Simplified REW Setup and Use (USB Mic & HDMI Connection) Including Measurement Techniques and How To Interpret Graphs - Page 499 - AVS Forum | Home Theater Discussions And Reviews
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post #14941 of 16017 Old 06-02-2015, 08:54 AM
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Originally Posted by AustinJerry View Post
TBH, given my room dimensions, the current layout may well be as good as it's going to get, and perhaps I should stop obsessing about it.
Aw, come on. The rest of us are having a blast watching you pull your hair out. Don't give up yet.


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post #14942 of 16017 Old 06-02-2015, 09:02 AM
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Originally Posted by lovingdvd View Post
Can you guys please have a look at my MDAT here: https://drive.google.com/open?id=0Bz...dWM&authuser=0 . It contains my pre-calibration sub only, post calibration sub only (via BFD) and post calibration sub plus L/R.

My take on it is as follows - I'm far from an expert so would like to know what you guys think:

1. The +13 dB peak is completely gone, thankfully. Side note: This nasty peak is what led me to see out a BFD years ago. When I first set up speakers in this room I thought my sub was bad - sounds so muddy and distorted. So I bought a new and highly regarded sub VTF3-MK2. Imagine my surprise when it sounded the same. That's when I researched and found REW and when I saw that I was like "there she is!". A BFD later and I was good to go.

2. Note that I purposely have my sub quite "hot". In addition, it has a house curve of +5 dB at 20 Hz down to +0 dB at 80 Hz. Still sounds very tight. As I posted a few days ago, if I try to balance it at the same SPL as the rest of the FR it sounds like the sub is completely missing...

3. Distortion looks completely fine - don't see any issues there. Right?

4. Waterfall shows great decay times. Considering this is an entirely untreated room that is great. Likely have this because it is a totally open room on one side.

5. ETC shows lots of issues; untreated room remember. I'm not concerned about it right now since I will be building a new room shortly. The purpose of this recalibration was because somehow my sub settings got changed and it didn't sound right, plus I wanted to sharpen my REW skills in preparation for getting an 88A when the new room is ready.

6. BIGGEST QUESTION: What do you guys thing is going on in the 110 Hz to 350 Hz range. It is so much lower than 350 Hz and up. My speakers are Pinnacle Gold Reference towers and are ported. They are each within a cabinet (with FR701 GOM in the front) that is big enough to fit them with about 6" of room in the front, side, back and top. Perhaps because these speakers are not meant to be placed into a cabinet doing so is messing up its response in the 110-350 Hz range?

7. Group delay (the subject of much discussion here lately) on the combined calibrated sub+LR looks looks pretty good, except for 60 ms at 162 Hz at mostly 25 ms and below, except for in the 225 - 350 Hz range as it peaks there near 50 ms and close to 100 ms at one point. Not sure what to do about it but not worth messing with given that I'll be rebuilding the room from scratch.

Other thoughts?

Thanks!
My overall comment would be that you are taking the measurements at too low a level. Of course the distortion looks good, because distortion increases as SPL increases, and your measurements are so low that I would be surprised if distortion would be an issue. The waterfall/spectrogram would be more meaningful if you had 40dB between the noise floor and the measurement peaks, which in your case are only 80dB. Do you have an issue with measuring at a higher level? There is quite a bit of ringing below 30Hz, but this is difficult to control. And finally, if you are going to measure reflections, measure one speaker at a time, not L+R.
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post #14943 of 16017 Old 06-02-2015, 09:03 AM
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Originally Posted by LastButNotLeast View Post
Aw, come on. The rest of us are having a blast watching you pull your hair out. Don't give up yet.

