Electronics > Metrology

Sound Meter Calibrators

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mendip_discovery:
I am looking for a Sound Meter Calibrator or noise generator that I could make DIY etc. Has anyone gone down this route?

Often then are 97db and 114db fixed, so I assume its fairly commen parts inside. Just wondered about them as I see them online new for £80 to £200.

Conrad Hoffman:
Many/most of the old ones used what looks like a telephone handset ear cartridge. IMO, the biggest problem is you have to machine an interface, and probably an adapter, to the mic you want to calibrate. The other problem is calibration of the calibrator itself. Reciprocity methods can work but are tough to DIY. The old and somewhat rare General Radio 1559-B made the job easier but only works with piezo and some capacitor mics. http://www.conradhoffman.com/GR1559B.htm The GR 1567 is typical of the small single frequency calibrator and often sells for very little. But, you still have to calibrate the calibrator. There's an interesting thread about spark-gap calibration here- https://www.diyaudio.com/forums/equipment-and-tools/353494-spark-calibration-microphones.html It's long and I don't know if they reached a practical method, but it addresses a lot of issues. IMO, the whole calibration thing is bypassed if you can find a pistonphone calibrator. It's more of a machine shop project to DIY, but I suppose it would be possible. You just need accurate dimensions and some way to verify the travel. I think this can be viewed by anybody- https://www.researchgate.net/publication/275157767_Acoustic_metrology_-_an_overview_of_calibration_methods_and_their_uncertainties And finally, one more here- https://www.mvaudiolabs.com/diy/microphone-calibration-pressure-chamber/

TimFox:
Back in the late 1960s, one of my friends (working towards his BSEE) had a summer job at an industrial firm that used sound meters.  I think their lab equipment was from Brüel & Kjær.  He described the calibration system to me, and it apparently was a reciprocating piston driven from an appropriate motor.  Knowing the displacement and frequency (whence, velocity) allowed one to calculate the absolute sound intensity from first principles.
B&K's current calibration equipment can be found at  https://www.bksv.com/en/calibration/calibration-systems/sound-level-meter-calibration-systems

GLouie:
I have a full vintage B&K 2209 SLM kit right here, with a 4220 pistonphone. OP is probably thinking of an electronic calibrator, being easier and cheaper. I think we are talking old-school field calibrators here, good enough for a quick check in the field, but only at one frequency and level.

As mentioned, after designing a suitable microphone adaptor/coupler, the trick will be calibrating the calibrator. I think the electronic calibrators basically just have a little speaker and oscillator feeding the adapter/coupler. For hobbyist level confidence, I think I might just try to borrow a calibrated SLM and go from there.

This pistophone largely predated the electronic calibrators IIRC. They require precision construction. This one resembles the handle of a D-cell Maglite (or the handle of a lightsaber). The coupler, for 1/2 inch measurement mics, slips over the mic capsule and is sealed with an O-ring. There is a little chamber in front of the mic, and 2 tiny plastic pistons move in-out into the chamber, driven by a central cam attached to a voltage regulated DC motor. It says 124dB. No frequency mentioned on the unit, but I recall it being around 400 Hz. I assume the volume and dimensions of the chamber make a difference depending on the piston size and travel, etc.

1audio:
I'm one of those playing with spark calibration. Its interesting but very much a work in progress. It won't give an amplitude sensitivity and really only response at higher frequencies. And still not confirmed for accuracy for that. I have 6 difference calibrators. The pistonphones are the reference for sensitivity. The GR reciprocity may replace them but much too cumbersome for any regular use. The pistonphone can be used from around 320Hz on down by controlling the motor speed. The oldest variations have mechanical governors. Later are electronic. It would be interesting to fit a fully servo'ed brushless motor. However most measurement mikes are flat to well below 20 Hz so its not that important.

The mechanicals are critical. You need to know the acoustic volume of your microphone to make the overall volume correct. Since the internal volume of all the calibrators is small and small change from the calibration volume can make a significant change in level.

The pistonphone is still a standards grade calibrator, which the ones based on microspeakers are not. The pistonphone is very stable and predictable with a calibrated accuracy to .1dB. (Nothing in the specs about confidence level, my docs all precede that concept.) Unfortunately speakers have a number of aspects that affect their sensitivity including temperature, time, moisture and even how long they have been operating. As the voice coil heats up the sensitivity changes. That is compensated in some designs.

The references in Conrad's link are very good but way beyond anything useful in the field. I would shop eBay for an older/distressed calibrator and repair/modify it. The GR's can be pretty affordable. Having access to a properly calibrated SLM to calibrate  the calibrator is the only option for diy.

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