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Reference spl level.

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gen0me View Drop Down
Young Croc
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    Posted: 26 November 2018 at 4:23pm
How was the first reference microphone calibrated?
Where was the reference level taken from?
Was it done by sweep, fft and gating or playing single frequencies and writing the reading? And how to believe that microphone I use as the reference was calibrated right? And how to know what was the method of first calibrations to believe its reading especially on bass?
Wink
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Post Options Post Options   Thanks (0) Thanks(0)   Quote APC321 Quote  Post ReplyReply Direct Link To This Post Posted: 26 November 2018 at 5:22pm
I am not sure how the first reference microphone was calibrated.

However if you type "Bruel and Kjaer Pistonphone 4228" into google then you can see a device that they make for calibrating their range of sound level meters. It clips over the microphone on the sound level meter.

It is described as "a battery powered acoustic calibrator which when used with a B and K microphone produces a constant sound pressure level of 124dB +/- 0.2dB at 250Hz at the microphone diaphragm".

I guess that the first reference microphones were calibrated in a similar way.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote gen0me Quote  Post ReplyReply Direct Link To This Post Posted: 26 November 2018 at 6:06pm
As I understand it just generates one sound of frequency 251.2Hz
Idea of the cam disc is beautiful. Basicly changing the speed of rotations we can control frequency having constant amplitude. Im curious on how high frequencies this idea could be used. Membranes would have to be stiff enough to get rid of break up modes. Best also flat. Which increases mms so the obvious question is how much power can it take with increease of frequency.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote toastyghost Quote  Post ReplyReply Direct Link To This Post Posted: 26 November 2018 at 11:42pm
Which is why you have different levels of microphones with different class ratings and diaphragm materials. Each level up is calibrated to the reference mic class or level above it.

Once you get to real lab level, you’re in a whole different league and some esoteric stuff. Which isn’t really of interest to our domain because you better believe it isn’t being used to measure loudspeakers at that point. Most of the acoustics world is far from interested with our piddly little cones and output.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote efinque Quote  Post ReplyReply Direct Link To This Post Posted: 27 November 2018 at 5:07am
Interesting topic.. the decibel scale is the most widely used method for referencing sound pressure levels and perceived loudness.

Most digital systems use the ground as a reference but then those are measured in volts (which, depending on the resolution are stored with certain accuracy as integers or floats). In an analog system you can essentially do the same too but they have very broad range of different scales (dBm, dBu, LUFS, VU etc) which are internally dependant scales and give no "real" hint of the perceived loudness or the overall amplification. I think impedance plays a role in microphone level matching too.

In an ideal system you'd have a zero potential to which you'd be referencing, however such system is, if not impossible, at least very complicated to build. You also mentioned FFT which is interesting because microphones, like speakers, have very different frequency responses. In general you're looking at how much power (in watts for example) a device is able to generate within a specific frequency range with a given sound source.


Edited by efinque - 27 November 2018 at 5:09am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote gen0me Quote  Post ReplyReply Direct Link To This Post Posted: 27 November 2018 at 10:11am
Its a nomenclature fail.
I didnt mean calibration in terms of reference spl level. I meant calibration in sense of frequency characteristic. Ofc it may require few different calibrations for different spl levels.
Example will make it easier:
1) Lets say we make a sweep 512k of frequencies on reference speaker flat from 20hz-20khz. Ideal anechoic chamber. Note the spl level on the mic compare to the frequency played at the sweep moment and we will get some curve of microphone frequency response.
2)For the next chart we can use random noise. Get impulse response and gate it.
3)Now lets play single sinusoidal note 20hz than 21hz,22hz and so on. Note the resaults.

So now at least in theory:
1) should fail for example if reference speaker had compression driver in it. It could skip its very narrow excitations.
2) bass rollout due to finite sample length used for fft.
3) should show up exactly the long term notes level. It should also contain very narrow excitations.

So the obvious question is how the first mic was calibrated in frequency domain because only the same approach will give the same calibration independently from methodology fails.

I understand that comparing mic to mic the same method we should get the same result. I am not sure about gating and starting phase of lower frequencies but there is ofc way around using different method.

I wouldnt even give a shit about higher frequencies as those compression driver excitations should be seen with ridiculous unproportions but figuring out how the microphone was calibrated on bass frequencies isnt that easy. I cant imagine myself situation where I heard same spl level on 35hz and 70hz without just trust that it is right. I can only tell what sounds good to my ear.
Is there any device like this calibrator from the second post that would allow for a change of frequency?

I dont care if mic is omni or cardioid. Small membrane measurement ones are omni. Bass frequencies interest me the most.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Pasi Quote  Post ReplyReply Direct Link To This Post Posted: 27 November 2018 at 4:00pm
Are you asking how the first ever in the world measurement mic was calibrated?

Nowadays you got devices for that, and 100% out of reach price wise.

Regarding your number 2, FFT and assumed bass roll out. That is not an issue as in any software you can set the FFT block lenght so long that you get enough resolution, like 0.18Hz in Systune. I think that's not even the longest block.

Block length is also dependant on sampling frequency, so if you think you need more for the bass, you can drop the sampling rate. 44kHz sampling rate does twice the resolution compared to 96kHz on same block size.

I just got my new measurement mic. Good up to 100kHz and 168dB :)

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Post Options Post Options   Thanks (0) Thanks(0)   Quote gen0me Quote  Post ReplyReply Direct Link To This Post Posted: 27 November 2018 at 4:58pm
Originally posted by Pasi Pasi wrote:

Are you asking how the first ever in the world measurement mic was calibrated?
Yes exactly it Wink
Nice point with fft.
No wonder why those devices are so expensive. Problem is really complex. Any more info?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote madboffin Quote  Post ReplyReply Direct Link To This Post Posted: 27 November 2018 at 10:29pm
Originally posted by Pasi Pasi wrote:


I just got my new measurement mic. Good up to 100kHz and 168dB :)



What's that then? Better than my 4133 and 4134.

But I am guessing it must be a 1/4" type with those specs. Are you recording bats, or doing room acoustics with scale models?

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