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Title: Adding delayed out of phase subs to decrease room modes
Post by: Herman Chigrin 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
Title: Re: Adding delayed out of phase subs to decrease room modes
Post by: Ivan Beaver 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.
Title: Re: Adding delayed out of phase subs to decrease room modes
Post by: George Friedman-Jimenez 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.
Title: Re: Adding delayed out of phase subs to decrease room modes
Post by: Ivan Beaver 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.
Title: Locked/Unlocked
Post by: Mac Kerr 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
Title: Re: Adding delayed out of phase subs to decrease room modes
Post by: Herman Chigrin 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
Title: Re: Adding delayed out of phase subs to decrease room modes
Post by: Chris Carpenter 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
Title: Re: Adding delayed out of phase subs to decrease room modes
Post by: Ivan Beaver 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.
Title: Re: Adding delayed out of phase subs to decrease room modes
Post by: Tim Padrick 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)
Title: Re: Adding delayed out of phase subs to decrease room modes
Post by: Brian Ehlers 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.
Title: Modeling the room modes
Post by: Herman Chigrin on March 21, 2011, 03:17:30 pm
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.

Is there software or any other feasible way to model the room modes for the subwoofer frequencies, so that I can figure out placement and quantity of subwoofers?

Thanks,
Herman
Title: Re: Adding delayed out of phase subs to decrease room modes
Post by: Ivan Beaver on March 22, 2011, 07:40:11 am
  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.
That is exaclty the point I was talking about in my last post here.  In a small rooms this can work well for "energizing the room modes".  It DOES NOT work in large rooms, where this technique will just case all sorts of new problems.
Title: Re: Modeling the room modes
Post by: Ivan Beaver on March 22, 2011, 07:42:34 am
Is there software or any other feasible way to model the room modes for the subwoofer frequencies, so that I can figure out placement and quantity of subwoofers?

Thanks,
Herman
Most of the models out there that do sub freq are only in 2D.  I am not aware of any that do 3D.  And that really starts to change things.

What is a hobby of many guys in small rooms is constantly moving the subs around untill they get what they like.  And that can be very opinion driven.  Do you want impact or depth for example?  What is more important?
Title: Re: Modeling the room modes
Post by: Herman Chigrin on March 22, 2011, 03:14:14 pm
Most of the models out there that do sub freq are only in 2D.  I am not aware of any that do 3D.  And that really starts to change things.

What is a hobby of many guys in small rooms is constantly moving the subs around untill they get what they like.  And that can be very opinion driven.  Do you want impact or depth for example?  What is more important?

I would say that impact is more important. As far as depth -- flat down to 30hz is good enough. The upper range of subwoofer coverage is 90 to 100Hz, with 18 db/octave crossover. But I am thinking to try a 12db/octave as well. So the subs will play into the midbass range, I presume.
Title: Re: Modeling the room modes
Post by: Chris Carpenter on March 22, 2011, 04:54:34 pm
flat down to 30hz is good enough
...
So the subs will play into the midbass range, I presume.

 ???
Title: Re: Modeling the room modes
Post by: Ivan Beaver on March 22, 2011, 06:28:52 pm
I would say that impact is more important. As far as depth -- flat down to 30hz is good enough. The upper range of subwoofer coverage is 90 to 100Hz, with 18 db/octave crossover. But I am thinking to try a 12db/octave as well. So the subs will play into the midbass range, I presume.
And what about music that has notes below 30Hz?

Are you saying that the musician has no purpose in putting them in there?

If the notes are in the music, then they deserve to be heard as the artist intended.

Just curious, why do you say 18dB/octave for the low pass?  Maybe that is what works for you, but people with different subs and full range cabinets may need very different crossover freq or slopes.
Title: Re: Adding delayed out of phase subs to decrease room modes
Post by: Randall Hyde on March 22, 2011, 06:29:41 pm
Your idea, 80Hz

Now put walls behind those two subs...
Title: Re: Modeling the room modes
Post by: Kevin Unger on March 22, 2011, 09:47:40 pm
Stick the sub in the center of the room.

Do a walk around, and see where the bass "sits" and sounds the best.

Place subs around the room in those areas.




You'll usually end up in corners, of coarse. I've done this in small rooms a few times with great success.
Title: types of subs
Post by: Herman Chigrin on March 23, 2011, 12:42:37 am
To clarify, I meant that the subs will cover the range below 80Hz. This is sort of a given because my midbass straight horns have a cutoff at 80Hz. I do want to hear any music material below 30Hz, to answer the question, I was just trying to make a point that for me personally, impact is more important than depth. If it is possible to get BOTH, I would love to hear the recommended approach. Also, as far as low pass crossover SLOPES, I am open to experimentation and I am in no way trying impose my opinion.

So far I can make a guess of how the subs shall be located, based on suggestions and the "CEDIA audio guide": in two or four corners, vertical arrays floor to ceiling. Plainly stated, two or four bass columns in corners, facing into room.

The question remains, what type subs provide both impact and good depth? Sealed, ported, tapped horns, bandpass?
Title: Re: types of subs
Post by: Ivan Beaver on March 23, 2011, 07:58:05 am
To clarify, I meant that the subs will cover the range below 80Hz. This is sort of a given because my midbass straight horns have a cutoff at 80Hz. I do want to hear any music material below 30Hz, to answer the question, I was just trying to make a point that for me personally, impact is more important than depth. If it is possible to get BOTH, I would love to hear the recommended approach. Also, as far as low pass crossover SLOPES, I am open to experimentation and I am in no way trying impose my opinion.

