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Author Topic: Adding delayed out of phase subs to decrease room modes  (Read 11847 times)

Herman Chigrin

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Adding delayed out of phase subs to decrease room modes
« on: March 17, 2011, 03:40:28 am »

In the search of SQ in the bass region (in a room) I've crossed the following approach:

To add a second set of subs, located on the opposite wall of main subs. Then to delay the signal to them by the time (a few mSec) it takes the sound to travel from the main subs to the opposite wall subs (the proposed second set). To invert the signal to the second set of subs.

Theoretically, this lets the sound travel through the room/venue then it is absorbed (almost completely) and does not get reflected to cause modes and all kinds of problems.

Has anyone tried this approach and what are your opinions on this? 

Thanks,
Herman
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Ivan Beaver

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Re: Adding delayed out of phase subs to decrease room modes
« Reply #1 on: March 17, 2011, 07:54:51 am »

In the search of SQ in the bass region (in a room) I've crossed the following approach:

To add a second set of subs, located on the opposite wall of main subs. Then to delay the signal to them by the time (a few mSec) it takes the sound to travel from the main subs to the opposite wall subs (the proposed second set). To invert the signal to the second set of subs.

Theoretically, this lets the sound travel through the room/venue then it is absorbed (almost completely) and does not get reflected to cause modes and all kinds of problems.

Has anyone tried this approach and what are your opinions on this? 

Thanks,
Herman
But consider what is happening in the opposite direction. The "delay subs" are producing sound-which travels TOWARDS the stage-creating cancellation at the stage.

You have to look at the entire picture-not just one spot.

And how would you feel if you were at the back of the room and the bass was cancelled out?  Not a very good seat.

The whole concept if flawed.

Yes delay and polarity is often used for steering of sub signals-but not in the method you describe.

It sounds like a person who has heard a little bit about it, and has no idea how the basic theory works and has come up with their own ideas-which are wrong.
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Ivan Beaver
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George Friedman-Jimenez

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Re: Adding delayed out of phase subs to decrease room modes
« Reply #2 on: March 18, 2011, 12:14:21 pm »

Moderator, thanks for unlocking this potentially interesting thread.
This is not such a naive and uneducated idea, just not completely appropriate for most of our live sound situations. In Floyd Toole's book Sound Reproduction: Loudspeakers and Rooms, in the chapter on Delivering Good Bass in Small Rooms, he quotes a 2002 paper from the 112th AES mtg by Welti, "How many subwoofers are enough?". The paper reports the results of a large set of mathematical simulations of subwoofers in small rectangular rooms with listeners grouped in a central area of the room. The model minimizes a Mean Spatial Variance (MSV) metric that describes the frequency dependent differences in SPL at 16 locations within the seating area. In other words small MSV means little frequency or spatial variability of the SPL over the 16 locations in the seating area. He reran the model for different configurations of 1, 2, 3, 4, up to 5,000 subwoofers in the room (obviously theoretical not experimental!). They reported better configurations of 2 and 4 subwoofers which lessened excitation of acoustically important room modes and wave interactions and minimized MSV. No configuration was perfect or even impressively homogeneous even in the small seating area, but the best configurations had 2 or 4 subs placed on front & back walls or front back and side walls.
Phase adjustments and delays, as well as boosts or cuts in SPL for different sub locations, are optimized by computer to minimize the MSV. Obviously, what works best in the small listening area could be completely wrong outside that area, in corners of the room, onstage, etc.

This body of scientific/commercial work is focused on small rooms with widely spaced modes and a few listeners near the middle (think home theaters) and is not necessarily relevant to large performance spaces with people packed in wall to wall, a band in front, dancers in various locations, and people trying to talk and be heard in the back. The analyses may also be relevant for small rehearsal spaces, recording studio control rooms, and other professional sound spaces.
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Ivan Beaver

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Re: Adding delayed out of phase subs to decrease room modes
« Reply #3 on: March 18, 2011, 12:48:38 pm »

. Obviously, what works best in the small listening area could be completely wrong outside that area, in corners of the room, onstage, etc.

This body of scientific/commercial work is focused on small rooms with widely spaced modes and a few listeners near the middle (think home theaters) and is not necessarily relevant to large performance spaces with people packed in wall to wall, a band in front, dancers in various locations, and people trying to talk and be heard in the back. The analyses may also be relevant for small rehearsal spaces, recording studio control rooms, and other professional sound spaces.
What happens in a small space that is operating in the pressure zone for a limited seating area is quite different than what happens for a large group of people who cover a much larger seating area and the spacing of the subs is large as compared to the wavelength.

It is a very common practice to use multiple subs spaced around a home theatre to basically energize all the room modes to flatten out the response.

Throwing a bunch of subs all over the place in a club or larger is a receipe for problems.

You have to look at the particular situation to come up with a solution that works best for that situation.  In audio, one size does not fit all.
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Ivan Beaver
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Mac Kerr

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« Reply #4 on: March 18, 2011, 12:54:44 pm »

Moderator, thanks for unlocking this potentially interesting thread.

The key to getting the thread unlocked is for the original poster to follow the clearly displayed posting rules. When someone starts a thread using an alias it gets locked and they get a warning via email. If they comply the thread gets unlocked as soon as a moderator notices the change. That might not be right away, we do have lives outside of these forums, although sometimes it seems otherwise. If the OP ignores the warning the thread gets deleted, and they get another warning.

Mac
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Herman Chigrin

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Re: Adding delayed out of phase subs to decrease room modes
« Reply #5 on: March 18, 2011, 05:07:51 pm »

Good to know people are thinking out there. Thanks for your opinions.

I am BUILDING a 500 sq.ft. multi use music room, rectangular shaped. Typical sound levels will be 110-115 db during loud passages, not often but happens. Music varies from Classical to Jazz to Techno.

I want the most even possible bass response over the entire room floor area for sitting and standing guests. Subwoofers will be covering the range below 90Hz. Has anyone any experience with such a task?

Cost considerations are secondary.

Thank you,
Herman
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Chris Carpenter

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Re: Adding delayed out of phase subs to decrease room modes
« Reply #6 on: March 19, 2011, 05:14:43 am »

In the search of SQ in the bass region (in a room) I've crossed the following approach:

To add a second set of subs, located on the opposite wall of main subs. Then to delay the signal to them by the time (a few mSec) it takes the sound to travel from the main subs to the opposite wall subs (the proposed second set). To invert the signal to the second set of subs.

Theoretically, this lets the sound travel through the room/venue then it is absorbed (almost completely) and does not get reflected to cause modes and all kinds of problems.

Has anyone tried this approach and what are your opinions on this? 

Thanks,
Herman

Your idea, 80Hz
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Ivan Beaver

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Re: Adding delayed out of phase subs to decrease room modes
« Reply #7 on: March 19, 2011, 10:14:12 am »

Good to know people are thinking out there. Thanks for your opinions.

I am BUILDING a 500 sq.ft. multi use music room, rectangular shaped. Typical sound levels will be 110-115 db during loud passages, not often but happens. Music varies from Classical to Jazz to Techno.

I want the most even possible bass response over the entire room floor area for sitting and standing guests. Subwoofers will be covering the range below 90Hz. Has anyone any experience with such a task?

Cost considerations are secondary.

Thank you,
Herman

It sounds like a typical home theatre setup.

IF you are building the room-be sure to pay attention to the floors, walls and ceiling construction.  Make sure they are THICK and sturdy.  If using sheetrock-use 2 layers with overlapping joints and glue and screw them together.

You will probably need more SPL than you are talking about-unless you are talking A weighted SPL.  Your numbers are pretty quiet for techno.

You best bet is NOT to attempt amy sort of delay or polarity inversion on the subs.

Use all the same model number (just make sure the model you choose goes low enough for the music style you describe) and spread them around the room.  Being sure to put them in different places.  IE NOT in the middle of the front and rear walls for example.  Maybe 1 in the middle of the front wall, 1 1/3rd of the way on the rear wall, and 1 each in the corner along the side walls-don't put them in opposite corners.

That would be a good start.
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Ivan Beaver
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Tim Padrick

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Re: Adding delayed out of phase subs to decrease room modes
« Reply #8 on: March 20, 2011, 12:14:16 am »

I am BUILDING a 500 sq.ft. multi use music room, rectangular shaped. Typical sound levels will be 110-115 db during loud passages, not often but happens. Music varies from Classical to Jazz to Techno.

Thank you,
Herman

Unless there's newer data, your room proportions should be as such:

Height | Width | Length
1.00 | 1.28 | 1.54
1.00 | 1.60 | 2.33
1.00 | 1.40 | 1.90
1.00 | 1.50 | 2.50
1.00 | 1.26 | 1.59
(from Master Handbook of Acoustics, 4th Ed. F. Alton Everest, McGraw-Hill, p.277)
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Brian Ehlers

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Re: Adding delayed out of phase subs to decrease room modes
« Reply #9 on: March 21, 2011, 02:20:34 pm »

I am BUILDING a 500 sq.ft. multi use music room, rectangular shaped. Typical sound levels will be 110-115 db during loud passages, not often but happens. Music varies from Classical to Jazz to Techno.

I want the most even possible bass response over the entire room floor area for sitting and standing guests. Subwoofers will be covering the range below 90Hz. Has anyone any experience with such a task?
It's been quite a while, but I believe the white paper refered to in the earlier post was published by an engineer with JBL and is availabe from their website.  If I remember correctly, one of the interesting (and somewhat intuitive) points made was that the multiple subs should not be positioned symetrically in the room.  For example, you don't want to have a left and right sub both equally spaced on the floor at the front wall.
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ProSoundWeb Community

Re: Adding delayed out of phase subs to decrease room modes
« Reply #9 on: March 21, 2011, 02:20:34 pm »


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