# Challenges with REW this time around



## lovingdvd (Jan 23, 2007)

Hi all,

Several years ago I used REW 4 to calibrate my sub with great results. It was a very long process because I spent hours tweaking the filters manually and did not have a MIDI interface to my BFD 1124 at the time, but eventually it worked out great.

Recently I've been wanting to redo things with the latest REW 5 beta because I thought the software could do better than my manual job and because I may change my receiver soon so I wanted to get confidence in my ability to do the calibration again.

I worked with REW 5 beta 7 for several hours today and really could not get good results. I am wondering what I am doing wrong...

I started by tuning to a new, blank memory bank on my BFD with no filters applied. I took a Measurement. This is my initial.jpg and initial.mdat file. Note the dip around 21hz - this was my primary concern.

So I pulled up the EQ window and hit Set Target Level, match range 10-80hz, individual boost 10, overall max boost 12, flatness target 1db. I hit Match Response to Target. Attached is the filter graph afterward (match response to target jpg). Also attached are the filters req file that it came up wit. As you can see, it didn't try to apply much. Why is it not addressing the dip at 21hz among other things?

I then took another measurement - see after.mdat. As you can see nothing really changed much. 

Now have a look at the bad-dip-rta.jpg. This is what my RTA looks like. The dip at 21hz is bothersome and one of the things I'm trying to fix.

OK, then I changed my BFD back to the memory bank that has the filters I created a year ago. I then ran an RTA with this. See great-previous-results-rta. jpg. Notice how beautiful this is in comparison. 

Can someone please tell me what I am doing wrong? I am at a loss for why my RTA looks so great with my old calibration I created by hand (manual filters) but with REW I cannot come close to making the RTA look anywhere near this good. 

Thanks!!


----------



## JohnM (Apr 11, 2006)

There are a few things going on.

The first is a bug in the auto EQ code. The match range starts at 10Hz, but the lowest frequency the BFD allows is 20Hz, so the filter assignment keeps trying to set filters below 20Hz but they get limited to the 20Hz the equaliser supports. To work around that set the lower end of the match range to 20Hz.

The dip at 20Hz is too narrow for REW to assign a filter as the filter's decay time would be too long, creating an artificial resonance.

The RTA results look pretty rough - are you using Pink PN as the test signal? Pink PN gives good low frequency results without averaging (or with minimal averaging), in contrast to Pink noise which requires a lot of averaging for useful low frequency results. In both cases the RTA window should be set to Rectangular.


----------



## lovingdvd (Jan 23, 2007)

JohnM said:


> There are a few things going on.
> 
> The first is a bug in the auto EQ code. The match range starts at 10Hz, but the lowest frequency the BFD allows is 20Hz, so the filter assignment keeps trying to set filters below 20Hz but they get limited to the 20Hz the equaliser supports. To work around that set the lower end of the match range to 20Hz.
> 
> ...


Thanks so much for the quick response John! I will try it again with the low end of the match range at 20hz.

Regarding the dip at 21hz - I am pretty sure that with my calibration from several years ago the dip is taken care of nicely. Please have a look at the 2nd and 3rd graphs I posted. The 2nd graph is a RTA with the new calibration (that I messed up by using 10hz as the low point). The 3rd graph is a RTA with my old calibration. Notice the significant dip at 21hz in the new calibration, which is pretty much non-existent with the old calibration. Does this show that indeed I am handling that dip with the old calibration? Or do you think I created artificial resonance issue (no idea what that means).

Yes I was using Pink PN. I will use the Rectangular option. I tried Pink Noise at first, but it didn't look like it was measuring anything (RTA seemed non-responsive). Sounds like I needed to give it a lot more time. Is one approach better than the other? This is the first time I was using the RTA so I wasn't sure what I was doing. 

OK and as a side note - here is the mother of all feature requests which possibly could be the holy grail... Is it possible for you to make REW do an automatic, real-time calibration? Something like sending an initial set of filters via MIDI to the BFD, playing pink noise, using the RTA, and the on-the-fly start sending filter adjustments to the BFD in real-time, then the software looks at the effect, and keeps tweaking watching how each change affects the others etc until it is dialed in as tight as it can get? This is probably a tall order but seems like it was be incredible.