So far I can make a guess of how the subs shall be located, based on suggestions and the "CEDIA audio guide": in two or four corners, vertical arrays floor to ceiling. Plainly stated, two or four bass columns in corners, facing into room.

The question remains, what type subs provide both impact and good depth? Sealed, ported, tapped horns, bandpass?
As usual there is more to it than just the type of sub.  What size and price are you willing to tolerate?

If you want a horn to go down low-it is going to have to be LARGE.  A ported box will go lower and be smaller.  HNOWEVER, it will not get as loud as the properly sized horn.

How loud do you need it to be? 

It is not just a simple question with a simple answer.  There are a lot of variables that have to be considered.
Title: Re: Adding delayed out of phase subs to decrease room modes
Post by: Iain.Macdonald on March 23, 2011, 03:07:24 pm
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

Herman,

500=22'≈ per side That's not a very big room! You are well within the guidelines for the multi-sub approach that Toole and Olive describe.

The most accurate sub for your listening room would be a closed box. But you mentioned techno, so the output levels and sound quality may not be to your taste.

Iain.


Title: loudness -- types of music
Post by: Herman Chigrin on March 23, 2011, 09:35:33 pm
First of all, I'd like to thank everyone for dissecting my problem.

The loudest I would need is 110 to 115db. However, I do NOT like to push the system to it's limits so at those levels I want the subs to be in "relaxed" mode and have some headroom.

If I choose to have two subs (placed in corners behind speakers) then I have 3'x2'x8' available volume for each sub = 48 ft^3 per sub

The music to be played is Classical, Jazz and Techno.

As far as HORN subs I have a big question: lets say that the internal folded horn path is 9 feet long (reasonable length). In a small room that would make the woofer be DOUBLE of my listening distance from the main speakers.
If the wavelength at 100Hz is about 11 ft, would not those extra 9 feet throw off the phase response grossly and mess up the sound at the 80Hz crossover point I intend to use???

SEALED subs: I can place stacked sealed subs in corners, lets say 4x15" in each corner. A total of 8x15" would provide enough quality vs output to outclass other types of subs for my application requirements (see above)?

I have only used sealed subs in my current system. And the problem is I cannot imagine how other types will sound compared to the sealed design.

Thank you,
Herman


Title: Re: loudness -- types of music
Post by: Ivan Beaver on March 24, 2011, 07:35:30 am
First of all, I'd like to thank everyone for dissecting my problem.

The loudest I would need is 110 to 115db. However, I do NOT like to push the system to it's limits so at those levels I want the subs to be in "relaxed" mode and have some headroom.

If I choose to have two subs (placed in corners behind speakers) then I have 3'x2'x8' available volume for each sub = 48 ft^3 per sub

The music to be played is Classical, Jazz and Techno.

As far as HORN subs I have a big question: lets say that the internal folded horn path is 9 feet long (reasonable length). In a small room that would make the woofer be DOUBLE of my listening distance from the main speakers.
If the wavelength at 100Hz is about 11 ft, would not those extra 9 feet throw off the phase response grossly and mess up the sound at the 80Hz crossover point I intend to use???

SEALED subs: I can place stacked sealed subs in corners, lets say 4x15" in each corner. A total of 8x15" would provide enough quality vs output to outclass other types of subs for my application requirements (see above)?

I
You state SPL levels-that is good-but what do they mean?  Is that A weighted? C Weighted?  No weighting?  Is that slow-fast-peak response?  All of those will give VERY VERY different readings on a SPL meter.

It is not just the length of the horn that you have to consider.  It is also the phase/"delay" response of the crossover filters, that will also affect signal arrivals.  It is not just the physical distance.

ALso you have to consider whether you are talking about electrical or acoustical crossover points.  Very often they are NOT (sometimes not even close) the same.  This is also affect the amount of "delay" in the system.

Putting the usbs in the corners is usually not a good idea-if you are trying to energize the room modes.  As said earlier, they need to be in different places-to excite different room modes.  The corners are easy, but often not the most effective.  But at least 1 sub should be ina corner.
Title: Re: loudness -- types of music
Post by: Herman Chigrin on March 28, 2011, 01:12:25 am
Hi,

the Spl measurement were taken at "slow" response, "c" weighing. Again, 115 db is not typical levels, but would be on the high side. Some of the options for a pair of subs are:
KV2 EX2.2 Active Bass Module
Danley sound 115

or a self built transmission line or sealed subs based on 12" or 15" woofers.
Title: Re: loudness -- types of music
Post by: Martyn ferrit Rowe on April 21, 2011, 02:49:06 pm
Hey Guys,
these might be of some use:
http://www.aes.org/tmpFiles/elib/20110421/13680.pdf
http://mehlau.net/audio/multisub_geddes/
http://lostinmusik.wordpress.com/2011/04/11/is-there-a-correct-key-to-write-dance-music-in-the-short-answer-is-yes/

the last one is interesting on freq content of music.
cheers,
ferrit