# How do I ask a question?



## Oldears (Nov 2, 2013)

I'd like to know how to calibrate a USB mic (Dayton Audio UMM-6) and a Cord Async USB controller? Where do I ask that question? I tried to search on "Calibrate USB mic" using the double quotes to get that phrase, but the search engine returned no hits. I don't know how to plug the analog output from the Cord DAC into the mic USB line.


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## tcarcio (Jun 27, 2007)

I would post your question in the calibration forum here..
http://www.hometheatershack.com/forums/spl-meters-mics-calibration-sound-cards/


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## mechman (Feb 8, 2007)

Thread moved.


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## Oldears (Nov 2, 2013)

OK, thanks for moving the post to the right thread. Looks like brucek may have answered my question several years ago.


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## Oldears (Nov 2, 2013)

*Using the USB mic*

Well, I thought this would happen. REW or RealTek failed to detect the USB mic. I have attached a screen shot of the error message. When I click on the Input Devices, the only device listed is the default device. I made sure the check box was unchecked on the mic/meter tab asking about SPL weighting. I've spent an hour trying to trouble shoot this to no avail. The output device, the built in sound card on my PC (Windows 7) is working because when I play a test signal from the mixer, accessed by clicking on the speaker icon in the lower right corner of the desktop, I hear it through the speakers.

How do I get the software to recognize the USB mic? Please don't assume I know anything about computers and audio. Although that is not true, my knowledge may be different than yours.


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## Oldears (Nov 2, 2013)

*No volume control*

I unplugged the USB cable from the mic and plugged it back in, and now the sound card recognizes it. Except now I have no volume control. I can hear the 1 kHz signal, but there doesn't seem to be a way to increase the input volume. It is greyed out and the RealTek volume is set for maximum. I have included another screen shot. FYI, I can crank up the volume on the preamp to the point of driving me out of the room, but there is no effect on the REW display. I've gone into control panels and looked at RealTek, but that is just for the speakers, and I've gone into the Sound panel. There the microphone is the UMM-6, and properties tabs are Level: set to 100; Advanced: 1 channel, 16 bit, 44100Hz (CD Quality).

I've now got 3 hours into this project and suspect anyone else would have taken about 30 seconds to get this far.


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## JohnM (Apr 11, 2006)

For USB mics it is best to use the latest beta version of REW, which has extra features to support them. You can find that here. The help files in the beta version (also available as a PDF at the beta download thread linked above) have been updated to cover USB mics. Make sure the mic is plugged in before you start REW, otherwise Java can take a couple of minutes to realise there is a new USB device present.


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## Oldears (Nov 2, 2013)

*How loud do I have to turn up the volume?*

Wow! That was a piece of cake. Took about two minutes to calibrate everything using a different computer, the latest version of REW, and using the external desktop speakers plugged into the computer. Looks like the little speakers don't go below 100Hz! It was great having the program ask me if the mic it detected, the UMM-6, was the one I wanted to use, and then asking for the calibration file, which I had. I felt confident everything was going to work. And it did. I've included a screen shot with a warning. BTW, it is really helpful having the additional information about using USB mics in all the steps.

When I was checking levels, I had to turn the volume up much louder than I ever have to get the input levels up to -30 dB. Is that normal? The UMM-6 mic is about 9' away from the right speaker and pointing straight up. I'm now going to try downloading the beta version of REW on the main computer downstairs, which is where I want to do the measurements and where I was having all the trouble last night. If I still have problems with that computer, I'll take this desktop down there.

Thanks for the link to the beta version. I didn't know where to find it on the website. When I opened REW last night, I saw a dialog box telling me there was a newer version available, and if I wanted it to click here. I did, and I was taken to the home page of the forum. I couldn't figure out what to do next.


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## jtalden (Mar 12, 2009)

That warning is for the soundcard calibration process. That process is not used with a USB mic like yours. 
The soundcard calibration file should be left as "none".

Your measurements should be made at about 75-80 dB SPL. If you get a low level warning when you take measurements just ignore it. Many USB mics have an output level which will trigger this warning, but it has no effect on the SPL or most other measurement types.


