# Measuring comparisons with two mics



## Phillips (Aug 12, 2011)

Hi please can someone tell me the best process to measure two mics?

I thought i would measure the center speaker.

Do i measure near field, e.g. 25 inches from the speaker?


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## robbo266317 (Sep 22, 2008)

If you want to measure just the speaker/mic response then, yes, put it close to the speaker. 
What are you trying to determine?


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## Phillips (Aug 12, 2011)

Thank you.

I am trying to compare mics only, I intend to make a sound card correction file for my mic.


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## Barleywater (Dec 11, 2011)

I've been working up an extended response to your microphone comparison question. This will get you started:

Omnimic and XTZ appear to have very similar bodies/tips. This eases comparison. Diffraction of tip and body effect microphone performance.

Slightest movement of microphones change results. A placement jig is almost mandatory.

REW All SPL tab and trace arithmetic tools are best way to make comparison.

First acquaint yourself with how sensitive comparison is to microphone placement:

A single driver is best. For starters though place microphone tip 1/4" from one of your speakers drivers, midrange or tweeter. Sweep volume should be kept low, since microphone is so close. 
Several sweeps should be done first to establish good level. Then run two sweeps back to back without touching microphone. Move microphone in any direction about an 1/8" and make a third sweep. 

In trace arithmetic do A/B function. Set B= 1st sweep, and A= 2nd sweep. Press Generate. Result will be a very flat over much of range. Set A= 3rd sweep and press generate again. Compare the two results and see difference. Now try to place microphone back in original position, sweep, generate result with A= 4th sweep. Is it identical to first comparison? Very touchy.

I do this with 25mm metal dome driver with no crossover and voltage of sweep <0.4V. Driver is 4ohm, so this is small fraction of 1 watt and poses no risk to driver. At distance of 9 inch results are less sensitive to tiny microphone movements. I also use 5" aluminum driver mounted in enclosure to get better low frequency results.

Drivers and microphones are >3ft from floor, wall, and furniture. For 200Hz up, gated measurements produce better results. For <200, big room high off floor, outdoors and elevated, or closed coupler(see below) is best.

Doing this with pair of Earthworks OM1 microphones shows microphones have very similar performance. Swapping in ECM8000 produces results similar to typical calibration curves for these microphones.

Good reading:

How Earthworks measures microphones

and

ARTA measuring box

Cross Spectrum uses similar methods, and they maintain laboratory standard reference microphones for comparisons.

Regards,

Andrew


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## Phillips (Aug 12, 2011)

Barleywater said:


> I've been working up an extended response to your microphone comparison question. This will get you started:
> 
> Omnimic and XTZ appear to have very similar bodies/tips. This eases comparison. Diffraction of tip and body effect microphone performance.
> 
> ...


Hi Andrew, thank you.

Is there any way i can measure the laptops sound card, i have a mic input & headphone out?

The XTZ mic is a Version 1 (goose neck)The question could be asked, how accurate is the XTZ mic?

I haven't got a single driver to measure.

I have attached (below) two txt files exported + the same data as jpeg. I have given it ago and placed the two mics straight against each other (side by side), pointing at the middle of the center speaker, measuring the center speaker full range, about 20 inches away.

What if i change positions for a average (think you mentioned) closer, further away, listening position etc?

Unfortunately i don't have the resources to do a really accurate result.

I will read the attachments.

Thank you very much.

If i could only measure the laptops sound card.

View attachment Omnimic.txt


View attachment XTZ Mic.txt


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## Barleywater (Dec 11, 2011)

> Is there any way i can measure the laptops sound card, i have a mic input & headphone out?


If purely microphone input, jack is likely mono. Small DC voltage is present across shield and signal to drive fet/jfet in electret microphone capsule. Nonetheless, a loopback measurement may be performed from headphone out to microphone in using appropriate stereo to mono cable. If you search forum, numerous posts exist about the dark side of laptop soundcards regarding this topic. If on PC device, recommend sample rate of 48k, but mostly recommend getting good external soundcard, and calibrated microphone.

Also highly recommend following outline of previous post for using Trace Arithmetic, and seeing just how sensitive measurements are to both placement and proximity.

