Synthetic vowel: the pulse train

[Paul C]

Hi all,

In the Simple Synthetic Vowel video, from around 0:38, Simon explains that the pulse train only has energy at periodic intervals (which I understand as representing the periodicity of the vocal fold vibration). I see how the regular peaks represent this over time on the x-axis but I don’t understand what the unit of the y-axis is here – its value is either 0 or 1, is this just an arbitrary on/off rather than some measurement of energy?

Thanks!

[Simon]

I should follow my own rule: Always label both axes!

The pulse train is just a waveform, so it’s in the time domain. You are correct that the horizontal axis is time. The vertical axis should be labelled “amplitude” (which we can think of as sound pressure).

The units of amplitude are arbitrary, and in this example the scale goes from 0 to 1 (all these pulses are positive). We could just as well have labelled it with the sample value (which would be from -32768 to +32767 for a 16bit waveform, and so the pulses would each have an amplitude of 32767).

[Paul C]

I (cautiously!) understand from Wikipedia that “Sound pressure is the difference, in a given medium, between average local pressure and the pressure in the sound wave” – so can we understand the value from 0-1 as an expression of the ratio between these two things? What would it mean for a pulse to have negative energy? Doesn’t negative amplitude => no sound at all?

[Simon]

Yes, what we are plotting on a waveform are deviations from the average pressure. These deviations can be positive (compression = air molecules are closer together than average) or negative (rarefaction = air molecules are further apart than average).

[Simon]

Don’t think of the instantaneous amplitude (i.e., the value of one sample of the waveform) as how loud the sound will be. That is not the case. The cochlea detects variations in pressure, not absolute pressure. So, it’s the movement of the waveform “up and down” that is important, and not the actual value of individual samples.

The pulse train could just as well be written as having the value -1 almost all the time and then going to 0 for a single sample at each pulse. It would sound the same.

[Author]

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