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Relationship Between Sound Wave Properties & Sound Perception

Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

After completing this lesson, you will be able to describe the properties of sound waves and explain how they relate to what you hear when you listen to a sound. A short quiz will follow.

What are Sound Waves?

You may have heard the saying, 'In space, no one can hear you scream.' But have you ever wondered why?

To answer that question, we first need to ask ourselves: What IS a sound wave? A sound wave is a vibration in the air that occurs when particles hit each other. If something makes a sound, then it must have moved in such a way that the air around it started to vibrate. Those air molecules hit each other and the sound spread throughout the room. Some of those vibrations make it into your ear, causing your eardrum to vibrate, and that's how you hear the sound.

But why do we say that sound is a wave? Molecules hitting each other doesn't sound very... wavy.

Well, if you measured the molecule density in the air (how tightly packed the air particles are) while a wave was passing through it, you would see that there are dense areas followed by sparse areas. They follow each other in an alternating pattern, like a multi-decker air-density sandwich. This happens simply because the sounds we make cause air particles to hit each other, creating a denser compression that travels through the air, collision by collision. If we continue to make sounds, we'll continue to create more and more compressions, with sparse areas behind them.

And if you plot a graph of that air-density sandwich, a graph of density vs. position, you get a perfect wave shape. So vibrations in the air are graphed like a wave, which is where we get the term 'sound waves.'

Air Vibrations are Waves
Air Vibrations are Waves

Sound Wave Properties & Perception

Sound waves, like any kind of waves, have various features or properties. Waves have frequencies, wavelengths, amplitudes, wave speeds, intensities, timbres and directions. Let's talk about the properties that have the biggest impact on how a sound sounds, the properties that affect how you perceive the sound in your brain.

Frequency is a pretty big deal as far as properties go. A high frequency wave is one where there are a lot of vibrations per second, and a low frequency wave is one where there are few vibrations per second.

High Frequency vs Low Frequency
High Frequency vs Low Frequency

But what does that SOUND like? Well, frequency is the same thing as pitch, and is measured in hertz (Hz). A higher frequency sound is perceived as a higher note, like a flute or violin playing a top note. A lower frequency sound is perceived as a lower note, like a cello or tuba.

A Tuba Produces Low Frequency Sound
A Tuba Produces Low Frequency Sound

Another important property is the amplitude. The amplitude is how tall the wave is. The amplitude is measured in meters, and looks like this:

Amplitude
Amplitude

A higher amplitude sound wave is a louder sound. So if you're standing in front of the speakers at a rock concert and your ears are hurting, your problem is the amplitude of the sound. Sound intensity is the same concept as amplitude, but measured in different units. Amplitude is the height of the wave itself, but intensity is a measure of how much energy the wave contains. Since larger amplitude waves contain more energy, it really tells you the same thing: how loud the sound is.

Timbre, or quality of sound, is about the combination of different waves and the way their frequencies are changing relative to each other in little ways. It's subtle and complex. But a high timbre sound will be richer and have more depth.

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