Help me understand my REW FR graph. - AVS Forum | Home Theater Discussions And Reviews
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post #1 of 7 Old 08-06-2017, 05:54 PM - Thread Starter
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Help me understand my REW FR graph.

Hi all,
I recently decided to experiment with REW a little bit, since on a small number of songs I would notice that the bass seemed to drop in and out quite a bit. Most of my music sounds fine to my ears but I figured I should familiarize myself with the program since I plan on adding a second sub for headroom and hopefully an improved response.
I'm using the BAFX advanced sound meter on the C weighting to take my measurements and not a UMIK-1.
When outputting 75db Pink noise from REW the SPL meter read ~73-75 db. I measured a 20hz sine wave at my MLP and the SPL meter read 79db while REW was outputting 86db. The two measurements got closer as I went up in frequency, so I presumed REW was applying the C weighted correction factors properly before taking these tests. I tested the sub with my mains (fr, fr and CC) in the 16hz and 20hz tuning about 12' away at my MLP. All the graphs per tuning looked pretty much the same so i just averaged them and put a 1/12th smoothing on.

The sub is a PB12-plus in a completely untreated room, around 5,500ft^3 with tiled floors over concrete (a basement). The crossover is at 80hz. Is there anything wrong with this graph that I should be specifically looking to fix or is this fairly normal for an untreated room?
From my understanding, there is a lot of room for improvement but I thought I'd leverage the expertise on this site.
If something seems off about the graphs, what can I do to improve it?

Thanks .
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post #2 of 7 Old 08-06-2017, 06:24 PM
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Originally Posted by DarkEnigma View Post
Hi all,
I recently decided to experiment with REW a little bit, since on a small number of songs I would notice that the bass seemed to drop in and out quite a bit. Most of my music sounds fine to my ears but I figured I should familiarize myself with the program since I plan on adding a second sub for headroom and hopefully an improved response.
I'm using the BAFX advanced sound meter on the C weighting to take my measurements and not a UMIK-1.
When outputting 75db Pink noise from REW the SPL meter read ~73-75 db. I measured a 20hz sine wave at my MLP and the SPL meter read 79db while REW was outputting 86db. The two measurements got closer as I went up in frequency, so I presumed REW was applying the C weighted correction factors properly before taking these tests. I tested the sub with my mains (fr, fr and CC) in the 16hz and 20hz tuning about 12' away at my MLP. All the graphs per tuning looked pretty much the same so i just averaged them and put a 1/12th smoothing on.

The sub is a PB12-plus in a completely untreated room, around 5,500ft^3 with tiled floors over concrete (a basement). The crossover is at 80hz. Is there anything wrong with this graph that I should be specifically looking to fix or is this fairly normal for an untreated room?
From my understanding, there is a lot of room for improvement but I thought I'd leverage the expertise on this site.
If something seems off about the graphs, what can I do to improve it?

Thanks .
I don't think you can put too much value into those measurements just glancing at the specs of your SPL meter I would say it accuracy is pretty questionable especially in the lower frequencies. The PB12 should not be dropping off that fast at 40hz so it is either the meter or something else is going on. I would guess the meter is not accurate down that low and needs a calibration/correction file.

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post #3 of 7 Old 08-06-2017, 06:29 PM
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Originally Posted by DarkEnigma View Post
Hi all, I recently decided to experiment with REW a little bit, ...
Although you have already spent money on an SPL meter, I would suggest you get a calibrated UMIK-1 from Cross Spectrum Labs.

Then read this guide.

Then read this thread.

Make some measurements and upload the .mdat file to a DropBox, OneDrive, or Google account so it can be downloaded.

