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Natural Science Forum / Physics / Acoustics / September 2006duct attenuation
hello, need some clarification... sound attenuation along a pipe is commonly 3dB per 60 diameters length for gaz and 3 dB per 175 diameters for liquid. Nevertheless acoustic impedance is lower for gaz... what is the reason for ? thanks > hello, > [quoted text clipped - 5 lines] > > thanks I take that you "measure" attenuation as what comes out from different lengths of ducts or as different pressure levels along the duct. If so, there are basically two "mechanisms" causing the attenuation: the losses in the fluid and vibroacoustics behavior of the duct (sound energy is also radiated by the duct). BR, ari www.machineryacoustics.fi One addition. If there is fluid flow, it also generates sound/noise. >> hello, >> [quoted text clipped - 18 lines] > > www.machineryacoustics.fi > One addition. If there is fluid flow, it also generates sound/noise. > >> jieme wrote: >>> hello, >>> >>> need some clarification... >>> sound attenuation along a pipe is commonly 3dB per 60 diameters length >>> for gaz and 3 dB per 175 diameters for liquid. Nevertheless acoustic >>> impedance is lower for gaz... >>> what is the reason for ? >>> >>> thanks >>> >>> >> I take that you "measure" attenuation as what comes out from different >> lengths of ducts or as different pressure levels along the duct. >> >> If so, there are basically two "mechanisms" causing the attenuation: the >> losses in the fluid and vibroacoustics behavior of the duct (sound >> energy is also radiated by the duct). >> >> BR, >> >> ari >> >> www.machineryacoustics.fi thanks Ari, I'm agree with you, there exist two paths for acoustic energy but my question was based on the ISO15665 assertion and if it is possible to reach analytical relation which explain this (difference with gaz and liquid based on speed of sound, acoustic coupling or something else..) Thanks Ok, maybe i was not clear enough. The vibroacoustics also includes sound absorption (or impedance if you prefer). Most of the phenomena are frequency (for example source and structure in real systems, fluid and dimension) dependent. Basically what i'm implying is that you can have an analytical solution. But the solution depends on the assumptions made (lossless media or something else etc.) And there might be easier ways out of this... It might be, that I still don't understand what is your point, since i don't have the standard and your background. BR, ari > > One addition. If there is fluid flow, it also generates sound/noise. > > [quoted text clipped - 32 lines] > > Thanks >hello, > [quoted text clipped - 5 lines] > >thanks According to the HVAC guys, your basic assumptions are wrong. Intuitively they are as well, because attenuation in a pipe is obviously going to depend on the wavelength of the sound. All sorts of useful numbers around this site: http://www.engineeringtoolbox.com/sound-attenuation-ducts-d_74.html d  SignaturePearce Consulting http://www.pearce.uk.com Hmn. Could somebody explain me the differences with round ducts, please. There are results in the stiffness controlled frequencies that i don't understand. Or does it have something to do with the system or measurement accuracy? BR, ari >> hello, >> [quoted text clipped - 14 lines] > > d A lower impedance for gas relative to liquid simply means that for equal sound pressure the gas will have a higher velocity amplitude than the liquid, since impedance is the ratio of wave pressure over velocity. So, higher impedance doesn't equal higher attenuation. Erik > hello, > [quoted text clipped - 5 lines] > > thanks |
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