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What about sound synthesis, how do we make sounds?

So, to create sound we apparently need to move an object. Conveniently enough, there are objects widely available that can be moved back and forth in a quick and arbitrary way. They're called speakers. A speaker is basically an electromagnet attached to a mobile cone, in such a way that when a varying electrical signal is fed to it, the cone will move to follow the signal.

The traditional way of creating sounds is to reproduce naturally occuring sounds. This is done using a microphone, which is essentially a reversed speaker, that generates a signal from the sounds it is exposed to. That signal is then either stored or passed directly through an amplifier to one or several speakers. When the speakers receive the signal, they move in the same way as the microphone membrane (cone) moved when it was exposed to the original sound, so the end result is yet again that same sound, but with a small time delay and a modified volume (typically louder).

Reproducing natural sounds is all well and good, but that's not what we're here for. If you want a specific type of sound you'd have to search for a way of generating it using actual physical objects, which can often be close to impossible. Old-school sound effect technicians in movie production have done some remarkable work with hands-on physical sound generation though. Popular examples are the Tie Fighter laser sound from Star Wars (hitting metal wires) and the T-Rex growl in Jurassic Park (large moving machinery, a flight simulator I believe). Obviously there are infinite other effects ranging from explosions to monster noises and of course the ever-popular fist fights.

Most of these aim to capture "life-like" sounds in some way though - things that could potentially be heard in the real world. For imaginary sounds that have no correlation to the real world, it would be handy to bypass the microphone stage completely and just create our own electrical signal for the speakers.

Once again, there are convenient systems in place for this. A computer soundcard has what's called a digital-to-analog converter (D/A or DAC), which is able to generate an electrical signal from a digital number that is given to it. The number must be within a certain range, say -1.0 to +1.0 (with arbitrary precision in between). Generally, zero gives us the rest/idle position of the speaker element, and positive numbers push it forward while negative numbers push it back in the other direction. It doesn't actually matter a whole lot which way it goes though, as the sound you hear will be more or less the same.

So this means our task becomes that of coming up with interesting sequences of numbers to feed into the soundcard. If we just put in one number and never change it, the speaker cone will move to that position and stay there, resulting in maybe a small pop and then silence. Interesting stuff happen only when we repeatedly change the number according to some pattern.

In practical terms, and somewhat ironically, you can't really expect a modern computer to maintain a steady enough timed loop for it to explicitly send out each number to the soundcard. Instead, the standard way of doing it is to supply a long sequence of numbers, maybe a few hundred, to the soundcard at once, and then the soundcard goes through that sequence with exact timing and feeds each number to the D/A converter. This all happens at a predetermined rate, usually at 44100 samples per second (a sample is one of these numbers) for CD-quality audio. This sample rate is sometimes referred to as "Hz", even though it's not technically describing actual back-and-forth (oscillating) motion.