is impossible to prevent. That’s why it’s difficult to design an acoustical room, such as a concert auditorium, that will propagate sound waves effectively at all frequencies for every listener.

32-1   Acoustic waves that follow reflected paths X, Y, and Z combine with acoustic waves that follow the direct path D to produce the sound that a listener hears.

Loudness and Phase

You do not perceive the loudness (also called volume) of sound in direct proportion to the power contained in the disturbance. Your ears and brain sense sound levels according to the logarithm of the actual intensity. Another variable is the phase with which waves arrive at your ears. Phase allows you to perceive the direction from which a sound arrives. Phase can also affect the perceived sound volume.

The Decibel in Acoustics

You’ve learned about the decibel as a unit of relative signal voltage, current, and power. Decibels are also used in acoustics to express relative sound power. If you abruptly change the volume setting in a hi-fi (high fidelity) amplifier so that you can just barely detect the difference in sound loudness, the increment equals plus-or-minus one decibel (±1 dB). If you use the volume control to abruptly double or halve the actual acoustic-wave power coming from a set of speakers, you perceive a change of ±3 dB.

If you expect decibels to have meaning in acoustics, you must have a reference level against which you can measure all other sound power levels. Have you read that a household vacuum cleaner produces 80 dB of sound? This figure represents a sound-power comparison with respect to the threshold of hearing, the faintest sound that a person with good hearing can detect in a quiet room specially designed to have a minimum of background noise.

Phase in Acoustics

Even if only one sound source exists, acoustic waves reflect from the walls, ceiling, and floor of a room. In the scenario of Fig. 32-1, imagine the baffles as two walls and the ceiling. The three sound paths X, Y, and Z will almost certainly differ in length, so the sound waves reflected from these surfaces won’t arrive in the same phase at the listener’s ears. The direct path D (a straight line from the speaker to the listener) always represents the shortest possible route. In this situation, there exist at least four different paths by which sound waves can propagate from the speaker to the listener. In some practical scenarios, there are dozens, hundreds, or thousands of different paths. In theory, there might be infinitely many!

32-2   A basic stereo hi-fi system.

Suppose that, at a certain frequency, the acoustic waves for all paths happen to arrive in exactly the same phase in the listener’s ears. Sounds at that frequency will seem exaggerated in volume. The same phase coincidence might also occur at harmonics of this frequency. This situation can cause problems because acoustic peaks, called antinodes, distort the original sound. At certain other frequencies, the waves might mix in phase opposition, yielding acoustic nulls called nodes or dead zones. If the listener moves a few meters (or even, in some cases, a few centimeters), the volume at any affected frequency will change. A new antinode or node might then present itself at another set of frequencies.

One of the biggest challenges in acoustical design is the avoidance of significant antinodes and nodes. In a home hi-fi system, this task might comprise nothing more than minimizing the extent to which sound waves reflect from the ceiling, the walls, the floor, and the furniture. Acoustical tile can be installed on the ceiling, the walls can be papered or covered with cork tile, the floor can be carpeted, and the furniture can be upholstered with cloth. In large auditoriums and music halls, the problem becomes more complex because of the larger sound propagation distances involved, and also because of the fact that some sound waves, especially at higher audio frequencies, reflect from balconies, chairs, lighting fixtures, and even people in the audience.

Technical Considerations

Regardless of its size, a good hi-fi sound system must have certain characteristics. Let’s look at two of the most important technical considerations: linearity and dynamic range. In acoustics, these terms have almost, but not quite, the same meanings as they do in communications.

Linearity

In acoustics, we define linearity as the extent to which the output waveform of an amplifier constitutes a faithful reproduction of the input waveform. In hi-fi equipment, all the amplifiers must be as linear as the state of the art allows.

If you connect a dual-trace oscilloscope (an oscilloscope that lets you observe two waveforms at the same time) to the input and output terminals of a hi-fi audio amplifier with good linearity, the output waveform shows up as a vertically magnified duplicate of the input waveform. When you apply an input signal to the horizontal scope input and the resulting output signal to the vertical scope input, the display shows a straight, but slanted, line. In an amplifier with poor linearity, the instantaneous output-versus-input function is not a straight line. The output waveform does not represent a faithful reproduction of the input, and distortion occurs. In some RF amplifiers, this state of affairs can be tolerated, but it’s unacceptable in a hi-fi audio system.

