# First REW Measurement... help needed interpreting results



## sdy101 (Feb 22, 2014)

So I finally got around to investigating my room acoustics, and it looks like I've got some issues from the start (as expected). Below is a plot of the frequency response measured with my HSU STF-2 subwoofer + Def Tech ProMonitor 1000 front speakers (left/right separately). Crossover for front speakers in Denon receiver is set at 90 Hz. Audyssey was turned off for this measurement (as was DynEQ and DynVol). I used a Sound Blaster x-fi external sound card (USB) and a Larson Davis 2541 free-field microphone (+/-1 dB from 20 Hz to 20 kHz) with preamp. I did not upload a mic cal file, but did apply the soundcard cal. I had the microphone connected to a Larson Davis 824 SLM to calibrate the SLM reading in REW prior to the FR measurement. The microphone was positioned at ear level (pointed up) at the main listening position about 16 ft from the center speaker. My room is about 16 ft wide and 30 ft long (family room that opens to the kitchen directly behind the sofa).









So I obviously have a large null (25+ dB) around 35 Hz and several peaks at 28, 41, and 70 Hz. Plus, I notice multiple large nulls in the front left speaker + sub curve that are not in the front right speaker + sub response (room acoustics?). Also, the transition into the front speakers (at 90 Hz) doesn't look all that smooth to me either, as the level drops 10+ dB from 80 Hz to 200 Hz.

So where should I go from here? I realize the HSU STF-2 is a bit undersized for my room, but I can I equalize this response out if I purchased a parametric EQ system (not sure what to get)? Should I invest in a 2nd HSU STF-2 sub to smooth things out? Should I try moving my sub to the other front corner of the room and see what I get? 

Running the same FR sweep with Audyssey turned on (DynEQ/DynVol still off) had little effect. Any comments would be well appreciated. Thank you!


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## JohnM (Apr 11, 2006)

Try measuring in a few different positions to see how the response changes, and try different sub positions if you have some flexibility in where it is placed. A quick way to do that is to use the REW signal generator set to 'Pink PN' and the RTA set according to the image below, then you can see the response change live as you move things around. Deep notches are generally the result of strong reflections, those reflections will appear as spikes in the ETC trace (on the Impulse graph). More on that here.


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## sdy101 (Feb 22, 2014)

Thanks, John. I will take some measurements adjacent to the MLP. Unfortunately, I can realistically only locate the subwoofer at one of the front corners of the family room. Currently it is on the left, so maybe I'll move it over to the right and re-measure. I never did the sub crawl since I felt limited in where I could place it.

I assume regardless of sub or listening position, I'll likely be stuck with some level of peaks and valleys due to typical room acoustics. If I were to move forward with an approach to try and improve and smooth out the bass response I have, which is typically performed first: single sub EQ or add a second sub? Or maybe I'm getting ahead of myself....


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## sdy101 (Feb 22, 2014)

So here are some more results....

I moved the subwoofer to the 4 places that I could (all along the front wall) and remeasured at the MLP: sub in front-left (FL) corner, under the front left satellite speaker, under the front right satellite speaker, and in the front-right (FR) corner. Since under the satellite placement provided only small changes from the corners, I have left them out of the attached plots. All plots are with the microphone facing up at ear listening height at the MLP and have Audyssey, Dynamic EQ/Volume turned off.

Below is a comparison of the frequency response between the sub in the front left and front right corners of the room. When the sub is located in the front right corner, frequencies below 40 Hz are up to 10 dB higher in level, the peak at 72 Hz has been somewhat tamed, and the null at 34.7 Hz is gone! Unfortunately a new null at 64.2 Hz emerged and it goes just as low (25+ dB). Measurements at the other end of the sofa lessened the null, but it was still there. So which position is "better"? I think sweeps and movie material "sound" a little better with the sub in the front right corner, but I keep thinking two subs or a PEQ may help here....









The next comparison is a frequency response with 1/3-octave smoothing out to 20 kHz for the left speaker with the sub on (FL vs FR corners). Here the FR corner sub placement seems to have less overall variation from 200 Hz to 20 kHz. Should I be concerned with the drop in level above 7 kHz for the sub in the FR corner? What about the 10-15 dB drop in level from 40-200 Hz? Should the mic be facing the front speakers (tweeters) for these measurements? My walls and pictures rattle some going through the low-frequency sweep -- does this elevate the low end? Should I lower my sub output?









And the last frequency response is the front left vs front right satellite speaker with the sub in the FR corner. What bothers me here is the front left speaker output being 5+ dB lower from 170 to 500 Hz. I saw this when the sub was placed in the FL corner as well. It's evident at the other listening spot on the sofa too. I'm assuming it has to do with the room layout, as the right wall is continuous from front to back for ~30 ft, but the left wall has a 6'x7' opening to the living room just 4' from the front wall and the hallway opening is on the left side of the room as well. Any thoughts on this would be appreciated.