----------



## lovingdvd (Jan 23, 2007)

John - I tried doing things over this time with the match range set to 20-100hz. However there results were the same as before. It concentrated all its filters in the 100hz area but did not attempt to fix any of the peaks and valleys in the lower area that I care most about. In particular the dip at 21hz remained untouched.

I know you mentioned you were not surprised that it left out the dip at 21hz. But I am confused by this. With REW v4 (or maybe it was v3 back then - this was probably 2006 or 2007) I had much better results for the filtering.

Is it possible that the older version of REW could handle my situation better and/or that there is a bug that is preventing REW from handling this?

When I put my BFD into the preset with my old calibration in it, the bass sounds smooth with the Pink PN generator and the RTA graph looks nice and flat (like the good-results RTA jpg I posted). When I do this same thing but with the BFD set to the REW 5 results, I can immediately hear the sub get noticeably muddier, and the RTA shows a big dip in the 21hz range - about 10 db or so. See the bad-results RTA jpg I posted earlier.

For reference, what I remember with the older REW used to make the BFD filters that are working for me - I did an auto filter calibration and then manually adjusted some of those filters and added a few to tweak things up beautifully.

Here are those filters that the older version generated with my tweaks and additions, which are in my BFD bank that are working very well. 

Frequency, Bandwidth, Fine, Gain:
1: 32, 14, -6, -12
2: 80, 6, -1, -9
3: 25, 5, -8, +4
4: 40, 4, +2, +4
5: 50, 4, +7, +5
6: 50, 2, +3, +2
7: 63, 7, +4, -5
8: 80, 10, -7, -2
9: 20, 4, +6, +2
10: 63, 10, +7, +4
11: 80, 2, -2, +2
12: 80, 4, 0, 0

Can you please let me know what you think may be going on to prevent REW 5 from doing the same great job the older version did? What else I can try to resolve it or to collect more info for you for troubleshooting?

These old filters are doing a nice job for me. But I will be replacing some equipment soon and before doing so I would like to have confidence that I can run REW later and get back to a beautifully tuned sub. 

Thanks so much in advance!


----------



## JohnM (Apr 11, 2006)

lovingdvd said:


> Is it possible that the older version of REW could handle my situation better and/or that there is a bug that is preventing REW from handling this?


No, REW V4 would only target peaks by applying cuts, it did not generate boost filters. Of the 11 active filters you list (the 12th has zero gain) 7 are boost, which REW would not have generated so those must have come from your manual adjustments.

Here is the effect of the filters listed on the "initial" measurement:









The 12dB cut of filter #1 at 29.55Hz makes a huge hole in the response and the 21Hz region is virtually unaffected. Since you do not see this kind of response when you use the RTA, I wonder about the "initial" measurement. Could that measurement have been made with the old filters active? I think you first need to make sure you are giving REW the correct starting point to work on.


----------



## lovingdvd (Jan 23, 2007)

Thanks John for the continued help. I really appreciate it. After reading your message I went back and started from scratch just to double check everything and the result was the same as before.

Here's a recap of everything:

- Started with my RadioShack SPL, dialed to 70db, C Weighting, Slow Response. C Weighting checked in Mic pref.

- Set my BFD set to channel 1.

- Use "main speakers to set/check levels", output meter shows -12db, input meter shows -20db, Radio Shack SPL read 75db with AVR volume level set where it is.

- Go to EQ Windows for the purpose of blanking BFD channel 1.

- Choose Filter Tasks, Reset Filters for Current Measurement, Send Filters to BFD, left only, Channel 1. All 12 leds off on BFD after this. BFD has taken settings. I verified using the buttons on the BFD to look. Nothing set.

- Close EQ window.

- Run Measure - starting freq 0, End Freq 150, level -12, length 512k, sweeps 2, estimated time 24s

- Headroom displayed during measure shows as 7.6db in green.

