Music is not a series of tones. Instead, it's a series of transients—momentary changes in current, voltage or frequency. What's interesting to me, given this fact, is our measurement equipment looks not at transients, but steady state tones—exactly what we don't listen to on all but the lowest of frequencies. One of the fundamental tests we perform on any piece of equipment is square wave response. Square waves have quick rising transients, then hang out in steady state, only to turn off quickly. Here's a picture of a square wave. Music doesn't normally contain square waves. However, they are handy to test how an amplifier responds to transient stimulus. We're happiest when square waves look like what we started with, unhappy when they have excessive ringing as in the next example. This is an extreme example of square wave ringing—it looks like a spring. This is an unhappy amplifier. An actual amplifier response to a transient stimulus might look more like this: There are other means of looking at transients called impulse responses, but they go beyond the scope of this post. The takeaway here is how sensitive the ear is to transients. We can't tell much from steady state tones, like the ones our test equipment uses to pass judgment, but we can hear tiny changes in transients. Music is far more complex than what might at first seem like simple duty to reproduce.
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