Calibrating microphone with air hose hiss.

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The other day I downloaded a trial version of Spectrogram 7 http://www.visualizationsoftware.com/gram/gramdl.html very useful even in the demo version BTW. It is an audio spectrum analyser with a wiggly line, 1/3 octave, and coloured mess display. For best results you need to use a calibrated microphone of course, and the software has an input equaliser to flatten the respone of the mic you are using. Ater thinking long and hard, I hit upon the idea of using the air hose from my compressor and blowing the air past (not into!!) the mic and recording the results. IMHO it is almost a perfect white noise source that rises at 12dB per octave as it should. Just a matter of fiddling the mic equalisation till you have a upward ramping straight line, then away you go. :cool:
 
I did it with the nozzle fully open.
First, this is proper white noise generated with CoolEdit, saved to a CD and played into the sound card from the CD player.
 

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This is the equalised microphone response. I don't really know that it is true white noise but it certainly sounds like it. The equalised microphone measurements now make a lot more sense than what they did before.

On this last pic the vertical scale is 30dB per division, not 20dB like the other two pics.
 

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One thing I should mention is that I used the bare microphone insert. It used to be inside this plastic housing complete with clip, and there was a 2mm hole about 2mm deep through which the sound travelled on it's way to the mic. This made the response just rotten. :mad: Shown below is the original response to white noise. :xeye:
 

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Circlotron said:
This is the equalised microphone response. I don't really know that it is true white noise but it certainly sounds like it. The equalised microphone measurements now make a lot more sense than what they did before.

On this last pic the vertical scale is 30dB per division, not 20dB like the other two pics.
Hey, Circlotron

So what is your conclusion?

Can we say that a hair drier can be used
as a poor mans - white noise generator?

Can you describe to all of us,
what use we can have of white noise.
Is this of no interest if we have no Oscilloscope
and other "fancy" test instrument to go along with it.
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Are you discussing with the tiny expert-panel?
or with the vaste majority, who have solder irons & a multimeter
but not much more.

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Please, Explain to us many readers:
What is it all about? This WHITE NOISE thing.

halo - limping his way - in half-truths ;)
 
Everyone here has a pc, most of us can afford a $2 electret microphone, all of us like audio. You can plot frequency response graphs of loudspeakers if you feed them pink noise and use the software mentioned in the first post. Trouble is, the results are only as good as the microphone you are using. You need a known source of wideband noise to apply to your microphone first and see how it behaves so you can set your spectrum analyser software so the microphone frequency response appears to be flat with pink noise. (actually rising in the case of white noise) High pressure hissing air (100+ psi) seems to be a reasonable and reproducible example of true white noise .


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/Circlotron - being most patient with halojoy.
 
Subject of This thread - is this?

So Bottom line is:
You can use a Hair Drier
and some "Spectrum-Anylyse" software
to calibrate your elektret-microphones.

Have I got this right or ....
... am I not in the truth, yet :D :xeye:

/halo - wants to be where everyone else seem to be:
- some place better ....
 
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Joined 2003
Fan noise

Fan noise is most definitely not white noise. Acoustical engineers and submarine sonar operators can analyse the spectrum of fan (or propellor) noise and determine the number of blades and rpm. They then quieten it. The acoustical engineers charge a consultancy fee and use baffles/lagging. The submariners blow it up. They do not charge a fee.

Bicycle tyres (not mountain bikes) operate at 100psi, so that might be a good source in lieu of a compressor.

Many thanks for an excellent calibration suggestion.
 
i just built up a tubular wheel set.... the thing about the glue is true. its horrible.

i've wanted to go to vinyl but a) cant afford a nice turn table say a 1200. b) wouldnt have any thing but mom and dads old music......

any suggestions? on other decent players? and cheap ways to get albums? (i think my last one was the alf records that burger king gave out)

-eric
 
:) ANY OF THEaforementiomed solutions are pretty iffy I REckon, the min distance for measurements should be 1m unless you are doing NF measurements on ports or bass drivers to see how low they go!( but I cAN"T BEleive there were no instructions with your system! otherwise If I wERE you I would not have bought it!!)the best and most reliable way is to get your mike caliberated by a proffessional who knows what he's doing, then they will give you a floppy with the "cal" file of your mike on it which you load into your measuring programme, which by selecting when taking measurements will automatically compensate for the mikes natural response:idea: that's what I did when I built my IMP with MLS.... because I WANTed to be able to correlate my measurements to manufacturers published graphs:devilr: to see how accurate my measurements is to theirs or vice versa, albeit with one hiccup MLS ( maximum length sequence) pulse signals are more accurate at the top end than sine which a lot of them still use so you see tweeter performance as it really is at the top end.... anyway that depends on measuring system and location, how well one can gate the response acquired te remove room reflections or other artefacts that can affect your results, :nod:
 
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Joined 2003
saletel said:
Cheap ways to get albums?

Charity shops. Trouble is, you have to sort through a lot of "Easy Listening No 4003: Bert Kaempfert's Orchestra Plays Variations on Mozart accompanied by Eric Postlethwaite playing the Brighton Cinema Organ in the Style of Liberace." Out of all the dross, there is likely to be 5% that's worth trying. Of this, 50% has been wrecked by crystal cartridges. But it's still worth a go.
 
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