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## Oldears (Nov 2, 2013)

The new version of REW was really easy to set up on the computer that was giving me fits before, and I spent about 8 hours making all kinds of measurements, checking different sitting positions, stacking pillows and cushions up to 5' high in every corner, draping quilts over walls, mirrors and pictures, wedging cushions between the tops of bookcases and the ceiling, and covering horizontal hard surfaces with blankets. (Yes, my wife is out of town for a week.) The best results seem to be just cushions in the corners so I'm a bit suspicious about the numbers.

I can upload the files if necessary, but the first thing that jumps out is the frequency response drops off pretty sharply around 10 kHz in every single measurement. When I play test tones from a CD that go from 20 Hz to 20 kHz, I don't hear anything above 8 kHz, but the free app on my iPhone registers a signal for every test tone up to 20 kHz. Why is the SPL meter showing such different results than the frequency plot in REW?

Do I need a better SPL meter or app? I read the thread "Spl meter/Worth it" which concludes with the statement that the iPhone is sufficient. (Unfortunately the link to the Cross Galaxy meter goes to a device for calibrating a meter, not the meter itself.) I'm using a free app from 10th Decibel. I picked it at random, and I have an external mic for the iPhone, the IMM-6 from Dayton Audio which I bought just to get free shipping since shipping was more than the cost of the IMM-6.

Of course I don't want to buy anything I don't have to, but I do want to get meaningful results. Using REW, I see almost no effect of different seat positions further away or closer to the speakers, nor do I see any effect of toe in, although the sound stage is much clearer when I listen with the speakers aimed at me rather than straight ahead, but maybe that's just my ears deceiving me. 

Ah, I just remembered REW is using only one speaker so of course it cannot detect sound stage effects, not to mention the test tones are probably equal in each speaker, but I would think the orientation of the speaker would have a noticeable effect on the room frequency response. We're talking about 30 degrees to the side from pointing straight ahead.

And remember, I really don't have much of a clue what I'm doing with an SPL meter or with REW. I understand an SPL meter measures the pressure of the sound waves which we associate with volume, and the frequency plot in REW shows how far from a perfectly horizontal line my room acoustics are. That's it.


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## jtalden (Mar 12, 2009)

Your REW setup using the UMM-6 mic is all that is needed. There is no need for any kind of SLM because REW provides everything that is needed for system setup. It even includes the features of an SLM, but they are not normally needed for room setup. 

If you have questions regarding the REW SPL chart it is best to post the chart, and better yet, the .mdat file on the REW Forum. It is difficult to comment on "it drops off pretty sharply" without seeing the chart. 

It is best to measure each main channel separately.


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## Oldears (Nov 2, 2013)

*Initial results*

For the measurements shown in the attached file, I used the default values in REW, not adjusting the volume when I checked levels both in preferences and in the measurement section since I was using a USB mic. The volume on the preamp was set for my normal listening level, which typically is around 75 dB.

Difference between first and second measurements is room position only. Position 1 is as far back as I can get, 2' from the back wall and 14' from the speakers and 3' above the floor. Position 2 is 7' from the back wall (38% of the distance between front and back walls) and 10' from the speakers and 4' above the floor (chair versus foot stool in position 1).

The difference between second and third measurements is removal of furniture between speakers and listening position (2 wing back chairs, end table and lamp). This is my usual configuration when no one is around.

I redid the SPL measurements with the iPhone on a stable platform the same height and position as the microphone and feel confident they are pretty good and, thanks to REW, my speakers are pretty bad!
20Hz 49 dB
40Hz 48 dB
100Hz 52 dB
400Hz 64 dB
1kHz 79 dB
2.5kHz 73 dB
4kHz 73 dB
8kHz 71 dB
10kHz 70 dB
12kHz 67 dB
15kHz 65 dB
18kHz 57 dB
20kHz 62 dB
When I listen to the speakers, I cannot hear a test tone above 8kHz, but when I use a pair of Grado Reference headphones, I can hear the 10 and 12kHz tones with no problem, and if I turn up the volume on the headphone amp to maximum, I can hear the 15kHz tone. And all this time I thought it was my ears. The roll off starting around 4-5kHz is noticeable in the REW frequency plots. I wonder just what frequency my speakers are putting out at 10-12kHz that I cannot hear but the SPL program measures.

Now I just need to figure out what all the REW data means.


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## jtalden (Mar 12, 2009)

This data is not correct yet.

