# DIY ('free') Microphone calibration (please comment)



## Chester (Feb 19, 2007)

I was thinking, and I *may* have stumbled upon a decent way to achieve a microphone calibration file for free (minus the cost of a few simple materials) with basically a given piece of metal rod used to generate a tone. When one strikes a metal rod (or any instrument I am aware of) two things happen, the fundamental frequency is generated along with harmonic (multiple) frequencies too. Since there is a science behind 1) what frequencies should be generated, and 2) what their amplitudes should be, if one were to strike a bell/chime/or some sort of simple harmonic instrument/generator, would we not be able to window the response with a fft to measure quickly both the spectrum and the amplitudes of said measured harmonics. If we take the difference between what the harmonics *should* be (their amplitudes) and what the measured amplitudes are, should we not be able to generate a pretty accurate calibration file with the points measured by the harmonics?

Thanks for your input!


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## keyboard (Jan 6, 2011)

Hi ,
Something like a tuning fork, with a known frequency?:scratch:


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## Chester (Feb 19, 2007)

right, i should have thought of a tuning fork, but instead of just looking at how accurate the tuning fork is (cycles per second) you would look at the level of the fundamental in relation to the level of the harmonics.

the levels of the harmonics should be predictable (not sure exactly how to predict those yet)

the difference between the prediction and the measurement would be the calibration file


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## JohnM (Apr 11, 2006)

The amplitudes and rates of decay of the fundamental and harmonics depend on the material properties of the object including the material damping and uniformity, its exact mechanical structure and dimensions, how it is supported and the spectrum of the force that strikes it. The rates of decay of the harmonics are faster than the fundamental and vary with the harmonic number, so the spectral content is time varying.


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## aceinc (Oct 24, 2006)

Two questions;

1) If one were to strike a tuning fork twice with differing amounts of force would the relative amplitudes and decay patterns of the primary and tertiary (harmonic) frequencies remain constant?

2) If one were to strike two different tuning forks of the same frequency with the same force, would the primary and harmonic frequencies be the same?

Both questions should be qualified by "close enough for the purpose intended."

If the answers to both questions are yes, then one approach would be to get a tuning fork, test it using a calibrated mic, and then everyone could use that type of tuning fork with the file created from the calibrated mic. You would probably need a few different frequency calibrated tuning forks to be able to calibrate a mic.

A concern that I would have with using a random tone source is that if I strike a pot lid with a particular primary frequency it sounds different than if I strike Marimba key, or a Xylophone key with the same primary frequency. I suspect the reason for the difference is the different harmonic frequencies generated.

Paul


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## robbo266317 (Sep 22, 2008)

This aproach, if it worked, would not allow calibration in the bass area. Unless you had a *Really Big* tuning fork. 
Also, as John said, the fundamentals and harmonics may decay at different rates and so would be difficult to use.


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## Chester (Feb 19, 2007)

in order to do this, i think we (whoever would use this method) would have to do as acenic mentioned and have a certain tuning fork or instrument that would be 'calibrated for' also. John mentioned that the harmonics decay at different rates, perhaps if we looked at x ms after the initial strike or impulse from the hammer or mallet... I just tried this and it appears that the 'plot spectrum' (under 'analyze') option in Audacity will plot the average frequency response for an interval of a recorded file, so that could be used to window out any room response.

If we were to get steel tuning forks, accurate to +/-0.05 hz of the same variety (same shape), it would stand to reason that they would be producing the same harmonics (especially if they were all the same brand tuning fork)

a standardized mallet system would be a good idea too (where to hit it, how hard, with what)

robbo266317: you are correct that we would not be able to calibrate the bass, response, however as far as I know, MOST measurement microphones have a pretty smooth roll-off response at low frequencies, it is the highs that get dicey, however there probably would be subharmonics generated too (i would think) just at lower levels, which we could use to get a general rolloff of the low frequency response

I think it would be fairly easy to do once all of the variables were considered (temp, type of tuning fork ex. brand, style, material, method of striking, angle of rotation of the tuning fork)...

also, as asked earlier, one last thing to figure out would be whether striking it hard vs soft would excite different harmonics more or less, again, a standardized way of striking helps here


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## lcaillo (May 2, 2006)

You don't have calibration unless you have a reference instrument and process. A tuning fork may be a useful reference for a single frequency, but there is no reference for amplitude as this is force variable. Mic calibration is a matter of determining levels over frequency changes. A tuning fork does not get you there.


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## glaufman (Nov 25, 2007)

That and getting a calibration curve custom made for a number of the popular mics is fairly inexpensive compare to the cost of the systems most of us are calibrating.


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