In normal usage measurement microphone tip is isolated from other objects. Body shape is typically designed to even help isolate tip from rest of microphone. Notwithstanding, deviations of two microphones performance is wild.

I just performed quick demo with 25mm tweeter at 24cm. Two measures with OM1, two measures with OM1 and ECM8000 placed with tips lined up and close together, and two measures with ECM8000.

All measurements are gated 4ms/6ms pre-peak/post-peak Blackman-Harris 4 windows.

Raw frequency response picture has ECM8000 trace offset to converge with OM1 traces <2kHz. Despite being two completely different manufacturers, the degree of correlation of two microphones below 2kHz is obviously very high:









For each double measurement Trace Arithmetic A/B is generated. Additionally A/B is also generated for OM1:ECM8000 pair compared to OM1, and for ECM8000 compared to OM1. These additional traces are offset to reference 0dB at 1kHz in plot displays.

Zoom of doublet comparisons:









Note in above pic the dB scale. Reference comparisons show measurement repeatability to fractions of 1/10th dB. This sort of repeatability is mandatory when generating calibration grade data.

Presence of ECM8000 next to OM1 causes roughly +1.3dB, -1.5dB:









Same with 1/3 octave smoothing:









The smoothed picture clearly shows typical ECM8000. This reflects well on old OM-1 microphones.

Compare to pic in Cross-Spectrum Microphone Calibration Service post#43

My upper end measurements of ECM8000 run with the pack, suggesting OM-1 has very flat response as indicated in calibration chart.

Back to your dilemma: 

This is same view as post above, but with plots referenced to cross at 1kHz:









A/B arithmetic referenced 0dB at 1kHz:









Something is truly amiss. You really need to do double measurements, and A/B arithmetic. If a very flat result isn't seen, then something is very amiss indeed. I would rerun your measurements, and with Omnimic also run with Dayton software. When you do this, which REW result looks closer to Dayton result?


Regards,

Andrew


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## Phillips (Aug 12, 2011)

Barleywater said:


> If purely microphone input, jack is likely mono. Small DC voltage is present across shield and signal to drive fet/jfet in electret microphone capsule. Nonetheless, a loopback measurement may be performed from headphone out to microphone in using appropriate stereo to mono cable. If you search forum, numerous posts exist about the dark side of laptop soundcards regarding this topic. If on PC device, recommend sample rate of 48k, but mostly recommend getting good external soundcard, and calibrated microphone.
> 
> Also highly recommend following outline of previous post for using Trace Arithmetic, and seeing just how sensitive measurements are to both placement and proximity.
> 
> ...



Thank you

I will buy a stereo to mono cable and measure the sound card. I tried to measure the laptop sound card earlier (without stereo to mono cable) and posted the graph and JohnM said it looked like a mic input measurement and that if so the measurement can't be done. But i will give it ago. Please see on "Mic Thoughts" thread post # 124, also there is mdat file on the sound card measurement.

I agree about the outboard sound card and mic, but trying to make the most what i have. 

With the XTZ mic the below 30hz is a significant difference in response and shape?

This might sound a silly question, but why would a "_very flat result isn't seen, then something is very amiss indeed"?_

On "Mic Thoughts" thread post # 89 there are mdat file on the comparison with REW & Omni software responses.

Thanks again


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## Barleywater (Dec 11, 2011)

With measurement microphones, differences of <1dB are significant. It isn't just below 30Hz, the two measurements are wildly divergent across most of range. The graphs can be shifted so that small regions line up fairly well, but there should be range as with my OM-1 and ECM8000 that line up near perfect over extended range.

My suspicion is that Omnimic has proprietary scramble filter. If measurement with XTZ microphone with REW very closely matches Omnimic measurement with Dayton software over extended range, then suspicion is likely true. In this case some manner of calibration file for the Omnimic may be generated for use with REW.

Old data is no good, it doesn't contain XTZ data. 


For laptop you need to split the headphone output so that either left or right channel may be fed as mono into microphone input. Where are you going with this? Are you thinking of using loopback as dedicated timing reference?

