Agenda - 11-22-2004-c1 - Laserfiche WebLink

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17 Aftachment 3 Environmental Sound Page 2 Environmental Sound and its Measurement Sound The human ear is a remarkable mechanism.. It can sense and give meaning to a wide range of sounds varying in loudness and pitch.. Perception is not directly related to the physics of sound. The effect perceived by the ear as sound is a very small and rapid change in air pressure. The variations aze very slightly above and below the barometric pressure. The perceived loudness of a sound corresponds primazily to the size of this variation in pressure. The ear is very sensitive and can detect, at some frequencies, variations less than 3 billionths ofa pound per square inch (psi). Variations greater than 0.001-.O1 psi can cause pain or discomfort. Notice that the pressure variation of the loudest sounds we hear is about a million times that ofthe quietest sounds, The energy in the sound is actually related to the squaze of the pressure. Thus, the energy in very loud sounds can be amillion-million times that in a very faint sound, For humans to heaz the sound, these air pressure variations must be rapid. They must complete a cycle at least 20 times per second and not more than 20,000 times per second, The repetition rate of these cycles is called the frequency of the sound. It has units of Hertz (Hz) where one Hz corresponds to one cycle per second, The bass sounds in music are low-frequency, and the treble sounds are high-frequency, Because of the wide range of sound pressures encountered, a special scale, the decibel scale, has been developed. The sound level increases by 3 dB (or decibels) when the actual sound energy present is doubled, Conversely, half the sound energy must be eliminated to reduce the level 3 dB. A six decibel change indicates four times as much sound, and a 10 decibel change 10 times the sound. However, the ear does not perceive these changes to be so large, Most people fmd a 3 dB change bazely noticeable, A 6 dB change is cleazly noticeable, and a 10 dB change is perceived as a doubling or halving ofloudness, This is very important, It means that 90% of a given sound energy must be eliminated before most people will,judge a sound half as loud. People do not hear very low-frequency and high-frequency sounds as well as they hear sounds in the middle speech frequencies.. A specialelectronic filter is usedm sound meters to simulate this characteristic ofthe eaz, For many environmental sounds it gives a simple, single-number descriptive level that correlates with the way people judge the loudness ofthe sormd. This measure is called the A-weighted sound leveh The units aze usually written dBA or dB(A).