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I'm designing a species of tiny humanoids. They are about 13cm tall. Normally that would make them have very high pitched voices but I want them to be able to also speak in a lower pitch.

Siamang gibbons have gular sacs; they are a throat pouch that can be inflated and is used for making resonating calls and songs.

non inflated sac

inflated sac

My design for the tiny humanoid's sacs is quite similar:

non inflated sac → inflated sac

Would it be possible for these tiny humanoids to speak with their sacs inflated and is the design of the gular sac plausible?

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Basic vocal pitch (in humans) is determined primarily by the size of the larynx. We modulate that base tone by changing things like the pressure of the air passing through the larynx and the tension of the vocal cords, but all you need to do to give your tiny humanoids low-pitched voices is give them oversized larynxes with heavier, stiffer vocal cords that resonate at a lower frequency. If you want them to have a broad range (from high voices down through bass tones) you'd need to give them larynxes that are more flexible that those of humans. Most people can manage 2 octaves, trained singers can sometimes manage 4, but it seems like you'd need to give these guys a natural 6 or 7 octave range. For comparison, a typical piano covers a little more than 7 octaves.

A gular sac would not affect pitch directly. If if were attached to the throat below the larynx it would certainly change both the power of the voice and the capacity for sustained production, by increasing the quantity of air available to move over the vocal cords. It might also work as a resonance chamber to give the voice a fuller, richer sound. I expect even low-voiced humanoids of this sort would sound a bit thin and nasally, because their chest cavity is too small: think of the difference in timbre between a ukulele (with its small sound box) and a guitar (with its large sound box). Thus one of your humanoids might speak normally (in a low or high voice) without engaging the gular sac, but then inflate the sac to shout, give a public speech, sing, or to sound more dominant.

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