Reflective Noise
Reflective Noise
(OP)
Is it possible to more than doule the noise gain from reflective sound.
We have some plant housed in an enclosure. Manufacturer rates the equipment at 78dbA +/- 3dbA at 1 meter free field. The measured value inside the enclosure is in excess of 85dbA.
Does this mean that the plant is generatin more than 81 dbA or is it possible for a gain of more than 3dbA from reflective noise.
Any help would be appreciated.
We have some plant housed in an enclosure. Manufacturer rates the equipment at 78dbA +/- 3dbA at 1 meter free field. The measured value inside the enclosure is in excess of 85dbA.
Does this mean that the plant is generatin more than 81 dbA or is it possible for a gain of more than 3dbA from reflective noise.
Any help would be appreciated.





RE: Reflective Noise
RE: Reflective Noise
M
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Dr Michael F Platten
RE: Reflective Noise
RE: Reflective Noise
I strongly dispute the first couple of modes figure you suggest, indeed one way of thinking about the high frequency response of a room is to consider the modal density, ie the number of modes per third octave. Even though we treat that as a lumped repsonse, in reality each resonance is still being excited.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Reflective Noise
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Reflective Noise
Greg,
The enclosure is approx. 10m x 5m x 4m, with duty/standby compresors inside. I am not sure of the construction of the enclosure, but it is skined with sheet steel, inside and out.
The microphone was approx. 1m from the duty compressor.
RE: Reflective Noise
RE: Reflective Noise
The factor of amplification by a resonance is the same whether you excite it with -160 dBA SPL or 160 dBA SPL. If there is energy at the frequency of the resonance then it will be excited (except in the academic case of a point source positioned at a nodal point).
How many modes should you consider? Well, all those whose natural frequencies fall within the range of your SPL measurement. At higher frequencies where there are many modes it no longer makes sense to consider each mode individually. You can get around this by treating all the modes in a particular 1/3 octave band as a single degree of freedom whose resonant frequency and damping properties are the mean of all the modes in the band.
redgdon,
The plant rating of "78dbA +/- 3dbA at 1 meter free field" is effectively a measure of the sound power of the plant. The average SPL in your enclosure will depend on the sound power (more power - more SPL), the absorption inside the enclosure (usually expressed in terms of the reverberation time, more absorption or shorter T60 - less SPL) and the volume of the space inside the enclosure (a bigger space means that the modal energy is spread out over a larger volume and hence the energy at a particular point is less, however a bigger volume also implies more resonant modes!)
The +/- 3 dBA bit is there to account for the fact that the plant does not emit sound equally in all directions. When you take your SPL measurement, you need to measure the SPL at a number (10 say) of RANDOM positions in the enclosure (steering clear of places close to walls or the plant) and then average them logarithmically. Don't feel that you have to take your SPL measurement at 1m from the plant.
M
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Dr Michael F Platten
RE: Reflective Noise
So as suggested above, in order to reduce the sound level within the enclosure, you need to introduce some means for dissipating the energy; foam or fibreglas for example.