Theoretical sound propagation from a 2D source (wall)
Theoretical sound propagation from a 2D source (wall)
(OP)
I have taken equivalent SPL measurements at various distances outside of a building which is housing a noise source. I am trying to prove that sound is leaking from the roof and reaching ground level by diffraction.
If only a single wall was propagating sound how would it decay? Obviously a 3D source should theoretically decay in intensity proportional to 1/r^2 but what of a 2D panel source?
Have not had much luck internet searching or even in text books.
The results do seem to indicate somewhat increasing lower frequency energy the further away from the walls which would indicate the diffraction but i dont think its enough evidence alone.
Any help would be much appreciated,
regards,
Paolo
If only a single wall was propagating sound how would it decay? Obviously a 3D source should theoretically decay in intensity proportional to 1/r^2 but what of a 2D panel source?
Have not had much luck internet searching or even in text books.
The results do seem to indicate somewhat increasing lower frequency energy the further away from the walls which would indicate the diffraction but i dont think its enough evidence alone.
Any help would be much appreciated,
regards,
Paolo





RE: Theoretical sound propagation from a 2D source (wall)
What's the source inside the housing and what's the wall? The reason I ask is why do you believe the wall is vibrating? Unless it's closely coupled with the source, there's no reason to believe the panel is generating the noise. Far more likely is that the noise is being transmitted through the panel (and any gaps on the roof-wall junction)
With respect to your question of a panel source "Engineering Noise Control" by Bies & Hansen has a section on an Incoherent Plane Radiator. It's by no means a simple calculation. It's based on a paper by Hohenwarter in Applied Acoustics #33 (1991).
If you do suspect there's gaps in the junction, then you would presumably get a peak if you do a line measurement at a fixed distance from the wall, along the Z axis, extending from above the roof height to the ground.
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Aercoustics.com
RE: Theoretical sound propagation from a 2D source (wall)
I managed to find the incoherent plane radiator you mentioned in a Hansen book, but am not sure if i can easily relate that to my measurements. I was hoping for some sort of empirical or simplified relationship but there does not appear to be much around.
Thanks a lot for your help.
RE: Theoretical sound propagation from a 2D source (wall)
Assuming that both are actually airtight, then you could measure the vibration on each and estimate the sound radiated by each.
The obvious way of getting what you want is near field intensity - but be warned, it is not by any means a straightforward process at low frequencies, where the wavelength is of the order of the size of the radiator.
Cheers
Greg Locock
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Theoretical sound propagation from a 2D source (wall)
My assumption is that as you move further away from the hall, all frequencies should decrease if there is a single 2D panel source. However once far enough from the hall wall the low frequencies tend to actually increase for a certain distance range which would seem to indicate diffraction from the roof.
Not sure if this is a legitimate analysis of the data.
RE: Theoretical sound propagation from a 2D source (wall)
How about calculating it for two piston in baffle radiators and see if, ecven in theory, your predcited method would work?
Cheers
Greg Locock
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Theoretical sound propagation from a 2D source (wall)
I'm afraid you have completely lost me at:
"calculating it for two piston in baffle radiators"
would you mind explaining what you mean?
Thanks
RE: Theoretical sound propagation from a 2D source (wall)
Cheers
Greg Locock
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Theoretical sound propagation from a 2D source (wall)
If you're comparing cinder block and brick combined, which would have a 1000 Hz transmission loss of 82 dB, to 18 ga. steel with a 1000 Hz transmission loss of 32 dB, guess where the noise is going to come from?
There is a formula relating noise reduction through a wall; this contains a term for the room constant of the receiving room, which, for an exterior wall becomes "very large", leaving the result almost independent of wall area; noise reduction of the wall is dependent entirely on the TL of the wall itself.
NR= TL + 6
Unless this building is quite high, it should be simple to show that most of the noise is coming from the roof, and the practical way of reducing outside noise is to treat the roof, to increase its barrier performance.