THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

19.3 Playback Audio Equipment 579cone and a dust cap or a one-piece dome, constitutes the second subsystem; thesuspension system, which includes the spider and the surround, constitutes theremaining subsystem.In the motor assembly, the back and front plates and the pole pieces are madefrom a highly permeable material, such as iron, which provides a path for the magneticfield of the ring-shaped magnet that is usually constructed of ceramic–ferritematerial. The magnetic circuit is completed at the gap, with a strong magneticfield existing in the air space between the pole piece and the front plate. The coil,which connects to a pair of input terminals, is wound around a thin cylinder thatis attached to the speaker cone that, in turn, is mounted at its outer edge througha flexible surround to the frame. A spider, essentially a movable membrane alsoattached to the frame, positions centrally the diaphragm at its inner edge and thevoice coil. The signal containing the program material feeds into the voice coil,which generates a change in the magnetic field imposed by the permanent magnetsurrounding the coil. If an alternating current is fed into the coil, the flow of thecurrent in one direction will cause the voice coil to move in one direction, and thereverse flow will cause the coil to move in the opposite direction. The cone underthe impetus of the moving voice coil acts as a piston in moving the air in frontof it.The ideal cone would act as a perfectly rigid piston pushing against the air. Thetransfer of the motion from the piston to the air is bound in terms of frequency byresonance frequency of the cone at the low end (here the ability to transfer energyto the air is limited by mechanical constraints) and by the radiation impedance atthe upper limit. This upper frequency limit occurs from the fact that it is a functionof both the nature of radiation impedance of the air and the radius of the radiatingsurface. Smaller radiating surfaces can reproduce higher frequencies moreeffectively than larger surfaces, which accounts for the smaller sizes of tweeters.Real-world cones, however, are not perfectly rigid and will flex depending on thetraits of the materials they are constructed from. Cone flexure has a critical effecton the high-frequency efficiency, the sound–pressure level output, and driver–polarresponse. Driver materials may differ in degrees of stiffness and transmit vibrationsat different speeds internally, but they tend to produce the same sort of flexuresor modes. In Chapter 6, these modes have been discussed for circular membranesfixed at their outer edges.The most important function of the speaker enclosure is to control transmissionof the driver’s rear-radiated sound energy in order to avoid its mutual cancellationwith forward-radiated energy at low frequencies. The enclosure also acoustically“loads” the driver by providing a suitable acoustic impedance to match the characteristicsof the driver and the requirements of the speaker system with regard tolow frequency and large-signal performance. The enclosure must be made sufficientlyrigid so that the cabinet vibrations and resonances do not add appreciablyto the program material. Two major classes of electrodynamic-driver enclosuresare (1) the infinite baffle/closed box in which the sole radiation source is the driverdiaphragm and (2) the bass reflex that is vented to augment the driver’s radiationat low frequencies. The bass reflex may be a vented system that incorporates a