Sound input to SPL level measure

[shjukheter]

Hello,

            First of all, I am not using any NI Devices for my project. I am acquiring a motor sound to .WAV file by the help of "sound input to file" example. I have connected my headset microphone near to motor and the correct jack is plugged to soundcard of my PC. I really want to calculate the SPL(sound pressure level) in dB of each motor to see weather it varies or not. I don't how to calculate this sound level in labview. Can some suggest me any VI for this operation. Please help me.

[Coq_Rouge]

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@shjukheter wrote:

Hello,

            First of all, I am not using any NI Devices for my project. I am acquiring a motor sound to .WAV file by the help of "sound input to file" example. I have connected my headset microphone near to motor and the correct jack is plugged to soundcard of my PC. I really want to calculate the SPL(sound pressure level) in dB of each motor to see weather it varies or not. I don't how to calculate this sound level in labview. Can some suggest me any VI for this operation. Please help me.

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The only way to do this is to use some sort of calibration with professional equipment. In your setup it is way to many unknown factors. What you could do is to do some normalization using a group of motors known to be OK. Then detect a bad motor as deviation from this standard

[shjukheter]

Yes, exactly I though the same way but first how should I calculate the SPL for the connected motor. Is there any VI for this operation.

[kuro-gane]

Hi, shjukheter,

Coq Rouge is correct that to get a real value for SPL you will need a sensor designed for that purpose.  Sound Pressure Level is a physical quantity just like pressure or temperature, which you need a specific sensor to measure.  Microphones convert sound pressure into electrical signals, but are generally not meant to measure an actual value for SPL.  If you want to get an actual value, buy a cheap SPL meter and take measurements at the location your microphone is at.  Also measure with your microphone, then relate the value from the SPL meter to those you get in your program with the mic.  I bought an SPL from RadioShack for about $70 about 10 years ago.  It actually has an analog output on an RCA jack, so you could potentially use that instead of your microphone to take your measurements.  If you are not in a location that has a radioshack, I'm sure you could find an SPL meter on line.  I did something similar with my SPL meter:  I wrote a program to send audio signals to my home theater then I measured the SPL at the listening spot to find and chart the resonances in the room.

Hope that helps

Craig

[shjukheter]

Thank you so much for good explanations.

Yes, I have a digital meter which calculates the SPL. So, I have taken the SPL values by using that meter in bad and good case. I need to check with my microphone in reality. After these discussions, I came to know that my microphone SPL values may vary from these readings. Though, I would like to calculate this value from my microphone. Is there any vi can do this?

I would like to calculate this value when my microphone is continuously acquiring the sound data.

[shjukheter]

If I get the TDMS signal from an microphone condenser, How can I convert that continuous waveform into SPL value. I just know the formula for SPL is 20 log(Prms/Pref). Pref is usually the air SPL value(20 µPa RMS) and what value should I have to take as Prms. Is that from "Cycle AVg RMS value VI" in labVIEW.

[kuro-gane]

That sounds like a good place to start.  The signal coming from your condenser mic should basically be an AC signal, so taking the RMS of that should give you a fairly constant value to relate to your SPL.  The tricky part will be scaling the RMS value you get from your mic so it matches the actual SPL you get with your meter.  There are generally a couple of approaches to do this.  The simplest one is to do a linear scaling where you take SPL and microphone measurements at 2 different points and plot a line between them.  Then to get an SPL value for your microphone, just use the equation of that line.  This won't be accurate if your microphone does not have a linear response, though, which most microphones don't.  The other thing you could do is take many measurements over the region of interest and then enter those values in an array constant in the program.  When you get a value from your mic, look at where that value falls in your constant array, then use the interpolation vi to get a closer value.  The more values in your look-up array, the more accurate the conversion from your mic will be.

Any way you look at it though, you need to correlate the value of your SPL meter with the value your condenser mic is reading in the program.  There are no formulas that will do this automatically for you, because you have no idea what the physical spec's of your condenser mic are.