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Dear Sarah, Eric and all others I am quite surprised to hear about these high levels at infrasonic frequencies. The levels seems to be higher in your buildings than at places, where I have usually measured. But no doubt that ventilation and air conditioning systems may be the source. Anyway, I wanted to add that we did a review a few years ago on the human hearing at low frequencies: H. Møller, C. S. Pedersen, "Hearing at Low and Infrasonic Frequencies", Noise & Health, 6(23), pp. 37-57 (2004). For the hearing threshold, newer data - including some of our own - were much in line with the 1974-data by Yeowart and Evans. There seems to be good agreement that the average threshold is around 93 dB at 12.5 Hz and around 110 dB at 5 Hz. Which means that what you measured, Eric, is only few dB below the average hearing threshold. So, if your measurements are correct, no doubt that some sensitive people will perceive the sound (not as a tone, but still with their ears). Best regards, Henrik Benjamin, Eric skrev: 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. -- Henrik Møller Professor Acoustics Aalborg University Fredrik Bajers Vej 7 B5 DK-9220 Aalborg Ø, Denmark Phone: +45 9635 8711 (direct) Phone: +45 9635 8710 (section office) Phone: +45 9635 8080 (switchboard) Fax: +45 9815 2144 (section office) mailto:hm@xxxxxxxxxxxxxxxx (personal) mailto:acoustics@xxxxxxxxxxxxxxxx (section office) http://acoustics.aau.dk |