- Result of Measures is attached as initial.mdat.

- After Measures Go to EQ Window.

- Press "Set Target Level", it chooses 77.7

- Match range set to 20-100, indv boost 10db, overall boost 12db, flatness 1db

- I click match response to target. It runs for a bit and assigns filters. Attached as filter file.

- Send filters to equilizer, left, channel 1. BFD has accepted the input.

- I use the buttons on the BFD to scroll thru the filters and verify it received and stored them OK. All is as expected with the stored filters.

- Remeasure with same params as first measure, headroom 8.4db.

- This second measure looks nearly identical to first. Doesn't appear to show any benefit at all from REW filters. This is attached as after.mdat

- Running a RTA with Pink PN and BFD confirms large dip at 21 hz and more muddled sound.

- Switch the BFD to channel 9 which is where I have my "good" filters from running REW several years ago. RTA smooths out, bass sounds much tighter. On channel 9 the red led dashes for the filters all line up on the left channel, so I know Left is the correct choice for which bank I am wired up to use.

Based on this does it seem I am doing everything correct? I verified I started with a clean slate, verified the assigned filters went into the BFD, verified the BFD was on the correct channel, remeasured and say no benefit, switched the BFD to the "good" channel with old calibration and results were great.

What could I be overlooking? Please let me know what steps you would like me to perform next. I am happy to gather whatever data or troubleshooting you think would be helpful and happy to experiment. I would love to get to the bottom of this so we can see what is going on and solve it! 

Thanks!!


----------



## LastButNotLeast (Sep 14, 2011)

Stupid thoughts from a rank amateur, since sometimes it's the obvious that's screwy:
Are you using the calibration files for your RS meter? Have you calibrated your sound card?
Are the filters on the BFD set to PA (as opposed to OF)?
Hope it turns out to be something simple.


----------



## lovingdvd (Jan 23, 2007)

LastButNotLeast said:


> Stupid thoughts from a rank amateur, since sometimes it's the obvious that's screwy:


Thanks for chiming in! Answers mixed in below.



> Are you using the calibration files for your RS meter?


Yes. I got the SPL meter calibration file from this page http://www.hometheatershack.com/forums/downloads-area/19-downloads-page.html . My meter is the Model 33-2050 as pictured on that page. And I've told REW to use the RadioShack-33-2050-CS.cal file I got from there.

This is the same exact meter I used to perform my calibration years ago (the "good" calibration I keep referring to). There are fresh batteries in it and it appears to be operating fine.



> Have you calibrated your sound card?


Yes. I followed the instructions and the graph from the sound card calibration looks as to be expected (sharp rise up from the left to right, then flat line across almost the entire range, then sharp drop off at the top end). I have told REW to use this calibration file.

This is the same exact sound device as I used previously with my "good" calibration. It is a SB Live 24 external USB.



> Are the filters on the BFD set to PA (as opposed to OF)?


Yes. In fact I just went and double checked and they are set to PA. In fact if they were set to OFF than the red LED dashes in the BFD display would not be lit.



> Hope it turns out to be something simple.


Thanks. Unfortunately so far its a brain teaser. I am looking forward to getting to the bottom of it though.

I keep thinking that maybe I am overlooking something simple. This is why I outlined all my steps in my recent post above, so that maybe something can be spotted. Then again, I had great success with REW v4 in the past without any challenges like this - all with the same exact room, same exact equipment (both AV gear and CPU gear) same laptop, same cables, same BFD - same everything! Hummmm.....


----------



## LastButNotLeast (Sep 14, 2011)

So keep using v.4?!
onder:


----------



## lovingdvd (Jan 23, 2007)

LastButNotLeast said:


> So keep using v.4?!
> onder:


Yea, I'm hoping to avoid going back to REW v4. I got good results with it, but I had to do a lot of manual work to get it as close as I wanted it. I think partially because it does not add gain filters but REW 5 does.

