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Using dB A or dB SPL

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Using dB A or dB SPL

Postby viral » Tue May 17, 2016 6:58 am

Hi everyone,

I have to do measurements with microphone. I got a sound calibrator ( GRAS Sound Calibrator type 42AB) and 5442A PicoScope. When I try to measure my calibrator, I got -46 dB which is normal because I have sinus of 5 mV.

Normally, the output of the calibrator is 114 dB SPL.

Do you know how I can have the value in dB SPL or dB A ?

Thanks everyone :mrgreen:
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Re: Using dB A or dB SPL

Postby Gerry » Thu May 19, 2016 12:40 pm

Hi Viral,

If the output of the calibrator is giving you -46dBu for your reference of 114dB SPL, then you can use the 'Arbitrary dB' setting for the 'Y-axis logarithmic unit' under the 'FFT options', and type in a reference of value of 1946V (which corresponds to 114dBu - 46dBu). This will convert the Y-axis to the dB SPL, or A weighted dB value that the microphone under test will be reading.

Regards,

Gerry
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Re: Using dB A or dB SPL

Postby viral » Fri May 20, 2016 8:23 am

Thanks you very much Gerry,

I didn't find the good value but it doesn't matter.
I need to measure sound pressure, my probe need to have Pascal unit for Y-axis. I didn't find how to do it

Do you know how to do it ?

My scope is the 5442A model
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Re: Using dB A or dB SPL

Postby Gerry » Tue May 24, 2016 1:36 pm

Hi Viral,

Strictly speaking, sound pressure is a different phenomenon to sound intensity. In your case the sound pressure of air molecules moving as a transverse wave is the cause and the sound intensity picked up by the acoustic transducer in your microphone and transformed into a voltage is the effect. The physical quantity of each of these vary differently, but they vary according to the acoustic impedance. We were able to convert sound pressure in dB SPL to sound intensity in dBu by assuming a reference acoustic impedance of 400N.s/m^3 (the actual acoustic impedance is likely to be slightly different, and changes with temperature as shown here: https://en.wikipedia.org/wiki/Acoustic_impedance) and by taking a common reference point. Unfortunately, the value that I gave you for the reference point was wrong. The actual reference would be the sound pressure level generated by the sound source in the microphone calibrator
(114dB SPL) for the corresponding voltage ratio read by the PicoScope (46dBu). So what i should have said was just type in a value of 501187V (114dBu) for the 'Y-axis logarithmic unit'. This is why you didn't have the correct value.

However, the above method works because the scales are changing at the same rate (decibels relative to a common reference). If you changed the scale to read Pascals then it would not be a logarithmic relative scale, it would be a linear absolute scale, which would not have any recognizable relationship with the decibel scale of the FFT plot. So there is no way to have the scale in units of Pascals.

Regards,

Gerry
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