Music and resonance

Musical instruments make tones in several different ways. Here's one. When you play a guitar, you move a string away from its resting position and let it go. Because the string is slightly elastic, it starts bouncing back and forth. It vibrates.

Every time a guitar string moves, it compresses the air on one side and decompresses it on the other. Compressions and rare factions, that's what makes a sound. Which tone is heard depends on the frequency of the vibrations, and that depends on what the string is made of, how much it has been stretched, and how long it is. When you play the guitar, you can make the string behave as if it were shorter by squeezing it down against the fret board. A shorter string vibrates with a higher frequency, giving a higher tone.

Here's another way to produce sound from an instrument. Here you use your mouth, and blow to set the air in motion. These instruments are called wind instruments. The vibrations are created by the player's lips. The tone of the sound is then shaped by the distance the sound travels through the brass tubing.

When the musician presses the buttons, valves open and modify the length the sound travels, and thereby the tone that is emitted. The longer the pipe, the lower the frequency, the deeper the tone. There are plenty of ways to make music. Some of the earliest instruments are those you beat on. Hit them with a stick, or slap them with the hand, and things will vibrate.

Just like a longer string or pipe produce a lower frequency, a bigger drum makes longer wavelengths and a deeper sound. In most musical instruments, the initial vibrations are weak. The guitar string alone vibrates with a low amplitude and can barely be heard. That's what the guitar's body is for. That wooden box amplifies the vibrations.

And that works without electricity, without adding any new energy at all. Here's how: when vibrations of the guitar strings reach the body of the guitar, the body starts vibrating too, with the same frequency but a greater amplitude. This is called resonance. Every object has its own inherent natural frequencies that it most readily vibrates with. And the guitar's body is built so its natural frequency matches those that are played on the guitar strings.

Resonance is the physical phenomenon that occurs when vibrations strike an object with matching natural frequencies and the object too starts to vibrate. It's like a swing. Give the swing a series of gentle pushes, carefully timed to match the natural frequency of the swing. Some of the energy you passed on to the swing is still there when you add your next push. Eventually, the swing will reach quite some height.

Your low amplitude pushes are transferred to the swing at a frequency matching the swing's natural frequency.