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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