Negative Impulse Train

[Max W]

I’m confused as to why the impulse train only has positive values. I believe every other audio signal I’ve seen has both positive and negative values.

[Simon]

To record sound, we measure deviations above or below mean (resting) air pressure using a microphone [*]. The vertical axis on an audio waveform plot corresponds to air pressure. That is why waveform samples during silence have values around to zero.

Our idealised impulse train is a waveform where all samples have a value of exactly zero, except for one sample per period which has a positive value. This is the simplest possible signal that contains energy at all multiples of the fundamental frequency.

As you have realised, this idealised signal is not physically possible – a real signal does indeed need to also have regions of below-mean-air-pressure. (It does not need to be symmetric though, so we don’t need “negative impulses” to balance the positive ones.)

[* Actually, microphones vary in precisely what they measure: pressure, pressure gradient across a diaphragm, velocity, …etc. This subtlety is not important to understand here.]

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