1) Why is frequency an element needed in the sound industry?

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2) When there is a sound pressure level of two different sources with different frequencies, why do sound pressure levels have to be determined in each separate frequency?

1) The sound industry sells equipment that produces, attenuates or amplifies sound. Acoustic design of auditoriums is a part of the industry. The performance depends upon the frequency (pitch) of the sound. The audible frequency range of 20-20,000 Hz is most important.

2) When there is more than one frequency present, the sound pressure does not vary as a simple sine wave, and the sound pressure levels of the separate frequency sounds are not additive. Sound intensity (power/area) is proportional to the square of the sound pressure level, for each frequency.