Engineers design hi-fi amplifiers to work with input signals up to a certain peak-to-peak amplitude. If the signal amplitude exceeds this limit, the system’s active components (usually transistors) become nonlinear, and distortion takes place. In a hi-fi system equipped with VU or distortion meters, excessive input causes the needles to “kick up” into the red ranges of the scales during peaks.

Dynamic Range

In a sound system, we can define the dynamic range as the ratio of the maximum power output to the minimum power output that the system can deliver while maintaining acceptable performance with low distortion. As the dynamic range increases, the sound quality improves for music or programming having a wide range of volume levels. We express dynamic range in decibels (dB).

At low volume levels, background noise limits the dynamic range of a hi-fi system. In an analog system, most of this noise comes from the audio amplification stages. In a tape recording, we also observe some tape hiss. A scheme called Dolby (a trademark of Dolby Laboratories) can minimize the effects of background noise in analog recording. However, digital recording and reproduction systems produce less internal noise than analog systems do.

At high volume levels, the power-handling capability of an audio amplifier limits the dynamic range. If we hold all other factors constant, we can expect a 100-W audio system to have greater dynamic range than a 50-W system. The speaker size is also important. As speakers get physically larger, their ability to handle high power improves, resulting in increased dynamic range. That’s why serious audio enthusiasts sometimes purchase sound systems with amplifiers and speakers that seem unnecessarily large.

Components

You can set up a hi-fi system in myriad ways. A true audiophile (“sound lover” or serious hi-fi enthusiast) assembles a complex system over a period of time. Following are some basic considerations that can serve as guidelines when choosing system components.

Configurations

The simplest type of home hi-fi system resides in a single box, with an AM/FM radio receiver and a compact disk (CD) player. The speakers are generally external, with short connecting cables. The assets of this so-called compact hi-fi system include small size and low cost. The main limitation is, as you might expect, limited audio output.

More sophisticated hi-fi systems have separate boxes containing individual devices such as:

•   An AM tuner

•   An FM tuner

•   An amplifier or pair of amplifiers

•   A CD player

•   A computer and its peripherals (optional)

The computer facilitates downloading music files or streaming audio from the Internet, creating (“burning”) CDs, and composing and editing electronic music. A high-end system can also include a satellite radio receiver, a tape player, a turntable, or other nonstandard peripheral. The individual hardware units in this type of system, known as a component hi-fi system, are interconnected with shielded cables. A component system costs more than a compact system, but it offers superior sound quality, more audio power, and greater versatility than a compact system. You can tailor the system to your preferences.

Some hi-fi manufacturers build all their equipment cabinets to a standard width and then mount the cabinets, one above the other, in a metal framework called a rack. A rack-mounted hi-fi system saves floor space and makes a system look professional. The rack can be equipped with wheels so that the whole system, except for the external speakers, can be rolled from place to place.

Figure 32-2 is a block diagram of a typical home stereo hi-fi system. You should ground the amplifier chassis to minimize hum and noise, and to minimize susceptibility to interference from external sources. The AM antenna usually comprises a loopstick built into the cabinet or mounted on the rear panel. The FM antenna can be an indoor type, such as television “rabbit ears,” or a directional outdoor antenna equipped with lightning-protection hardware.

32-3   Methods of tone control. At A, a single potentiometer/capacitor combination (X) provides treble attenuation only. At B, one potentiometer/capacitor combination (X) attenuates the treble, and the other (Y) attenuates the bass.

The Tuner

A tuner is a radio receiver capable of receiving signals in the standard AM broadcast band (535 to 1605 kHz) and/or the standard FM broadcast band (88 to 108 MHz). Tuners don’t have built-in amplifiers. A tuner can provide enough power to drive a headset, but it usually takes an external amplifier to provide enough power for speakers.

Modern hi-fi tuners employ frequency synthesizers and digital readouts. Most tuners have numerous programmable memory channels that allow you to select your favorite stations with a push of a single button, no matter where the stations fall within the frequency band. Most tuners also have seek and/or scan modes that allow the radio to automatically search the band for any station with reasonable signal strength.

The Amplifier

In a hi-fi system, an amplifier delivers medium or high audio power to a set of speakers. An amplifier always has at least one input, but more often three or more: one for a CD player, another for a tuner, and still others for auxiliary devices such as a tape player, turntable, or computer. A few milliwatts of input power can produce audio output up to the amplifier’s limit, in some cases hundreds of watts.

Amplifier prices rise with increasing power output and improvements in the dynamic range. A simplified hi-fi amplifier forms the basis for a public-address (PA) system. Popular music bands use massive amplifiers, some of which employ vacuum tubes that offer electrical ruggedness and excellent linearity. Tube-type amplifiers require bulky, massive power supplies that deliver dangerous DC voltages.

