Characteristics of Sound

Sound is a longitudinal wave that is transmitted through the air and detected by our ears. Good human ears can detect sound from about 50 Hz to about 20,000 Hz (20 kHz); this can be called the audible range of frequencies. As people grow older, we loose our ability to detect high frequencies so the highest frequencies that a middle-aged person is able to detect may be about 15,000Hz or even 12,000Hz.

Sound with frequencies below about 50 Hz may be felt by our bodies rather than heard by our ears. Such very low frequencies are called infrasonic.

Sound with frequencies above our hearing sensitivity are known as ultrasonic. Dogs can hear much higher frequencies than humans; that is why a dog whistle may not be heard by humans. Sound with these ultrasonic frequencies are used for such things a sonograms that give a picture of the inside of a person's body without requiring surgery.

Sonograms, like the one in here, are common during pregnancies but they are also used in diagnosing tumors and other conditions.

Bats emit ultrasound with a frequency of about 50,000 Hz (50 k Hz) to 100,000 Hz (100 kHz) and then listen for an echo or sound reflected from flying insects. This is known as echolocation.

Dolphins use a similar method of echolocation to find their way underwater.

The speed of sound in air is about 345 m/s at a temperature of 23oC (typical room temperature).

For each increase in air temperature of one degree, the speed of sound increases 0.6 m/s.

Speeds greater than the speed of sound are called supersonic.

Noise--like the crash of a trash can--is sound that is not periodic. That means that the sound wave does not repeat itself. A musical sound is periodic; there is a repeatable pattern.

Noise is sound that is not periodic.

Musical sound is periodic.

The loudness of a sound is connected to the amplitude of the sound wave. A louder sound means the air molecules are moving with a greater amplitude and they, in turn, cause your ear drum to move with a larger amplitude. A louder sound carries more energy per time across some area (like your ear drum). This value of energy per time per area, or power per area, is called the intensity of the sound wave.

Loudness is connected to amplitude.

What is similar -- and what is different -- when you hear different instruments play the same melody?

Recall the standing waves on a rope which we have seen and discussed before. Only particular frequencies create these standing waves -- these are called resonances.

The lowest frequency (and the longest wavelength) is known as the fundamental frequency. Remember,

speed = frequency x wavelength

The pitch of a sound is determined by this fundamental frequency, the lowest frequency that is present.

Changing the pitch usually means changing the length of a string or a pipe or a column of air.

The lowest frequency, the fundamental frequency, will determine the pitch of the sound that you hear.

Which additional harmonics are present and their amplitudes will determine the quality or timbre of the sound--what the instrument sounds like.

| Sound Module |

Longitudinal Standing Waves

Beats
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(c) 2002, Doug Davis; all rights reserved