Sound Meter Calibrators - Page 1
Forum section: electronics / general

author: mendip_discovery
author_rank: Super Contributor
timestamp: July 25, 2021, 04:04:53 pm
content: 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.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: July 26, 2021, 07:34:07 pm
content: 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.htmThe 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.htmlIt'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_uncertaintiesAnd finally, one more here-https://www.mvaudiolabs.com/diy/microphone-calibration-pressure-chamber/

author: TimFox
author_rank: Super Contributor
timestamp: July 26, 2021, 07:51:27 pm
content: 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 athttps://www.bksv.com/en/calibration/calibration-systems/sound-level-meter-calibration-systems

author: GLouie
author_rank: Regular Contributor
timestamp: July 27, 2021, 07:40:24 pm
content: 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.

author: 1audio
author_rank: Frequent Contributor
timestamp: July 27, 2021, 09:15:14 pm
content: 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.

author: srb1954
author_rank: Super Contributor
timestamp: July 28, 2021, 06:39:00 am
content: Quote from: GLouie on July 27, 2021, 07:40:24 pmNominal frequency of a 4220 pistonphone is 250Hz but can be adjusted over the range 30Hz to 320Hz by feeding an external DC supply to the motor.QuoteSound level of a pistonphone is also dependent to a small extent an the barometric pressure. The Bruel & Kjaer pistonphone kit included a circular barometer which had a calibrated scale directly showing the dB correction to be applied.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: July 28, 2021, 12:49:23 pm
content: My GR reciprocity calibrator has a scale for barometric pressure. It's a small effect but it leads you down an interesting rabbit hole learning about local station pressure vs pressure normalized for altitude, which is what's normally reported, plus temperature compensation. For those used to precision in metrology, you'll find barometric pressure is remarkably less precise.

author: TimFox
author_rank: Super Contributor
timestamp: July 28, 2021, 01:29:16 pm
content: Barometric pressure: I had a similar issue with the calibration on an ionization chamber used to measure high-intensity x-ray flux, since it is vented to equilibrium with local atmospheric pressure. I could only find average values for absolute barometric pressure at about 6000 feet altitude. (The current in this chamber is proportional to absolute pressure.)

author: mendip_discovery
author_rank: Super Contributor
timestamp: July 31, 2021, 06:34:57 pm
content: I was looking at the basic in-the-field calibrators. We get asked all the time to calibrate customers meters and they are usually used in a factory environment to check that the CNC machines are not making too much noise etc. So not into VoltNut standards of precision.Often see them at 94 and 114db @1kHzMuch like this,https://www.ebay.co.uk/itm/132949032346

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: July 31, 2021, 08:30:13 pm
content: One hopes that, when you buy a new one, it's calibrated and comes with certs. The problem is that, sooner or later, you have to calibrate the calibrator.

author: mendip_discovery
author_rank: Super Contributor
timestamp: August 01, 2021, 05:32:59 pm
content: They often come with a CoC but that isn't much traceability. I do have a traceable sound level meter, what we have had to do is compare the Customer's and our Meter using a sound generator app on the phone. I would like to buy a nice meter but when for the same cost we could by some other item we need more than that then its a tough call.