Speakers and Headsets

No amplifier can deliver sound having better quality than the speakers allow. Speakers are rated according to the audio power they can handle. It’s a good idea to purchase speakers that can tolerate at least twice the maximum RMS audio output power that the amplifier can deliver. This precaution will ensure that speaker distortion won’t occur during loud, low-frequency sound bursts, and will prevent physical damage to the speakers that might otherwise result from accidentally overdriving them.

Good speakers contain two or three individual units within a single cabinet. The woofer reproduces bass frequencies. The midrange speaker handles medium and, sometimes, treble (high) audio frequencies. A tweeter is designed especially for enhanced treble reproduction.

Headsets are rated according to how well they reproduce sound. Of course, that’s a subjective consideration. Two different headsets that cost the same at retail can, and often do, exhibit huge differences in the quality of the sound that they put out. Not only that, but two different people will likely disagree about the quality of the sound from a given headset.

Balance Control

In hi-fi stereo sound equipment, the balance control allows adjustment of the left-channel volume versus the right-channel volume.

In a basic hi-fi system, the balance control consists of a single rotatable knob connected to a pair of potentiometers. When you turn the knob counterclockwise, the left-channel volume increases and the right-channel volume decreases. When you turn the knob clockwise, the right-channel volume increases and the left-channel volume decreases. In more sophisticated sound systems, you can adjust the balance with two independent volume controls, one for the left channel and the other for the right channel.

Proper balance is important in stereo hi-fi. A balance control can compensate for such factors as variations in speaker placement, relative loudness in the channels, and the acoustical characteristics of the room in which the equipment is installed.

Tone Control

You can adjust the amplitude-versus-frequency characteristic of a hi-fi sound system with a so-called tone control. In its simplest form, a tone control consists of a single knob or slide device. The counterclockwise, lower, or left-hand settings of this control result in strong bass and weak treble audio output. The clockwise, upper, or right-hand settings result in weak bass and strong treble. When you set the control to mid-position, the amplifier exhibits a more or less flat audio response; the bass, midrange, and treble sounds emerge in roughly the same proportions as they existed in the original recorded or received signal.

Figure 32-3A shows how a single-knob tone control can be incorporated into an audio amplifier. The amplifier is designed to exhibit an exaggerated treble response in the absence of the tone control. The potentiometer attenuates the treble to a variable extent.

A more versatile tone control has two capacitors and two potentiometers, as shown in Fig. 32-3B. We insert the series resistance-capacitance (RC) circuit in parallel with the audio output; it attenuates the treble to a variable extent. We insert the parallel RC circuit in series with the audio path; it attenuates the bass to a variable extent. We can adjust the two potentiometers separately, although some interaction occurs.

Audio Mixer

If you simply connect two or more audio sources to the same input terminals of a single amplifier, you can’t expect good results. Different signal sources (such as a computer, a tuner, and a CD player) will likely have different impedances. When connected together, the impedances appear in parallel, causing impedance mismatches for most or all of the sources, as well as at the amplifier input. As a result, you’ll get degradation of system efficiency and poor overall performance.

Another problem arises from the fact that the signal amplitudes from various sources almost always differ. A microphone produces almost no AF power all by itself, whereas some tuners produce enough to drive a pair of small speakers. Connecting both of these components directly together will cause the tuner signal to obliterate the microphone signal. In addition, the tuner output terminals might “see” the microphone as a miniature speaker or headset and damage it by forcing audio energy into it.

An audio mixer eliminates the problems involved with connecting multiple devices to a single channel. First, it isolates the inputs from each other, so that no impedance mismatch or competition exists among the sources. Second, it allows you to independently control the gain at each input, facilitating adjustment of individual amplitudes so that the signals blend in the relative proportions you want.

Graphic Equalizer

A graphic equalizer allows for adjustment of the relative loudness of audio signals at various frequencies so that you can fine-tune the amplitude-versus-frequency output characteristic of hi-fi sound equipment. Serious hi-fi stereo enthusiasts and recording engineers employ these devices, which act as high-level, precision tone controls.

A typical graphic equalizer comprises several independent gain controls, each one affecting a different part of the audible spectrum. The controls are usually slide potentiometers with calibrated scales. The slides move up and down, or, in some cases, left to right. When you set the potentiometers so that the slides are all at the same level, the audio output or response is flat, meaning that no particular range is amplified or attenuated with respect to the whole AF spectrum. By moving any one of the controls, you can adjust the gain within a certain frequency range without affecting the gain outside that range. The positions of the controls on the front panel provide an intuitive graph of the output or response curve.

