pucho812

Mic calibrator, shooting rooms, volts pascal
« on: October 10, 2016, 08:27:58 PM »
What exactly is a mic calibrator and how does it work?

I asked the boys over at AP as I  just started to use the acoustic measurement portion of the APX software.

I am trying to ascertain correct level of what the preamp on the measurement mic should be.

The recommended a mic calibrator? which I am not Familiar  with.  What does it do and how does it work?

I just need to make sure the DBX measurement mic and subsequent preamp we are using with it, is set to allow a proper level back into the AP.  So far I got useable results in that it did register level enough to map things out but want to be sure I am at a correct nominal level.

Then what are the v/pa equations so I can covert that into dBu? I can't seem to find any direct information or online calculators. 

Would love to learn more about this... If anyone has some suggestions as such.
Every mic has a purpose it might be a door stop or a hammer, but every mic has a purpose.


Audio1Man

Re: Mic calibrator, shooting rooms, volts pascal
« Reply #1 on: October 10, 2016, 11:19:03 PM »
Quick overview: The CALIBRATOR is an acoustic signal generator that gives a known dBSPL. This is connected to the microphone and the amplifier output voltage goes to the AP input. The measuring signal is set to dBu and the OFFSET is adjusted to the dBSLP  the VALUE.
The CALIBRATOR take a few different forms. Loudspeaker or a mechanical piston. @ a single frequency.
Duke :)

pucho812

Re: Mic calibrator, shooting rooms, volts pascal
« Reply #2 on: October 11, 2016, 01:09:03 AM »
thanks duke....
Every mic has a purpose it might be a door stop or a hammer, but every mic has a purpose.

abbey road d enfer

Re: Mic calibrator, shooting rooms, volts pascal
« Reply #3 on: October 11, 2016, 03:55:43 AM »
Typical mic calibrator
https://www.amazon.com/Extech-407744-Sound-Calibrator-Microphones/dp/B001AFFVR6?th=1
It needs to be acoustically coupled to the mic, that's why it's specified for 1" and 1/2" mics, and is correct only for omni (pressure) mics.
There are less expensive models on eBay...
Who's right or wrong is irrelevant. What matters is what's right or wrong.
"The important thing is not to convince, but to give pause for thought." (B. Werber)
Star ground is for electricians.

arnyk

Re: Mic calibrator, shooting rooms, volts pascal
« Reply #4 on: October 11, 2016, 11:48:13 AM »
What exactly is a mic calibrator and how does it work?

I asked the boys over at AP as I  just started to use the acoustic measurement portion of the APX software.

I am trying to ascertain correct level of what the preamp on the measurement mic should be.

The recommended a mic calibrator? which I am not Familiar  with.  What does it do and how does it work?

I just need to make sure the DBX measurement mic and subsequent preamp we are using with it, is set to allow a proper level back into the AP.  So far I got useable results in that it did register level enough to map things out but want to be sure I am at a correct nominal level.

Then what are the v/pa equations so I can covert that into dBu? I can't seem to find any direct information or online calculators. 

Would love to learn more about this... If anyone has some suggestions as such.

Reading this  thread so far, I find enough useful information about mic calibrators so that we can proceed to the second half of your question:

"Then what are the v/pa equations so I can covert that into dBu?"

The key piece of information is that  " An atmospheric sound which generates a root-mean-square pressure of 1 Pascal has a power measurement 94 dB SPL"

Now the fact that so many mic calibrators have an available 94 dB SPL output starts making sense.  If you correctly mate a correctly operating mic calibrator with a mic (even one that is questionable),  the calibrator's  94 dB SPL output will cause the output terminals of the microphone, to have the same voltage as a 1 Pascal  root-mean-square acoustical pressure causes.

Mic calibrators are generally  built robustly so that they reliably generate a tone reasonably close the SPL they are marked to produce. Most calibrators are generally far more reliable than the inexpensive electret microphones that are generally used for technical acoustical measurements. 

To be more specific, I've been using ECM 8000 mics for acoustical measurements for a decade or more. I have a number of them that still give the expected results with my mic calibrator. I have another bunch of them that put out signals that read 10 dB or more low.  I should just throw them away.

