Subject: Re: acoustic shielding From: Yuvi Kahana <yuvikahana(at)HOTMAIL.COM> Date: Wed, 7 Jul 2004 14:41:16 +0000Dear 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(at)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(at)ARCH.USYD.EDU.AU> >Reply-To: Densil Cabrera <densil(at)ARCH.USYD.EDU.AU> >To: AUDITORY(at)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(at)arch.usyd.edu.au _________________________________________________________________ MSN 8 helps eliminate e-mail viruses. 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