Posted by: "Eric Benjamin"
> As a general rule of thumb, walls reduce noise
> transmission by an amount proportional to
> 20*Log(frequency) and proportional to the mass of the
> wall. There are certainly variations depending on the
> structural resonances of the wall, but the rule of
> thumb holds pretty well. Commercially available sound
> booths usually give about 40 dB of attenuation in the
> mid-band (500Hz), and get 6 dB worse for each octave
> that you go down in frequency. The Army may have been
> able to do better than that, but the rest of us have
> to depend on what we can afford to do.
The booths we used were commercial booths, I have no idea the brand
anymore.
When I was doing EIS work and doing sound work that might have to stand
up in court, we did a bunch of measurements to try and get a handle on
sound attenuation at outdoor sites. One thing we found was the mass
between us and the source was not very predictive of what we would measure.
> The saving grace is that the threshold of hearing gets
> worse towards low frequencies at a rate greater than 6
> dB/octave. The threshold is about -5 dB SPL at 4 kHz,
> but +76 dB SPL at 20 Hz. There can be a lot of low
> frequency noise by an objective measure and have it
> still be inaudible.
I'd take those figures with a grain of salt or two. That might be a
average, but after measuring the hearing of literally thousands of
military recruits and draftees, I'd say it's far from universal. I had
plenty of folks who's low frequency hearing (50Hz) was the same as their
1000Hz & 2500Hz hearing. Plenty that got down to -10dB at 50Hz. We did
not measure at 20Hz.
Sure there were those who had ruined their hearing with loud noises,
who's hearing graphs were awful to look at, but it was not universal.
Walt
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