1. 8 sweeps were used for the measurements. With USB mics multiple sweeps are often (always?) a problem. REW loses the timing between the sweeps and that corrupts the result. Use only 1 sweep for all your future measurements. There is no problem using single sweeps for all common setup needs.

2. I believe we may still have a problem after using single sweeps as it appears that the freq range was filtered to 600-6k Hz. 

REW reports that the measurement sweep was from 2-20k Hz so that was not the problem. [You probably should change the sweep to 15 - 20k Hz so as to not stress the SW as much when measuring.] Your sample rate was set to 48 kHz. That should be okay so I am not sure what may have caused the limited freq range in the measurements.

Please post another measurement .mdat file using a single sweep so we can see if we still have a problem.


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## Oldears (Nov 2, 2013)

_2. I believe we may still have a problem after using single sweeps as it appears that the freq range was filtered to 600-6k Hz._

I'm not sure why you think that. I have attached a photo of what I see when I open the attachment, and I see wiggly lines from 2 Hz to 20 Hz, but maybe I'm looking in the wrong place. This is what I see when the file opens.

I redid the last measurement from the previous file, and I repeated it 3 more times to look at the reliability of the measurements. That file is attached as well. Hopefully this time I did it correctly. Why is there so much variability? If I had been moving traps around, I would have thought I was really making a difference.

Position of mic was 7' from back wall, 10' from speakers and 4' above floor.


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## jtalden (Mar 12, 2009)

The dB scale of your settings is misleading you. See the  *"Posting a Graph"* sticky. Since your levels are a little lower than "standard" I shifted the scale but kept the 60 dB range as shown below:









Everything <200 Hz is just noise floor. There is no significant output until 600 Hz. The highs fall off rapidly above 7 kHz and are at/near the threshold of hearing from 9-20 kHz. I would think even a 3" desktop computer speaker would do better than this.

So we still have problem with the measurement. You switched to single sweeps, but it had no effect on the distorted REW Impulse Response (IR).

Before anything else I would suggest you try setting the REW sample rate to 44.1 kHz for a measurement. I thought that the Dayton UMM-6 will work at either rate, but the IR results look similar to a sample rate mismatch.

It that doesn't solve it then I have no other good ideas for what to change. Possibly someone with experience with USB mic setups will know what is wrong.


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## JohnM (Apr 11, 2006)

Note that you can apply smoothing to the measurements using the entries in the Graph menu, or the ctrl+shift+number key combinations (e.g. ctrl+shift+6 gives 1/6th octave smoothing). 

The impulse response looks OK to me, but you can check the sample rate the USB mic is using by looking at the default format on the Advanced tab of the properties (right click the volume icon, select recording devices, select the USB mic in the list, click properties, select the Advanced tab). Probably set as 2-channel 48 kHz and probably not something you can change.

Do make sure you have only a single speaker playing when you make your measurements, as mentioned earlier.


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## Oldears (Nov 2, 2013)

*Getting better, I hope*



jtalden said:


> Everything <200 Hz is just noise floor. There is no significant output until 600 Hz. The highs fall off rapidly above 7 kHz and are at/near the threshold of hearing from 9-20 kHz. I would think even a 3" desktop computer speaker would do better than this.


This gave me the idea to raise the volume to see if I could get the lower frequencies up off the "noise floor" to register consistently because these speakers definitely sound better than my 3" desktop computer speakers. I've attached the new mdat file, RTA5. Actually I couldn't upload the original 19.7 MB file so I broke it into two files. RTA5a contains the first 4 measurements. Several things to note here. For the first 4 samples, I changed the sample rate to 44.1 kHz in the preferences box. Second, I unplugged the left speaker so only the right speaker was emitting sound. Third, I raised the volume on the preamp to "65" which is almost twice my usual setting of "35." In the "Check Levels" button on the Measurement window, that resulted in a message of "-31.6 dB Level is Low." The previous measurements in RTA4 were done with a reading of "-64.2." Using my iPhone and a SPL app with a plug in mic, the meter recorded 98 dB peak with this new setting. I was glad I was only doing 1 sweep. I am impressed with the results. The first 4 measurements have identical settings, and they are impressively consistent, to my eye. Do these results seem reasonable, or am I still missing something?

