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My understanding is that a distinct sound will generate waves of identical amplitude and frequency, but that most things (such as people's voices, music intsruments, etc) produce multiple distinct "sounds" (waves) at the same time.

My understanding is that the base wave generated by something that does this (something like a human voice) is called the fundamental, and that all the other "secondary" sound waves/signals generated are referred to as the harmonics. And that the set of waves as a whole (the fundamental and any accompanying harmonics) is referred to as the timbre.

Hence, my understanding is that the "sound" we hear with our ears is really the timbres of the things making the sounds (some type of aggregation/composition of its fundamental plus any other harmonic waves generated).

So to begin with, if my understanding above is incorrect or misled, please begin by correcting me! Assuming my understanding is more or less correct however...

In this video of a digital oscilloscope in use you can see (around 0:40 seconds in) that when the author speaks, only a single wave is produced. Whereas, given my understanding of "sound" as being the timbre, I expected to see multiple waves displayed on the screen (one for the fundamental and any others for the respective harmonic waves).

My question: is this oscilloscope displaying the author's "timbre" wave (some type of aggregating or composition of his voice's fundamental plus any accompanying harmonic waves)? Or is it somehow filtering out the harmonics and only showing his fundamental wave? Or is my understanding of sound waves inherently flawed?

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You have a slight misunderstanding of what a wave is. Your ears (or an oscilloscope) will hear the sound of pressure increasing and decreasing. That's what sound is. The air in your ear can really only move in or out, so there is only one "waveform" it is sensing, no matter how many frequencies are in that waveform. No aggregation is required. It is "aggregated" by default.

If you have a single frequency sine wave, it will be very easy to see a sine wave on the oscilloscope.

If you add other frequencies that are related to that one as integer multiples, then it may look like a sine wave with wiggles on it.

If you add frequencies that are unrelated, the wiggle may look much less like a wave and more like random noise.

But in all cases, it is still 1 wave. You can only see those frequencies in the wave by using something like a spectrum analyser or fourier transforms to extract them. Our ears do this very well, which is how we understand things like octaves (where one frequency is double another one, etc)

Here's an example from cauk.tv

enter image description here

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  • That's an awesome explanation @Rory Alsop (+1) -- thank you! A few quick followup questions to what you just said, if you don't mind: (1) just to confirm, in the diagram above, you're saying that the "Total" signal/wave is what I'd be seeing on the oscilloscope, and to decompose it into its constituent waves (its fundamental and 1+ harmonics), I would need to perform (like you say) Fourier transforms or similar methods, correct? (2) is there a name for the "Total" wave? How does the "Total" wave compare to the "timbre"? Dec 24 '20 at 17:08
  • And finally (3) if there were multiple noise sources (say 2 different people talking at the same time, and perhaps a dog barking in the background as well) is it fair to say that each noise source would have its own fundamental and its own harmonics? Meaning, 2 humans + 1 dog would be a total of 3 fundamentals, 3+ harmonics and all of them combined in a single wave to produce the "Total" wave in the oscilloscope? Thanks again! Dec 24 '20 at 17:08
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    @hotmeatballsoup 1) yes, 2) the total waveform is indicative of the timbre...a particular timbre will have a unique waveform. and 3) yes. Generally our ears are reasonably good at separating out the 2 humans + 1 dog by using the phase difference between our ears to locate each group of sounds
    – Rory Alsop
    Dec 24 '20 at 17:20
  • Just a wee bit of an addition to the last comment of Rory Alsop. It is actually our brain that manages to distinguish the three sources (each with its distinct timbre/waveform). The ear more or less performs the "analysis" and the data are processed at a higher cognitive level. So, Interaural time and level differences, as well as other psychoacoustical mechanisms are utilised at a higher level in our neural system to separate sources and do other functions constituting phenomena such as "best ear", "cocktail party", etc. Still, the comment is correct and I am just trying to add some info.
    – ZaellixA
    Dec 24 '20 at 22:40
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    You are correct @ZaellixA - thank you. I was over simplifying a bit too much.
    – Rory Alsop
    Dec 25 '20 at 23:08

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