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Thanks! This reminds me, I've just recently read about old oscillators and the cycles, periods and hertz of electric signals. In oscillators, or clocks, that are used in computers, the signal switches between current - no current. Which isn't the same as switching polarity in AC, but still.
It also reminded me of how insane I find it, that the membranes of speakers - whose vibration is controlled by an electromagnet, if I understand them correctly - are able to vibrate in a fashion that not only makes one sinus wave of one frequency, but sometimes a complex, intricate mixture of sounds, such as when watching a scene from a movie that has soundtrack, ambient sound, speech, explosions, whathaveyou. How on earth can one membrane do that... A piano commonly needs 88 keys whose combination can produce complex harmonies. Speaker membrane: hold my beer.
Yes, in digital logic, ones and zeros are almost always represented as LOW (negative pole of power supply) and HIGH (positive pole of power supply), such as 0 V and 3.3 V, referenced to ground. This is based on properties of both bipolar and CMOS transistors, fundamental logic elements, where zero base-emitter current or zero gate-source voltage means they're non-conductive (I hate using open/closed for obvious reasons). However, high-speed and/or long-distance communication pretty much requires differential signalling, which is pure AC measuring between the two conductors. (Just compare SCART (coaxial analog baseband signal, RGB+bidirectional composite SD A/V) and DP or HDMI (shielded twisted digital differential pairs, unidirectional 4k+ A/V) cables by thickness and bandwidth.) And much like sound, radio waves can only be AC.
A piano/guitar string is plucked and then vibrates at its own natural frequency (plus in practice, higher modes aka harmonics/overtones defined by where it's plucked and mechanical design). Wind instruments are designed to create continuous oscillation from constant flow of air by amplifying reflected waves with incoming air pressure energy (blowing straight into a cylinder won't work, hence the weird pipe shapes, holes and reeds). Either way, they resonate at their design frequency. So do self-oscillating piezo buzzers, they have a tap on the crystal that provides delayed feedback (electrical pulse of the mechanically reflected wave) feeding into a transistor that "kicks" the crystal again at the right time. Meanwhile, a speaker membrane, ideally does not have a resonant frequency (responds equally to disturbances at any frequency between 20 Hz and 20 kHz) and needs to be pushed constantly to create sound. Like the membrane of a mechanical phonograph/turntable, the shape of the wave it should create is delivered to it in real time, except electromagnetically. That's why player pianos need very little data (literal punch cards: one bit per beat and string (ignoring dynamics), so up to about 240 ร 88 โ 2.6 kB per minute, uncompressed, or 240 B/min per channel) to reproduce entire songs as opposed to audio recordings that require samples at decent precision (16 bits is generally good enough) at at least 2x the highest frequency to be reproduced (about 5 MB/min for one CD-quality channel, uncompressed). Yes, if you feed a pressure wave into a tube, string, drum membrane etc., you will be able to hear any sound out of it (although its frequency response will cause distortions unless it's specifically designed with a flat one, like a phonograph horn).