And here are some addition plots with the sub in the front right corner. The RT60, waterfall, and spectrogram plots show the room decay. Should I be concerned that the decay takes up to 0.6s in RT60, or is over 450ms under 40 Hz in the spectrogram and waterfall plots? I will say I have a very large open room (30 ft long), so maybe these decay times aren't terrible, but how would you go about reducing them? Room treatments?

























What I really would like to get out of all this analysis (if at all possible) is: 
1) Better speech intelligibility from the center speaker. I usually can't understand what people are saying on TV when others are talking in the kitchen behind me).
2) Smoother transition/crossover from sub to front speakers (currently set at 90 Hz by Audyssey). It just doesn't sound like it blends as good as I think it should.
3) Flatter overall bass response. When I go through low frequency sweeps sitting at my MLP, you here the level go up, then down, then up, then back down, etc.

Thank you in advance for reading and helping with this stuff. I realize I asked a lot of questions here, but I do feel like I am learning a lot!


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## JohnM (Apr 11, 2006)

Does reversing the sub polarity help the responses any?


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## sdy101 (Feb 22, 2014)

JohnM said:


> Does reversing the sub polarity help the responses any?


John, are you asking if changing the sub polarity from 0 to 180 helped increase the frequency response level near the crossover frequency (90 Hz) or helped reduce the observed valleys/nulls or both? 

Although I forgot to mention it in my previous post, I did run RTA with a Pink PN signal and moved both the microphone and sub around to evaluate the effects. I will say that did not notice any significant improvement at the nulls when changing the polarity, and I recall that the FR level dropped noticeably around the crossover frequency for 180. I will say that using RTA is what led me to move the subwoofer over to the other corner of the room.

Can you provide any overall comments regarding the plots above? Does the FR level drop off too much from sub to front speakers (10+ dB from 40 to 200 Hz) or is this normal? I'm still a little unsure of what I should be shooting for here with no active EQ other than Audyssey...


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## sdy101 (Feb 22, 2014)

An update: I went back and rechecked the effect of sub polarity on the bass response and crossover with the sub located in the front-right corner of my room. John was correct (not a big surprise), switching sub to 180 deg definitely helped the response at the crossover frequency (levels increased by up to 15 dB from 80-100 Hz) and reduced the 64 Hz null somewhat! The plot below shows the difference, it is a FR sweep for the sub and front right speaker with a PCB calibrated mic at the MLP pointing up toward ceiling (90 deg incidence angle) and Audyssey MultEQ Off. It also shows that changing the crossover freq from 80 to 100 Hz in the Denon had a slight effect as well.









I also re-investigated the 7 dB dip in the left speaker (with sub) response at 250 Hz from the post above. Well the left-right speaker level discrepancy appears to be gone as well! I am using a different mic this time around with its cal file loaded (although the plots in the posts above were acquired with a calibrated Larson Davis sound level meter that had +/- 1 dB variation from 20 Hz - 20kHz). See FR result below.









The last comparison I did this morning was looking at 1) Audyssey MultEQ effects and 2) Pointing the microphone directly at the front left speaker when measuring the FR. 
1) The results (blue vs magenta curves) show that Audyssey MultEQ does appear to really help smooth out the peak at 750 Hz, increase the valley around 200-500 Hz, and bump up the bass response below 40 Hz.
2) The results (magenta vs green curves) show that pointing the microphone directly at the speaker (0 deg incidence, which is what the calibration entered was at) reduces the response roll-off above 7 kHz by up to 8 dB (a good thing).









Overall, I'm feeling much better about my frequency response curves now. I'd still like to reduce the 65 Hz null some more, but maybe a second sub in the front-left corner would rectify that.

My last plots are my frequency response with Audyssey MultEQ On and the mic pointed at each speaker. MultEQ seems to have added some peaks and valleys near the crossover, so I'm thinking I likely need to rerun Audyssey with the sub now at 180 deg phase. Hopefully it smooths my bass response even more and closes the gap between the left and right speakers from 300 to 1 kHz!!
















Thanks again John for your help and for the REW software!! :T


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## Wayne A. Pflughaupt (Apr 13, 2006)

sdy101 said:


> The microphone was positioned at ear level (pointed up)


I wish I knew where people keep getting the idea to do this! For starters, you can’t legitimately use an upright mic orientation unless you have a specific calibration file to accommodate such a drastic off-axis measurement. You mentioned you are using no calibration file. 

Even with the proper 90° calibration file, upright measurements are unreliable because they depend on ceiling reflections for upper-frequency information (above 2 kHz or so [depending on the mic], they are more directional than below that point). So you won’t get the same results in a room with a flat 8” ceiling vs. one with a steep, sloping cathedral ceiling. Or in the case of the former, if a ceiling is has a dense, course texture (that would tend to disperse the soundwave) vs. one with that’s relatively smooth with virtually no texture.




> Also, the transition into the front speakers (at 90 Hz) doesn't look all that smooth to me either, as the level drops 10+ dB from 80 Hz to 200 Hz.


It’s not an issue of steeply-dropping levels. You have a peak in response at 70 Hz. Eliminate that and the transition to the front speakers would look a lot better. :T




> So where should I go from here? I realize the HSU STF-2 is a bit undersized for my room, but I can I equalize this response out if I purchased a parametric EQ system (not sure what to get)?