Figure 32-4 is a block diagram of a hypothetical graphic equalizer with seven gain controls. (Seven isn’t a “magic number”; we use it here only as an example.) The input goes to an audio splitter that breaks the signal into several paths of equal impedance, and prevents interaction among the circuits. The individual signals are fed to audio bandpass filters, each one having its own gain control. In this schematic, the slide potentiometers appear as variable resistors following the filters. Finally, the signals pass through an audio mixer, and the composite goes to the output.

32-4   A graphic equalizer comprises an audio splitter, several selective filters, several gain controls, and an audio mixer.

Engineers face multiple challenges in the design and proper use of graphic equalizers. Each filter gain control must operate independently of all the others. Judicious choice of filter frequencies and responses is important. The filters must not introduce distortion. The active devices, if any, must not generate significant audio noise. Graphic equalizers aren’t designed or built to handle high power, so they must be placed at low-level points in an audio amplifier chain. In a multichannel circuit such as a stereo sound system, a separate graphic equalizer can be installed in each channel path.

Specialized Systems

Mobile and portable hi-fi systems operate at low DC voltages. Typical audio power levels are much lower than in home hi-fi systems. Speakers have smaller sizes as well. In portable systems, headsets often replace speakers.

Mobile Systems

Mobile hi-fi systems, designed for cars and trucks, usually have four speakers. The left stereo channel drives the left front and left rear speakers; the right stereo channel drives the right front and right rear speakers. The balance control adjusts the ratio of sound volume between the left and right channels for both the front and rear speaker sets. Another control adjusts the ratio of sound volume between the front and rear sets.

A mobile hi-fi system has an AM/FM receiver and the capability to reproduce music from recorded or downloaded media. Some older vehicles have systems with cassette tape players. Some new cars and trucks have satellite radio receivers in addition to all the other standard media. One note of caution: Tapes and CDs are heat-sensitive, so you shouldn’t store them in a car or truck and then leave the vehicle out in the sun.

Portable Systems

Portable hi-fi systems operate from batteries. The most well-known is the headphone radio or Walkman. Dozens of designs exist. Some include only an FM radio; some have AM/FM reception capability. Some have a small box with a cord that runs to the headset; others are entirely contained in the headset. Some have multiple media players and satellite radio receivers. The sound quality is usually excellent, although you’ll need a good headset if you want to get the best from them.

Another form of portable hi-fi set, sometimes called a boom box, can produce several watts of audio output, and delivers the sound to a pair of speakers built into the box. A typical boom box has about the same dimensions as a desktop computer tower. It includes an AM/FM radio and various media players. The system gets its name from the loud bass acoustic energy peaks that its speakers can deliver.

Quadraphonic Sound

Quadraphonic sound refers to four-channel audio recording and reproduction. Audiophiles often call it quad stereo or four-channel stereo. Each of the four channels operates independently of the other three. In a well designed quad stereo system, the speakers should be level with the listener, equidistant from the listener, and separated by angles of 90° from the listener’s point of view. If the listener faces north, for example, the left front speaker will lie to the northwest, the right front speaker will lie to the northeast, the left rear speaker will lie to the southwest, and the right rear speaker will lie to the southeast. This geometry provides optimum balance, and also facilitates the greatest possible directional sound contrast.

Hard Recording Media

Methods of recording sound, particularly music, have evolved dramatically since the ascent of digital technology. Existing and historical hard recording media include the compact disk, analog audio tape, digital audio tape, and vinyl disk. Computer flash drives and variants thereof can store digital audio data as well, but they’re more appropriately called soft recording media.

Compact Disk

A compact disk (sometimes spelled “disc”), also called a CD, is a plastic disk with a diameter of 4.72 in (12.0 cm), capable of storing sound, images, computer programs, and computer files. Digital sound, when recorded on the surface of a CD, suffers little from the hiss and crackle that bedevil recordings on analog media because the information on the disk exists in binary form. A bit (binary digit) has either the 1 (high) or the 0 (low) logic state. The distinction between these two states is more clear-cut than the subtle fluctuations in the level of an analog signal.

When an engineer records a CD for hi-fi use, the sound initially undergoes analog-to-digital (A/D) conversion, changing the continuously variable AF waves into digital logic bits. Microscopic craters called pits are then physically burned into the surface of the disk, one pit for each bit. The pits are arranged in a spiral path called a track that would measure several kilometers long if straightened out. Digital signal processing (DSP) minimizes the noise introduced by environmental factors, such as microscopic particles on the disk or random electronic impulses in circuit hardware. A scrambling process can “smear” recordings throughout the disk, rather than burning the pits in a direct sequence.

