Thursday, May 14, 2009

Defining noise

Sound is what the human ear hears; noise is simply unwanted sound. Sound is produced by vibrating objects and reaches the listener’s ear as pressure waves in the air or other media. When the amount of sound becomes uncomfortable or annoying, it means that the variations in air pressure near the ear have reached too high an amplitude. The human ear has such a wide dynamic range that the decibel (dB) scale was devised to express sound levels. The dB scale is logarithmic because the ratio between the softest sound the ear can detect and the loudest sound it can experience without damage is roughly a million to one or 1:106. By using a base-10 logarithmic scale, the whole range of human hearing can be described by a more convenient number that ranges from 0 dB (threshold of normal hearing) to 140 dB (the threshold of pain). There are two dB scales: A and L. The dB(L) unit is a linear scale that treats all audible frequencies as having equal value. However, the human ear does not experience all sound frequencies as equally loud. The ear is particularly sensitive to frequencies in the range of 1,000 to 4,000 Hz (cycles per second), and not as sensitive to sounds in the lower or higher frequencies. Therefore, the A-weighting filter, which is an approximation of loudness, is used to correct the sound pressure levels to more accurately reflect what the human ear perceives. This frequency-weighting dB(A) scale was adopted by OSHA in 1972 as the official regulated sound level descriptor.

by Dennis Aaberg