Converters
Two blocks wide, an ear on each half. Tune the ear (like any tunable block) and the output in hertz (sneak + quartz, or a fork set to infrasound). The second ear sits output hertz above the first: a Wave Detector reads both.
Play a note at each ear (A4 and A4 +45 ct give about 12 Hz). The mixer takes the same power from both, as much as the weaker gives, and sounds half of what it took as infrasound. One note alone does nothing: it is the difference that rings.
A quartz crystal in a copper clamp. Tune the ear to a note and the output in kilohertz (sneak + quartz, or a fork on ultrasound). It picks the whole multiple nearest your note and listens exactly there (B5 +20 ct x40 = 40 kHz).
Its efficiency falls with the multiple: 75% up to x36, about 55% at x68, 37% at x150, 18% at x600. High notes pay off. Ultrasound dies in air, so lead it away with a Waveguide touching the multiplier.
Both converters take from their inputs only what is taken from their output; idling, a tenth.