RTA5b contains the last 4 measurements. The next two measurements, #5 and #6 (#1 and #2 in RTA5b), were done with both speakers plugged in, otherwise everything else was the same. Because both speakers were being used, I believe that explains the higher volume compared to the 4 previous tests. Again I was impressed with the consistency between the two tests.

The last two measurements were done with the volume reduced to my normal listening level, which is what I thought I used in RTA4, but the amount of "Headroom" just before the test ended was 37.8 and 36.8 compared to 16.1 and 16.2 for measurements #5 and #6 above and 46.3 or so for the 3 measurements in RTA4. The iPhone measured 80 dB at this volume, which is about as loud as I normally listen.

For measurement 6 I switched the sample rate back to 48kHz in the preferences tab, and that had no effect on the outcome. I forgot to look at the sample rate of the mic so I don't know if I really did change it for first 5 tests or if everything was done using 48 kHz. I'm including a screen shot which shows summaries of the 3 types of measurements, 1 speaker at high volume in red, 2 speakers at high volume in green, and 2 speakers at normal volume in purple. I did read the link on posting a graph, but I forgot to resize the axes. I find the double dip between 45 Hz and 55 Hz and again at 200 Hz consistent when both speakers are playing.



JohnM said:


> Note that you can apply smoothing to the measurements using the entries in the Graph menu, or the ctrl+shift+number key combinations (e.g. ctrl+shift+6 gives 1/6th octave smoothing).
> 
> The impulse response looks OK to me, but you can check the sample rate the USB mic is using by looking at the default format on the Advanced tab of the properties (right click the volume icon, select recording devices, select the USB mic in the list, click properties, select the Advanced tab). Probably set as 2-channel 48 kHz and probably not something you can change.
> 
> Do make sure you have only a single speaker playing when you make your measurements, as mentioned earlier.


Is there somewhere I can read about interpreting the various graphs in REW? I read the How to Use REW information several times, and I read the first 100 pages of the help manual that was on the thread with the latest beta version, and I still don't know what to look for, or how to interpret any of the graphs except maybe the SPL graph. What I see in the photo below is my speakers seem to be good down to about 25 Hz but start to roll off, if that is the correct term, at about 1 kHz. I think that is consistent with the SPL meter results I presented in my previous post which showed a peak value at 1 kHz.

Why should I use only one speaker if I am interested in the room dynamics? If I wanted to know about the frequency response of the speakers, I can understand using only one speaker, but I'm trying to decide what room treatment I need. Of course, you probably didn't know that.


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## jtalden (Mar 12, 2009)

> You still need to correct the vertical scale on the SPL chart (45-105 dB). Click the link I posted above or find the sticky thread at the top of the forum for more info and instructions.

> All the measurements now look normal. Both the 44.1 kHz and 48 kHz sample rates resulted in charts that look normal. I don't know what changed, but the issues are now resolved. 

> You should continue to test each speaker separately. [We should be interested the reproduction of the signal to each speaker. We are not as interested in the combined result of playing a mono signal through 2 or more speakers as the interaction is disruptive and changes greatly with slight mic position changes.]

> Test volume level is 75-80 dB per the chart. This is a good test level. However you seem to feel that this is louder than you think it should be and it does not seem to agree with the iPhone. So:

● I wouldn't think a phone app would know the sensitivity of an external mic unless they are specifically designed to go together (possibly they are?) but the accuracy of such a marriage is highly suspect. I would suggest that REW will be far more accurate if properly setup.

● There is not much that can be wrong when using a USB mic with REW. The proper mic sensitivity value should be within the supplied calibration file you loaded (it's in first few lines). If this value is correct then REW SPL levels are correct. That said, I read some of the early sensitivity values were wrong on one of the brands of USB mics. I don't recall which one. You may want investigate this and see if was with the UMM-6. If it was, then you can determine if your calibration has a value similar to the corrected ones. Once you confirm that your value is in line with the values that others are using then all is well. The "Mic" forum will be able to help quickly resolve this question if someone here doesn't confirm the expected range of values.

