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The Science of Sound

How do sound waves work? How are they measured? And why can't we hear dog whistles? Everyday Einstein explains all about sound. 

By
Lee Falin, PhD
March 23, 2014
Episode #093

                      soundwave

When I was in high school, a fellow student brought in a whistle one day. He claimed it was a dog whistle. Everyone knew that humans couldn’t hear dog whistles, only dogs could. He then said “Watch, I’ll prove it.” Then he blew the whistle. It turned out that the whistle wasn’t actually a dog whistle, just a regular one. But when people would say they could hear it just fine, he’d say “I know. It’s a dog whistle.” 

You might have guessed already that this kid wasn’t very popular, but this story gives a great introduction to talk about sound. 

Catch a Wave

If you’ve ever seen a picture of sound, you’ve probably seen it represented as a series of waves. Sound waves are produced when an object vibrates, causing vibrations in whatever is surrounding that object.

So if you pluck a guitar string, that string will vibrate, and the vibrating string pushes against the surrounding air molecules, causing them to vibrate. Those molecules push against the ones next to them, and so on, until the vibrating molecules reach your ear drum, and you can determine whether or not the guitar is in tune. 

What’s the Frequency, Kenneth?

There are a few different properties of sound waves that determine exactly what they sound like. The first of these is frequency. If you imagine sound as a series of waves traveling through the air, the frequency would be how many waves pass a point in a given amount of time. With sound, we usually measure frequency in Hertz, which is the number of waves passing a fixed point every second. 

The more waves passing per second, the higher the frequency. Dog ears can detect sounds with a higher frequency than human ears can. As you get older, the range of frequencies you can hear starts to drop off. This process is accelerated if you damage your ears by listening to loud music. (For the biology behind this, check out this article).

Sometimes people refer to frequency as “pitch.” The difference is that frequency is an absolute measurement while pitch is a relative one. A given sound may seem to have a higher or lower pitch than another sound, but which sound seems higher or lower can sometimes vary from person to person. In contrast, frequency is an exact measurement.

Crank Up the Amps!

Another measurement that involves sound is something called amplitude. The amplitude of a sound wave is the height of the wave. Amplitude can also be thought of as the amount of energy a wave contains.

If you push a key on the piano, it causes a small hammer to strike a wire inside the piano. That wire vibrates, producing vibrations in the surrounding air, which our ears interpret as sound. The harder you push the piano key, the harder the hammer strikes the wire. The added force causes an increased amount of energy in the resulting vibrations. If you could see the waves coming off the wire, you would see that the waves generated by the harder strike have a larger amplitude, or wave height.

Amplitude is important in sound, because the higher the amplitude, the louder the sound seems to be when it reaches our ears.

Deci-Who?

Sound amplitude, or intensity, is often measured in decibels, named for Alexander Graham Bell. (For details on why it’s a deci-bel and not just a bel, see this article). Decibels can be a little confusing, because they’re often used differently in different contexts. 

The first thing to know is that on the decibel scale, the quietest sound the human ear can hear is set as a reference of 0 decibels (or dB). As you go up the decibel scale, sounds get louder. For example a lawnmower has a sound intensity of about 90 dB. 

If you have a digital amplifier, you might notice that your volume is measured in negative decibels. How does that relate? For most amplifiers, the highest intensity output the amplifier can generate before you hear distortion is marked as 0 dB. So a reading of -60 dB on your amplifier means an output volume or intensity of 60 dB below the level where distortion would occur. That’s why turning up your volume gets you closer to zero decibels. 

Conclusion

So now you know more about how to talk about sound waves. To review, frequency is the measurement of how many waves pass a given point in a certain amount of time, and the frequency of a sound determines if you can hear it or not. 

The amplitude of a sound wave is how high the crests of the wave are, and tells you how much energy or intensity the wave contains. Waves with higher amplitudes have a higher volume than waves with a lower amplitude. 

Sound waves: Low pitch image, Tess Watson at Flickr. CC BY 2.0.

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