Re: low frequency noise in sound booth

[Barry Blesser <bblesser@xxxxxxxxxxxx>]

Hi All,

As someone who has some experience with physical acoustics, I would like to
offer my perspective. 

Infra-sonic sound, essentially anything below 20 Hz, is basically vibration
that is transmitted almost entirely through various structures. And such
vibrations can travel very long distances through the ground and building
frames. A sound booth does little to suppress this kind of vibration. Sound
booths suppress airborne sound. 

To actually isolate an environment at very low frequencies requires mounting
the room within a room on springs such that the mass-spring-damping serves
as a high pass filter. If you have ever seen a spring loaded microphone
mount, you will be observing the same principle. But in your case the mass
is the room within a room, and the springs have to be strong enough to
support the structure, and the springs also must have enough compliance to
create the required high-pass filter cutoff frequency.  

In other words, either the cost is excessive, or one adapts to what one has.


Regards,
Barry

-----Original Message-----
From: AUDITORY - Research in Auditory Perception
[mailto:AUDITORY@xxxxxxxxxxxxxxx] On Behalf Of Benjamin, Eric
Sent: Friday, December 14, 2007 5:06 PM
To: AUDITORY@xxxxxxxxxxxxxxx
Subject: Re: low frequency noise in sound booth

Sarah,

I have similar problems with low-frequency noise affecting calibration
in the building that I work in.  But mine is worse, with the acoustic
levels being about 90 dB SPL at 12.5 Hz, and greater than 100 dB SPL at
5 Hz.  I believe that these levels aren't particularly uncommon in
buildings located in large cities.  But it's all inaudible (referring to
Yeowart and Evans, "Threshold of Audibility for Very Low-Frequency Pure
Tones," J. Acous. Soc. Am., Vol.55, pp. 814-818,) and thus only a
problem in measurement of transfer functions or in calibration.

I have observed a problem in the use of couplers wherein vibration,
apparently coupled through the floor, causes the device under test to
vibrate up and down (invisibly) on the coupler, creating pressure
signals in the coupler due to the modulation of the internal volume of
the coupler.  In that particular case I was able to reduce the amount of
low-frequency noise considerably by re-orienting the device and the
coupler so that it was vertical.  Try experimenting with the
orientation.

I don't know of any way to eliminate the low-frequency noise in my
location.  If I have to do a measurement that requires that the noise be
considerably lower I do it at home, since I'm fortunate enough to live
in a very low noise location!

Eric Benjamin