Compact-disk players recover the sound from a disk without any hardware physically touching the surface. A laser beam scans the disk. The pits scatter the incident beam, but the unpitted plastic surface reflects the beam like a mirror. A digitally modulated beam, therefore, emerges from the disk; a sensor picks up the beam and converts it into electrical currents. These currents proceed to a descrambling circuit, then to a digital-to-analog (D/A) converter, then to a DSP circuit, and finally to the audio amplifiers. Speakers or headphones convert the AF currents into sound waves.

With a CD player, the track location processes are entirely electronic, and they can all be done quickly. Tracks are assigned numbers that you select by pressing buttons. You cannot damage the CD, no matter how much you skip around among the songs. You can move instantly to any desired point within an individual track. You can program the system to play only those tracks you want, ignoring the others.

Analog Audio Tape

We can classify analog audio tape recorders and playback devices as cassette type or reel-to-reel type. Although these systems are obsolete in hi-fi applications, we’ll still encounter them once in a while. A typical audio cassette plays for 30 minutes on each side; longer-playing cassettes allow recording for as much as 60 minutes per side. The longer tapes are thinner and more subject to stretching than the shorter tapes.

A reel-to-reel tape feed system resembles an old-fashioned movie projector. The tape resides on two flat spools called the supply reel and the take-up reel. The reels rotate counterclockwise as the tape passes through the recording/playback mechanism. When the take-up reel fills up and the supply reel empties out, both reels can be flipped over and interchanged for recording or playback on the “other side” of the tape. (Actually, the process takes place on the same side of the tape, but on different paths called tracks.) The speed is usually 1.875, 3.75, or 7.5 inches per second (in/s).

In the record mode, the tape moves past the erase head before anything is recorded on it. If magnetic impulses already exist on the tape, the erase head removes them before recording anything else. This process prevents doubling, or the simultaneous presence of two programs on the tape. In sophisticated tape recorders, you can disable the erase head if you want doubling to occur. The recording head comprises a small electromagnet that generates a fluctuating magnetic field, whose instantaneous flux density varies in direct proportion to the instantaneous level of the audio input signal, thereby magnetizing the tape in a pattern that duplicates the waveform of the signal. The playback head is normally deactivated in the record mode. However, you can use the playback head while recording to create an echo effect.

In the playback mode, neither the erase head nor the recording are activated. The playback head acts as a sensitive magnetic-field detector. As the tape moves past it, the playback head is exposed to a fluctuating magnetic field whose waveform duplicates that produced by the recording head when the audio was originally recorded on the tape. This magnetic field induces weak AF currents in the playback head. These currents are amplified and delivered to a speaker, headset, or other output device.

Digital Audio Tape

Digital audio tape (DAT) is magnetic recording tape on which binary digital data can be recorded. In digital audio recording, tape noise remains minimal because it has an analog nature, and the digital system doesn’t respond very much to analog fluctuations. Some electronic noise arises in the analog amplification stages following D/A conversion, but to a far lesser extent than the noise generated in older, fully analog systems. The reduced noise in DAT equipment provides more true-to-life reproduction than analog methods could ever offer.

With DAT, you can make multi-generation copies with practically no degradation in audio fidelity. (For the same reason, a computer can repeatedly read and overwrite data on a magnetic hard drive.) On DAT, distinct magnetized regions on the tape represent the data bits. While analog signals appear “fuzzy” in the sense that they vary continuously, digital signals come out “crisp”; they’re either totally present or else totally absent. Imperfections in the recording apparatus, the tape, and the pickup head affect digital signals less than they affect analog signals. A well-designed DSP system can eliminate the minute flaws that creep into a digital signal when you record it or play it back.

Vinyl Disk

Vinyl disks were superseded years ago by CDs and Internet downloads, but some audiophiles still harbor a “vinyl love affair”! Some vinyl disks, and the turntables that can play them, have attained value as collectors’ items. The main trouble with vinyl is its susceptibility to physical damage. Even if you try hard to preserve a vinyl disk, it will gradually acquire imperfections that lead to a “scratchy” sound when you play the contents back. In addition, electrostatic effects can produce “crackling” noises when the atmospheric humidity drops. Friction between the stylus, or “playback needle,” which physically rides along a groove in the disk, gives rise to charge buildup and subsequent small discharges that produce the noise.

