Acoustic Filters to Aid Digital Voice*
James L. Flanagan · Bell System Technical Journal · 1979
Digital conversion of speech requires that the signal be band-limited. 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 band-limit specifications for the D-channet bank.