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SOUND LEVELS
Recently someone said they were at a concert where the sound levels were 130 dB. Someone else asked how that was measured. The reply was to the effect that the person had experienced 125 dB at a concert and this was louder than that!
Now maybe that concert did have levels of 130 dB, or maybe the levels were 110 dB or maybe they were 140 dB or higher (unlikely but possible depending on how the levels were measured). The ear is not good at judging absolute levels, so without an actual measurement it is hard to say what the levels really were.
So how are sound levels measured? What is this dB that people talk about? Here is a chart that can help you start to understand these issues.
The dB scale is not linear, and was intended to correspond to the way human hearing works. 0 dB Sound Pressure Level (dB SPL) was selected to correspond to the softest sound a young person with perfect hearing could hear. A 3 dB increase is level is just noticeable to human hearing but is twice the power. A 10 dB increase in level sounds twice as loud to the ear, but is 10 times the power.
Another factor in sound level measurement is the "weighting curve" used. A weighting curve is merely a way to make the measurement less sensitive to some frequencies. The "C" weighting curve is equally sensitive to most frequencies, except for the very low and very high frequencies. The "A" weighting curve is mostly sensitive to mid frequencies which annoy the ear the most, and less sensitive to lower and higher frequencies. Neither one perfectly corresponds to the potential for hearing damage of a given sound, but all the sound level regulations use "A" weighting and a "Slow" level detector (how fast the meter responds to changes in sound level). If you wish to make measurements and compare them with the Standards you must take the measurements using A weighting and Slow response time.
Notice that some of the levels on the chart specify a distance. For example a whisper at a distance of 1 meter is around 20 dB SPL. This is important because sound levels die out with distance. In general the further you get from a sound source the softer the level gets.
In the concert example we started with the listener was closer to the loudspeakers then the location of the mixing console. That means he was probably exposed to a higher level than the person mixing the sound. Well designed sound systems will provide sound coverage that does not vary a lot depending on where you are seated, but not all sound systems are designed for uniformity of coverage. Those doing the mixing should be aware of how representative of the average seat in the house the mixing position is.
At the concert in our example, the listener complained that the levels caused his ears to hurt. While people vary in their sensitivity to loud sounds, in almost all cases if sound is so loud it hurts, then it is dangerously loud at least for you. Pain is the body's way of warning us that we are overdoing things. Hearing loss is a one way street. It is not like weight lifting where pushing the limits makes you stronger. Every time you feel pain from sound levels you are doing damage, and that damage can't be reversed.
Short of painful sound levels, if you leave a concert and notice you are not hearing as well as normal, you may have temporary threshold shift. This you will usually recover from, but why take chances?
The chart shows that the typical person experiences pain starting at around 120 dB SPL. Older people with hearing loss may be more sensitive to sound levels. Certain drugs may make you more sensitive to loud sounds.
Sound does not have to be painful to cause hearing loss. Studies by the House Ear Institute (HEI) of hearing loss were the basis for first the OSHA noise regulations, and the more recent NIOSH standard. HEI remains one of the best places to learn about protecting your hearing:
http://hei.org/
Ray A. Rayburn
Ray@K2Audio.com
http://www.K2Audio.com
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