Amplitude is the magnitude of vibration. Sound is vibration,
so its amplitude is is closely related to volume / loudness.
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The getLevel() method takes an array
of amplitude values collected over a small period of time (1024 samples).
Then it returns the Root Mean Square (RMS) of these values.
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The original amplitude values for digital audio are between -1.0 and 1.0.
But the RMS will always be positive, because it is squared.
And, rather than use instantanous amplitude readings that are sampled at a rate
of 44,100 times per second, the RMS is an average over time (1024 samples, in this case),
which better represents how we hear amplitude.