So anyway I am anticipating that REW 5 can do a better job than REW 4 so I don't want to give up on it. I am waiting to hear what John has to say and then follow his plan of attack for getting to the bottom of this. If it does turn out that v4 is much better at setting filters for my situation I want to help John figure out why so he can resolve the issues in v5.


----------



## JohnM (Apr 11, 2006)

Please make a measurement with your preset 9 filters in use to see what that response looks like. Bear in mind that the RTA comes a distant second to making a measurement, don't use the RTA to judge results.

REW is making little change to the response because it is pretty smooth to start with, there is little for it to do. Your "after" response, however, is not a good match to the Predicted response which suggests that not all of the signal being measured has passed through the BFD and the sub. What speakers are running when you make your measurement, sub only or sub + mains or sub + 1 main? Sub only is required for the filters to be meaningful but you mention using the main speakers test signal to check levels?

What are the Target Settings in the EQ window? Do you have a house curve loaded?


----------



## lovingdvd (Jan 23, 2007)

JohnM said:


> Please make a measurement with your preset 9 filters in use to see what that response looks like. Bear in mind that the RTA comes a distant second to making a measurement, don't use the RTA to judge results.


OK, good to know. Although I can say for sure that when playing the Pink PN noise while the RTA is going that the base sounds much smoother on the channel 9 with the good input. When I switch the BFD to channel 1 with the new filtered results or with no filters the base turns muddy in comparison.



> REW is making little change to the response because it is pretty smooth to start with, there is little for it to do. Your "after" response, however, is not a good match to the Predicted response which suggests that not all of the signal being measured has passed through the BFD and the sub. What speakers are running when you make your measurement, sub only or sub + mains or sub + 1 main? Sub only is required for the filters to be meaningful but you mention using the main speakers test signal to check levels?


Allow me to clarify, as it sounds like I am doing this part wrong...

On the Preferences for Soundcard I have selected "Use Main Speaker to Check/Set Levels". Then I run Check Levels and set the volume on the AVR so that the Radio Shack SPL reads 75db when it checks the sound.

It sounds like I should be using "Use subwoofer to Check/Set Levels" instead? I do recall briefly choosing that and checking the levels and the Radio Shack meter was pinned on the 70db setting (so this level did not match with the 75db level on the AVR). However I did not keep the setting this way and I didn't use it this way, which it sounds like I should?

When I make my measurements I have the sub + front left main + front right main. My rationale there was that all these speakers will be playing together for real, so I need to take the bass they are providing into account in the calibration. This seems logical to me, but from what you are saying I gather I should not be doing this - and instead I should disconnect the mains so that it is the sub only? 

So when my RTA is going I am also having the sub + left/right mains going.

Based on all of the above do you think that is causing these problems? I can't recall for sure but I thought I did my original calibration this way.



> What are the Target Settings in the EQ window? Do you have a house curve loaded?


Speaker type: Subwoofer
Crossover: 24db/octave
Cutoff: 80hz (which matches what my AVR is set at)
LF Slope: 24 db/octave
LF cutoff: 10hz
Target Level: (set by clicking on "Set Target Level")

Yes I have a house curve set. 20 +8, 80 0 (I like my sub hot!  ). I attached the file.

I also uploaded my soundcard calibration file just in case there is anything funny about that (I don't think so, it looks normal).

Please let me know what you think and what would be good next steps. Thanks John I really appreciate the great help!!


----------



## lovingdvd (Jan 23, 2007)

Forgot to attach files to the last post. Attaching them here... (Oh I see I cannot attach the .cal for the soundcard because its too large 43Kb but I don't think you need it anyway - as I mentioned the graph of it looks just like what your example sound card shows it should look like). But the housecurve is attached. Thanks.


----------



## lovingdvd (Jan 23, 2007)

*Good progress! Still requires manual filters for dip.*

John,

I went back and switched things so that the sound check was sub only, and so that the sub was the only speaker on during calibration.

Then I ran an initial Measures with no filters in the BFD channel. See subonly-initial.mdat. Note the 5db dip at 21.5hz.