Yes, Jim trained me well!
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post #14944 of 16017 Old 06-02-2015, 09:41 AM
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Originally Posted by Alan P View Post
...3. Agreed. But you need to measure higher to see when the distortion starts becoming an issue. I'm assuming you listen higher than you measured....maybe not...
Great point. How can I measure higher without causing clipping? As it stands now I am measuring at around 75 dB for the mid and high range, which is about 85 dB for the sub. IIRC REW says I have about 7 to 10 dB of Headroom after a measurement. My AVR master volumen is like around -25 dB at this measurement level, maybe -20 dB. If I turn the AVR up to say -10 or 0 dB to see what distortion does at these higher levels, most certainly REW is going to complain that the input is clipping.

So that brings me to my question - Is there a way to configure REW or input levels that lets me raise the AVR master volume and still be able to get clean readings without it complaining about running out of headroom during a measurement? I have all my levels for input and output at the defaults (and don't really understand when and how to use them, so perhaps that holds the key?). Thanks!
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post #14945 of 16017 Old 06-02-2015, 09:42 AM
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Originally Posted by AustinJerry View Post
My overall comment would be that you are taking the measurements at too low a level. Of course the distortion looks good, because distortion increases as SPL increases, and your measurements are so low that I would be surprised if distortion would be an issue. The waterfall/spectrogram would be more meaningful if you had 40dB between the noise floor and the measurement peaks, which in your case are only 80dB. Do you have an issue with measuring at a higher level? There is quite a bit of ringing below 30Hz, but this is difficult to control. And finally, if you are going to measure reflections, measure one speaker at a time, not L+R.
Yes good point. I am not sure how to measure at higher levels without REW complaining about running out of headroom/clipping (of the mic input?). Please see my post to Alan which I made just before seeing your post. Simplified REW Setup and Use (USB Mic & HDMI Connection) Including Measurement Techniques and How To Interpret Graphs
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post #14946 of 16017 Old 06-02-2015, 09:53 AM
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Originally Posted by lovingdvd View Post
Yes good point. I am not sure how to measure at higher levels without REW complaining about running out of headroom/clipping (of the mic input?). Please see my post to Alan which I made just before seeing your post. Simplified REW Setup and Use (USB Mic & HDMI Connection) Including Measurement Techniques and How To Interpret Graphs
I assume you are using a mic that has a sensitivity parameter and doesn't need manual calibration. Follow these steps:

1. Set REW output to L+R+Subs, and AVR to Stereo.
2. Set up the mic at the MLP at ear level.
3. Open the REW SPL Meter and click the red button to start measuring SPL.
4. Open the REW signal generator, select Pink Noise, Subwoofer Cal, and set the output to -12dBFS.
5. Click the play button. You should hear the pink noise test tone.
6. While observing the level in the SPL meter, adjust the AVR master volume until you are registering at least 90dB.

That is the proper setting for the AVR MV (on my system, I get 90dB with MV= -15). Now run a measurement sweep (make sure the sweep is also at -12dBFS). Are you getting clipping?
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post #14947 of 16017 Old 06-02-2015, 10:26 AM
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Originally Posted by lovingdvd View Post
Great point. How can I measure higher without causing clipping? As it stands now I am measuring at around 75 dB for the mid and high range, which is about 85 dB for the sub. IIRC REW says I have about 7 to 10 dB of Headroom after a measurement. My AVR master volumen is like around -25 dB at this measurement level, maybe -20 dB. If I turn the AVR up to say -10 or 0 dB to see what distortion does at these higher levels, most certainly REW is going to complain that the input is clipping.

So that brings me to my question - Is there a way to configure REW or input levels that lets me raise the AVR master volume and still be able to get clean readings without it complaining about running out of headroom during a measurement? I have all my levels for input and output at the defaults (and don't really understand when and how to use them, so perhaps that holds the key?). Thanks!
What I did (from advice I got here) is to lower the mic level in Windows. When I turned it down from 100 to 30 I was able to measure up to 137db.
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post #14948 of 16017 Old 06-02-2015, 10:27 AM
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Originally Posted by lovingdvd View Post
IIRC REW says I have about 7 to 10 dB of Headroom after a measurement.
That's how much you should increase the AVR volume (assuming you've also followed Jerry's advise about calibration).
As long as it's not clipping, you'll get accurate graphs.
I use the delay feature and run out of the room when I'm taking seriously loud measurements.

