×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

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

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

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

(OP)
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.

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

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

Quote:

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.

Quote:

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)

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

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.

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

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

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

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.

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

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

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

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

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

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources