Room Resonance - help needed

[JeremyinCalgary]

I have gone for Soffit mounting for my main speakers (ATC SCM 100 Actives). This has improved the look dramatically and also helped eliminate any cancellations from rear wall reflections.

However, I now have a stronger standing wave problem than before.

My room is 26 by 16 by 8 feet high with an irregular bay window which widens the room to on one side to about 26 feet for about 8 feet. The speakers are mounted on the short axis (16 feet) wall.

I have tried various sub woofer positions (ATC SCM 0.1/15 sub) and I get about two to three strong suckouts/resonances (loss of bass at particular frequency) when the sub and speakers are in phase (varying positions just alters the frequency).

I have found that by positioning the sub next to one of the speakers along the side wall and with an out of phase polarity that I get the best linearity.

I believe the sub (15" woofer) when positioned next to the left speaker 912" woofer) and driven with opposite polartity is creating the same LF response as the speaker but in a negative way and hence reducing the room resonance. (The sub is driven to up to 90Hz (12 db filter above that) while the main speakers are driven full range with same source from a CD player.)

My question: Is it acceptable to have a sub driven in opposite phase from main speakers in order to cancel out LF room resonance? Is there any drawback

 

[ScottMayo]

I have gone for Soffit mounting for my main speakers (ATC SCM 100 Actives). This has improved the look dramatically and also helped eliminate any cancellations from rear wall reflections. ...
I believe the sub (15" woofer) when positioned next to the left speaker 912" woofer) and driven with opposite polartity is creating the same LF response as the speaker but in a negative way and hence reducing the room resonance. (The sub is driven to up to 90Hz (12 db filter above that) while the main speakers are driven full range with same source from a CD player.)

My question: Is it acceptable to have a sub driven in opposite phase from main speakers in order to cancel out LF room resonance? Is there any drawback

Sometimes it's all you can do. Some people argue that it gives an unnatural sound to bass; others argue that phase in bass is inaudible, so running bass out of phase can't possibly damage the sound. If it solves your problem, do it.

Another option is to stop driving the speakers full range, and let the sub handle everything. At least then you have a single source to contend with.

It sounds like you mounted your main speakers right against a wall. That more or less maximizes bass output, which you generally don't need to do if you have a sub. It can also maximize bass problems. If you need this configuration, consider adding diffusion on the long walls, and absorption on the short ones. It may help some.

 

[JeremyinCalgary]

Humps

Scott,

Thanks for the advice. I will try your suggestion to cut off the LF from the mains....my only fear is that the separation of the two LF signals may give me the best humps and troughs from both! the mains above 80 Hz and the sub below 80Hz....

Since the WAF factor prevents me from going for mega acoustic panels...how about parametric equalization? Anyone using the Rane PEQ 55 - it has 5 parametric bands per channel for two channels? I looked at the specs for the very highly regarded Rives PARC, however, I fear that three bands is not quite enough for my humps, my humps, my humps, my humps. (Yeah that is four humps....and BTW I am not much of a Black Eyed Peas fan, although I do like their number about the audiophiles frequency response problems and the cost to maintain a good response curve....or were they singing about something else? As an audiophile nerd I will probably never know, LOL)

 

[rynberg]

Given the width of your room, I would cross-over the mains at 60 Hz and place the sub at the centerline of the room at the front wall. This will at least greatly reduce modes across the width of the room. You need to provide more detail about geometry and flexibility for us to help you more.

A PEQ cannot be used to fix nulls.

 

[JeremyinCalgary]

Room

Here is the room there are two chairs and a large sofa in fabric as well as carpeting on the entire floor.

 

[JeremyinCalgary]

More info

I did some measurements last night using Ethan's pure tone 10 sec test CD (previously I had used a 1/3 octave test CD and I now realize I was missing the clarity I needed in my original post for others to help);

Main speakers alone have a broad 10 db suckout from 60 to 80 Hz and a couple of sharp deep notch (2 or 3 Hz wide and 18 db deep) suckouts at 105 Hz and 145 Hz. Otherwise they are roughly +/- 3db flat as far as I tested (160 Hz).

Sub is worse: flat from 18 to 25 Hz, a 15 db dip from 25 to 35 Hz, a 5 db bump from 40 to 50 Hz, a 10 db dip again from 50 to 60 Hz, and then fairly flat until it tails off after 90HZ as the filter kicks in.

The above was made with no filtering apart from the sub cut off above 90 Hz.

I am nervous about applying excessive (more than 5 db) adjustment with a PEQ as I think this is inherently a band aid solution (after all my reverb problem still remains even if I artificially force a flat response). Therefore, I am, after all, going to need some acoustic treatment. I may have to make a temporary arrangement (install when in use and hide away in cupboard after use for WAF factor reasons).

My questions;

Can anyone reocommend the best options for "portable" acoustic bass treatment for my room - how much surface area and corner placements do I likely need. Note there are bay windows all down the left side and along the left speaker wall.

[For assumption purposes lets say I want to get 5 db improvement from temporarily installed acoustic treatment and then I plan to use a PEQ to do no more than another 5 db adjustment and whatever non-linearity after that then I will live with.]

Oh, and one other point, need I worry about the very sharp suckouts...I don't plan to correct these as I suspect that it is inherent with a pure single frequency tone in any enclosed space....?

 

[Ethan Winer]

Jeremy,

> I am nervous about applying excessive (more than 5 db) adjustment with a PEQ as I think this is inherently a band aid solution <

Agreed, especially above 60 Hz or so which is where the skewed response and ringing are most damaging musically.

> Can anyone reocommend the best options for "portable" acoustic bass treatment for my room <

Several companies sell bass traps with stands. So you can move them out of the way when friends come over, or other times WAF is an issue.

> how much surface area and corner placements do I likely need. <

As much as you can manage. It's impossible to make any small room perfectly flat, or to get rid of all ringing. So it comes down to how much skewing and ringing you're willing to accept, versus the effort and expense to obtain a given amount of improvement.

> need I worry about the very sharp suckouts. <

Yes, in my opinion nulls are more damaging than peaks, or at least as damaging anyway. The good news is bass traps raise nulls even more than they reduce peaks. Traps also reduce ringing, and the improvement in all three extends everywhere in the room.

--Ethan

 

[ScottMayo]

Main speakers alone have a broad 10 db suckout from 60 to 80 Hz and a couple of sharp deep notch (2 or 3 Hz wide and 18 db deep) suckouts at 105 Hz and 145 Hz. Otherwise they are roughly +/- 3db flat as far as I tested (160 Hz).

Sub is worse: flat from 18 to 25 Hz, a 15 db dip from 25 to 35 Hz, a 5 db bump from 40 to 50 Hz, a 10 db dip again from 50 to 60 Hz, and then fairly flat until it tails off after 90HZ as the filter kicks in.
...Oh, and one other point, need I worry about the very sharp suckouts...I don't plan to correct these as I suspect that it is inherent with a pure single frequency tone in any enclosed space....?

First off, this isn't too bad. An untreated toom with only 10db drops is arguably better than average. (The 15db drop from 25-35Hz is not terribly critical musically, though I'd go after it anyway. The deep narrow nulls can be worked around.) Imaging is another matter. I'm going to hazard that you aren't getting much.

You might want to update the diagram with the placement of your sofa and other seating.