Then I used the EQ Window to generate filters using same params as I previously wrote about and saved these to the BFD. The filters REW came up with on its own (untouched by me) are attached as filters.req.

Then I remeasured. The results are in subonly-afterfilters.mdat. Note the 5db dip was not addressed by the auto-generated filters. REW did, however, nicely clean up the area from 60hz - 100hz!

So then I decided to add a couple of filters manually to address the dip at 5 dB. I had to take a couple filters away from what REW did in Auto (since I was out of slots otherwise) so I picked the two that seemed to have the least significance.

Please have a look at the two manual filters I added to filterstweaked.req. I then remeasured and the results were excellent (I think)! See subonly-after-manual-fix.mdat.

I also added a sub-and-mains-after-manual-fix.mdat file which is a measurement of my tweaked filters with the sub and left/right front mains back on (when I was finished).

So I have two remaining questions at this point:

1) Do you agree that the subonly-after-manual-fix.mdat shows an excellent response and there is no work left to be done here? Or does it look good but I've created a problem without realizing it?

2) What do you make of me having to manually create these filters in order to fix the dip myself? Is there something about this particular scenario that REW is currently not designed to handle? Is there an enhancement needed to REW in order for it to be able to come up with a better curve then it was doing on its own and to replicate (or beat) what I came up with manually? I don't mind making tweaks like this, but I am worried that I am breaking something or that if I got lucky with this calibration I may not with a future calibration attempt...

Please let me know what you think. Thanks so much!


----------



## lovingdvd (Jan 23, 2007)

Hi John - I figure you have been very busy with other things. Was just hoping you may have some time tonight or this weekend to provide your thoughts on the latest info I posted a couple nights ago above. Thanks!


----------



## JohnM (Apr 11, 2006)

Sorry, was travelling yesterday. Measuring sub only was the right thing to do, especially when you are capturing a measurement you want REW to equalise. Your equaliser is only in the path to the sub, so REW can only alter the sub response. If you make a measurement of sub+mains and ask REW to try and work out filters for that the result will not be as predicted, because part of that measurement came via paths the filters will not affect.

On the auto filters, there can be two reasons for REW not generating filters for the lowest part of the response where your (small, by the way!) dip is. The first is if the response below the dip (i.e. at lower frequencies) does not rise above the target level. This is to prevent the lowest part of the response being boosted beyond the capability of the driver. The second is if the dip would require a filter that is too narrow, because narrow filters act like modal resonances, they generate ringing in the response. More on that shortly.



> Do you agree that the subonly-after-manual-fix.mdat shows an excellent response and there is no work left to be done here?


Unfortunately, no.



> Or does it look good but I've created a problem without realizing it?


Yes, that is exactly what has happened. Let's take a look at some waterfall plots to see this more clearly. Here is the response with the REW filters:









There is some damped ringing just below 20Hz, likely a modal resonance of the room.

Here is the response with the manually tweaked filters:









Note that the resonance at around 20Hz has become much worse, with a very slow decay. That is because of the very sharp manual cut at 20Hz (gain -10dB, BW 1/60th octave). If you look at the RHS of the EQ filters window there are columns labelled Mode T60 and Filter T60. Mode T60 is the decay time of a resonance that the filter settings would be suitable to counter. For that filter it is 8.3 seconds. Filter T60 is the decay time of the filter itself, 2.6 seconds. Because there is no modal resonance with an 8.3 second decay at 20Hz we are left with the ringing of the filter, hence the newly created resonance in the waterfall. When manually adjusting filters it is a good idea to make sure the Filter T60 values do not get too high and to look at the Predicted waterfall plot REW generates in the EQ window, that lets you quickly spot whether your adjustments are making the waterfall better or worse.

I've attached a set of filters to try, that I have manually tweaked at the low end to better reduce the ringing at 20Hz and smooth out the response.

View attachment jm-filters.req


----------



## lovingdvd (Jan 23, 2007)

John,

Thank you so much! I did not know anything about waterfalls and delays prior to your message and never paid attention to it. It took me a little bit to fully digest what you are saying and I did some studying up on it and now I can say I totally get it. 

