What Determines The Pitch Of A Sound?
A sound’s pitch is determined by the speed at which its soundwaves vibrate. When you pluck a guitar string, for example, it vibrates at a given frequency. The faster the frequency, the higher the pitch.
The frequency of a soundwave is measured in hertz (i.e., cycles per second). That’s why an 80Hz tone, for instance, is lower in pitch than a 440Hz tone. The 80Hz tone vibrates 80 times per second, while the 440Hz tone vibrates 440 times per second.
That’s what pitch is all about: the more cycles per second (or hertz) the higher the pitch; the less, the lower the pitch. It sounds simple, but is it enough?
To truly understand what determines pitch, one needs to know a little bit about the physics of sound, take a deep dive into how soundwaves behave and analyze other aspects that can influence the character of a sound.
How does sound work?
Sound is produced whenever an object vibrates. That object’s vibration then causes the surrounding air molecules to vibrate, giving way to a chain reaction. Air molecules bump into one another continuously and eventually reach the human ear. The sound captured by the ear is then processed by the human brain.
Inevitably, sound doesn’t exist without vibration. Even after entering the ear canal, sound only reaches the brain because the vibration of the eardrum causes the malleus, the incus, and the stapes (three tiny bones) to vibrate. In the video below, you can see how it all works.
The way humans perceive sound isn’t exact. We can determine the precise frequency of soundwaves with the help of equipment such as a multimeter, but our ears are inherently biased and perceive sound unequally.
In fact, humans are incapable of hearing frequencies under 20Hz (subsonic frequencies) and over 20.000Hz (supersonic frequencies).
Soundwaves: how does pitch work exactly?
A pitch is merely a specific frequency of a soundwave. But what does this mean?
To have a better understanding of how soundwaves (and, consequently, pitch) work, humans rely on visual representations such as the diagram below.
In the image, the x represents the duration. For convenience, let’s assume that the diagram shows a full cycle of one second. Since there are two “bumps” in there, representing one pulse each, we know that the soundwave in the diagram is a 2Hz soundwave (which vibrates twice per second).
The y represents the amplitude, which is the energy of the vibration. The more energy, the louder the sound. Loudness doesn’t determine pitch, but it influences how pitches are perceived by humans. It also changes the character of the sound significantly.
Finally, the λ represents the wavelength, i.e., the duration of each pulse. If the wavelength is longer, then the pitch of the soundwave is lower. The musical note A, for instance, has a frequency of 440Hz, meaning you’d need 440 λ per x to accurately show it in the diagram.
In the example above, the represented wavelength is extremely long because the soundwave is only vibrating at 2Hz per second. If the soundwave was vibrating at 440Hz per second, then the wavelength would have to be 220 times shorter.
Soundwave diagrams are perfect for understanding the technical aspects of what determines pitch. The only way to truly understand pitch is to see these concepts in action. This video should help to clear things up:
With the help of a high-definition digital camera, this musician managed to accurately capture the vibration of his guitar strings. Notice how the thicker strings (which play lower notes) vibrate at a slower frequency, while the thinner strings (which play the higher notes) vibrate much faster. You can see the wavelength in action in both cases.
The amplitude is well represented too. When the musician strums the guitar with more velocity, you can see clearly that the wavelength is wider. Close to the strings, the camera’s microphone gets to capture the pitches produced by the guitar because the air molecules around the strings vibrate against one another.
Why are sounds with the same pitch different?
Sounds with the same pitch are different because of the overtones. An overtone is any soundwave with a frequency higher than the fundamental frequency, which determines pitch. They’re like “extra pitches” that don’t influence pitch itself (i.e., the note you’re hearing) but the timbre of a sound.
That’s why an E played on the piano sounds different than an E played on the guitar. While both instruments can produce the same fundamental frequency, their overtones are different, as a result of which they sound differently.
Even though the lowest soundwave in a sound determines pitch, virtually all sounds, except pure sinewaves, are composed of multiple soundwaves vibrating at different frequencies. This is why they have different textures and timbres.
As discussed before, the amplitude of a soundwave can also impact the character of a sound. An E played softly on a piano, for instance, is perceived radically differently than an E played strongly on a piano.
What does it mean to have perfect pitch?
People with perfect pitch (also known as absolute pitch) are capable of correctly identifying which musical note they’re hearing instantly. Some people with perfect pitch can even accurately recognize the pitches of non-musical sounds, such as a burst of wind or a car engine.
Many experts believe that perfect pitch is one of the best qualities a musician can have. However, absolute pitch per se doesn’t make you a good composer. In the video below, the YouTuber Adam Neely makes a strong case in favor of the value of relative pitch, which is how people ordinarily perceive pitch.
To sum up, the pitch of a sound is determined by the frequency of the vibrations of the soundwave. The amplitude of the soundwave determines the loudness of the sound. The overtones of a sound determine its timbre.
Can knowing the physics behind how sound works and the technical aspects of how a pitch is produced make you a better musician? Well, maybe not instantly. However, it’s important to know as much as possible about sound when you work as an audio professional.