I do not recommend equalization if your subs are marginal or undersized for the room. Equalization requires considerable headroom, and if you don’t have it you’re going to just trade poor response for a sub that bottoms out all the time.




sdy101 said:


> Should I be concerned with the drop in level above 7 kHz for the sub in the FR corner? What about the 10-15 dB drop in level from 40-200 Hz? Should the mic be facing the front speakers (tweeters) for these measurements?


 The drop in level above 7 kHz has nothing to do with the sub. It’s because your mic isn’t pointing at the speakers while measuring.  The drop between 20-200 Hz is definitely a big problem, though. Offhand I’d say it looks like your sub is way too hot. A rule of thumb for sub level compared to the mains, at least a place to start, is for the sub to be 10 dB hotter. It looks like more like 20 dB in your graphs.




> And the last frequency response is the front left vs front right satellite speaker with the sub in the FR corner. What bothers me here is the front left speaker output being 5+ dB lower from 170 to 500 Hz.
> When the sub is located in the front right corner, frequencies below 40 Hz are up to 10 dB higher in level... I'm assuming it has to do with the room layout, as the right wall is continuous from front to back for ~30 ft, *but the left wall has a 6'x7' opening* to the living room...


Yes, both problems are caused by the opening in the wall. I’d definitely stay with the right-corner location for the sub. You might not be able to eliminate the 65 Hz null, but at least you’ll have greater SPL and better extension.





> What I really would like to get out of all this analysis (if at all possible) is:
> 1) Better speech intelligibility from the center speaker.


We’d have to see a dedicated center-channel graph to make a judgment on that one. However, with the high sub level I expect you’re getting some “rumbling” in voices.




> 2) Smoother transition/crossover from sub to front speakers (currently set at 90 Hz by Audyssey). *It just doesn't sound like it blends* as good as I think it should.


I’d attribute that to the huge gaping hole centered at 200 Hz, and possibly the hot sub level. Your speakers have really tiny woofers, so in a room is as large as yours they just aren’t going to put out enough low end to blend well with the sub. I doubt Audyssey will be able to totally fix it a problem like that.

Regards, 
Wayne


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## sdy101 (Feb 22, 2014)

Thank you Wayne for taking the time for the detailed response and answers to my questions! I am glad to see how much effort you and John put into to answering newbie's posts, it is very much appreciated!

As you probably didn't read my latest post this morning since you were in the process of answering the questions in my first post, I did (finally) take measurements with a calibrated microphone at 0 deg incidence (pointed at each speaker). This raised the levels above 7 kHz as you mentioned (which makes perfect sense for more directional sound waves at that high frequency). I then combined the 0 deg incidence mic orientation with an Audyssey MultEQ measurement and sub at 180 deg phase, and the gaping hole at 200 Hz isn't quite as bad now and the bass response was improved a bit more. The front speakers only have 5-1/4 inch woofers, so like you said they are a bit inadequate for my very large and open room, but they were a step up from my Bose AM speakers and the wife "approves" them mounted on the wall with the wires concealed inside the walls.

My sub volume/gain is currently at 11 o'clock and Audyssey put it at +3 dB trim. I could try lowering the gain knob a bit more since I want to rerun Audyssey with the new 180 deg phase setting, so I'll report back. Thanks again for the comments!


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## sdy101 (Feb 22, 2014)

So I went back and made more freq response measurements and still can't get past the dip/valley in my FL and FR satellites from about 100-400 Hz. So I decided to take my center speaker which has two 5-1/4" drivers and put it in place of my left speaker (which has only one 5-1/4" driver) and remeasure. The plot shows my PC2000 center speaker in it's typical center location (Black), my PM1000 front left speaker in it's typical left main location (Red), and the PC2000 speaker moved to the left main location (Green). Crossover for the mains is set at 80 Hz.









The PC2000 in the left speaker position definitely improved the valley in the response with a 7 dB increase at 190 Hz. It also really increased the levels above 1.5 kHz. It looks good on paper, but it's hard to evaluate with music/movie material. I don't really want to go out and purchase two more center satellite speakers to replace my mains, but this does seem to be the root of my problem (i.e. too small a main speaker for my large open room). Not really a surprise I guess, but I was hoping for an alternative solution to improve my response from 100-400 Hz without having to replace speakers....


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## Phillips (Aug 12, 2011)

Have you swapped the left with the right main speaker to try and see any difference (might have missed this in this thread)?
Move the mic forward or backward, watching you don't interfere with the other speaker and center.


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## sdy101 (Feb 22, 2014)

Phillips said:


> Have you swapped the left with the right main speaker to try and see any difference (might have missed this in this thread)?
> Move the mic forward or backward, watching you don't interfere with the other speaker and center.


Yep, I thought the same thing, so I swapped the left and right speakers and got the same response. I did play with moving the mic around (fwd, back, up, down), and it does change things slightly, but not as much as putting the center speaker where the left main is...


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