Here is a *Link to general setup info*


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## Oldears (Nov 2, 2013)

jtalden said:


> >● I wouldn't think a phone app would know the sensitivity of an external mic unless they are specifically designed to go together (possibly they are?) but the accuracy of such a marriage is highly suspect. I would suggest that REW will be far more accurate if properly setup.
> 
> ● There is not much that can be wrong when using a USB mic with REW. The proper mic sensitivity value should be within the supplied calibration file you loaded (it's in first few lines). If this value is correct then REW SPL levels are correct. That said, I read some of the early sensitivity values were wrong on one of the brands of USB mics. I don't recall which one. You may want investigate this and see if was with the UMM-6. If it was, then you can determine if your calibration has a value similar to the corrected ones. Once you confirm that your value is in line with the values that others are using then all is well. The "Mic" forum will be able to help quickly resolve this question if someone here doesn't confirm the expected range of values.
> 
> Here is a *Link to general setup info*


First, I don't know very much about dB measurements. I have read that normal listening levels are around 75-80 dB. That is what my iPhone measures when I play pink noise at my normal listening levels. It seems sometimes as sound gets louder, the dB value goes up, as on my iPhone app, and sometimes it goes down, meaning less negative, as in the Check Levels windows in REW.

Second, I have no idea how to use the mic as a SPL meter. I tried clicking on the SPL button in REW and got a nice display of a SPL meter that measured something, but I have no idea what it was measuring because nothing was coming out of the speakers, and changing the volume on my preamp had no effect on the dB values. Whatever it was measuring had a value of 50.5 dB.

When I open Preferences and click on check levels and when I open Measure and click on check levels, I hear pink noise coming out of the speaker, but as I increase the volume, the number, in dB, becomes less negative, approaching 0 I guess. The reference sound is -12 dB. When I set the preamp volume to 35, the input dB is -49 and the SPL meter on my phone is 78. When I set the preamp volume to 40, louder than I normally listen, the input dB is -45.5 and the SPL meter reads 82. I have no idea how the values of -49 or -45 relate to my iPhone SPL meter reading of 78 or 82, other than in both cases the numbers differ by 4 dB. In any case, neither -49 or -45 is very close to -12.

Based on the fact that the plots of the frequency response in my last post required a setting of "65" which registered as 98 dB on my iPhone, and was uncomfortably loud, I think the mic is defective, meaning it is not putting out as much voltage as it should, and I will attempt to send it back to Parts-Express. Unfortunately I threw out the box it came in.

I read the entire thread on the UMM-6 mic and didn't see anything about this problem. For what it's worth, here is the first line of the calibration file associated with my mic's serial number.
"Sens Factor =-19.139dB, SERNO: 1381011".

I really don't know where to go from here.


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## JohnM (Apr 11, 2006)

Here are a few articles to help with understanding measurements:

MiniDSP UMIK-1 setup article,
MiniDSP loudspeaker measurement article using REW and UMIK-1,http://www.realtraps.com/art_measuring.htm, 
http://www.hifizine.com/2011/06/bass-integration-guide-part-1/,
http://redspade-audio.blogspot.com/2011/05/quickstart-guide-to-bass-measurements.html

If you make measurements with both speakers playing then any slight difference in the distance from the mic to each speaker will result in phase cancellations at high frequencies, the signals combine acoustically at the mic location and the one which is further away arrives a little later in time, at high frequencies that small delay can mean an increase in pressure from one speaker corresponds to a decrease from the other and the net result is a cancellation, which adds extra notches to the measurement. 

Input levels of around -45 dB FS from a USB mic are normal. The sensitivity figure in the calibration file is the input level from the mic when it is fed a 94 dB signal with the windows mic input volume control set to 100, you can set the mic input level from the Levels tab of the properties for the mic or using the volume control on REW's soundcard preferences, both act on the same control.


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## Oldears (Nov 2, 2013)

Thank you for the links. I am working my way through them. The photos on the set up link are really impressive!



JohnM said:


> Input levels of around -45 dB FS from a USB mic are normal. The sensitivity figure in the calibration file is the input level from the mic when it is fed a 94 dB signal with the windows mic input volume control set to 100, you can set the mic input level from the Levels tab of the properties for the mic or using the volume control on REW's soundcard preferences, both act on the same control.