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post #14949 of 16017 Old 06-02-2015, 10:59 AM
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Originally Posted by AustinJerry View Post
I assume you are using a mic that has a sensitivity parameter and doesn't need manual calibration.
Correct. Using a UMIK-1 with sens data included in calibration file.

Quote:

Follow these steps:

1. Set REW output to L+R+Subs, and AVR to Stereo.
2. Set up the mic at the MLP at ear level.
3. Open the REW SPL Meter and click the red button to start measuring SPL.
4. Open the REW signal generator, select Pink Noise, Subwoofer Cal, and set the output to -12dBFS.
5. Click the play button. You should hear the pink noise test tone.
6. While observing the level in the SPL meter, adjust the AVR master volume until you are registering at least 90dB.

That is the proper setting for the AVR MV (on my system, I get 90dB with MV= -15). Now run a measurement sweep (make sure the sweep is also at -12dBFS). Are you getting clipping?
From what I recall, I am almost certain that I will be way short of headroom. I think the most the volume can be set to without REW complaining about clipping is with the MV set so the REW SPL meter measures about 80 dB. Hence my original question of how am I suppose to measure with it louder than in the MDAT I posted, because much more than about 5 dB louder will cause it to clip. See my question in the response to Alan below.

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Originally Posted by Alan P View Post
What I did (from advice I got here) is to lower the mic level in Windows. When I turned it down from 100 to 30 I was able to measure up to 137db.
OK, great. I was wondering whether something like that would work. I think that was the key I was missing. Jerry does this sound like a plan to you?

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That's how much you should increase the AVR volume (assuming you've also followed Jerry's advise about calibration).
As long as it's not clipping, you'll get accurate graphs.
Well that's the issue. I will have clipping as soon as I turn the AVR up about 5 dB, which isn't enough to play it loud enough to see at what point distortion sets in.

Quote:
I use the delay feature and run out of the room when I'm taking seriously loud measurements.
Lol I can picture that. Myself, I use hearing protection ear muffs which are ok up to the levels I've been testing. But once I start going higher to test distortion I will likely need to run out too!
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post #14950 of 16017 Old 06-02-2015, 11:15 AM
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^^^

Meh, I just plug my ears with my fingers.
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post #14951 of 16017 Old 06-02-2015, 11:26 AM
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Originally Posted by lovingdvd View Post

From what I recall, I am almost certain that I will be way short of headroom. I think the most the volume can be set to without REW complaining about clipping is with the MV set so the REW SPL meter measures about 80 dB. Hence my original question of how am I suppose to measure with it louder than in the MDAT I posted, because much more than about 5 dB louder will cause it to clip. See my question in the response to Alan below.
We are all using the same mic, the same software, etc. so there should not be a big difference in our results. You should not have to turn down the mic gain in Windows. And you should be able to measure at 90dB without clipping.

One thing we haven't discussed--is your UMIK-1 set to -12dB sensitivity, or -18dB sensitivity? Some of the newer mics are shipping with the dip switch settings at -18dB.
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post #14952 of 16017 Old 06-02-2015, 11:59 AM
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@AustinJerry I don't know whether you'll find this useful but perhaps it might start to explain what I was on about with windows and so on, I'll use your measurement as an example. This is intended as a laymans guide to windows in REW hence the annotations within each pic. Let me know if this makes sense or is just as confusing as the REW help

Lets start with your C measurement, here's the IR view zoomed out to show the window. By default, REW uses a 125ms left (hann) window with a 500ms right (hann) window. The details aren't important right now, just that it's a v wide window. What does it mean to say we have a wide window? It means we are including a lot of sound that arrives at the mic at many different points in time, i.e. we have the actual sweep itself played by the speakers plus all that sound that then bounces around the room for the next 500ms.