Vinyl disks require a turntable that spins at selectable speeds of 33 and 45 revolutions per minute (r/min). Common systems include rim drive, belt drive, and direct drive. In the rim drive, a small wheel rotates while in contact with the turntable, thereby causing the turntable to spin at a slower, regulated speed. A belt drive works in much the same way as the fan belt in a motor vehicle functions; the motor drives the belt, which in turn drives the turntable. In a direct drive system, the motor shaft goes straight to the turntable shaft without any intervening gears, wheels, or belts.

Electromagnetic Interference

The term electromagnetic interference (EMI) refers to unwanted phenomena in which appliances, circuits, devices, and systems upset each other’s operation because of EM fields they produce or pick up. When EMI results from an electronic system’s improper response to nearby RF transmitters, engineers call it radio-frequency interference (RFI). Audio systems are particularly vulnerable to EMI effects.

EMI from Computers

A computer will produce wideband EM energy, especially if it has a CRT monitor. The digital pulses in the central processing unit (CPU) can also cause problems in some cases. Hi-fi tuners in close proximity to computers or their peripherals can pick up EM energy from these sources. The EM fields escape the computer through the interconnecting cables and power cords, which act as miniature transmitting antennas.

When you place a hi-fi system and a computer next to each other, such as when you want to amplify streaming audio from the Internet or you want all your electronic systems in one place, you should anticipate EMI when you set the tuner to certain frequencies. Some Internet connection devices, such as cable modems or wireless routers, can also cause EMI. Even cordless telephone sets can sometimes cause trouble.

RFI from Radio and Television Transmitters

Hi-fi sound equipment can malfunction because of strong RF fields from a nearby radio or television broadcast transmitter, even when the transmitter functions according to its specifications and in compliance with the standards set by the Federal Communications Commission (FCC). In these cases, and also in cases involving Citizens Band (CB) radios and amateur (“ham”) radios, the root cause of the problem is rarely a transmitter malfunction. Instead, the trouble usually arises from inferior home-entertainment-equipment design. The EM energy can enter through speaker wires, power cords, the tuner antenna, and cables between an amplifier and externals, such as a CD player or tape deck.

As the number of connecting cables in a home entertainment system increases, the likelihood of interference from an RF field of a given intensity and frequency also increases. In addition, the risk of RFI increases as interconnecting cables grow longer. Good engineering principles dictate minimizing the number of connecting cables, and keeping them as short as possible. If you have excess cable for a given interconnection and you don’t want to cut it shorter, you can coil it up and tape the coil in place. You should also ensure that the entire system has a good electrical ground connection.

Amateur Radio and RFI

If you’re an amateur radio operator (“radio ham”) with a sophisticated or high-powered station, you might find yourself taking the blame for interference to home entertainment equipment, whether the problem is technically your station’s fault or not. In situations of this kind, you should use the minimum amount of transmitter output power necessary to maintain the desired communication. (That’s the law according to the FCC, anyway!) You should make certain that your transmitters are aligned properly, so that they radiate signals only at the frequencies intended. Antenna systems should be located and installed so as to radiate as little energy as possible into nearby homes and other buildings.

Unfortunately, a large proportion of RFI cases result from inadequate or nonexistent built-in protection for home entertainment equipment, particularly a lack of EM shielding. Therefore, you’ll likely have difficulty solving an RFI problem solely on the basis of modifications to your radio station. You may nevertheless mitigate the trouble by reducing your transmitter power, switching to another frequency band, or operating only when the home entertainment equipment is not in use. A compromising attitude can help to secure the cooperation of a neighbor who is experiencing RFI from your amateur radio station.

As an amateur radio operator—even a highly qualified one—you should never attempt to modify a neighbor’s home-entertainment equipment. If something goes wrong with the neighbor’s hardware later, he or she might blame you. Once in a while, a manufacturer of home-entertainment equipment will offer technical support in RFI cases, but as most of us already know, technical-support departments often leave a lot to be desired even when equipment completely breaks down. If you’re contemplating the purchase of a large home-entertainment system from a particular vendor, you might do well to do some research on the Internet and find out how previous users have rated the vendor’s technical support department.

EMI from Appliances and Power Lines

Hi-fi tuners can pick up EMI from appliances, such as vacuum cleaners, light dimmers, heating pads, electric blankets, hair dryers, and television sets. All of these devices contain components that generate electric sparks and/or produce harmonics of the AC utility wave because of nonlinear operation. Utility lines can also radiate considerable EM energy. These fields rarely get strong enough to interfere with consumer electronic systems directly, although they often cause trouble for shortwave radio listeners and amateur radio operators.