Attached are my results from this latest round. First is initial.mdat which is what I started with tonight as a baseline, no filters in BFD, initial reading.

Then I loaded in your tweaked filters and uploaded these to BFD channel 1. See with-john-filters.mdat for the results. The filter I added was #12 - see john-m-filters-tweaked.req.

Then I decided to try and smooth out the hump in the low 30s. I was careful to do so in a way that did not cause a problem with the waterfall. I think the results were an improvement - the hump flattened out a bit and the waterfall actually improved a little. See with-john-filters-tweaked.mdat.

And then for reference I took at look at my original calibration from a few years ago. The waterfall in the latest tweaked version I think is better by a little, although the curve/response seems a bit better in the original. It is attached as original-ch.mdat.

A few follow up questions please:

1) the calibration I am now running is john-m-filters-tweaked.mdat. Do you agree that this is a very good result (considering it is an untreated room with one open wall!) and that I should be satisfied and stop here? Or once again am I overlooking something and it really leaves a lot of room for improvement?

2) do you feel this is a better result than my original-ch.mdat?

3) After I was done with john-m-filters-tweaked.mdat I needed to adjust the subwoofer's amplifer level out to balance the bass with the rest of the speaker system. After I did this I measured the subonly with this new amplifier setting and the response curve changed. See with-john-filters-tweaked-after-subvol.mdat. Is this OK or did I mess something up by touching this control?

4) Now that I understand the waterfall I am wondering if there is a room treatment I can do in order to cut the resonance in the 15-20hz range (or whatever you call the issue where the waterfall is lingering way to long). How would I go about figuring out what is causing this and what type of treatment is typically needed to improve on that?

Thanks John you are the best!


----------



## JohnM (Apr 11, 2006)

Note that you can save multiple measurements in a single mdat file using the "Save All" button, but remember to change the trace names and/or add some text in the notes box to identify what each trace represents.



lovingdvd said:


> Then I decided to try and smooth out the hump in the low 30s. I was careful to do so in a way that did not cause a problem with the waterfall. I think the results were an improvement - the hump flattened out a bit and the waterfall actually improved a little. See with-john-filters-tweaked.mdat.


Whilst this is an improvement, there is something a little unusual going on between 25 and 40Hz. The extra filter should not have been needed, and even with it in place the level at 28.9Hz, where your -4dB cut is placed, has only changed by approx 1.5dB. That suggests that some of the signal in this region is not directly related to the signal going to the subwoofer. Changes in the measuring environment might be a factor (doors in different positions, external noises such as traffic or air conditioners) or there may be something in the room or its construction that is reacting at those frequencies in a somewhat non linear way. Any other change in the measurement setup, such as a slightly different position for the meter, would also have an effect.



> 1) the calibration I am now running is john-m-filters-tweaked.mdat. Do you agree that this is a very good result (considering it is an untreated room with one open wall!) and that I should be satisfied and stop here? Or once again am I overlooking something and it really leaves a lot of room for improvement?


Yes, it looks very good, nothing stands out as needing attention.



> 2) do you feel this is a better result than my original-ch.mdat?


Yes, a little.



> 3) After I was done with john-m-filters-tweaked.mdat I needed to adjust the subwoofer's amplifer level out to balance the bass with the rest of the speaker system. After I did this I measured the subonly with this new amplifier setting and the response curve changed. See with-john-filters-tweaked-after-subvol.mdat. Is this OK or did I mess something up by touching this control?


The only effect of changing the sub volume setting should be to shift the trace vertically, not alter its shape, so it is strange to see the shape has been altered. For comparison I adjusted the trace levels so that the region from 100-150Hz, which is very similar in all the measurements, is at the same level, that plot is below. The response actually looks better, but it should not have altered. Could any other control on the subwoofer have been moved? Does the processor have any form of dynamic EQ or loudness control active?











