Harmoni from 777

Figure 1. A mechanical filter made by the Kokusai Electric Company intended for selecting the narrow 2 kHz bandwidth signals in SSB radio receivers. It operates at 455 kHz, a common IF for these receivers, and is dimensioned 45×15×15 mm (1+3⁄4×7⁄12×7⁄12 in).

A mechanical filter is a signal processing filter usually used in place of an electronic filter at radio frequencies. Its purpose is the same as that of a normal electronic filter: to allow a range of signal frequencies to pass, but to block others. The filter acts on mechanical vibrations which are the analogue of the electrical signal. At the input and output of the filter there are transducers which convert the electrical signal into, and then back from, these mechanical vibrations.