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Anechoic said:
The standard uncalibrated CM-140 tends to be within 1-3 dB of the actual SPL value out-of-the-box. That said, the verified meters I sell have been adjusted to the correct value and sell for about the same price you can get a CM-140 elsewhere (and a little cheaper with the HTS discount).
So in other words, 75 dB on the CM-140 might be 77-78 dB in reality?
 

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Just a question, as I heard a member talk about a calibrated CM-140 in the UMM-6 thread (not sure if was referring to the cross-spectrum calibration or not) but :

"i was previously considering the calibrated galaxy cm-140 (just to skip phantom power and mic pre) but after looking at the posted calibration result i decided to cancel. the meter have too much attenuation at the low frequency that it's not exactly precise anymore. -6db down at 20hz, -12db down at 10hz and -22db down at 5hz. it's only flat to 100hz."

Is this true? I just received my CM-140 calibrated by cross-spectrum. Is it still inaccurate? I'm confused by the above comments.
 

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The CM-140 is a sound level meter that only has A- and C-weighted settings. Both of those weightings roll off the high and low frequencies, with the A-weighting setting producding sharper rolloff, but C-weighting is still down 14 dB at 10 Hz. The correction curves I ship with those meters (in theory) will correct for those extreme rolloffs and flatten the response. However boosting the levels that much (as much as 25 dB at 5 Hz) can create distortion problems similar to what you would get if you boosted the level of a graphic equalizer by 20 dB. That's the potential issue one might have to deal with.

The meter and the calibration aren't inaccurate, it's just a matter to the degree that it needs to be corrected to produce a flat response.
 

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How do I know how much you boosted my curve by so that I know that there is low distortion as opposed to a lot of distortion?

I'm just trying to understand this. You mention distortion, but how do I know that my meter doesn't have gross distortion due to huge correction? What does that mean for me? How does this distortion influence the results?

I'm not clued up, clearly, but I wasn't aware there would be potential problems. I just thought I would get my meter calibrated for accurate results. Please try to clarify these things for me ...
 

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Are you the Vaughan from SA? If so, you got a "Verified" meter, so I didn't provide you with a correction curve (as opposed to the "Verified+" meters which are provided with correction curves). If you use your meter as a microphone with a computer program (REW, etc), the low and high frequencies will be rolled off. There's not much you can do about it except to find a generic correction curve someplace and use that to compensate. But as a sound level meter reading absolute C- or A-weighted SPL's, it will be accurate.
 

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Hi there,

Yes, that's me. I'm using this meter purely for setting levels, so you say that everything is accurate for that application and definitely more accurate than the standard meter? Correct me again, what does the "Verified" option do? I've clearly gone off the rails here as I must have assumed something else.
 

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The CM-140 is a sound level meter that only has A- and C-weighted settings. Both of those weightings roll off the high and low frequencies, with the A-weighting setting producding sharper rolloff, but C-weighting is still down 14 dB at 10 Hz. The correction curves I ship with those meters (in theory) will correct for those extreme rolloffs and flatten the response. However boosting the levels that much (as much as 25 dB at 5 Hz) can create distortion problems similar to what you would get if you boosted the level of a graphic equalizer by 20 dB. That's the potential issue one might have to deal with.
There aren't any distortion problems if using a meter with software like REW, since the signal isn't actually being boosted in a hardware sense, the displayed level in measurement responses is simply scaled up to reverse the attenuation of the C weighting curve. The main problem is that signal and noise both get scaled up, so the measurement result at the very lowest frequencies has more noise than if measured with a mic. None of this is an issue if using a meter to measure SPL, however.
 

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I just need reassurance that a) this verified meter is more accurate than the standard meter b) more accurate than the RS meter for measuring levels. That's it.

For measuring REW I'll get a UMIK-1 at a later date.
 

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I just need reassurance that a) this verified meter is more accurate than the standard meter b) more accurate than the RS meter for measuring levels. That's it.

For measuring REW I'll get a UMIK-1 at a later date.
a) if by "standard meter" you mean "Galaxy CM-140 meter purchased at Amazon or someplace else, then the answer is "yes, the Verified meter is more accurate" by virtue of me putting an acoustical calibrator on it and setting the proper level. It's not going to be as accurate as a Bruel & Kjaer 2250, but then again is doesn't cost $8,000.

b) yes, it's more accurate than the Radio Shack meter for the same reason, plus the CM-140 has a smoother frequency response.
 

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Ok, sorry if this is totally obvious to everyone, but I want to make sure. Tried doing a search but couldn't find the answer. I have a EMM-6 from CrossSpectrum and wanted to be sure exactly where I put the calibration file. So I put it under preferences -> Mic/Meter -> then browse to the calibration file, correct? and make sure the C-weighted box is not ticked? And do I use the narrow band 90 degree file?