> 4) Now that I understand the waterfall I am wondering if there is a room treatment I can do in order to cut the resonance in the 15-20hz range (or whatever you call the issue where the waterfall is lingering way to long). How would I go about figuring out what is causing this and what type of treatment is typically needed to improve on that?


That resonance is too low to be meaningfully altered by treatments that would leave any space in the room for you to sit  and is not bad enough to be concerned about anyway, just ignore it.


----------



## lovingdvd (Jan 23, 2007)

JohnM said:


> Whilst this is an improvement, there is something a little unusual going on between 25 and 40Hz. The extra filter should not have been needed, and even with it in place the level at 28.9Hz, where your -4dB cut is placed, has only changed by approx 1.5dB. That suggests that some of the signal in this region is not directly related to the signal going to the subwoofer. Changes in the measuring environment might be a factor (doors in different positions, external noises such as traffic or air conditioners) or there may be something in the room or its construction that is reacting at those frequencies in a somewhat non linear way. Any other change in the measurement setup, such as a slightly different position for the meter, would also have an effect.


Thanks. I am sure everything was the same between measures. Same session so exact same meter placement, perfect silence, no running A/C or appliances etc, no doors or room differences. Not sure why that is happening. Is it possible that something is off in REW?



> Yes, a little.


Because you think this extra filter helped a little, do you think it makes sense for me to go back and try an even greater filter - like maybe -8 instead of the -4? The -4 definitely seemed to help the waterfall stop its echo/resonance earlier. Maybe the -8 would help too? Or do you think I should just leave well enough alone at this point?



> The only effect of changing the sub volume setting should be to shift the trace vertically, not alter its shape, so it is strange to see the shape has been altered. For comparison I adjusted the trace levels so that the region from 100-150Hz, which is very similar in all the measurements, is at the same level, that plot is below. The response actually looks better, but it should not have altered. Could any other control on the subwoofer have been moved? Does the processor have any form of dynamic EQ or loudness control active?


Indeed this is quite puzzling. And I remember years ago experiencing the same thing...

Basically what I do to set levels in the beginning is I put the Radio Shack meter on 70, then use Check Level for Sub-only and turn up the AVR volume until the Shack meter reaches 75. Then I use the REW Sound Meter and hit Calibrate. That seems to measure about 10db lower for some reason. So I calibrate the meter that way.

Then I run through the EQ and Filter stuff. OK now that its all done and measures out and I'm happy, the next thing I do is put all my speakers back on and the sub is way too strong. So I use my AVR levels balance feature to send pink noise to all 5.1 speakers and set the volume so that at 75bd each speaker measures good.

Now in this case the sub is pegged over at the 70db dial position. I guess this may have to do with my +8 20 0 80 house curve?

So then I lower the levels out from the AVR and also may adjust the level out for the subs amplifier (dial on a sub). There is no dynamic EQ or loudness applied that I know of.

Now in this particular case I can say that two things happened. I'm pretty sure I wound up RAISING the levels out for the sub (.1) channel on the AVR output so that it was now at the center point (before it was likely nearer the bottom of the range adjustment). And then I LOWERED the sub's amp output (dial on back of sub). 

So now basically the sub measures about 80db or low 80db-ish while the rest of the speakers measure 75db. At this point when I disconnect all speakers and remeasure the sub, that's when you see that nicer curve. Any ideas? Well I guess its good that the changes went in my favor at least...

BTW, is my approach to setting these levels wrong? It seems like I set it up initially to be around 75db but by the time all is said and done, the sub is too over powering and requires me to dial it back. The end result is very nice. But I am not 100% confident it couldn't be better or that I've somehow created a sub par environment. All I can say is that it sure sounds nice and tight. I don't know, maybe the house curve of +8 is too aggressive? I remember coming up with that years ago when I first used REW and was bored and unsatisfied with a evenly EQ'ed sub... 

Thanks again John. I really appreciate all the help!


----------



## JohnM (Apr 11, 2006)

What range is your meter set to when you measure? Does the needle end up pegged during the measurement? If so it would be better to measure on the 80dB range, when the meter needle is pegged the analog output from the meter is also reaching its limits within the meter circuitry so your readings become unreliable.