Power-line interference arises from high-current electric sparks between points that lie in close proximity on the power line, and that differ greatly in voltage. Engineers call this phenomenon arcing. A malfunctioning transformer, a failing street light, or a defective insulator can arc, generating EMI that can prove difficult to locate and eradicate. Sometimes, the utility company will offer their help. If you live near an amateur radio operator and you suspect that power-line interference is causing trouble with your hi-fi system, chances are good that the “radio ham” is also having trouble with it. The radio amateur’s technical expertise may help you track down the source of the power-line noise if the utility company won’t cooperate.

Gasoline-powered internal combustion engines used in lawn mowers, weed trimmers, snow blowers, cars, trucks, farm implements, and road construction equipment occasionally cause EMI to hi-fi systems. This type of interference resembles power-line EMI, but usually constitutes a less severe problem because of its intermittent or infrequent nature. In most situations of this kind, the offending device is easy to locate.

Other Potential Problems

Unwanted RF mixing can occur in the most unsuspected places. (This type of mixing, also called heterodyning, is not the same thing as the process that takes place in an “audio mixing” console.) At RF, mixing products arise at the sum and difference frequencies of other signals when they combine in a non-linear device. Heterodyning can give rise to a species of RFI that affects radio receivers and hi-fi tuners, and that can be almost impossible to track down and correct. Poor electrical connections in house wiring, plumbing, and exterior metallic structures, such as fences and rain gutters, can generate mixing products and harmonics in the presence of RF fields from multiple radio transmitters in the vicinity.

Intermodulation, a particularly obnoxious form of RF mixing, sometimes occurs in the downtown areas of large cities, where many powerful wireless transmitters operate simultaneously. The number of mixing products and harmonics in these areas can become so great that they constitute broadband RF noise. Intermodulation (sometimes informally called “intermod”) causes false signals in radio receivers, often sounding “hashy” or broken-up. In the worst cases, intermod can completely ruin FM stereo reception.

Interference to hi-fi tuners can sometimes result from harmonic emissions or spurious emissions (output signals at frequencies other than the design frequency, but not harmonically related) from a nearby broadcast, CB, or amateur-radio transmitter. Emissions of this type don’t occur often with well-designed CB and amateur-radio transmitters because these systems employ relatively low power. However, if you live in the shadow of a broadcast or cellular communications tower, you might experience trouble from the harmonics and/or spurious signals generated by the attendant transmitters.

Preventive Measures

You can employ various tactics in your quest to keep stray RF energy out of home entertainment equipment. You can install multiple RF chokes and bypass capacitors in power cords and interconnecting cables. However, you must ensure that these components won’t interfere with the transmission of power, signals, or data through cables. For advice, consult a competent engineer or the manufacturer of the equipment in question. Here’s a word of caution: If you install RF chokes or bypass capacitors, you’ll probably void the equipment warranty if the installation involves internal modification or the cutting of built-in cords or wires.

RF shielding can help to prevent sensitive electronic apparatus from picking up stray RF fields. The simplest way to provide RF shielding for a circuit or device is to surround it with a metal sheet, mesh, or screen and connect the metal to a good electrical ground. Because metals constitute electrical conductors, an external RF field induces electric currents in them. These currents oppose the currents in the RF field. If the metal enclosure (known as a Faraday cage) is well-grounded, it electrically short-circuits the RF energy. Obviously you won’t wrap all your hi-fi gear in aluminum foil or window screening, but if you’re shopping for a new system, particularly a tuner or amplifier, you might also want to buy a metal cabinet for the hardware.

In addition to using metallic enclosures, you should also shield all of your interconnecting cords if you want your system to have optimum protection against RFI. In a shielded cable, all the signal-carrying conductors are surrounded by a tubular copper braid that’s electrically grounded through the connectors at the ends of the cable. The most popular form of shielded cable is coaxial cable. It can replace two-wire cords between an amplifier and other parts of a system.

More Help

The American Radio Relay League (ARRL), 225 Main Street, Newington, CT, publishes books about EMI and RFI phenomena, their causes, and ways to deal with problems when they occur. These publications are intended mainly for amateur radio operators, but high-end audiophiles might also find them useful.

Quiz

Refer to the text in this chapter if necessary. A good score is 18 correct. Answers are in the back of the book.

