What Is Pitch?

Introduction

As the text describes, sounds are produced by variations in air pressure. The pressure waxes and wanes, and so we can think of these pressure changes in terms of cycles — from maximum pressure to minimum, and then back to maximum again.

Scientists can, with appropriate instruments, measure how many of these cycles occur in each second of a sound, and this measurement — of cycles-per-second — is the frequency of the sound.

Frequency is measured in units called "Hertz" — abbreviated "Hz" — in honor of the man who figured out many of these facts, but Hertz simply means: cycles per second. (And, again, each "cycle" is simply the change in pressure from maximum to minimum, back to maximum.)

The Experiment

The frequency of a sound is a major determinant of the sound's perceived pitch. The higher the frequency, the higher the pitch.

 Press to hear a 400 cycles-per-second tone — 400 Hz. (This is the pitch many orchestras tune to.) Press to hear a higher frequency tone — 500 Hz. Press to hear a lower frequency — 300 Hz.

Usually, though, moving objects (such as someone's vocal chords, or the vibrating strings of a guitar, or the reed of a clarinet) do not produce a single frequency of sound. Instead, they produce a complex mix of frequencies, and it's this mix that determines the quality (or "timbre") of the sound.

But, with a mix of frequencies, what determines the pitch of the sound? Usually, perceived pitch is determined by the lowest pitch in the group, and this pitch is referred to as the fundamental frequency for that group of sounds.

 Press to hear a mix of frequencies: 200 Hz + 400 Hz + 600 Hz. Press to hear a single frequency, of 200 Hz.

These tones have a very different sound quality, but they have the same perceived pitch — a pitch corresponding to 200 Hz (the fundamental frequency in the first tone, and of course the only frequency in the second sound).

But now we need to add a twist to our theory: Press the buttons below to hear the tones. Immediately below the button is a list of the frequencies we've mixed together to produce each tone.

 Tone 1 Tone 2 Tone 3 200 Hz 200 Hz 600 Hz 400 Hz 800 Hz 600 Hz 1000 Hz

Does Tone 1 sound higher or lower than Tone 2? Or are they the same pitch? If you're not sure, try humming each of the notes. Do you have to go up, go down, or stay the same as you move from one note to the other?

What about Tone 1 and Tone 3?

Conclusion

For most people, these three tones all sound like they have the same pitch, but that doesn't fit with the idea that pitch depends on the lowest frequency in the mix. According to that theory, Tones 1 and 2 should sound like they have the same pitch (both contain a 200 Hz tone). But Tone 3 should sound higher (its lowest component is 600 Hz). But that's not what people perceive.

For the first two tones, pitch is determined by the lowest frequency in the mix. Both contain a 200 Hz tone, and both sound like the same pitch.

For the third tone, things are different. Your ear detects a pattern: One element at 600 Hz, and the next at 800 Hz, a 200 Hz difference. The next element at 1000 Hz; again a 200 Hz difference.

It turns out that (for reasons of physics) most sound-producing bodies end up creating regular patterns of frequencies, with each frequency in the mix a simple multiple of the base (just like in Tone 2 — the first component has a frequency that's one times 200, the second component has a frequency that's two times 200, and the third is three times 200).

Therefore, your ear can make a simple inference: Given the way sounds usually work, if the steps are each 200 Hz, then the base must be 200 Hz. And so your ear "fills in" the "missing element" and hears a frequency in the group that, in truth, isn't there. Your ear "fills in" what is called the "missing fundamental."

In this way, it is possible to mix together two or three high tones, and, from these ingredients, create a very low tone!

As a further illustration of how this works, try pressing the buttons shown below:

 400 Hz 400 & 600 600 Hz 600 & 800 All four! 800 Hz 800 & 1000 1000 Hz

All four, played together, should produce the same pitch as a 200 Hz tone, which, all by itself, sounds like this:

 200 Hz

In this way, we create a low pitch by mixing together high pitches. In essence, this is a ‘recipe' in which the final product (the low pitch you perceive) resembles none of the ingredients we have mixed together.