Air cooled Exhanger Fan- Noise Data 3

[SENGUTTUVAN]

In the Noise Data sheet submitted by a vendor for an air cooled Heat Exchanger FD fan , the sound pressure level at various octave band centre frequecies is ranging between 93dBA to 64dBA and the average is mentioned as 85dBA. Also it is indicated that this is measured one metre below the equipment. Should the SPL be within 85dBA at various frequency bands or the average value is O.K.? Whether we should insist for the SPL at one metre beside the equipment also? Pl advise

 

[GregLocock]

You should ask for an OVERALL SPL, or SWL, not an average. It is very hard to justify an AVERAGE SPL, mathematically.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[SENGUTTUVAN]

Greg, sorry they have indicated only Overall, not average. Is this O.K. ? Similarly whether we must insist for 1 metre beside the equipment ?

 

[GregLocock]

The overall SPL must be higher than any of the individual band SPLs.

If you need the SPL where you are indicating then ask for it, but don't be surprised if they charge you for measuring it. I believe the EU standard suggests using sound power (SWL) rather than SPL at 1m.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[SENGUTTUVAN]

Thank U Greg. Why should the OASPL be higher than individual band SPLs? Can U please explain in simple terms.

 

[Tmoose]

Each octave band contributes something to the overall, but the nature of dB is that an octave band 10 dB lower than another does not add much. If the 2 bands highest bands were equal (say 90 dB) their "total" would only be 3 dB higher, or 93 dB.
One 90 dB band, with the rest 65 dB (wildly unrealistic) would result in 91 dB OAL

http://www.sengpielaudio.com/calculator-spl.htm

 

[SENGUTTUVAN]

Thank U Tmoose and for ur calculator too.

 

[Strong]

The most common data presentation is to list or plot the unweighted octave band levels. The energy sum of these levels is the overall unweighted or Linear value. The A-weighted sound level is an overall level that can be measured directly or it can be calculated from the octave band levels. The A-weighting corrections (attenuation) are applied to each octave band, and then they are energy summed to get the total or overall A-weighted level. The word "average" in your question is not relevant, since no averaging math is used in the calculations. Averaging is used for time-averaging or measurement point or area averages where several measurements may be combined.

Walt