Then do I do the same process of SPL calibration using the REW SPL meter button as described in the REW help files but with the microphone instead of an SPL meter? Anything else I need to do?

Thanks
 

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I have a EMM-6 from CrossSpectrum and wanted to be sure exactly where I put the calibration file. So I put it under preferences -> Mic/Meter -> then browse to the calibration file, correct?
Correct.

and make sure the C-weighted box is not ticked?
Correct
And do I use the narrow band 90 degree file?
If you plan to point the mic straight up, yes.

Then do I do the same process of SPL calibration using the REW SPL meter button as described in the REW help files but with the microphone instead of an SPL meter?
Yes.

Anything else I need to do?
Nope.
 

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A few questions for you Anechoic:

1. Can you calibrate a measurement mic that isn't one of the usual ones you sell or advertise calibration for if it appears to have the same or similar body?
2. Would putting it vs. an EMM-6 in your hearing aid chamber provide any useful information as to whether its self-noise is lower? The mic I'm looking at has a self-noise rated similarly to the LinearX microphone we discussed before but at a much lower cost, also it can use 48V rather than 9V phantom power.
3. I'm looking at measuring lateral fraction with ARTA, which requires a bidirectional mic. However I can't seem to find any bidirectional measurement mics easily. Any ideas, or suggestions where to go to find an answer?
4. Same question as 3 but with IACC and binaural in-ear mics (with myself as the dummy, given that I come a lot cheaper than the proper dummy heads). Any suggestions on what is a decent mic set for this purpose and what isn't?

Thanks,
John
 

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A few questions for you Anechoic:

1. Can you calibrate a measurement mic that isn't one of the usual ones you sell or advertise calibration for if it appears to have the same or similar body?
Yes. I can and do measure mics of all kinds of shapes and types, including ribbons and large diameter condensers, as well as various mics with the ECM8000 body-type.

2. Would putting it vs. an EMM-6 in your hearing aid chamber provide any useful information as to whether its self-noise is lower? The mic I'm looking at has a self-noise rated similarly to the LinearX microphone we discussed before but at a much lower cost, also it can use 48V rather than 9V phantom power.
I can measure down to about 16-17 dBA, so as long as the mic noise floor is above that (typical EMM-6 noise floor is around 32 dBA), I can give you a number. For mics with a noise floor below 17 dBA, all I can report is a value of "<17 dBA."


3. I'm looking at measuring lateral fraction with ARTA, which requires a bidirectional mic. However I can't seem to find any bidirectional measurement mics easily. Any ideas, or suggestions where to go to find an answer?
"Bidirectional measurement mic"? Does such a thing exist? Usually when one is trying to make a directional sound measurement, you use a sound intensity probe, mic array, or two mics with a hard planar surface between them.

4. Same question as 3 but with IACC and binaural in-ear mics (with myself as the dummy, given that I come a lot cheaper than the proper dummy heads). Any suggestions on what is a decent mic set for this purpose and what isn't?
For this, I would just recommend using a pair of any reasonable omnidirectional microphones (this is a decent one, so long as you don't need data below ~ 50 Hz) and attach them to a pair of glasses near your ear, and use a cal file to correct any frequency response deficiencies of the mic (the Radio Shack mics are fairly flat up to about 10 kHz).
 

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"Bidirectional measurement mic"? Does such a thing exist? Usually when one is trying to make a directional sound measurement, you use a sound intensity probe, mic array, or two mics with a hard planar surface between them.

For this, I would just recommend using a pair of any reasonable omnidirectional microphones (this is a decent one, so long as you don't need data below ~ 50 Hz) and attach them to a pair of glasses near your ear, and use a cal file to correct any frequency response deficiencies of the mic (the Radio Shack mics are fairly flat up to about 10 kHz).
Thanks for the replies. I will contact you through the website if I get the mic.