The FR is just another way of looking at that IR, it looks v ragged and becomes increasingly hard to interpret at frequency rises. It's just very noisy.



The usual reaction is therefore to apply some sort of fractional octave smoothing to make it less noisy/easier to read. This is the same measurement with 1/6 octave smoothing.



However there is another way to interpret this data and that's by changing the shape of the window, i.e. by changing what data (recorded sound) we include in the analysis. We can look at what this does in practice.

Lets first zoom in on the first 25ms or so of the IR so we can see in a bit more detail what is going on



As we shall see, all that low level fuzz is what makes the full range FR so noisy. Lets change the window to see what happens.

This is a 1ms left (hann) window, 20ms right (blackman harris 4) window. Notice the light pink line and how it is closer to a flat line as time passes (the graph moves to the right). The blue line shows the shape of the window, if you think of the previous straight line as 100% to mean "include all of the content in the windowed response" then that line sloping down means "take less and less of the actual recorded content into account when analysing the response".



Look at what happens to the FR as a result, not so hairy but we lose LF information. Basically the shorter window means we have reduced frequency resolution *and* eliminated some of that late arriving futz.



We can continue and make it even shorter, this is a 10ms right window.



and the resulting FR



The lost LF resolution is a problem. We can use frequency dependent windowing (FDW) to try to recover this but that's for another post (as well as explaining how the spectrogram or waterfall view uses windows).
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post #14953 of 16017 Old 06-02-2015, 12:36 PM
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Originally Posted by AustinJerry View Post
We are all using the same mic, the same software, etc. so there should not be a big difference in our results. You should not have to turn down the mic gain in Windows. And you should be able to measure at 90dB without clipping.

One thing we haven't discussed--is your UMIK-1 set to -12dB sensitivity, or -18dB sensitivity? Some of the newer mics are shipping with the dip switch settings at -18dB.
Great call Jerry! Yes indeed, my sensitivity is -18 dB. I noticed that it said this in REW where you select the mic, and it stood out at me a bit since I recalled seeing in the guide that it was shown there as -12 dB. Where is the dip switch and should I change it? Does it make any difference in the calibration file or sens data (and downside to changing it to -12 dB)? Why did they start shipping them at -18 dB? And is this the reason why I am clipping so early? Can I just compensate for this in the REW input or Windows input level instead of messing with the dip switch?
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post #14954 of 16017 Old 06-02-2015, 12:42 PM
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Originally Posted by AustinJerry View Post
Some of the newer mics are shipping with the dip switch settings at -18dB.
As far as I know ALL of the UMIK's are now shipping with -18dB sensitivity. 12dB is a thing of the past.

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post #14956 of 16017 Old 06-02-2015, 03:47 PM
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As far as I know ALL of the UMIK's are now shipping with -18dB sensitivity. 12dB is a thing of the past.
I got my UMIK-1 on Friday and indeed it is set to -18dB. What is the advantage of this (why do you think they started shipping them this way instead of at -12dB)? And what are the implications of this when it comes to REW? Is this why I do not seem to be able to measure too high without clipping? I want to measure at a high SPL so I can see at what point distortion starts to pick up. How can I do this when it wants to clip around 80-some dB?
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post #14957 of 16017 Old 06-02-2015, 04:02 PM
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As I said, just turn down the mic level in Windows.

Worth a shot........
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post #14958 of 16017 Old 06-02-2015, 04:08 PM
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As I said, just turn down the mic level in Windows.

Worth a shot........
Well that might "work", but I don't know if doing so jeopardizes any of the accuracy or resolution of the reading, or has any other implications. Afterall I think the instructions say to maximize the input so I don't want to make a chance without knowing what it means. Likewise I'd like to understand more about the benefits of a -18dB gain mic vs -12dB, and why MiniDSP has made this change. I contacted the boss to get his thoughts: http://www.hometheatershack.com/foru...18db-gain.html
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post #14959 of 16017 Old 06-02-2015, 04:49 PM
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Originally Posted by Alan P View Post
As I said, just turn down the mic level in Windows.

