History of Multi-Sub & Sound Field Management (SFM) for Small Room Acoustics

[gene]

The problem with the small rooms we listen in is that they so often present us with offensive bass booms, and the bass is different at different seating locations. The traditional solutions for taming room resonances have relied on low-frequency absorption (a.k.a. bass traps), system layout, and manipulating room dimensions. In homes these may or may not be practical. So Dr.Floyd Toole and and his team investigated alternative solutions, leading to multiple subwoofer strategies and a sophisticated DSP process called Sound Field Management (SFM).

This article describes the evolution of these methods, that can replace or supplement bass traps in some situations.

Read: History of Multi-Sub & Sound Field Management (SFM) for Small Room Acoustics

 

[TheWarrior]

This is only a sample of the knowledge derived from the lifetime of research found in Dr. Toole's book, 'Sound Reproduction - Loudspeakers and Rooms'.

It is important to understand the relationship between sound Production and Reproduction.

'It is science, in the service of art', and Dr. Toole has spent more years than I've been alive, defining, and illustrating that! Even a casual reader would gain new insight, and possible redirection, as they endeavor to achieve their own slice of audio bliss.

Thanks for sharing!

 

[shadyJ]

This, in addition to Gene's recent article, just drives home the point about multiple subs. So many people think there should be just one sub in the system because that is all they have ever seen and are used to it; it is convention. But it rarely sounds good. Perhaps that can change if businesses start selling speaker/sub packages with multiple subs instead of just one, and also if AVRs can independently calibrate multiple subs instead of just one.

 

[mtrycrafts]

..., and also if AVRs can independently calibrate multiple subs instead of just one.

Can you expand on this thought? Do you mean calibrate one sub then the 2nd, then both together?

 

[TheWarrior]

Can you expand on this thought? Do you mean calibrate one sub then the 2nd, then both together?

From 'Sound Reproduction': "It remains a perplexing dilemma that there are no truly reliable technical standards for control room sound, making the reference a moving target."

In any domestic living space where you might find speakers, the boundaries of the room determine a varying transition frequency at which low bass frequencies stop being resonant, and become directional - Bouncing all over the room until they run out of energy.

Most of us do not have a simple rectangular 'shoe box' room in which we can easily calculate the couple of room modes that will be excited at low frequency. Conversely, the listening position could also be centered in a room node, absent of specific sounds. In either case, part of the solution can come from placement, where inches make a difference, various types of absorption, or, employing multiple subwoofers to either balance the sound seat to seat, or to work together to cancel-out a room mode.

But without measurements, you are stabbing in the dark. Level matching the subs does not solve anything if you do not know what might be missing. And it is that narrow perspective, that causes most auto-eq programs to fall short of actually solving problems. In recognizing that, it appears @Floyd Toole and his successors Dr. Sean Olive and Todd Welti have helped Harman create a new DSP process that actually accounts for this and minimizes seat to seat variation.

Since this technology is in its infancy and limited in availability, REW and an Omni Mic will have to suffice for room set up. But the knowledge is transferable!

 

[shadyJ]

Can you expand on this thought? Do you mean calibrate one sub then the 2nd, then both together?

I mean independently set all subs so that they are working together for the best overall response at listening positions. Audyssey's SubEQ is a start at this, but it only does two subs. I would love to see it do four subs, and I would love to see to handle deeper frequencies better as well, but deeper frequencies might need a better mic and be more expensive.

 

[TheWarrior]

I mean independently set all subs so that they are working together for the best overall response at listening positions. Audyssey's SubEQ is a start at this, but it only does two subs. I would love to see it do four subs, and I would love to see to handle deeper frequencies better as well, but deeper frequencies might need a better mic and be more expensive.

It does produce some odd results when trying to run quad subs with y-plugs...

 

[mtrycrafts]

I mean independently set all subs so that they are working together for the best overall response at listening positions. Audyssey's SubEQ is a start at this, but it only does two subs. I would love to see it do four subs, and I would love to see to handle deeper frequencies better as well, but deeper frequencies might need a better mic and be more expensive.

I asked because I have two subs and use a 2 ch Behringer EQ with steady state tones at 1 Hz intervals.
Flattened one sub, flattened the other individually but then both at the same time was an issue.
Hopefully the software in the receiver can compare the two back and forth and both to get the best results. Manually, I was not successful.

 

[shadyJ]

I asked because I have two subs and use a 2 ch Behringer EQ with steady state tones at 1 Hz intervals.
Flattened one sub, flattened the other individually but then both at the same time was an issue.
Hopefully the software in the receiver can compare the two back and forth and both to get the best results. Manually, I was not successful.

You don't want to flatten them each necessarily. You want their response to compliment each other for the flattest response at the listening position overall. Use sub #2 to compensate for nulls left by #1 placement. After you have addressed the nulls with placement, take down the peaks with equalization. Then you should have a good room response.

 

[mtrycrafts]

You don't want to flatten them each necessarily. You want their response to compliment each other for the flattest response at the listening position overall. Use sub #2 to compensate for nulls left by #1 placement. After you have addressed the nulls with placement, take down the peaks with equalization. Then you should have a good room response.

Yes, but you need a software to do that as you have to compare the two and try appropriate corrections at each significant frequency that show up.

 

[Floyd Toole]

Just a clarification. Multiple subwoofers combine in a room according to their frequency AND phase responses - the transfer function. It is this that must be measured if superposition is to work. It is the performance with all subs simultaneously working that matters, and by starting with transfer function measurements from each sub to each seat, superposition works - namely it is possible to predict the room steady-state room curve very precisely when multiple subs are added in the computer with or without DSP in the signal path. That is what the optimization program does - it modifies the transfer functions from subs to listening locations until the target performance is attained: i.e. the least variation among the seats. This done, then global EQ, if it is needed, can be applied.