Regards,

Andrew


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## Phillips (Aug 12, 2011)

Barleywater said:


> My suspicion is that Omnimic has proprietary scramble filter. If measurement with XTZ microphone with REW very closely matches Omnimic measurement with Dayton software over extended range, then suspicion is likely true. In this case some manner of calibration file for the Omnimic may be generated for use with REW.
> 
> 
> For laptop you need to split the headphone output so that either left or right channel may be fed as mono into microphone input. Where are you going with this? Are you thinking of using loopback as dedicated timing reference?
> ...


Thanks Andrew

The reason for measuring the laptop sound card is to load in to the sound card calibration (REW) to allow for correction, like the mics calibration file. What does the dedicated timing reference do in simple terms?

I will do some more measurements with the XTZ, Omnimic & Omnimic software and import the file into REW. 
Also i will keep mics away from each other.

Have you or do you know anyone that has used a Steinberg C1 Pre / Soundcard?

Thanks again


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## Barleywater (Dec 11, 2011)

I don't know anybody with Steinberg interface. Are you describing this one:









Specs for it are USB1.1, THD+N <0.05%, dynamic range 96dB, and 24bit. With its distortion and dynamic range figures, it might as well be 16bit.



Does your laptop have USB2.0?

Regards,

Andrew


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## Phillips (Aug 12, 2011)

Barleywater said:


> I don't know anybody with Steinberg interface. Are you describing this one:
> 
> View attachment 37690
> 
> ...


Hi Andrew yes that is the one.

Is the speacs not good enough?

I am sure that my laptop has USB2.0.

Thank you


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## Barleywater (Dec 11, 2011)

If sole purpose of acquisition is for measurement, it may be fine. 

Likewise if we sort out Omnimic response mystery, a suitable calibration may be derived for it. This is matter of finding microphone for reference that is trustworthy. It may be XTZ, but an additional reference would be good. Do you have access to a calibrated ECM8000?

Andrew


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## Phillips (Aug 12, 2011)

Barleywater said:


> If sole purpose of acquisition is for measurement, it may be fine.
> 
> Likewise if we sort out Omnimic response mystery, a suitable calibration may be derived for it. This is matter of finding microphone for reference that is trustworthy. It may be XTZ, but an additional reference would be good. Do you have access to a calibrated ECM8000?
> 
> Andrew


Hi Andrew

The Steinberg would be only for REW, is that ok?

Unfortunately i have no access to a reference mic. I asked around to see if i could get the Omnimic calibrated but no one could help. To send overseas the cost would be close to a new mic.

The laptop sound card measurement loaded into REW would this be worth doing, as suggested in earlier post? 

Attached is the mdat sound card file. Which one looks the best to use?

On graph 2 & 3 i "Disabled All Sound Effects" in the "Microphone Properties" enhancements in the control panel. Do i able this again or leave disabled?

View attachment Soundcard.zip


Thank you


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## Barleywater (Dec 11, 2011)

Graph 2 & 3 are perfectly acceptable, use those settings. For this setup distortion is <0.03% above 100Hz, and rises to about 0.09% going down to 20Hz. Very good for built in. This beats published spec for Steinberg device over most of range. Tweaking levels could drop distortion at 20Hz, good to know but for your purposes built in will work.

I would explore raising levels and checking distortion. It's good to know level that maximizes dynamic range, without clipping.

Also do several loopback measurements with system set for 44.1kHz, and post. I've seen Windows do horrible things to signals running at CD rate.

Doing soundcard cal will compensate for low frequency roll off and high frequency ripple.

If you have generic microphone that uses 3.5mm plug, some comparison measurements are in order. DIY measurement style microphone easy to build. These range from simply adding cable and plug, or using battery to boost performance and allow use with standard line in, up to building battery powered with balanced drive, or fully phantom powered setups.


Referencing is still issue. Microphone performance will have more impact on measurements than soundcard.

Please follow up with Omnimic/Dayton v XTZ/REW comparisons.

Regards,

Andrew


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## robbo266317 (Sep 22, 2008)

Do you have the ID for your Omnimic.
If so the software here:- http://www.libinst.com/Omnimic/OmniMicInstall.exe may be of use.


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## Phillips (Aug 12, 2011)

Also do several loopback measurements with system set for 44.1kHz, and post. I've seen Windows do horrible things to signals running at CD rate.