Calibration of the REW meter only needs to be done once unless you change either the input volume on the soundcard or the meter range.

To see how the sub level sits versus the mains make a measurement over a wider range.


----------



## lovingdvd (Jan 23, 2007)

JohnM said:


> What range is your meter set to when you measure? Does the needle end up pegged during the measurement? If so it would be better to measure on the 80dB range, when the meter needle is pegged the analog output from the meter is also reaching its limits within the meter circuitry so your readings become unreliable.


I've watched the Radio Shack meter enough during measures to answer that without checking.  The meter does not get pinned. What happens is in the very beginning as the sound starts to "roll up" you see the meter come up and it does maybe bang off the top of the meter but it is only temporary for maybe a split second. Then it backs off and pretty much smoothly rolls off as the sound rolls off.



> To see how the sub level sits versus the mains make a measurement over a wider range.


This brings up an interesting question that I have been thinking about... and maybe this is what you are refering to in your question - isn't it possible that the sub has different response at different volume levels? For instance maybe at a volume level of 75db, 30hz plays at 80db, but if we then lower the volume to 65db, maybe 30hz plays at 75db. And further that there may be a different response like this at different hz. So just as an example maybe 30hz measure 5db less at 75hz when the volume level is dropped by 10db, and maybe 35hz measures 10db less when its dropped 10db. And further yet, maybe different hz have a different rate of change?

Such a thing must be a known phenomena? Is there anything in REW that would measure this? If not would adding such a feature be valuable? I'm thinking it would run a sweep at decreasing levels in steps of Xdb x number of times and show some sort of a plot that records the characteristics?


----------



## JohnM (Apr 11, 2006)

lovingdvd said:


> isn't it possible that the sub has different response at different volume levels?


Not within the linear operating range of the sub, by definition. Eventually your sub will run out of output and start to compress, the curve will then no longer retain its shape if level is increased further, and distortion will increase rapidly (if the curve does not keep its shape there will be distortion). You could also have problems if clipping occurs elsewhere in the path from receiver output to subwoofer drive unit, such as at the subwoofer amp input or, if the amp is driven too hard, at the subwoofer amp output. At very low levels the noise floor will start to affect the measurement.


----------



## lovingdvd (Jan 23, 2007)

OK thanks John. Is there a recommended way for ensuring that the levels I am calibrating with are the levels to use once mixed in with the other speakers? I set my input level to the BFD based on documents describing how to do this on the Shack (high but not to the point of clipping). Then I turn up the volume on my AVR with just the sub connected until Check Levels gives me 75db on my Radio Shack meter. Then I calibrate the REW SPL which usually reads about 10db lower. Then I make all my REW EQ filters. Then when I bring the sub in, its too loud. Is this the result of my 20 +8, 80 0 house curve maybe?


----------



## JohnM (Apr 11, 2006)

lovingdvd said:


> Then I calibrate the REW SPL which usually reads about 10db lower.


No reason it should read lower unless the last time you calibrated it the RS meter was on a 10dB lower range setting.



> Is this the result of my 20 +8, 80 0 house curve maybe?


It wouldn't be particularly related to the house curve, but rather to where the target level ended up compared to the sub response and so whether your filters ended up mostly cutting or mostly boosting. If you apply filters that on average boost the output then the result afterwards would be higher levels, and vice versa.


----------



## Dr. Spankenstein (Aug 4, 2006)

It may be an issue with the types/bandwidth of the signal that your are using to calibrate. Aren't some filtered at 30Hz whereas others are full bandwidth. So looking back at the jm filter-tweaked-subvol trace (in pink), everything under 30Hz would not be checked. That accounts for nearly +10dB.

Secondly, I had an embarrasing mix-up in my BFD wherein I neglected to check which Engine mode (Left, Right or Coupled) I was entering my filters into. So when I went to "clear" one for a new set, I zeroed out everything in the Right Engine not realizing that you can also store separate settings in the Left Engine even under the same Program. Maybe something to look into...


----------