Worth a shot........
I started doing this after reading your comments on it recently. It does drop your measurement SPL, but allows you to measure with your AVR/Amp volume higher. My issue though, is that unless we all agree to the exact same volume level, we're all going to have measurements that are incompatible with each other's, and none of us can really be sure that our system is outputting what it says it is. For example, if I put the windows mic volume at 100 and my AVR at -20 and get 90db readings, then put the mic volume down to 30 and measure 60db, my system is still putting out the same SPL as before, I just made the mic less sensitive to it.

So I suppose you could say that absolute measurements become meaningless, but as long as you use the same setting for all your measurements, the relative differences are still just as valid as ever. Thoughts?
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post #14960 of 16017 Old 06-02-2015, 05:10 PM
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Originally Posted by Soulburner View Post
I started doing this after reading your comments on it recently. It does drop your measurement SPL, but allows you to measure with your AVR/Amp volume higher. My issue though, is that unless we all agree to the exact same volume level, we're all going to have measurements that are incompatible with each other's, and none of us can really be sure that our system is outputting what it says it is. For example, if I put the windows mic volume at 100 and my AVR at -20 and get 90db readings, then put the mic volume down to 30 and measure 60db, my system is still putting out the same SPL as before, I just made the mic less sensitive to it.

So I suppose you could say that absolute measurements become meaningless, but as long as you use the same setting for all your measurements, the relative differences are still just as valid as ever. Thoughts?
The level of the measurement is only significant in several types of measurements. For example, the waterfall should be 40dB above the noise floor. Otherwise, the level has no significant effect. And if there is a level inconsistency among us, that doesn't make much difference either. I wouldn't worry about it.
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post #14961 of 16017 Old 06-02-2015, 05:12 PM
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Originally Posted by 3ll3d00d View Post
@AustinJerry I don't know whether you'll find this useful but perhaps it might start to explain what I was on about with windows and so on, I'll use your measurement as an example. This is intended as a laymans guide to windows in REW hence the annotations within each pic. Let me know if this makes sense or is just as confusing as the REW help

Great stuff, Matt. Thanks for taking the time to share this!
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post #14962 of 16017 Old 06-02-2015, 05:19 PM
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I spent some time today working on the null I reported earlier at around 250Hz. The problem was most pronounced with the center channel, so that is where I focused my attention. The theory was that if SBIR is causing the dip, then moving the speaker should affect the measurement of the dip.

I moved the center speaker forward several feet, and back several feet, but none of the movements resulted in any improvement. Most of the positions were worse, and the current location actually seems to be best.

Then I tried moving the MLP froward 12-24", and back 12-24", and the results were the same. The current MLP produces the best result.

So, after all this work, I am no closer to alleviating the 250Hz dip. If you look at it closely, the dip is less than 5dB, so I guess I will just live with it.
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post #14963 of 16017 Old 06-02-2015, 05:37 PM
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Originally Posted by AustinJerry View Post
I spent some time today working on the null I reported earlier at around 250Hz. The problem was most pronounced with the center channel, so that is where I focused my attention. The theory was that if SBIR is causing the dip, then moving the speaker should affect the measurement of the dip.

I moved the center speaker forward several feet, and back several feet, but none of the movements resulted in any improvement. Most of the positions were worse, and the current location actually seems to be best.

Then I tried moving the MLP froward 12-24", and back 12-24", and the results were the same. The current MLP produces the best result.