Thanks Andrew

Do you mean in the laptop control panel or REW?

I "Disabled All Sound Effects" in the "Microphone Properties" enhancements in the control panel for graph 2 & 3. Do i able this again or leave disabled for further measurements?


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## Phillips (Aug 12, 2011)

robbo266317 said:


> Do you have the ID for your Omnimic.
> If so the software here:- http://www.libinst.com/Omnimic/OmniMicInstall.exe may be of use.


Thank you

Did you want it for Andrew or me? I have the software loaded on my laptop.

Do you have a Omnimic, if so how accurate do you think the mic is?

My ID # 1100052


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## Barleywater (Dec 11, 2011)

> Do you mean in the laptop control panel or REW?


Both.

Since disabling all sound effects in Microphone Properties produced desirable results, leave disabled from here out.

Regards,

Andrew


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## Phillips (Aug 12, 2011)

Hi Andrew i have attached info on the Steinberg C1.

It has ASIO.

The manual is to large for this forum (3.2mb)

View attachment CI1_-_Technical_Specifications.pdf


View attachment ci1_en_om_b0.pdf


Thank you


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## Phillips (Aug 12, 2011)

Hi Andrew attached are all the measurements, labelled. 

The mics were separated. 

Both Omnimic measurements (REW & Omni Software) are the closest.

View attachment Comparison.zip


Thank you


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## Phillips (Aug 12, 2011)

Barleywater said:


> Referencing is still issue. Microphone performance will have more impact on measurements than soundcard.
> 
> Please follow up with Omnimic/Dayton v XTZ/REW comparisons.
> 
> ...


Hi Andrew is there anything that you can suggest on the above zip files (previous post)?

Thank you


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## Barleywater (Dec 11, 2011)

Omnimic results with REW and 1/6th octave smoothing appear closest match with Omnimic using Dayton software.

I don't know what your needs are, but seems that measurements with Omnimic/REW will certainly reveal relative changes to your system configuration regarding EQ changes (at least with current 1 octave band EQ), and changes in speaker/boundary interactions with changes in speaker and listening position.

Once again, what sounds good/acceptable to listener is more important than squiggly plot lines on a computer. If you are after refined measurement capability, calibrated microphone is still the answer.

For sub/main integration with REW, loopback timing reference is not the only real option. Omnimic software help describes use of small outrigger speaker run with split signal and optional attenuator. This speaker is always set at fixed distance off to side of microphone and provides a fixed secondary impulse response in sweep results, allowing calculation of relative flight times during different measurements. There are other approaches with periodic impulse techniques possible with other software that make use of single channel, but this is altogether a different topic.

This is too, but was inspired by your persistence with your laptop soundcard:

I fired up c. 2000 Compaq Presario 17XL260 with mono mic and headphone jack. Microphone jack has about 5V for electret microphone capsule. Loopback with careful selection of output level and input level result in THD across most of range <0.025% response is -3dB at 25Hz, and -6dB at 15Hz. Using such measurements for soundcard calibration file would work well for many applications. I imagine limiting performance is more on microphone preamplifier side, but haven't taken time to do multi-soundcard extended performance testing of headphone output. 

I don't currently have functional 3.5mm electret microphone to test with, but will dig/rig something up soon. Then I will perform a DIY substitution calibration against my OM-1 microphones. It shouldn't be too hard to equal or surpass typical Radioshack SPL meter performance.

Regards,

Andrew


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## Phillips (Aug 12, 2011)

Barleywater said:


> Omnimic results with REW and 1/6th octave smoothing appear closest match with Omnimic using Dayton software.
> 
> I don't know what your needs are, but seems that measurements with Omnimic/REW will certainly reveal relative changes to your system configuration regarding EQ changes (at least with current 1 octave band EQ), and changes in speaker/boundary interactions with changes in speaker and listening position.
> 
> ...


Hi Andrew thanks again.
My needs are room acoustics (no speaker design etc at this stage). My eq will change later. 

With the info you know about the OmniMic + laptop (Acer 4315) soundcard do you think this is sufficient for my needs?
With files i sent the OmniMic/REW didn't measure as low as the XTZ, any idea why?

