Science In Real Life

Whether it’s of the “Ah, what lovely music!” or the “What is that awful noise?” variety, sound is an integral part of our lives. Most of us don’t go a day without it. We sound out, sound off, turn the sound on, sound the alarms, make sure things are safe and sound, and do or don’t like the sound of things. We listen up, listen in, let me begin. I came to win, battle me that’s a sin and listen to reason when it comes to things like referencing 90s rap songs.

Before we talk about sound, we have to talk about waves. A wave is a way for a thing to transmit energy without the thing itself, or any part of it, actually moving. Suppose I throw a baseball. While in flight, the baseball has what we call kinetic energy (the energy of motion), and that energy is moving along with the ball. The ball breaks a window – I transmitted energy from my hand to the window via the baseball, and then I transmit energy to my feet as I run away. By contrast, if I’m holding one end of a metal rod and the other end is touching the window, I can break the window by giving my end of the rod a good hard whack with a mallet.

What happened? When the mallet connects with the end of the rod, that end of the rod is compressed. The molecules that make up the very edge are pushed in, and so they push on the molecules next to them, which in turn push on yet a third bunch. By the time the second group is pushing on the third group, the first group is back to a normal distance from the second group, so it just sits there. Meanwhile the push, the compression, is still moving down the length of the rod. When it reaches the other end, the compression travels smoothly from the metal to the glass. But when the glass tries to compress itself it merely shatters like the average American Idol contestant’s illusions of talent.

A sound wave is essentially this same process. Only now instead of a mallet and a metal rod I have a taut, vibrating membrane and an atmosphere full of air. Instead of a glass window, I have the human ear. When I send a jolt of electricity into a speaker, the membrane expands out into the air, creating that very same compression. If the membrane contracts and then expands over and over again, you now have a repeating pattern of compressed air layers alternated with rarefied air layers. This auditory layer cake is what we hear as a single tone.

(Note: all of this pushing I’ve been talking about is actually the same electromagnetism that I mentioned in previous essays. Molecules, just like atoms, are surrounded by a cloud of negatively charged electrons. If two molecules get too close, they repel. Follow this to its logical conclusion: all interactions between ordinary sized objects on planet Earth are just groups of molecules repelling each other. In the case of sound, you can recreate the compression wave by using people and personal space instead of molecules and electrostatic repulsion. I will leave this as an exercise to the reader.)

But real life sound is much more complicated than a single tone. To create a single tone, I have to vibrate my speaker membrane back and forth at a single frequency. More complicated sounds result when we hear several frequencies at once. We can also vary amplitude (loudness) and duration, or all three at once, to create sound patterns as crazy and complicated as we wish them to be, limited only by our imagination. Willy Wonka eat your heart out.

Another effect we’ve all experienced is the Doppler effect – this is what happens when the source of sound waves is moving relative to someone hearing the sound. For simplicity let’s go back to a single frequency, a single repeating pattern of compressed/rarefied air. I put that speaker far away from you. The vibrating membrane in the speaker sends out compressed chunks of air at a certain speed, the speed of sound in air. This speed is a property of whatever material the sound is passing through, and for gases like air is dependent primarily on temperature, molecular weight, and humidity.

So the chunks of compressed air hit you at a certain rate. But now you start running towards the speaker. You’re going to impact the compressed air layers at a faster rate. A faster rate is just another way of saying a higher frequency – it will seem to you as though the speaker is emitting a tone of higher pitch. If you start running away, the higher tone having activated bad memories of that time you spent in a government lab, the compressed air layers can’t catch up to you as often, which is a lower frequency.

This is why a car’s engine doesn’t keep the same sound as it drives by. If you had superhuman pitch identification, and a knack for mental math, you could calculate exactly how fast the car is going relative to you just by hearing the doppler shift of the roar of its engine. This is analogous to what astronomers do with light to calculate the relative speed of stars and galaxies; however, the physics of the doppler shifting of light is significantly more complicated, involving not compression of air molecules but of the geometry of space and time. Whoa.