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Old 2nd February 2006, 04:26 AM   #11
kevinkr is offline kevinkr  United States
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One simple thing no one here has apparently considered is the response of the microphone at low frequencies, certain patterns can exhibit a strong proximity effect boost at low frequencies. Also the roll off in the response may be quite deliberate based on one of several psychoacoustic weighting curves used for measuring spl. I would recommend leaving it alone for this reason.

I checked one of rat shack analog meters against a B&K (model forgotten it was long ago) and the calibration was quite close on both A and C weighting. (within a dB or so from about 100Hz up)
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Old 2nd February 2006, 04:32 AM   #12
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Do you have to do it yourself? Eric Wallin has done much the same previously: Radio Shack SPL Meter mods.
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Old 2nd February 2006, 04:48 AM   #13
kevinkr is offline kevinkr  United States
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The wallin mods defeat the weighting applied in these meters which is exactly what you need if you are trying to measure spkr frequency response or unweighted spls, or particularly in order to determine the flatness of the speaker/room response. If you are trying to correlate what sounds loud to human hearing you do need the weighting, so in the end it depends on what the meter will be used for.

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Old 2nd February 2006, 12:29 PM   #14
percy is offline percy  United States
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Let me provide some more info...

Quote:
Originally posted by kevinkr
One simple thing no one here has apparently considered is the response of the microphone at low frequencies
The RMAA response posted above does NOT include the mic. The mic was removed and signal was applied straight to the pads where the mic goes. So that response is only of the 'electronics' of the meter.


Quote:
Originally posted by NetRunner
Do you have to do it yourself? Eric Wallin has done much the same previously: Radio Shack SPL Meter mods.
True but like I mentioned earlier I also want to find out if its worth changing the IC or not. And I also wanted to find out more about pole analysis and how its done.

Quote:
Originally posted by kevinkr
T..if you are trying to measure spkr frequency response or unweighted spls, or particularly in order to determine the flatness of the speaker/room response.

...and that is the purpose.


Look at these two charts. These are of a woofer and a tweeter measured with a PC based speaker measurement setup (BLUE). Setup is a ECM8000 mic, UB802 mixer/preamp, Turtle Beach Santa Cruz soundcard and Speaker Workshop. Then the mic+preamp was simply swapped for the radioshack meter (RED). The meter has an rca/phono out so it could be essentially used as a mic+preamp.

Both measurements are nearfield (very close to the cone/dustcap).

I would have expected the shape of curves(not the amplitude for now, just the shape) to be same atleast in the frequency range where the meter electronics is flat. Notice the rise in response between 2k-5khz. My guess is that this behaviour could be because of the mic capsule used because that is the only thing different between the RMAA and acoustic tests.

First the Woofer -


Click the image to open in full size.


...and the tweeter..


Click the image to open in full size.


..
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Old 2nd February 2006, 12:33 PM   #15
SY is offline SY  United States
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What's the frequency limit where near-field curves cease to be valid?

Are the mike diaphragms the same diameter?
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Old 2nd February 2006, 01:43 PM   #16
percy is offline percy  United States
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while that is true for measurements intended for xover modeling, I fail to understand how that would matter in this particular case.

I have not measured the physical dimensions of the mic capsules but they seem to be very similar in size.
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Old 2nd February 2006, 08:51 PM   #17
percy is offline percy  United States
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oh yes I remember now, I did repeat the same measurements for "farfield" also and got the same results.
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Old 9th March 2006, 09:33 PM   #18
percy is offline percy  United States
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Do multiple poles (or zeros) add up to a net result or are they all independent of each other ?
for example if there are two or three poles in a circuit that have -3db points of say 8hz, 16hz and 6hz then will they somehow interact or add up to some final pole frequency ? or just the pole with the highest frequency(16hz in this case) simply becomes the most restrictive pole ?
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Old 9th March 2006, 09:51 PM   #19
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In the case of poles the lowest valued one tends to be considered the dominant one... the converse applies to zeros.

A 3rd order low pass filter with poles at 10, 100, and 1000 Hz would be considered a 10 HZ low pass filter.

A 3rd order high pass filter with poles at 10, 100, and 1000 Hz would be considered a 1000 HZ high pass filter.

This is somewhat of a word game though... and in the course belittles the real math behind it all.
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Old 10th March 2006, 10:54 AM   #20
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Greetings from Norfolk

Having in the distant past worked closely with engineers developing special microphones, I would say that the difference between the red and blue curves could be explainable just by the different microphones used. Mics do vary considerably, even between samples of the same type, especially near their 'limits' of frequency operation.

A plot of the ACTUAL performance of each of the mics would be very illuminating. This needs to hbe done very carefully, under controlled conditions.

Whilst the actual size of the mic capsule is a guide to its low and high frequency limits, the actual internal design modifies the detailed frequency performance - the type of damping and pressure equalisation material used for example.

Plot the curves for the two mics under identical conditions, using, ideally, a flat amplifier, with the correct loads on the mics.

Richard
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