Re: low frequency noise in sound booth

["Benjamin, Eric" <EMB@xxxxxxxxx>]

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

-----Original Message-----
From: AUDITORY - Research in Auditory Perception
[mailto:AUDITORY@xxxxxxxxxxxxxxx] On Behalf Of Ferguson, Sarah Hargus
Sent: Friday, December 14, 2007 1:36 PM
To: AUDITORY@xxxxxxxxxxxxxxx
Subject: low frequency noise in sound booth

Hello list - I've run into a very annoying problem in my (double-wall)
sound booth. When I try to calibrate, I'm getting floor noise of 75-80
dB SPL WITH the earphone (which isn't connected to anything) on the 6 cc
coupler, WITH the door closed. The spectrum analyzer shows a peak around
65-70 dB SPL at 16 Hz, with energy about 20 dB below that at 12 and 20
Hz - and nothing else, really. 

I have two questions:

1) Does any one have any ideas about what could be causing this? Three
gentlemen from Facilities Operations worked on this for over an hour
this morning, turning off lights and air handlers and exhaust fans -
they even went out on the roof to turn things off. Nothing that they did
had any impact on the noise. 

2) How low can I reasonably expect my floor noise to be? While trying to
pin down the onset of this noise, I realized that I never write down the
level of the floor noise, just the levels of the sounds I'm calibrating
(which have often been at levels much lower than 75 dB SPL). With all
this troubleshooting I've completely lost my bearings about what I
SHOULD be getting.  

~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ Sarah Hargus Ferguson,
Ph.D., CCC-A Assistant Professor Department of Speech-Language-Hearing:
Sciences and Disorders University of Kansas Dole Center 1000 Sunnyside
Ave., Room 3001 Lawrence, KS  66045
office: (785)864-1116
Speech Acoustics and Perception Lab: (785)864-0610
http://www.ku.edu/~splh/Faculty/FergusonBio.html 
 

-----------------------------------------
This message (including any attachments) may contain confidential
information intended for a specific individual and purpose.  If you
are not the intended recipient, delete this message.  If you are
not the intended recipient, disclosing, copying, distributing, or
taking any action based on this message is strictly prohibited.