Last night I tried again, this time using the signal generator in REW set for Pink Noise and then opening the SPL meter. And this time I was able to affect the SPL meter by changing the volume on the preamp. In order to get the REW SPL meter to register 70 dB, I really had to crank up the volume to a level louder than I listen to normally. When I used the iPhone SPL app to measure the sound, it recorded 85 dB. The level of sound in the room fit the written description of 85 dB a lot better than the description of 70 dB so my conclusion is the mic is about 15 dB low. I will try again tomorrow, after company leaves, because I realized today I have the mic pointed at the ceiling whereas I have the iPhone mic pointed at the right speaker. Maybe that explains the 15 dB difference.



JohnM said:


> If you make measurements with both speakers playing then any slight difference in the distance from the mic to each speaker will result in phase cancellations at high frequencies, the signals combine acoustically at the mic location and the one which is further away arrives a little later in time, at high frequencies that small delay can mean an increase in pressure from one speaker corresponds to a decrease from the other and the net result is a cancellation, which adds extra notches to the measurement.


In REW, in the Preferences window, there is a box to select Left or Right Speaker. Why doesn't that just send a signal to one speaker? Why do I have to disconnect either one speaker or the input on one channel to make a measurement?

Also, if I am trying to use REW to measure the effect of various room treatments, I would think I would want both speakers playing because it is precisely the interaction of the waves from both speakers that I am trying to control. Lots of stereo CDs have one or more instruments or singers spatially located in the center of the stage, in the middle between the two speakers, which is basically a mono signal, I would think.


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## JohnM (Apr 11, 2006)

Mic orientation will make very little difference to the SPL reading. It is possible that REW is not converting the UMM-6 input level to an SPL correctly, you can change the REW reading by altering the Sens Factor figure in the cal file, for example changing from -19.9 to -29.9 will cause the REW SPL meter to read 10 dB higher.

The soundcard prefences offers a selection of left or right _input_, that tells REW which channel of the stereo input to listen to. 

When using the Java audio drivers REW places the test signal on both channels of the output. There are a few reasons for this - it removes the possibility of the user connecting up a channel with no signal and thinking the system isn't working; it allows both channels to be driven without a splitter, which is useful in some circumstances; and in some measurement configurations (not applicable for USB mics) the other channel loops back to the soundcard to provide a timing reference. ASIO drivers are inherently mono, if the ASIO drivers are being used an individual output channel can be selected to carry the test signal.

It is better to make full range measurements with each speaker separately, for the reasons already explained. At low frequencies it is useful to also make measurements with both speakers running, as some modal resonances which would be excited by a single speaker would not be excited if both speakers are playing a mono signal (often the case for bass content) depending on the speaker position - an example would be odd order width modes with speakers placed symmetrically across the width. Above the modal range (above a few hundred Hz) this is no longer relevant.


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## Oldears (Nov 2, 2013)

JohnM said:


> Mic orientation will make very little difference to the SPL reading.


Yes, this is what I discovered. The SPL meter on my iPhone, however, seems very sensitive to orientation. Having read through the links above, I understand waterfall, decay, and impulse plots much better now. I also discovered after entering the room dimensions and speaker and listening positions in the Room Sim tab, I can play with different listening positions and see how it is affected by the room modes. That's pretty neat!



JohnM said:


> It is possible that REW is not converting the UMM-6 input level to an SPL correctly, you can change the REW reading by altering the Sens Factor figure in the cal file, for example changing from -19.9 to -29.9 will cause the REW SPL meter to read 10 dB higher.


I changed the value from -19.9 to 34.9 and now the SPL meter in REW is within about 1 dB of the iPhone app. Also, based on a couple of the links, I understand I should try and have the measurement signal about 50 dB above the room noise. Today I was recoding about 47 dB in my listening position with nothing playing so I made the measurements at 97 dB, which was pretty loud! Hopefully the measurements are worthwhile. If so, I will repeat them and include one more measurement with both speakers.

Now I just have to figure out where to get some 703 acoustic panels and start playing with placements. Because they have to be removable, I am planning to enclose each one in a plastic bag (from the dry cleaners). I trust LF waves can go right through very thin plastic.


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## JohnM (Apr 11, 2006)

Oldears said:


> I understand I should try and have the measurement signal about 50 dB above the room noise. Today I was recoding about 47 dB in my listening position with nothing playing so I made the measurements at 97 dB, which was pretty loud!


Ignore that advice, it is wrong. High levels are bad for your equipment and your ears.


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