Options:
1. Get the Omnimic re-calibrated and use with computer soundcard. I wonder if cross spectrum would calibrate.

2. Buy a calibrated mic from Cross Spectrum + a Pre/Soundcard.

Any suggestions on a pre/soundcard? Does the Steinberg stack up?

Thanks again


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## Barleywater (Dec 11, 2011)

Isn't Omnimic strictly USB?

What is first stumbling block that you will encounter with current setup?

Are you interested in digital room correction?

Steinberg specs may be overly conservative, since our craptop soundcards beat THD 0.05%.

Gears in head still turning... Cross Spectrum is about half an hour from where I am.

Andrew


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## Phillips (Aug 12, 2011)

Barleywater said:


> Isn't Omnimic strictly USB?
> 
> What is first stumbling block that you will encounter with current setup?
> 
> ...


Hi Andrew

Yes the Omnimic is strictly USB.

First stumbling block is the calibrated Omnimic, accuracy, restricted to usage of REW, other than those not totally sure what else, hopefully you can help.

Yes i am interested in digital room correction, trying to source, reviews etc currently.

It would be great if the Steinberg was up to it.

Thanks again


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## Phillips (Aug 12, 2011)

If i was able to get a reference mic (for a very short time) would i go through the same process?


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## Barleywater (Dec 11, 2011)

I dug up an old microphone, it used to belong to an AudioControl EQ with analyzer, from well over 20 years ago. EQ became disused, dysfunctional, and tossed. Somehow this microphone has followed me about as something to sort out or cannibalize. It's been in attics and basements, broiled, frozen and otherwise totally neglected. Capsule housing is oxidized and mildly pitted. Once it was shiny and smooth. It has RCA plug and long cable. Adapting it to Compaq Presario 17XL260 a few days ago, I found it mildly surprising that it works. Installing Sound Blaster Extigy, I find functionality as well. Previously with 17XL260 soundcard, loopback setting were found demonstrating acceptable baseline performance, including looping to external soundcard.

A series of sweep measurements using REW on the 17XL260 of 2" full range speaker where made with external soundcard plugged into microphone input, and headphone output routed to amplifier via external soundcard. Earworks OM-1 and ECM8000 microphones where both used, with care taken to place microphone tips to within 1mm of same location. Multiple practice measures where done with OM-1, removing and replacing microphone to confirm repeatability. Tiniest movements produce measurable, repeatable differences. REW trace arithmetic A/B comparisons show that very careful placement/replacement provides repeatability of +/- 0.1dB:









A pair of measurements both with ECM8000 and with ADC microphones where made, with tips located to within 1mm of OM-1's tip placement. A/B of 1st and 2nd measurement with each microphone verifies basic stability. Extensive runs where not done at this time, to save time. Being different microphones, with different body types, a couple of quick comparisons is good place to start. For each measurement A/OM-1 arithmetic are performed.

Here is overview overlay of results:










Within practicable performance limits of 2" full range driver above 100Hz, overview shows extended similarities of three microphones over much of range. 

In following picture ADC/OM1 and ECM8000/OM1 traces are offset to 0dB at 1kHz:










The three microphones match +/- 1dB 50Hz-3kHz. A second set of measurements with sub will allow extension of results to <10Hz.

I also dug up a beat up microphone dating back to a Pentium P90 that appears to need cord replaced. Currently it's a bare 10mm electret microphone capsule. I have some expectation that results, if possible, from it will also have extended range of +/- 1dB match to above results. 

My expectations where that Omnimic and XTZ microphones would have similar extended range of +/- 1dB match. Your repeated results are highly similar, run to run, but the two microphones appear to have vastly different equalization.

If you get a hold of reference microphone with calibration, what microphone(s) are you attempting to calibrate? In going this route I recommend finding inexpensive 3.5mm microphone that works with laptop soundcard, and practicing technique. This also provides opportunity for further insight to Omnimic and XTC microphones peculiar performance together.

Regards,

Andrew


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## Phillips (Aug 12, 2011)

Barleywater said:


> If you get a hold of reference microphone with calibration, what microphone(s) are you attempting to calibrate?
> 
> 
> 
> ...


