dB and sound pressure

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http://www.google.com/search?hl=en&source=hp&q=microphone+sensitivity&btnG=Google+Search&rlz=1R2GPEA_en&aq=f&oq=&aqi=

"H"

** That electret mic is not handling the pressure, 2.5 volts is well outside its linear range. ** Would be over 146 dB SPL, if genuine.

( 8 dB + 44 dB + 94 dB )

...... Phil

=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D I thought the 0dB pressure reference was 1mw/10 dynes/cm^2 ?? (I know that 1mw across 600ohms is .775v. Sound guys have a head full of numbers like that)

"BobG"

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I thought the 0dB pressure reference was 1mw/10 dynes/cm^2 ??

** Mic sensitivity used to be quoted in mV per microbar, where 1 microbar = 1 dyne per sq cm = 0.1 Pa. 1 bar is very close to the standard atmospheric pressure and air pressure was for a long time quoted in millibars by weather bureaus.

The above mic works out as 0.63mV per ub - which is to say that a 74 dB SPL at 1 kHz would cause the mic to output 0.63mV rms.

..... Phil

This is indeed the instantaneous SPL (assuming, as you note, that the 2.5V is correct). Just a note to the OP, however, that this number will not correlate well with "loudness" since the sound is very brief. It will certainly sound a *lot* softer than (say) a rock band whose output measures 146 dB SPL on a sound level meter.

Another issue that seems to be generally overlooked by the general public is that SPL measurements give the loudness at the specific location of the measuring mic. In most cases, SPL falls off greatly with distance (unless you are very close -- in the "near field" -- to a very large sound source, for example).

I suspect the OP's microphone was closer to the target than a typical listener's ear. And since the listener will surely not be in the near field (which in this case is probably an inch away or less), the sound fall-off will be roughly a square law.

Some formulas for working with sound:

Best regards,

Bob Masta DAQARTA v4.51 Data AcQuisition And Real-Time Analysis

Scope, Spectrum, Spectrogram, Sound Level Meter FREE Signal Generator Science with your sound card!

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Since we are on the topic of sound units, I would like to know what is the output level (in volts, db, etc.) from a walkman earphone, compared to, say, a small pocket radio.

Thanks

Steve

=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D Go get a radioshack db meter and make some sort of acoustic coupler from the earbuds to the mic, note spl level for some particular music clip.. long sustained power chord??. Then measure volts during same passage, and resistance of coil. Since 8 ohm speakers have about 6 ohm resistance, I claim if the resistance is say 24 ohms, you can use 32 ohms as the estimated impedance. Calc power. V^2/Z. Repeat same calcs for other transducer. Now you can compare sensitivites by repeatedly doubling the power and adding 3dB of the weaker signal until the mWs are the same. I think the earphone transducer is 'more efficient' at delivering a certain spl per mW because of the inverse square law relationship of transducer to eardrum.

Harbor Freight has a similar meter:

$22.95

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You can\'t have a sense of humor, if you have no sense!

he topic of sound units, I would like to know what is

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Yes, thanks.

The problem is that I am in India, which is thousands of miles away from a Radio Shack.

There are db meter suppliers here, but could be rather expensive (for El Cheapo, like me..?).

During better times, I had acquired a frequency meter, along with a volt meter, which could also be used.

Since I have access to computers, an alternative could be to use software.

Steve

"Bob Masta" "Phil Allison"

** The OP clearly asked for a SPL in dB.

Learn to read sometime - pal.

** You bet it was.

And THAT is THE place he wants to read the SPL.

( snip rest of this wanker's irrelevant drivel )

.... Phil

"steve"

My Daqarta software includes a sound level meter option, but please understand that you can't measure an absolute sound level (like SPL) without a complete calibrated system. The first thing you will need is a calibrated microphone. A properly calibrated microphone is expensive. Some

3rd-party suppliers will sell inexpensive mics that they have calibrated, but you will pay for their labor. (In other words, you may pay $100 for a $10 mic that they have calibrated.)

If you don't really need precision and are just looking for comparisons (as you mention in your original post), you can use an uncalibrated microphone. The SPL reading won't really be true SPL, but you can compare the *difference* between SPL values obtained from different sources measured with the same mic... with a few caveats:

First, note that microphones don't usually have flat frequency responses. Since most people want to measure the total sound over a particular frequency range (A-weighted, etc), you might get bogus comparison results if one sound source has a different frequency response than another... one might be putting out more power in a region that the mic doesn't respond well to, while the other might be emphasizing a region that the mic is boosting.

If you are selecting a microphone for this, look for "omnidirectional" as opposed to "unidirectional" (or "cardioid", etc), since the latter sacrifice frequency response to obtain directionality.

You may be able to buy an inexpensive raw mic cartridge for a few dollars, and make your own mic. The Panasonic WM-61A is a good choice for flat response over a wide frequency range, though it is not especially sensitive.

Also, with software sound level meters you must either always run with the sound card sensitivity at maximum (so you know where you are) or you must calibrate the sound card input controls. Daqarta can do that automatically, using a loopback connection from the output. It may take 10-15 minutes or more as it scans over the control range, but when you are done you can change ranges and Daqarta will keep track of the attenuation so its displays compensate.

The above refers only to the *relative* attenuation of the sound card mixer controls, so that Daqarta knows when you've set (say) 20 dB of attenuation that the input must have been 20 dB larger than what it is measuring. This is a completely separate issue from *absolute* calibration, so that it knows what reading constitutes one Volt, etc. There is a separate procedure for that, which requires that you either have a known source or a known measuring device. You'd need both relative and absolute calibration, along with a microphone calibration, to measure true SPL.

Best regards,

Bob Masta DAQARTA v4.51 Data AcQuisition And Real-Time Analysis

Scope, Spectrum, Spectrogram, Sound Level Meter FREE Signal Generator Science with your sound card!

I just did the measurement again. It is correct. I also tried changing the distance between impact and microphone. At about a foot (the size of the sheet of paper I am using for a target), I still get about 1V peak.

I understand that. The pulse is about 40us long, so it'll probably sound like tiny pop, just like you'd expect when something pierces a sheet of paper at high speed.

I can't hear it at all over the mechanical noise from the rifle itself. The important thing here is that the signal is very strong, so it should be easy to detect it and feed to a microcontroller for timing measurements.

Thanks everyone for the comments.