A quote from the ARTA manual:
Code:
Early lateral energy is being measured with two close spaced microphones: omni-directional and 
bidirectional (with figure of eight directional pattern). Interaural cross correlation coefficients are 
measured with either a dummy head, or a real head, and with two small microphones placed at the 
entrance to the ear canals.
and later
Code:
To measure spatial parameters a sound system with two microphone input channels is required. For 
measurement of IACC two small identical microphones must be used at the entrance of the ear 
channel of a dummy or a real head. For measurement of lateral energy fraction, an omnidirectional 
microphone should be connected to left channel and bidirectional microphone to right channel. 
Microphones should be calibrated, at least average difference in sensitivity have to be known.
I'll investigate the devices you mentioned since I'm not familiar with them. Anyway I figured a bidirectional measurement mic didn't exist since I couldn't find anything specifically called that. However a mic array or two mics with a plate between can't be used since it is only one channel of acquisition (unless possibly it's two mics with a mixer I guess). Anyway for this one :dontknow:
 

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aackthpt;585217 I'll investigate the devices you mentioned since I'm not familiar with them. Anyway I figured a bidirectional measurement mic didn't exist since I couldn't find anything specifically called that. However a mic array or two mics with a plate between can't be used since it is only one channel of acquisition (unless possibly it's two mics with a mixer I guess). Anyway for this one :dontknow:[/QUOTE said:
The book "Room Acoustics" uses almost the same wording as the ARTA doc. It's a weird thing to recommend, but I suppose that is the technique that's used. I'll post a message to the National Council of Acoustical Consulting forum and see what those folks have to say about the matter.

In that context "bidirectional" refers to the mic's polar pattern, see http://en.wikipedia.org/wiki/Microphone#Bi-directional
Right, but so far as I know, no one makes a measurement microphone in anything other than an omnidirectional pattern. I've measured by share of figure-of-eight recording mics and have been hard-pressed to find ones that are flat outside of 200 Hz to 7-8 kHz (usually by design).


edit: just found this link which discusses using a dummy-head measurement to determine the lateral fraction (end of sec 5.2.1)
 

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Hi all.

I recently bought an Earthworks M30 in order to use it for acoustic measurements and subwoofer testing and maybe DIY projects in the future.
I am using a Focusrite 2i2 as a mic pre amp that is flat to 10Hz from measurements I have found but will also be replaced later by an even more accurate preamp.


Now, I want to be able to measure absolute SPL levels for subwoofer max output testing and I was hoping to use the M30 in conjunction with REW or ARTA at very low frequencies down to 10Hz and later on, as subs are added, even lower.

My questions are:


A) The Earthworks M30 came with a calibration file but only down to 770Hz. Contacting earthworks their engineer told me to get the calibration value form the 775Hz (not there) and copy it down to the desired frequency (10 or 5Hz) at appropriate intervals or leave it blank as the mic is accurate +-0.25d db down to 5Hz.

Is there a point to send the mic for an extra calibration to cross spectrum labs? Would extra information such as polar response be of value? What a bout placement when measuring? (Vertical, facing)


B) Is REW appropriate for measuring max spl levels for subwoofer testing? Will providing it with a reference 75db point while calibrating its SPL be enough to then measure accurately the rest of the frequency band with the M30?


C) My SPL (US Blaster) crapped out so I bought one for my iphone 4 (studio six digital) for the moment. I was thinking of buying the Galaxy from Cross Spectrum Labs with verified + calibration. Will this be appropriate then as a ref point at 75db or any other software? Is there something better I could get from cross spectrum if needed?


Thank you in advance.
 

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A) The Earthworks M30 came with a calibration file but only down to 770Hz. Contacting earthworks their engineer told me to get the calibration value form the 775Hz (not there) and copy it down to the desired frequency (10 or 5Hz) at appropriate intervals or leave it blank as the mic is accurate +-0.25d db down to 5Hz.

Is there a point to send the mic for an extra calibration to cross spectrum labs? Would extra information such as polar response be of value? What a bout placement when measuring? (Vertical, facing)
The M30 spec page shows typical frequency response being flat to 10 Hz and down 3 dB at 5 Hz. Based on that, if you are looking for high accuracy at 5 Hz, having the microphone calibrated might be wise. Polar response is only an issue above 1 kHz or so with a good measurement omni, and there are guidelines in the REW help file and online help threads for mic placement and angle. For some measurements, off axis-angles are useful. For off axis accuracy at the higher frequencies, the additional info from an off-axis calibration would be needed.

B) Is REW appropriate for measuring max spl levels for subwoofer testing? Will providing it with a reference 75db point while calibrating its SPL be enough to then measure accurately the rest of the frequency band with the M30?
Yes, and yes. Calibrating the soundcard frequency response will be necessary for the accuracy you want at those low frequencies. For measuring max SPL levels, recalibrating REW at a higher SPL might be needed.

C) My SPL (US Blaster) crapped out so I bought one for my iphone 4 (studio six digital) for the moment. I was thinking of buying the Galaxy from Cross Spectrum Labs with verified + calibration. Will this be appropriate then as a ref point at 75db or any other software?
Yes. Absolute SPL accuracy is not super critical, you just need a repeatable calibration source in case the S EL calibration in REW gets lost and needs to be redone.
 
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