When a guitar string vibrates, it collides with the air molecules around it. Molecules near the string are alternately compressed, then rarefied (spread out). The range of sound frequency that the human ear can hear is called the audible frequency range. This range is about 20 Hz to 20,000 Hz. Sounds below 20 Hz are called infrasonic sounds, and sounds above 20,000 Hz are ultrasonic sounds.
The pitch of a sound is our subjective impression of the frequency of the sound. Sounds waves are longitudinal waves and can be visualized on an oscilloscope as transverse. Click one of the links below to learn more about these kinds of waves.
The frequency of sound vibrations are what determine the pitch of the sound. If you watch an oscilloscope, you will notice that if the sound was louder the height of the wavees was greater. The amplitude is the measure of the loudness of a sound. The greater the wave amplitude, the louder the sound.The loudness of a sound depends on its intensity. The intensity is the amount of sound energy arriving per second on a square meter of area. It is measured in watts per square meter (W/m^2). 1 W/m^2 is equal to 12 bels, or 120 decibels. (loud!!) Bels are named after Alexander Graham Bell (1847-1922), Canadian inventor of the telephone.
Sound Quality
Even if two instruments play tons of equal pitch, you can still tell a difference between them. Each instrument has a unique timbre, or quality to it. Both instruments may produce the same fundamental frequency but the overtones they produce are different. Overtones are frequencies that are many times the fundamental frequency. The quality of a sound depends on the number of overtones and their relative loudness.
The Doppler Effect
When a fast car passes you, you notice that as it approaches its pitch increases, and as it goes by the pitch lowers. Austrian physicist C. J. Doppler (1803-1853) was the first to explain the effect in terms of waves. This effect has been called the Doppler Effect.
