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

578 19. Sound ReproductionINNER SUSPENSION(SPIDER)OUTER SUSPENSIONCENTER DOME(DUSTCAP)FRAMEVOICE COILPOLE PIECESCONEMAGNETMAGNETIC GAPFigure 19.4. Elements of an electrodynamic speaker. (Courtesy of JBL Professional.)combines on a single chassis the preamplifier (which acts as a controller) andthe amplifier. A stand-alone amplifier is referred to as a power amplifier and itfunctions in conjunction with a separate preamplifier. A chassis that combines aradio tuner with a preamplifier and a power amplifier is called a receiver. This typeof construction makes for more economical production, with a single-power supplyserving all of these sub-components, and conservation of rack or shelf space; itsprincipal disadvantage is potentially increased heat output from the electronicsand the lack of flexibility in upgrading individual sub-components.The third stage consists of loudspeakers and or headphones. A tremendous varietyof loudspeakers are available in the market. The most common type is theelectrodynamic speaker, which can range from a single-cone type to more elaboratemultidriver units consisting of woofers to reproduce low-frequency sounds,midrange-drivers to handle the frequencies between the low frequencies producedby the woofers and the high frequencies produced by tweeters. A two-way loudspeakersystem divides its program material between a woofer and a tweeter. Athree-way loudspeaker system incorporates mid-range drivers, and four and fivewayunits have also been constructed. In order that the individual drivers receivethe proper frequencies to the exclusion of the program contents outside of theirrespective optimal operating ranges, special types of bandpass filters, or crossovernetworks, are employed to separate out the high frequencies from the signals beingfed into the woofers, to band-pass the mid-frequencies into the mid-range drives,and to channel only the high-frequency portion of the signals into the tweeters.Because much of the acoustical energy is contained within the low-frequency portionof the signals, passage of unfiltered signals into the tweeters can destroy thesedrivers.Figure 19.4 shows the construction of an electrodynamic driver. Three separatebut interrelated subsystems constitute the driver. The motor system consists ofthe magnet, pole piece, front plate, and voice coil. The diaphragm, generally a