If you are measuring the steady state room curve (i.e. amplitude but no phase) for each sub separately at a listening location, the summation cannot work predictably. I may be wrong, but I sense that some people, and maybe some algorithms are doing this. Maybe somebody knows . . .

Floyd

 

[gene]

Just a clarification. Multiple subwoofers combine in a room according to their frequency AND phase responses - the transfer function. It is this that must be measured if superposition is to work. It is the performance with all subs simultaneously working that matters, and by starting with transfer function measurements from each sub to each seat, superposition works - namely it is possible to predict the room steady-state room curve very precisely when multiple subs are added in the computer with or without DSP in the signal path. That is what the optimization program does - it modifies the transfer functions from subs to listening locations until the target performance is attained: i.e. the least variation among the seats. This done, then global EQ, if it is needed, can be applied.

If you are measuring the steady state room curve (i.e. amplitude but no phase) for each sub separately at a listening location, the summation cannot work predictably. I may be wrong, but I sense that some people, and maybe some algorithms are doing this. Maybe somebody knows . . .

Floyd

Floyd;

Audyssey used to measure level, distance and optimize EQ for each sub individually. Years ago I had them over my place to show how that doesn't work well for multi-sub as it's very unpredictable. They have since then updated the algorithm (though I haven't tested it) to set level, and distance of each sub individually and then apply a target EQ of the combined response. Since my AVP doesn't have that version of Audyssey, I did it all manually myself using mDSP and just bypassed Audyssey all together

 

[lovinthehd]

I believe Audyssey SubEQ does have the capability of 4 subs but no avr manufacturer has implemented such. Audyssey can also work at higher sample rates. Both are subject to the available processing power.

Anyone check out miniDSP's new HD 2x4 with the capability of time based correction? https://www.minidsp.com/products/minidsp-in-a-box/minidsp-2x4-hd

 

[Olivier Schmitt]

Dear all,

You may be interested in new technology for room mode correction, developed by a Swiss team of both universities and industrial companies:
https://lts2.epfl.ch/software/electroacoustic-absorbers/interacts-project/

More on this from last AES convention from last year:
http://www.aes.org/events/139/

And during incoming AES convention in June in Paris:
http://www.aes.org/events/140/papers/

Best regards,

Olivier

 

[Floyd Toole]

Thank you Olivier. I was not aware of this new work.

I could not find the aes 139 convention preprint - was there one?

In any event, since we started publishing methods of bass control there have been several "competitors", some of which are best described as subsets of SFM, which in theory embraces many alternative solutions.

In addition to multiple-sub strategies there is the active-absorber approach, initiated by Olson in 1957 and briefly discussed in my book, p. 220. I think BagEnd has such a product on the market. They work.

My interpretation of this new work is that it is a frequency-selective active absorber, which if coupled to room analytical measurements and software should be even more effective than the broadband, absorb everything, approach. I look forward to learning more.

 

[mtrycrafts]

.... It is the performance with all subs simultaneously working that matters, ...

If you are measuring the steady state room curve (i.e. amplitude but no phase) for each sub separately at a listening location, the summation cannot work predictably. I may be wrong, but I sense that some people, and maybe some algorithms are doing this. Maybe somebody knows . . .

Floyd

I am just a simpleton so I didn't measure the phase, just steady state. As you indicated, ended up doing both on and both channel filters sinked. That gave a very smooth result.

Doing one, then the other and then check both was a way different curve from each one before the filters and certainly wasn't very nice when combined to two.

 

[Olivier Schmitt]

Thank you Olivier. I was not aware of this new work.

I could not find the aes 139 convention preprint - was there one?

In any event, since we started publishing methods of bass control there have been several "competitors", some of which are best described as subsets of SFM, which in theory embraces many alternative solutions.

In addition to multiple-sub strategies there is the active-absorber approach, initiated by Olson in 1957 and briefly discussed in my book, p. 220. I think BagEnd has such a product on the market. They work.

My interpretation of this new work is that it is a frequency-selective active absorber, which if coupled to room analytical measurements and software should be even more effective than the broadband, absorb everything, approach. I look forward to learning more.

The work will be presented with full paper support in AES 140th convention in Paris in June. The absorber is set for a range of frequencies, typically 30 to 130 Hz for instance. We hope to be able to demonstrate the results as well during the convention. Will you attend it ?

 

[Mctwins]

Hallo! My first post here.

I just want to say that I have great result with Varitunes V4 and V6 tunable Helmholtz resonators in both of my rooms. This devices is passive for the treatment of low freq. So no need for cables or so..

https://www.gearslutz.com/board/11697747-post393.html

https://www.gearslutz.com/board/11144503-post389.html

There are some measurements some years ago in this thread.

Have fun

 

[Floyd Toole]

Olivier,
Paris is one of my favorite cities - I proposed to my wife there in 1960! We were post-graduate students in London, unable to afford more than a hotdog and coffee on the deuxieme etage of the Eiffel Tower. But, sadly, I won't be at the AES this time.
As for the bass absorbers, if the preprint has adequate measurement data, I really don't need to hear it

And including Mctwins: Absorption works, and frequency selective absorption - active or passive - should work especially well to address the resonance problems without overdamping the rest of the spectrum.

 

[Goliath]

Hi Floyd/Gene,

How are you guys doing? Floyd, I wanted to ask you if you were perhaps familiar with MiniDSP products for room correction.

In particular this :

Dirac live

Wanted to get your thoughts on the product. It seems like a very powerful tool. You can shape the response curve in real time, and it can addresse phase, polarity, levels etc.

What do you think? Gene, have you used this product before or heard of it? Thanks for your time.