Acoustic Filters to Aid Digital Voice

Acoustic Filters to Aid Digital Voice1" By J. L. FLANAGAN (Manuscript received September 19, 1978) Digital conversion of speech requires that the signal be bandlimited. The band limitation customarily is accomplished by electrical filtering. Typically, the cost of electrical filtering is a noticeable fraction of the total cost of voice digitization. We propose here that a large part, if not all, of the requisite filtering can be economically accomplished through acoustic design of the transducer housing (both at transmitter and receiver). We derive a design technique for voice-band acoustic filters, and we compute and construct several prototype filters. We make free-space acoustic measurements on the prototypes, used in combination with electret microphones. Finally, we demonstrate that the acoustic filters can approximate the bandlimit specifications for the D-channel bank.