Intensity and loudness are often mistaken as having the same meaning. Although related, they are not the same. Intensity is a measure of a sounds energy, while loudness is the effect on the detector. If sound intensity is increased, the loudness is increased but not in direct proportion. To double the loudness of sound requires about a tenfold increase in the sounds intensity. Sound intensity is measured in decibels (dB). A decibel is the unit used to express relative power (intensity) differences between acoustic signals (sounds). Decibel levels are assigned based on a sounds intensity compared to an established standard. Some common intensity levels are as follows: a whisper, 10 to 20 dB; heavy street traffic, 70 to 80 dB; thunder, 110 dB. 

Learning Objective: Define energy loss or spreading loss as it pertains to sound waves.

As a sound wave moves away from its source, it spreads out. The energy within the wave decreases as the wave spreads through an increasingly large area. Thus, the wave energy per unit area decreases as the distance from the sound source increases. This loss of energy due to distance is known as spreading loss.

Learning Objective: Define Doppler effect, and recognize how it effects the pitch and frequency of sound.

The Doppler effect is the apparent change in a sound due to motion. It is a change in pitch (a detector variable) without a frequency change occurring (a sound source variable). The change in pitch is brought about by the relative motion of a sound source and a detector. For example, we hear the whistle of an approaching train. The frequency of the whistle does not change as the train approaches, but our ears detect an increase in the pitch. The increase in pitch is caused by the compression of sound waves. The train acts to "push" the sound waves toward us. The sound waves arrive at a faster rate than they would if the train was not moving. Then, as the train goes by, the sound waves arrive at a much slower rate. The train is now pushing the sound waves away from us. The sound waves to the rear of the train spread farther apart as the train moves farther away from our position, and the effect is one of lower pitch.

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