Factors Influencing Fundamental Frequency

Is it body size?
The most obvious influence on pitch that comes to mind is the size of the sound-producing apparatus; we can observe from the instruments of the orchestra that smaller objects tend to make higher-pitched sounds, and larger ones produce lower-pitched sounds. So it's logical to assume that small people would make high sounds, and large people would make low sounds. And this assumption is borne out by the facts, at least to an extent. Baby cries have a fundamental frequency (hereafter referred to as Fo) of around 500 Hz (roughly corresponding to the note B₄). Child speech ranges from 250-400 Hz (notes B₃ to G₄, adult females tend to speak at around 200 Hz on average (about G₃), and adult males around 125 Hz (or, B₂).

So, it does seem to make sense to relate body size to Fo, at least at first. But if we think more about it, we know that big opera singers don't always make low sounds; there are very large sopranos, and some rather short, slender basses. So body weight and height can't be a sole determining factor.

Is it laryngeal size?
Perhaps a measurement of something more relevant to the voice source itself, such as the size of the larynx, would be more helpful. Men, on average, have a larynx which is about 40% taller and longer (measured along the axis of the vocal folds) than women, as seen below. But this does not explain all of the difference between male and female Fo. But there is a size difference inside the larynx which explains the full difference...

Vocal fold length
If we assume that the vocal folds are 'ideal strings' with uniform properties, their Fo is governed by this equation:

The key variable here is the length of the part of the vocal folds that is actually in vibration, which we call effective vocal fold length. If we examine this quantity for men and women, we find that men have a 60% longer effective fold length than women, on average, which fully accounts for the difference we see in Fo between the sexes.

Vocal tract length
Along with pitch, another variable used to classify voices is their quality, or timbre. A fuller treatment of Voice Qualities is available in a later section. In the tutorials, Rules for Modifying Vowels, and How the Vocal Tract Filters Sound, we explained that formant frequencies and timbre are related to the length of the vocal tract. Different vocalists have different vocal tract lengths, of course, and speakers and singers can control the tract length to a limited extent by raising or lowering the larynx. A longer tract results in a 'darker'-sounding voice, while shorter tracts will make the voice sound 'brighter'. One must also keep in mind, however, that the length of the vocal tract is largely determined by nature. Some of us are born with necks that are swan-like, while others are quite short.

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