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## Phillips (Aug 12, 2011)

Hi

Is it best to measure to compare mics 
1. With a Center or Floorstander main speaker at about 2 feet?
2. Include a subwoofer to get full range?
3. If it is best to use a Floorstander main speaker at what height should i use?

Thanks in advance


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## Barleywater (Dec 11, 2011)

To get full range:

Measure Center or Floorstander <9 inches. Use measure or other method to insure each microphone is located in same position, hopefully to within 1/16th of an inch.

Repeat measurements using Sub. For Sub place each microphone about 1/2 inch to 1 inch from center of driver. Run Sub fairly loud. Idea is to get good excursion from driver, while keeping microphone from being hit. 

Small differences in microphone placement have less impact on results with long wavelengths.

Since you've got 3-way Floorstander?, you may want to do measurements first with microphone (still <9 inch) located midway between mid and tweeter, and then at <1" for woofer of main. If main is run full range woofer may produce enough signal to get low end, otherwise do sub measurements too.

Results from measurements are level adjusted to match, at desired merge frequencies and merge function used to combine results. Merge frequencies are chosen where frequency responses of measurements ( sub v woofer, woofer v mid/tweeter) are both smooth, and hopefully similar in shape.

Using A/B math, level matching and merging may be used to produce calibration data. Saved data as text become calibration file.

Regards,

Andrew


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## Phillips (Aug 12, 2011)

Barleywater said:


> To get full range:
> 
> Measure Center or Floorstander <9 inches. Use measure or other method to insure each microphone is located in same position, hopefully to within 1/16th of an inch.
> 
> ...




Hi Andrew thank you.

Yes my speakers are 3 way Floorstanders (Energy Veritas 2.3i). 
I run them full range.

Do i have to turn off the e.g. woofer or can i measure with all speakers operating, but different mic position?

Thanks again


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## Barleywater (Dec 11, 2011)

The less going on with each measurement, the better.

You are comparing now Omnimic V1 v V2? They have identical bodies? The closer they are to exact same location for each measurement, the better the A/B math works.

Do Energy Veritas have bi-amp terminal setup? Or by woofer are you referring to sub?

Andrew


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## Phillips (Aug 12, 2011)

Hi Andrew

Yes hopefully will be able to.

The Veritas are bi-wired currently.

I have moved REW on to another laptop.

I am trying to do a soundcard calibration file for the other laptop. 

I have attached some print/screen shots of the settings, please can you advise.

Thanks again


View attachment Laptop Settings.zip


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## Barleywater (Dec 11, 2011)

Laptop Settings.zip is second Laptop? Does it have separate line in and mic input as seen in recording controls?

Master Volume Control: This is output. Master: not muted, wave out: not muted, line: muted.

Loopback calibration shows good signal management, and gross behavior, but is combo of output amplifier and input amplifier, actual input for measurement is USB and the amplifier built into Omnimic.

Andrew


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## Phillips (Aug 12, 2011)

Barleywater said:


> Laptop Settings.zip is second Laptop? Does it have separate line in and mic input as seen in recording controls?
> 
> Master Volume Control: This is output. Master: not muted, wave out: not muted, line: muted.
> 
> ...



Hi Andrew yes it is the second laptop.

No it is only single in and out.

Looking last night there was stereo mix and mono mix as well (hidden in the advanced settings)

The laptop is XP and about 8-10 years old. I run it on power not battery.

Would it pay to download the Realtek High Definition Drivers, currently using ALi, old?

The general error i get is Excess Variation 

You mentioned that Omnimic might have "different equalization", what do you mean, the calibration file?

Any thoughts?

Thanks again.


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## Phillips (Aug 12, 2011)

Barleywater said:


> if we sort out Omnimic response mystery
> 
> Andrew



Hi Andrew i have attached a mdat file with a REW measurement + Imported Omni Wav file 32 bit and a imported Omni software FRD file, all labelled. 

All measurments were taken with the same mic / position, just different software.

Please could you look at these, very interesting, the REW + Omni Wav files look similar.

It is a generic mic.

I have seen something about response correction/calibration and not having sample clocks locked. Would this make a difference?

Thanks in advance

View attachment REW Comparison.zip


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