WARNING:
AudioControl Industrial assumes no liability or responsibility for hearing loss
WARNING
incurred directly or indirectly by the use of the SA-3051/SA-3052.
• If the peak straddles two equalizer controls, then split the difference between them. Unless the system
and/or space is really poor, you won't need much equalization to smooth things out.
• Don’t try to equalize out dips in the overall response curve, unless they are quite shallow (less than
3 dB). Anything more than this is a decided waste of power (3 dB = twice power). Fortunately for us,
our hearing is more sensitive to presence rather than absence. Thus, we'll hear the peaks in the response
long before we notice the absence caused by the dips.
• Once you've gotten the overall curve flattened out, listen to the system with music and speech. You'll
probably want to put in some controlled high-frequency rolloff...say 1 to 3 dB per octave starting
somewhere between 1 and 8 kHz.
• If you're equalizing an autosound system, you'll probably need to introduce some carefully contoured
bass boost to overcome the low-frequency ambient noise level in the car, and to help compensate for
the Fletcher-Munson effect.
• If the system is installed in a car, drive around at varying speeds to assess the effect of engine and road
noise on the performance of the sound system. You may need to trim the overall equalization to suit.
• Listen to the system for an extended period of time, at varying volume levels. If the overall system
equalization curve isn't right, listening fatigue will let you know that you haven't finished yet. It may
take several tries to get it just right.
Body Effects
When measuring, it’s important to keep any reflecting surfaces away from the microphone. This includes
your body if you are hand-holding the microphone. If you aren't careful, the reflection can cause response
errors because of the multiple paths into the microphone. Ideally, the microphone should be suspended in
free space. In reality three feet should be adequate spacing.
In an automobile, the three foot rule may turn into science fiction. On the other hand, that's the
environment that the system is going to be listened to in. In this case, the rule goes out the window.
You may notice more variation in the response as you move nearer to the windows. That is just another
good reason for using a multiplicity of measurement points, and taking an average of all of them.
Sound Pressure Level and Hearing Loss
Although it’s great to have a sound system that will achieve levels of 130+ dB SPL, it’s also great to be
able to hear it for a while. It's a proven fact that one of the causes of hearing loss is prolonged exposure to
excessive sound levels.
Research has shown that prolonged exposure to average levels under 90 dB SPL will not cause harm to
our hearing. As the level exceeds 90 dB SPL, the capacity for damage increases, and the damage becomes
more and more permanent. The amount of exposure that our ears can withstand without damage is related
to intensity and time. We can tolerate 105 dB SPL for less time than we can 95 dB SPL.
This is because of the way our ears respond to stimulus outside of the normal range of sounds in our
environment. For example, if you enter a building where there is loud music playing, it may seem loud
to you at first, but after 20 minutes or so, the music will seem less loud. That is because your hearing has
acclimated itself to the new ambient environment. Unless you're conscious of this, you may not notice the
change until you leave the building and go outside. Suddenly everything seems very quiet. You have just
experienced what is known as threshold shift. After a while, your threshold will return to a level that is
close (but not quite) to your original threshold. This is one mechanism of permanent hearing loss.
Overview
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