Standing Waves

A standing wave is a wave which stays in one constant position. This can occur when the medium is traveling in the opposite direction as the sound, for example, when you yell into a strong wind. It can also be caused when two waves are traveling in the opposite direction, resulting in interference with each other.

Every object has a natural frequency or frequencies at which it is said to "resonate." We also know that a sound wave is a mechanical wave traveling through a medium and that a sound can constructively or destructively interfere with another sound. What do you think happens when you create a frequency in a room that constructively interferes with its echo? If you guessed that the sound becomes much louder you would be right.

Sounds bounce and reflect off of flat surfaces. Most sounds, when they are reflected, destructively interfere with the other sounds in the room. However, when a sound is created with the right frequency for a given sized room, the sound will bounce off the walls in such a way that it will constructively interfere with itself, thus magnifying its amplitude in places and becoming resonant.

This is exactly what happens in the pipes of an organ. Each of the pipes is designed as a specific size in order to setup a standing resonant wave in the organ pipe. The sound inside the pipe constructively bounces from end to end. This standing wave can become very loud.

Perhaps the best way to picture this is to look at the standing wave that is created in a stringed instrument. The wave is held fast at the two ends so the string will vibrate back and forth. The frequency depends on the length of string, its weight, tension and other similar factors.

Have you ever thought about why it is so much fun to sing in the shower? You may have noticed that the sound quality is unusual, being more resonant at certain frequencies. You may also have noticed the same sound in an empty room because sounds will echo loudly, especially at certain frequencies.

This effect is more prevalent in a "bright" room or a relatively empty space with hard flat parallel walls that do not absorb the sound. Many bathrooms are good examples of resonant bright rooms. Carpets, furniture, textured walls all serve to breakup and absorb sound, making them a less potent resonant chamber.

Nodal Points

When two waves with the same amplitude and frequency travel towards one another, an interesting phenomenon occurs. At certain points the two waves cancel out each other, creating a quiet zone or nodal point. At other points, the two waves create the opposite effect, becoming twice as loud. These points are called anti-notes.

Designing a Theater

There is an amazing science behind the design of large concert halls and theaters. You want to reflect the actors' voices from the stage all the way to the back of the theater, while making the theater space as absorbent of the sound as possible. Many theaters use specially designed decor specifically to absorb as much of the sound as possible. In addition to helping cut down the "resonant ringing," it also helps to reduce or eliminate nodal points in the room.