Then post your questions on the above thread.
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post #4 of 7 Old 08-06-2017, 07:13 PM
 
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I don't think you can put too much value into those measurements just glancing at the specs of your SPL meter I would say it accuracy is pretty questionable especially in the lower frequencies..
No C weighted meter is going to be accurate below 50Hz. This is why:


Also keep in mind that's with a perfect meter, and most aren't down below where they're intended to operate.
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post #5 of 7 Old 08-08-2017, 08:08 PM
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Rescale your graph to make it meaningful. 2dB increments gives a false visual impression of your frequency response. Usually a 60 dB scale will give the standard 5 dB increments. So 45-105 or 55-115. Also 10-200 Hz for x-axis. Unless you are crossing your sub over above 200 Hz.
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post #6 of 7 Old 08-11-2017, 03:15 PM
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Below 80 Hz LFE Crossover, your measured Room Frequency Response is +/- 2 dB (Very Good), except for the Primary Room Resonance near 40 Hz....and the overall Sub-Woofer Level is somewhat higher than the average ABOVE the Cross-over Frequency....which is fairly irregular....and MIGHT be improved using AVR's Room Equalizer Capability.

Drop-off below 40 Hz is VERY Typical, some of it due to the ROOM and some of it the Sub-Woofer(s)...and lack of any Bass Compensation....You should enter your SPECIFIC Room Dimensions into REW's "ROOM SIM" App to see an approximation of what JUST the ROOM is doing....I would guess that it shows a Room Mode BOOST at about 40 Hz, with a Rapid Dropoff below that.

You should try some "What If" exercises, moving the entered HEIGHT and X, Y Location of your EARS to get a feel for how sensitive the various Room Modes can be. However, you will likely find that one or more of the Room Dimensions needs to be increased to reduce the Primary Room Resonance BOOST Frequency. What you CAN do is to try different SW Location(s), Location(s), Location(s)....

Also FWIW: Moving my Ears Forward just a few inches made quite a big change in the Level of my 34 Hz Primary Room Resonance....and the Rapid Dropoff was MUCH Faster if I entered 8-ft Room Height vs 15-ft Height.....so YMMV....

Last edited by holl_ands; 08-11-2017 at 03:29 PM.
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post #7 of 7 Old 08-11-2017, 06:21 PM - Thread Starter
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Originally Posted by holl_ands View Post
Below 80 Hz LFE Crossover, your measured Room Frequency Response is +/- 2 dB (Very Good), except for the Primary Room Resonance near 40 Hz....and the overall Sub-Woofer Level is somewhat higher than the average ABOVE the Cross-over Frequency....which is fairly irregular....and MIGHT be improved using AVR's Room Equalizer Capability.

Drop-off below 40 Hz is VERY Typical, some of it due to the ROOM and some of it the Sub-Woofer(s)...and lack of any Bass Compensation....You should enter your SPECIFIC Room Dimensions into REW's "ROOM SIM" App to see an approximation of what JUST the ROOM is doing....I would guess that it shows a Room Mode BOOST at about 40 Hz, with a Rapid Dropoff below that.

You should try some "What If" exercises, moving the entered HEIGHT and X, Y Location of your EARS to get a feel for how sensitive the various Room Modes can be. However, you will likely find that one or more of the Room Dimensions needs to be increased to reduce the Primary Room Resonance BOOST Frequency. What you CAN do is to try different SW Location(s), Location(s), Location(s)....

Also FWIW: Moving my Ears Forward just a few inches made quite a big change in the Level of my 34 Hz Primary Room Resonance....and the Rapid Dropoff was MUCH Faster if I entered 8-ft Room Height vs 15-ft Height.....so YMMV....
Interesting feature. It does show some room boost at 39hz. It also shows a pretty significant boost at 19hz. It doesn't show a significant drop off below 40hz though, so it may be the fault of the SPL meter.
The receiver i'm using is the Yamaha RX-V381 and AFAIK, the manual PEQ only goes down to 63hz, so there probably is no EQ below that (unless YPAO goes lower anyway?). That could also be an explanation for the problems down low.
As for the other posts, i scaled the graph based on Alan's posts here: https://www.avsforum.com/forum/113-su...raph-good.html.
I do intend to purchase a better receiver and a UMIK-1. I bought the spl meter first because it at least gives me an idea of what output levels I'm looking to obtain, and if any changes i make are actually affecting output.
I posted because i was just playing around with REW and was just wondering if there was anything I could take out of these graphs. The consensus seems to be that they're useless so i'll report back when I've made the additional upgrades.
Thanks for the input.
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