So, after all this work, I am no closer to alleviating the 250Hz dip. If you look at it closely, the dip is less than 5dB, so I guess I will just live with it.
That's frustrating I'm sure. Do I recall that you have Dirac Live? Maybe you can bump that a bit in that area in your target curve to try and address it via EQ?
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post #14964 of 16017 Old 06-02-2015, 05:55 PM
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That's frustrating I'm sure. Do I recall that you have Dirac Live? Maybe you can bump that a bit in that area in your target curve to try and address it via EQ?
You are correct, I am using Dirac Live. But if Dirac doesn't correct it routinely, trying to fix it manually is not the best idea.
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post #14965 of 16017 Old 06-02-2015, 06:23 PM
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You are correct, I am using Dirac Live. But if Dirac doesn't correct it routinely, trying to fix it manually is not the best idea.
Perhaps you can shape your target curve to have it add a few dB peak in that area and see if that helped? You tried moving your MLP and speaker locations and that didn't help. So that leaves perhaps testing with a different speaker to rule out the speaker itself (I assume near-field tests of the speaker show flat FR?) or perhaps trying to add some room treatments but without a clear understanding of how and where. Seems like Dirac would be the easiest thing to try. 5 dB dip in that range seems like it would be important to try and work out.
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post #14966 of 16017 Old 06-02-2015, 06:26 PM
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For example, if I put the windows mic volume at 100 and my AVR at -20 and get 90db readings, then put the mic volume down to 30 and measure 60db, my system is still putting out the same SPL as before, I just made the mic less sensitive to it.
Does calibrating the mic make any difference at this point?

Did you really need to quote that entire post in your reply?
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post #14967 of 16017 Old 06-02-2015, 06:45 PM
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Perhaps you can shape your target curve to have it add a few dB peak in that area and see if that helped? You tried moving your MLP and speaker locations and that didn't help. So that leaves perhaps testing with a different speaker to rule out the speaker itself (I assume near-field tests of the speaker show flat FR?) or perhaps trying to add some room treatments but without a clear understanding of how and where. Seems like Dirac would be the easiest thing to try. 5 dB dip in that range seems like it would be important to try and work out.
I am not obsessing about what is a minor dip.
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post #14968 of 16017 Old 06-02-2015, 06:47 PM
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Does calibrating the mic make any difference at this point?
It's a UMIK-1 from Cross Spectrum Labs, so it uses its own calibration file and doesn't require any calibration by the user. But I see what you're saying - it would make sense to me that the input volume should be at max if you want accurate SPL measurements. At least that's what it would be calibrated for.

You could try to recalibrate so that a mic volume of 30 would give you accurate results again. But wouldn't that put you back to square one? Someone want to test this and see if they lose the headroom they gained?
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post #14969 of 16017 Old 06-02-2015, 06:53 PM
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It's a UMIK-1 from Cross Spectrum Labs, so it uses its own calibration file and doesn't require any calibration by the user. But I see what you're saying - it would make sense to me that the input volume should be at max if you want accurate SPL measurements. At least that's what it would be calibrated for.

You could try to recalibrate so that a mic volume of 30 would give you accurate results again. But wouldn't that put you back to square one? Someone want to test this and see if they lose the headroom they gained?
You forget that the CSL calibration file does not contain the sensitivity parameter, which must be downloaded separately from the MiniDSP web site and inserted manually into the calibration file.

So, the question is, for the mic with dip switches set for 18dB sensitivity, is the sensitivity parameter correct? One could test this out with an external SPL meter (if you have one). Simply generate a REW test tone, measure the SPL output using the USB mic, and measure with the external SPL as well. The readings should match.
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post #14970 of 16017 Old 06-02-2015, 07:03 PM
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I have the sens factor from MiniDSP.

I don't have an SPL meter, but I do have the Audyssey mic which, with my X4000, mandates that I have my sub set to 75db before starting Audyssey. When I got the UMIK all set up, with the help of your guide, I put the Audyssey screen on the TV with the sub tone, and had REW going on my PC. I remember that they were within 1-2db. So if I run the mic at only 30 in windows, it's no longer going to give an accurate SPL reading.

My question is, if you calibrated the mic so that 30 gave you accurate readings again, do you lose the headroom you gained (the sole reason for the tweak in the first place)?
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