Correction Factors for Measured Noise Level

[homoly]

Hello,

I have received a task to measure the noise according ISO 3744.

The is a requirement to log temperature,relative humidity and atmospheric pressure, there are also given intervals of measurement validity ( 975 hPa - 1025 hPa for pressure and 3C-27C for ambient ). However there is not mentioned how should the correction factors calculated for some unified conditions if I would like to compare 2 different measurements.

Could anybody recommend me some material dealing with the effects of temperature,relative humidity and atmospheric pressure on measured acoustic noise level.

Any help will be appreciated.

Gabriel.

 

[GregLocock]

I imagine you'd find the corrections in the technical literature accompanying your microphone cartridge, and your calibrator.

What class instrument are you using?

Cheers

Greg Locock

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

 

[homoly]

Hello Greg,

I am using Bruel&Kjaer 2236 device. In the manual I have found some minimum / maximum values where the device is working with the defined precision, but not find some curve with the correction coefficients.

 

[GregLocock]

B&K usually have lots of correction curves. The 2236 is a SLM not a cartridge. It uses a 4188 cartridge. What calibrator are you using?

Sorry if I come over as rude, you don't even seem to be trying to figure the issue out. Read the manual.




Cheers

Greg Locock

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

 

[homoly]

Greg,

Do not worry I do not think you are rude in any way.

I am using Multifunction Acoustic Calibrator Type 4226.

In the user manual I have ( Precision Integrating Sound Level Meter Type 2236 Brüel & Kjær User Manual ) for correction there is only following information : "At 40C and 95% RH, add 2 dB to typical values and 3 dB to maximum values.Includes preamplifier’s electrical noise and microphone’s thermal noise.".And " Effect of Temperature: <0.5dB (–10 to +50 °C)Effect of Humidity: <0.5dB for 30%<RH<90%(at 40 °C, 1kHz)". But there is no correction per centigrade or per percent of relative humidity. However maybe there are some concrete curves in separate documents.

Nevertheles from you last entry I have a feeling that the temperature and RH have effect on noise sensing by the device rather than the noise propagation itself. I am primarly an electrical engineer and my naive idea was that it would be possible to unify the measured noise level obtained by 2 devices to same condition. Now I can see that probably there is not a magic formula that includes temperature and RH that should be applied on the measured values, but the only way is to use concrete data on concrete measuring devices and try to find a condition on which both devices have information how the reading out should be corrected.

In my case where the effect on each "normal" enviromental condition is lower than 0.5dB when measuring noises on cca 55 dB level could be practically neglected - I think this will be the way .

The first impulse that gave me an idea that I was thinking about the correction was when the ISO standard I have to use which prescribes the recording of the temperature, RH and atmospheric pressure. But after your information I am thinking now, these data are recorded rather to prove the device was in its working condition regarding environment during the measurement not to evaluate some corrections afterwards.

 

[GregLocock]

That sounds like a reasonable interpretation - and of course having recorded the environmental conditions, if at a later date you find a correction curve, you can then use them.

FWIW I know someone who set up a sound level meter and checked its calibration every day for 6 months, it didn't shift by more than 0.1 dB - we used to calibrate them every day and set the level, this was actually reducing our accuracy, the calibrator should be used as a check.





Cheers

Greg Locock

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

 

[SomptingGuy]

I always found this basic approach key in NVH measurement. Keep ALL raw data, never discard anything. Write down the ID's of all pieces of equipment used too.

- Steve