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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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Re: Using dB A or dB SPL

Postby KARTHIKEYAN » Wed Mar 08, 2017 5:21 am

Sir
I have a query.. I am using signal conditioner 482c series from PCB and picoscope 6.. iam measuring only dBu/dBv/dBm values.. is there any way of measuring dB SPL values?
Please help me...
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Re: Using dB A or dB SPL

Postby Gerry » Thu Mar 09, 2017 4:15 pm

Hi Karthikeyan,

I'm assuming that you're also using a PCB microphone, and looking at their site they appear to have a large range of microphones with non weighted scales for relatively high level signals, and a few microphones with lower range (A) weighted scales for lower level signals. As these are all analog output systems the handling of the sensitivity is very straightforward (the (A) weighting is treated the same, it's just that the low level signal reference is a filtered one to mirror the low level frequency response of the ear).

To display the measurement in the form of the decibel scale used for the sensitivity of the microphone you need the sensitivity of the microphone plus the conditioning circuit at the SPL reference level. If you don't have that then you need to calculate it (for example, if the sensitivity of the microphone is 50mV/Pa, and the gain of the conditioning circuit is x 4 then the sensitivity of the microphone and conditioning circuit is 200mV/Pa).
You can then use this voltage to set up the scale in our Picocope 6 software. So, in Spectrum View, if you select the 'Spectrum options' by clicking on the 4th icon from the left under the menu, then click on the arrow in the 'Logrithmic Unit' window to get the drop down list, and select 'Arbitrary dB', you will then get the text box to enter your sensitivity value. Having done that, your scale should now be displaying the Sound Pressure Level in dB with reference to the specified microphone frequency range (which could be a weighted or non-weighted bandwidth or single frequency) at a 94dB Sound Pressure Level (i.e. 1 Pascal, or Pa).

Regards,

Gerry
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