 Pick an indication on your audio test gear that you want to use a  to your  reference point for 94 dB SPL.  Attach the calibrator to the mic, and the mic to the test gear, and adjust the mic preamp's gain until your test gear provides the desired indication.   

Whether the mic's output is accurate or not, calibrating your test setup to a standard acoustical souce like a mic calibrator ensures that your chosen reference indication  is as accurate as that of your mic calibrator.

Some calibrators have two SPL output levels such as 94 and 104 dB, so that you can estimate the linearity of your test setup over that range.  I have found cheap SPL meters that fail this kind of test pretty badly.

pucho812

Re: Mic calibrator, shooting rooms, volts pascal
« Reply #5 on: October 11, 2016, 12:36:01 PM »
wow guys a lot of info to digest. I typically deal in electronic measurements so this is new territory for me.  Thanks for the details.
Every mic has a purpose it might be a door stop or a hammer, but every mic has a purpose.

abbey road d enfer

Re: Mic calibrator, shooting rooms, volts pascal
« Reply #6 on: October 11, 2016, 01:36:09 PM »
Most microphone mfgrs specify the sensitivity in mV/Pa, sometimes in dBV/Pa.*
If a mic has a sensitivity of 1mV/Pa, it should measure 1mV when submitted to a sound pressure of 1Pa, which turns out to be teh equivalent of 94 dBspl.
On older spec sheets, the pressure unit the microbar, so the sensitivity was in mV/microbar.
The microbar is 0.1 Pa, so a sensitivity of 1mV/Pa translates in 0.1mV/microbar

*For example JLI's TSB165A
http://www.transsound.net/electrets/tsb-165a.htm is specified at -45 +/-4 dB referenced to 1V/Pa
That means a typical sensitivity of 5.6mV/Pa, with a possible variation in the range 3.6 to 8.9 mV/pa
Who's right or wrong is irrelevant. What matters is what's right or wrong.
"The important thing is not to convince, but to give pause for thought." (B. Werber)
Star ground is for electricians.

moamps

Re: Mic calibrator, shooting rooms, volts pascal
« Reply #7 on: October 14, 2016, 05:51:03 PM »
Just to add that 94dB SPL comes from

SPL (dB)=20log (1Pa/0,00002Pa) where
p=0,00002Pa  is threshold of hearing at 1kHz.


Typical mic calibrator
https://www.amazon.com/Extech-407744-Sound-Calibrator-Microphones/dp/B001AFFVR6?th=1

It's amazing to me that some manufacturers use upper case for "kilo". I will never buy theirs product.

abbey road d enfer

Re: Mic calibrator, shooting rooms, volts pascal
« Reply #8 on: October 15, 2016, 02:53:56 AM »

It's amazing to me that some manufacturers use upper case for "kilo". I will never buy theirs product.
Very common mistake; note that many PCB and simulator packages don't care.
I don't see that in said ad, though...
Who's right or wrong is irrelevant. What matters is what's right or wrong.
"The important thing is not to convince, but to give pause for thought." (B. Werber)
Star ground is for electricians.

Andy Peters

Re: Mic calibrator, shooting rooms, volts pascal
« Reply #9 on: October 16, 2016, 06:22:54 PM »
It's amazing to me that some manufacturers use upper case for "kilo". I will never buy theirs product.

What about manufacturers that use a lower-case 'm' for mega?
"On the Internet, nobody can hear you mix a band"


JohnRoberts

Re: Mic calibrator, shooting rooms, volts pascal
« Reply #10 on: October 16, 2016, 06:26:49 PM »
and use buss instead of the correct word (bus). >:( >:( >:(

JR
John Roberts
http://circularscience.com
Tune it, or don't play it...

benb

Re: Mic calibrator, shooting rooms, volts pascal
« Reply #11 on: October 17, 2016, 02:02:40 AM »
What about manufacturers that use a lower-case 'm' for mega?
I presume these offenses are current or recent, which makes them that much more egregious, since there has been a SI prefix standard for, I dunno, surely two or three decades now.

I remember capacitors marked MMFD for picofarads, so get off my lawn!