> Posted by: "Rob Danielson"
> > I obtained the attenuation range by recording
> inside (with my body
> > between the door and the mics) and outside by the
> door and comparing
> > the files.
> > The low frequencies are the rub. I could stand in
> the booth 150 yards
> --- Walter Knapp <> wrote:
> Just seemed awfully little for the structure you
> were describing.
>
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 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.
Eric Benjamin
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