Re: low frequency noise in sound booth (Barry Blesser )

Subject: Re: low frequency noise in sound booth
From:    Barry Blesser  <bblesser@xxxxxxxx>
Date:    Fri, 14 Dec 2007 22:06:53 -0500

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@xxxxxxxx On Behalf Of Benjamin, Eric Sent: Friday, December 14, 2007 5:06 PM To: AUDITORY@xxxxxxxx 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

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