Sounds tell us a great deal about our environment. Hearing is one of our most highly developed senses, providing information about what is happening in the world around us, including things we can't see. Sound also enriches our lives. Precisely because so much brainpower is devoted to processing what we hear, we are able to distinguish between and appreciate a wide variety of sounds.

Car alarms, singing birds, a series of notes on a piano, and even running water all have the potential to convey important information. And yet all sound is created in essentially the same way: vibration. A vibrating object creates pressure waves in the surrounding air. These waves travel in all directions away from their source. When they reach a person's ear, they cause the eardrum and small bones in the middle ear to vibrate. Nerve receptors in the inner ear translate these vibrations into electrochemical signals that are sent to the brain, which interprets the signals as particular types of sound.

Sounds vary primarily in pitch and amplitude. Objects that vibrate rapidly produce sound waves at higher frequency (more waves over a given period of time) than do objects that vibrate slowly. High-frequency sounds are interpreted by the brain as higher in pitch than are low-frequency sounds. Our ears interpret sounds that have larger, or higher-amplitude, sound waves as louder than sounds with lower-amplitude waves. This is because amplitude is related to how much energy the wave is carrying. The greater the amplitude, the louder the sound.

However, it takes more than differences in pitch and amplitude to distinguish among the countless different types of sounds. For example, how do our brains tell the difference between a trombone and a saxophone if they each play a note at the same pitch and amplitude? Musicians use a number of terms to describe the nuances of various types of sound, such as quality, timbre, tone, spectrum, and envelope. These characteristics are influenced by a number of factors, such as the shape of an instrument, the type of material from which it is constructed, and the origin of the instrument's vibration. For example, a guitar's sound begins with a vibrating guitar string, while a clarinet's begins with a vibrating reed. These characteristics result in added frequencies known as overtones, which the instrument emits along with the fundamental frequency that determines the pitch of the sound.

In the construction of new instruments, such as those created by the Design Squad team, instrument makers sometimes strive to create an instrument that can play what is called an octave. As the prefix "oct" implies, there are eight notes in an octave. A note that is one-half or twice the frequency of another note is said to be an octave lower or higher, respectively, and has the same letter designation on the musical scale. Our ears hear two notes that are an octave apart, as well as several other combinations, as harmonious and pleasant. The remaining combinations of notes are dissonant and less harmonious. Music is all about harmony and dissonance.

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