6.2. Construction [2]
A cross-sectional sketch of a typical loudspeaker drive unit is shown in Fig. 6.1. The diaphragm (1) is a cone made from a suitably light and stiff material, although most of the stiffness comes from the fact that it is curved. In the center is a dust cap (2), which guards against metallic dust fouling the magnetic gap and prevents sound from the back of the diaphragm leaking through to the outside world. If the loudspeaker were mounted in a bass-reflex enclosure, such leakage could seriously reduce the Q of the port resonance. Attached to the top of the cone is a coil former on which the coil (3) is wound. This coil is located in the gap of a magnetic path, comprising a pole piece (4) and pole plate (5), where the magnetic flux is produced by a permanent magnet (6), which is
held in place by a basket structure (7). The diaphragm is supported at the perimeter and near the voice coil by a surround (8) and spider (9), respectively, so that it is free to move only in an axial direction. The name “spider” originates from the early electrodynamic loudspeakers in which the cone was supported by a spider-like slotted disk that was anchored to the pole piece in place of the dust cap. Apart from this modification and the switch from electromagnets to alnico (aluminum–nickel–cobalt) permanent magnets in the 1930s and then to ferrite magnets in the 1970s (for economic reasons, not performance related), there has been very little change in the construction of electrodynamic loudspeakers since the Rice–Kellogg [3] patent of 1924. We will refer to the spider and surround as the suspension. In general, sound from the back of the cone exits through holes in the basket (7), whereas sound from the back of the dome (2) leaks through the magnetic gap and spider (9), which is often made from a phenolic resin-impregnated textile, before exiting through the basket.
When an audio signal is applied to the electrical connections (10), the resulting current through the voice coil creates a magnetomotive force which interacts with the air-gap flux of the permanent magnet and causes a translatory movement of the voice coil and, hence, of the cone to which it is attached. The movement of the cone in turn displaces the air molecules at its surface thus producing sound waves. Usually, the cone is sufficiently stiff at low frequencies to move as a whole. At high frequencies, however, vibrations from the center travel outward toward the edge in the form of waves. The results of these traveling waves and of resonances in the cone itself are to produce irregularities in the frequency–response curve at the higher frequencies and to influence the relative amounts of sound radiated in different directions. Unless treated, metal cones have relatively low internal damping and tend to produce high Q resonances, but at frequencies higher than paper or polymer cones due to their high ratio of flexural rigidity to density. Care needs to be taken in the choice of surround material and means of attachment to the coil former to minimize such resonances. Paper and polymer cones have greater damping so that the compression waves propagating through the cone from the coil are mainly absorbed at higher frequencies. This leads to an interesting phenomenon whereby the effective radiating area of the cone decreases with frequency, which is beneficial for maintaining a widely dispersed sound field. Eventually, only the dust cap radiates and the stationary cone acts as a horn. We will discuss the vibration modes of the cone later in this chapter.
In Fig. 6.1, the drive unit is shown mounted in a flat baffle (11) assumed to be of infinite extent. Obviously, this is not possible in practice, but it is an ideal configuration which simplifies our analysis of the drive unit. By definition, a baffle is any means for acoustically isolating the front side of the diaphragm from the rear side. For purposes of analysis, the diaphragm may be considered at low frequencies to be a planar piston of radius a moving with uniform velocity over its entire surface. This is a fair approximation at frequencies for which the distance b on Fig. 6.1 is less than about one-tenth
wavelength. The piston in an infinite baffle is the only sound source which gives a uniformly flat far-field on-axis response under constant acceleration, and this phenomenon is explained in Section 6.6.
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