1.  You measure the maximum output power P_max that a hi-fi system can deliver while maintaining acceptable performance, and then you measure the minimum output power P_min that the system can deliver with good audio audibility and quality. What does the ratio P_max/P_min represent?

(a)  Distortion tolerance

(b)  Dynamic range

(c)  Linearity factor

(d)  Signal-to-noise ratio

2.  You have a pair of hi-fi speakers that can handle up to 150 W RMS at all audio frequencies without producing unacceptable distortion, even on the loudest sound bursts. You want to buy a hi-fi system. What’s the maximum RMS power that your system can produce without overstressing the speakers?

(a)  50 W

(b)  75 W

(c)  100 W

(d)  150 W

3.  Despite their nostalgic value to collectors, vinyl disks

(a)  cause intermodulation distortion.

(b)  can be easily damaged.

(c)  produce unreliable audio.

(d)  All of the above

4.  On a compact disk (CD), sound is recorded in the form of

(a)  tiny pits in the plastic.

(b)  grooves like those on a vinyl disk.

(c)  variations in the color of the plastic.

(d)  tiny bumps on the plastic.

5.  In air at sea level, the wavelength of a pure audio tone at 335 Hz is approximately

(a)  2.0 meters.

(b)  1.4 meters.

(c)  1.0 meter.

(d)  50 centimeters.

6.  What do you call the extent to which the output wave from an audio amplifier “looks like” the input wave (as seen on an oscilloscope, for example)?

(a)  Similarity

(b)  Shape factor

(c)  Linearity

(d)  Amplitude

7.  In a hi-fi amplifier, you can adjust the relative loudness of the bass, midrange, and treble using the

(a)  tone control.

(b)  balance control.

(c)  linearity control.

(d)  audio mixer.

8.  You can minimize a hi-fi power amplifier’s susceptibility to interference from external sources by

(a)  avoiding the use of batteries for power.

(b)  using a graphic equalizer.

(c)  placing a capacitor in series with the input line.

(d)  grounding its chassis.

9.  Most digital AM/FM tuners have

(a)  frequency synthesizers.

(b)  programmable memory channels.

(c)  seek and/or scan modes.

(d)  All of the above

10.  Most children can hear sounds ranging between frequencies of roughly

(a)  40 Hz and 10 kHz.

(b)  20 Hz and 20 kHz.

(c)  10 Hz and 40 kHz.

(d)  10 Hz and 60 kHz.

11.  In a vinyl disk player, which of the following objects, if any, physically touches the disk during sound playback?

(a)  Stylus

(b)  Laser

(c)  Tweeter

(d)  Nothing physically touches the disk.

12.  An audio mixer allows you to

(a)  connect two or more devices (such as a tuner and CD player) to an amplifier that has only one input.

(b)  combine multiple audio outputs to work with a single pair of speakers or a single headset.

(c)  control the tone levels independently for several different AF ranges.

(d)  connect a radio transmitter to the output of an audio amplifier to broadcast AM or FM programs.

13.  When direct and reflected sound waves arrive at a certain point in phase coincidence, then a listener at that point is

(a)  at an antinode.

(b)  in a dead zone.

(c)  at the threshold of hearing.

(d)  at a nonlinear point.

14.  Which of the following devices will convert low-frequency AF current to bass sound?

(a)  Woofer

(b)  Mixer

(c)  Tweeter

(d)  Inverter

15.  What do some people call a portable hi-fi receiver designed for private listening through headphones?

(a)  Stroller

(b)  Sound box

(c)  Walkman

(d)  Boom box

16.  In air at sea level, sound waves travel at roughly

(a)  1100 feet per second (ft/sec).

(b)  550 ft/sec.

(c)  335 ft/sec.

(d)  670 ft/sec.

17.  Shielded connecting cables between the amplifier and speakers help to protect a hi-fi system against

(a)  variations in AC line voltage.

(b)  transients on the AC line.

(c)  ripple in the power supply output.

(d)  EMI from nearby wireless devices.

18.  A sophisticated tone control that allows you to adjust loudness within specific AF ranges is called

(a)  an audio mixer.

(b)  a linearity control.

(c)  a graphic equalizer.

(d)  a level adjuster.

19.  If you halve the frequency of a pure audio tone in air at sea level, then its wavelength

(a)  quadruples.

(b)  doubles.

(c)  stays the same.

(d)  becomes half as great.

20.  If you quadruple the wavelength of a pure audio tone in air at sea level, then its propagation speed

(a)  quadruples.

(b)  doubles.

(c)  stays the same.

(d)  becomes half as great.