Sound is energy in motion

Suppose you're sitting in a room with a friend who has a large drum that she bangs from time to time with a large stick so, every so often, you hear a drum beat. What sort of science is going on here? Playing and hearing the drum actually involves a series of steps in which energy is converted from one from to another.

A marching navy drummer hits a drum with his sticks

When your friend lifts her drum-stick, she gives her arm (and the stick) potential energy (the ability to do something). When she lowers her arm, moving it back toward the drum skin at some speed, it has kinetic energy (the energy something has because it's moving). As the drum stick contacts the taut drum skin, the skin soaks up most of the energy and starts to vibrate. In other words, it has the kinetic energy now. As the skin vibrates, it pushes the air molecules that are in contact with it. The air molecules vibrate too, with each molecule causing neighboring molecules to start vibrating as well. Before long, all the air molecules in the room are vibrating. Some of them vibrate right next to your ear; others vibrate in your ear canal. Inside your ear, the vibrating air molecules make tiny hairs vibrate. The hairs stimulate nerve cells, which send signals to your brain—and your brain perceives these signals as sounds.

Photo: A marching drummer is firing sound energy off in all directions. Photo of drummers from from the Royal Australian Navy by William R. Goodwin courtesy of US Navy.

In short, we can think of sounds as waves of energy traveling from something that is moving back and forth (vibrating or oscillating) to our ears. The waves travel by alternately squeezing and stretching the air; if there's no air in the room, they cannot travel at all. That's why you can't hear sounds in space, where there's no air, or traveling in a vacuum. If you could see sound waves moving, you'd see the air squeezing and stretching all over your room like an old-fashioned concertina. In science, the squeezed parts of the air are known as compressions (because the air molecules are pressed together) and the stretched parts are called rarefactions (because the air molecules are thinned out and less dense).

Two key features of a sound wave control what it sounds like to us. The frequency (how many times the wave vibrates in one second) is broadly related to the pitch of the sound we hear. So we hear a high-frequency sound as having a higher pitch. In other words, a choir boy's voice produces a mixture of sound waves of generally higher frequency than an adult man's voice. The amplitude (volume) of a sound is related to the amount of energy that the sound waves carry. When you bang a drum hard, you make more energetic sound waves with more amplitude that you hear as louder sounds.

Read more in our main article about sound.

What makes one instrument sound different from another?

When two instruments play exactly the same musical note, at roughly the same volume, they can sound completely different. How can that be if they're producing the same sound waves? The answer should be obvious: they're not producing the same sound waves! We can use an oscilloscope (an electronic graph-drawing machine, a bit like a cathode-ray TV, only it shows pictures of what waves look like) to see the difference.

If we play a pure musical note with a tuning fork, the oscilloscope shows an undulating hilly pattern called a sine wave. But if we play the same note with a trumpet, the wave will look more zig-zagged, like the teeth of a saw (it's usually called a saw-tooth wave). Now, if we play the same note again on a flute, we will see triangular waves. The shape of the sound waves , which is controlled by how the instrument pumps energy into the world around it—in other words, how it vibrates and makes the air around or inside it vibrate in sympathy—is one of the things that makes instruments sound different from one another.

There are other factors too. An instrument doesn't just produce a single sound wave at a single pitch (frequency). Even it's playing a steady note, it's making many different sound waves at once: it makes one note (called a fundamental frequency or first harmonic) and lots of higher, related notes called harmonics or overtones. Playing together, the harmonics make a dense, complex sound a bit like a barber's shop choir, with low voices and high voices all singing in tune. The more harmonics there are, the richer the sound.

A third factor that makes instruments different is the way the sound waves they make change in volume (amplitude) over time. Instruments don't make sounds the way lamps make light: it's not "all" or "nothing." If you press a piano key and release it, the sound changes volume gradually over time. First, it rises quickly (or "attacks") to its maximum volume. Next, the sound "decays" to a lower level and stays there or "sustains." Finally, when we let go of the key, the sound "releases" and dies down to silence. Graph of ADSR sound amplitude envelope In a piano, the attack phase is fairly slow and the sustain phase can be really long as the notes take a long time to die away. But with a flute, the attack phase is quicker and sharper, there is little decay, the sustain continues for as long as the flautist keeps blowing, and the release is also very fast. The changing pattern of sound volume plays a huge part in what makes one instrument sound different from another. We call the pattern of attack, decay, sustain, and release the ADSR envelope shape.

Picture: An ADSR envelope shows how the volume of a musical note changes with time. When a sound plays, it attacks to a maximum volume, decays to a lower level, sustains or holds at that level for a while, then releases back to silence.

How synthesizers work

Now we understand the theory of how sound works, and how different instruments produce it in different ways, we know enough to build ourselves a synthesizer. You can probably see already that a machine that can copy the sounds of virtually any other instrument would need to be able to:

Generate sound waves of different shapes.
Generate more than one sound tone at once to produce a fundamental frequency and harmonics.
Make the volume of the sound change over time to produce different ADSR envelope shapes.

That's pretty much what an electronic synthesizer does in a nutshell. It has a number of different voices or oscillators (sound tone generators), each of which can produce waves of different shapes (sine wave, square wave, saw tooth, triangular wave, and so on). It can combine the waves to make complex sounds, and it can vary the way the sounds attack, decay, sustain, and release to make the sounds mimic existing instruments like pianos.

To make a synthesizer sound somewhere between a piano and an organ, you could select a square wave generator (which gives an organ-like sound) and set the ADSR values to be like those of a traditional piano (slowish attack, quickish decay, long sustain and release). Modern synthesizers have "presets" (ready-programmed settings) or "modes" that let you select particular instruments at the flick of a single switch. Of course, you don't have to copy traditional instruments with a synthesizer: you can change the settings to whatever you like—and create all kinds of sounds no-one has ever heard before.

Analog and digital synthesizers

The original synthesizers achieved all this using laboratory-style electronic equipment that generated and manipulated actual sound waves. Instruments like this are known as analog synthesizers because they work directly with the sound waves themselves. Many of these synthesizers had lots of separate, sound-creating modules that could be connected together in different ways; that's why they were called modular synthesizers.

Screenshot of Timidity software synthesizer

Modern synthesizers do everything digitally, by manipulating numbers with computer chips. Not surprisingly, they're called digital synthesizers. They're essentially computers that have been specially programmed to generate and manipulate sounds. Most synthesizers can be connected up to personal computers, so the computer can be used to store and record the sounds the synthesizer makes or play it automatically. To make this sort of thing easier, computers and synthesizers use a standard way of connecting together known as MIDI (Musical Instrument Digital Interface).

Another kind of digital synthesizer, the sampler, lets you feed in a recorded sound (maybe the noise of a sparrow singing) and then manipulate it in various ways by changing the sound settings. So you can make the sparrow sing more quickly by speeding up the sound, or play the bird-song on your keyboard, so the low notes sound like older, heavier birds and the high notes like younger, smaller, and chirpier ones!

Photo: Timidity is a digital synthesizer that runs on the Linux operating system. There's no electronic equipment here: Timidity is purely a piece of computer software.