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Discussion Starter · #1 ·
This is a sweep of my subwoofer with the REW output connected directly to the sub itself (SVS PB12-NSD). The sub gain was set to ~ 9 o'clock and the Low Pass was set to "disabled."

The in-room response looks remarkably good except for a bump at 45 Hz and a major null at 62 Hz.

Microphone was about 11 feet from the sub and the room is a rectangle with an L-shaped opening at the rear (about 2700 cu ft).

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My first question is: why is there so much information in the 100 to 200 Hz range? Remember, REW was connected directly to the sub and not to my AVR. Should I change the low pass setting on the sub?

My second question is: how can I correct the null at 60 Hz?

[Note, if this belongs in a different forum, please move]

Thanks!
 

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I am not an expert at the use of REW (yet!), but you probably are getting "so much information in the 100 to 200 Hz range" because the LP filter is disabled. The sub is trying to reproduce the entire range of sound being fed to it. Engaging the LP filter will cut-out the higher frequencies; a subwoofer is designed to reproduce the lowest frequencies.

I'm sure that someone with more knowledge of REW will add some useful information and address the bumps/nulls (that are probably room modes). Good luck!! :)
 

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Discussion Starter · #3 ·
I think you are exactly right. I had the sub's lowpass filter set to "disable" because the sub is normally connected to my Onkyo 709 which handles bass management duties. In normal operation, the sub sees only LFE information and any bass information below 70 Hz (for my mains), 80 Hz for my center channel, and 90 Hz for my surrounds. If I run the 15 to 200 Hz sweep signal through my processor with my mains disconnected (instead of directly through the sub), I suspect I'll see the AVR's lowpass filter in operation.

I'll try that when I have a chance later this week.
 

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Discussion Starter · #5 ·
Thanks for the input!

Here is a full-range sweep of the Left main speaker and subwoofer. I ran the sweep through an analog input on my AVR in stereo mode with bass management on and the crossover set to 70 Hz.

Unfortunately, I don't have REW here at work, otherwise I would capture a graph with an overlay of this measurement on the subwoofer only measurement and would set them to the same frequency range. As it is, this graph cover the full range from 15 to 20k Hz, so it's hard to compare the two graphs.

At first glance, however, it appears that the mode at 45 Hz and the null at 60 Hz is still there. The range from 100 to 200 Hz looks bad, too, with a significant peak at 150 Hz.

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Thoughts?
 

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Discussion Starter · #7 ·
Good question. I've run sweeps with Audyssey on and off and, frankly, I don't remember which one this was! I believe it was with Audyssey on in "movie" mode. Hopefully tonight I can run more sweeps and take better notes on each and then do some overlay graphs to make comparisons easier.

One goal is to hone in on the best crossover point for my mains.

The next goal is to see how good of a job Audyssey does.

Ultimately, I am thinking about adding a second sub to increase the bass output in my room and even out the dips and nulls to the extent possible. I'd also like to DIY for the sub, but wanted to take some measurements first to see what I'm working with in terms of in-room response.

Ken
 

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Discussion Starter · #9 ·
How did you go about integrating the second sub? Is it identical to the first? I've read tons of material on how to integrate multiple subs and some of it, frankly, is conflicting and confusing. Of course, I'm making it more complicated by attempting a DIY sub, but that's half the fun, right? :bigsmile:
 

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I have front and back IB 'ceiling' manifold install's. The front IB holds 4 x 15" drivers, and the rear manifold 2 x 18".

Using REW, I found reversing the front manifold drivers phase, and keeping the rear manifold drivers in phase with my main speakers gave best results. I then ran Audyssey (1 in 2 out) to combine the two manifolds together.

You certainly will get a lot more bang for your buck going DIY..
 

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Based on the sub curve and the combined curve, I would set the crossover point as low as possible - even 40 Hz if that is available, presuming that the main speakers are capable of 40 Hz. A full range measurement of the L&R speakers with no crossover would also be useful.
The null which appears to be centered at about 72 Hz in the combined curve, is surely a room cancellation and you cannot eq that away. If you boost 72 Hz, the cancellation will just be that much stronger and, although the amp and speakers will be working harder and producing more distortion, there will be no net benefit in terms of flatter frequency response. You can move either your sub or the mains to try to achieve flatter response, or you can install bass traps to absorb some of the bass energy at the point of cancellation.
Be careful if you purchase bass traps, as many products out there that are promoted as bass traps are effective at mid range or mid bass frequencies at best.
 

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Discussion Starter · #12 ·
I will try lowering the crossover point. From a purely subjective viewpoint, I liked the sound better with lower crossovers (having tried 60 Hz to 80 Hz with the mains). I will see whether my AVR will let me set it even lower. The speakers will definitely go that low. From the manufacturer: Frequency response: 42-20kHz +3db anechoic, 60-18Khz +1.8db (usable in-room bass response extends cleanly to 36-40Hz)

I will be much more methodical with my next set of measurements: changing only one variable at a time, properly labeling each measurement, and generally ensuring consistency from one measurement to the next.

Does this sequence make sense:
- Each sweep from 15 to 200 Hz only.
- Mains only (no crossover or bass management in AVR)
- Sub only (just disconnect the main speaker)
- Mains + sub, no Audyssey (separate run for each crossover point, from 40 Hz to 80 Hz)
- Mains + sub, with Audyssey (separate run for each crossover point from 40 to 80 Hz)

That's a total of 12 runs. Luckily they are easy to do! [Thank you REW]

I suspect that the 72 Hz null is a room cancellation. I realize that it can't be EQ'd out. In terms of WAF, I may have more luck adding a second subwoofer than installing a bass trap. From what I've read, bass traps capable of absorbing low frequencies are pretty big and unsightly!

If I go the second subwoofer route, preferably DIY, what should I be looking for in terms of complementing the SVS PB12-NSD?

Thanks!
 
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