Sound pressure level (dBA) correction for air temperature /humidity

[cef3rd]

Is there a correction factor for sound pressure level if we want a value at 35°C instead of 25°C which is the actual environment it was measured? Say if we get 45dBA at 25°C, what should be the value at 35°C? Assume no change in air humidity.

 

[SomptingGuy]

Quoting from my handy "Measuring Sound" booklet from B&K:

Humidity

In most cases relative humidity levels up to 90% will have a negligible effect on the sound level meter and the microphone. ... Even if the windscreen becomes very wet, measurements will still be accurate.

Temperature

At Bruel & Kjaer sound level meters are designed to operate accurately over the -10 to +50 C range. However, care should be taken to avoid sudden temperature changes which may lead to condensation in the microphone.

So the short answer is no, No correction required.

(Note, I do not work for Bruel & Kjaer)

 

[GregLocock]

Even more confusingly... the ambient temperature where I work varies from 12 to 42 degrees C. An SLM calibrated with the same 1/2 " calibrator will vary by less than 0.3 dB over that range.

So, either the calibrator varies bin the same way as the SLM (quite possible), or it isn't making much differnece.

So, having sorted that out, how come the old pistonphone calibrators used to come with a barometer?



Cheers

Greg Locock

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

 

[cbrn]

Hi,
all I can guess is that it may depend on the construction of the microphone:
- if it's differential (air pressure acts on both sides of the membrane), it shouldn't be affected by air properties
- if it's sealed-chamber (air pressure acts on one side of the membrane, the other getting to a "sealed" chamber"), then a way must be provided to correlate external air properties with the ones of the fluid (could be air, but not only) in the chamber.
Note: ALL the microphones I dealt with in my life were of the first type...

Regards

 

[SomptingGuy]

Atmospheric conditions are important when measuring AI using a two-microphone approach. Air density forms part of the expression used to calculate AI from the crosspower between the two microphones.

 

[Rob45]

Variations in temperature produce changes in the acoustic impedance of air, lower temperatures prodcuing higher impedance, thus affecting the measured sound pressure levels.

 

[amorrison]

Googling “sound air frequency attenuation” got

http://www.sfu.ca/sonic-studio/handbook/Sound_Propagation.html

Which seems to be part of this site/book ??

http://www.sfu.ca/sonic-studio/handbook/

I will let you do the legwork from here on .

 

[Strong]

Greg,
The old B&K Pistonphone was a fixed volume displacement device, so barometric pressure would affect the dynamic pressure it produces. I guess the electrostatic (speaker-type) calibrators in common use today do not have this issue. I still feel compelled to document the temperature, RH, barometric pressure, wind speed, and wind direction when making outdoor sound pressure measurements. Atmospheric parameters have a greater effect on sound propagation than on the measurement instrumentation.

Walt

 

[GregLocock]

Makes sense. I guess B&K have optimised the drive impedance 1/2" calibrator so that it compensates for density, to some extent.

Cheers

Greg Locock

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