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Re: acoustic shielding

Dear Satra,
The following might not answer your question or can be helpful in your
immediate need, but I thought you might find it useful as you required non
magnetic, non metallic material:

Although not commercially yet,  Phone-Or is proposing an advanced slim
active noise control headset to remove ambient noise, such as found in fMRI.
The amount of noise reduction using the active system is much higher than by
any passive means.

Here is the abstract of a paper just submitted mutually with IHR to the next
ACTIVE conference in Virginia.


Optoacoustical Ear Defenders with Active Noise Reduction in an MRI
Communication System


by
Yuvi Kahana1, Alexander Kots1, Sever Mican1, John Chambers2, Dave Bullock2

1 Phone-Or, Or-Yehuda, Israel
2 The MRC, Institute of Hearing Research, Nottingham, UK


During Magnetic Resonance Imaging (MRI) intense acoustic noise is generated
due to the altering Lorentz forces, which are exerted on the coils of the
gradient magnet. The noise levels can be in excess of 130 dB(A) in 3T
scanners. This level of sound causes anxiety and discomfort in MRI patients,
has safety implications for anyone in the scanning room, and affects
functional MRI studies.

In the development of an ANC system for MRI, two main difficulties exist:
(1) due to the presence of strong electromagnetic and radio frequency
interference (EMI/RFI), conventional speakers and microphones are not
recommended due to interference with the imaging (2) there is a large
variety of noise patterns differing both in time (with short or long duty
cycles) and frequency (varying resonance amplitudes at low or high
frequencies).

In this paper we present an optoacoustical technology that can be used in an
active noise controller for significant attenuation of ambient noise
received at the ears. We describe the principle of operation of the system
sub-components, including: optical microphone, light powered speaker,
associated electro-optic units, as well as the DSP algorithm. We then
present the acoustical measurements obtained in an MRI acoustic simulator
using the ANC system. Measurements of noise spectra show attenuation of
35-50 dB of the fundamental frequency at the reference microphone. These
values are in addition to 15 dB of passive attenuation resulting from a slim
ear defenders headset.


Best regards,
Yuvi Kahana, Ph.D.
CTO
Phone-Or Ltd.
17 Hatasia St. P.O Box 323
Or-Yehuda 60212
Israel
Tel:  +972 3 634 4488 (ext. 163)
Mobile: +972 54 733951
Fax: +972 3 634 9292
yuvik@phone-or.com
www.phone-or.com


Dear List members,

We are trying to increase the external attenuation of a set of headphones.
Do you know of any non-metallic, non-magnetic, thin [less than 8mm],
pliable material that can provide noise suppression upwards of 10dB upto
10KHz?

Thanks,

Satra

--
Satrajit Ghosh
Department of Cognitive and Neural Systems, Boston University




From: Densil Cabrera <densil@ARCH.USYD.EDU.AU>
Reply-To: Densil Cabrera <densil@ARCH.USYD.EDU.AU>
To: AUDITORY@LISTS.MCGILL.CA
Subject: Re: [AUDITORY] acoustic shielding
Date: Wed, 7 Jul 2004 22:16:51 +1000

Satra,
a couple of points:
* the difficulty with sound insulation is blocking low frequencies -
high frequencies are generally solved if you meet your low frequency
criterion.
* you might be able to estimate the characteristics of the desired
material using the simplistic assumptions of the 'mass law' for sound
insulation:
the mass per square metre of material = 10^((attenuation+42)/20) /
frequency.
That equation assumes normal sound incidence (not random incidence,
for which you probably need a bit more mass per square metre).  It
also has many other assumptions which may not apply, but it might be
a good start for a flexible material.
* one more hint - it is important to seal all air paths - the
material must be airtight.
densil


Hi Satra,

you could try with small pieces of thick foam should do the
trick. this foam which is used for acoustic treatment: pads,
bass-traps and so one. not sure what it would give in only 8mm
though.

Paul

On Fri, Jul 02, 2004 at 01:01:55AM -0400, Satrajit Ghosh wrote:

 Dear List members,

 We are trying to increase the external attenuation of a set of
headphones.
 Do you know of any non-metallic, non-magnetic, thin [less than 8mm],
 pliable material that can provide noise suppression upwards of 10dB
upto
 10KHz?

 Thanks,

 >
 > Satra
 >
 > --
 > Satrajit Ghosh
 > Department of Cognitive and Neural Systems, Boston University
 >


--

Dr Densil Cabrera, Lecturer in Audio and Acoustics
School of Architecture, Design Science and Planning
Wilkinson Building G04
The University of Sydney, NSW 2006, Australia
Telephone +61 2 9351 5267
Fax +61 2 9351 3031
densil@arch.usyd.edu.au

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