Multiple Small Subs - Geddes Approach

[john k...]

Originally posted by soongsc

I tend to agree. Normally CDF is used to solve very specific problems. Very seldom if any is it used to model blanket range scenarios. Even with todays PC computing power, it takes too much computing time could take a few days; and then if the results are not good enough, human intelligence still is needed.

I won't argue about what CFD can or can not do. Earl has already pointed to a JAES article that shows the direction room simulation is going.

[john k...]

Originally posted by markus76
I don't care how exact the model is as longs as it's exact enough. Auralization ist the key point to me. The conventional approach of measurement, construction and measurement is time consuming, error-prone, expensive and allows no A/B comparison - or even A/B/C/D/E/...n comparison.

So how would you propose to validate any model without comparison to measurement? How would you know when your model is exact enough? The reality of the situation is that with a multi-subwoofer system you can probably slide the woofers around the room and while making real time measurements and find a solution faster than any simulation will. Even if you have a model which is exact you would still need to run many cases to find an optimum positioning of the woofers. This could probably be handled by numerical optimization of the woofer positions, but still represents a lot of computational effort. Even with exact simulation it’s not like you will be able to tell it, ' I want +/- 3dB from 30 to 200 Hz over this listening space and the code will tell you that you need X many woofers places in these (x,y,z) locations.

If you want to aim for the optimization problem, fine, but you still need to address the simulation tool and validate it accuracy before moving forward. An accurate tool without such optimization only allows you to perform simulation based experiments as opposed physical experiments.

[gedlee]

Originally posted by john k...
Form a mathematical point of view room acoustic is a simple interior problem for which the solution is governed by boundary conditions at the bounding surfaces.

The interior is easy. The wave equation is suitable.

John

Why is it that the work that others do "is easy" but our own is so difficult and complex? Maybe its because we don't see all the complexities of what they do?

There are "easy" interior problems (a rectangular room with rigid walls) and there are difficult ones (like diffraction in ray models or wall coupling in modal models). NO approach to solution is going to work over the entire bandwidth of the problem - there are different issue with each approach in different regimes of the wavenumber. If this is "easy", then I've made it far too complex all these years.

[john k...]

Easy is a state of mind. Being complicated doesn't necessarily make it hard, just tedious.

Anyway, could you post the title/author of that AES article on FDA of room response. I'm not an AES member so I would have to purchase a reprint to look at it.

[markus76]

I'm not a fan of lobbies but every serious person in the audio business should be a member. Take a look at the valuable information available:
http://aes.org/journal/toc/

[gedlee]

Originally posted by john k...
Easy is a state of mind. Being complicated doesn't necessarily make it hard, just tedious.

Anyway, could you post the title/author of that AES article on FDA of room response. I'm not an AES member so I would have to purchase a reprint to look at it.

And looking "easy" from the outside does not make the problem any easier when actually doing it.

Here it is (DID NOT require AES membership to get this) from October:

Low-Frequency Loudspeaker–Room Simulation Using Finite Differences in the Time Domain—Part 1: Analysis
Adrian Celestinos and Sofus Birkedal Nielsen 772
A computer simulation using finite-difference time-domain techniques has been used to model the low-frequency transfer function between a loudspeaker and virtual microphones in a rectangular room. The model for the analysis was validated with measurements in an IEC standard listening space. Using the mean sound field deviation, the cumulative spectral energy decay, and the clarity metric, the properties of a sample room can be examined mathematically and visually. At frequencies between 30 and 100 Hz, sound pressure varies as much as 30 dB from one position to another.

Here is another interesting paper from November:

Controlled Acoustic Bass System (CABS) A Method to Achieve Uniform Sound Field Distribution at Low Frequencies in Rectangular Rooms
Adrian Celestinos and Sofus Birkedal Nielsen 915
A new method for improving the low-frequency response in small- and medium-sized rooms uses a pair of front loudspeakers for initiating a traveling plane wave and a canceling rear pair of loudspeakers that effectively remove the acoustic reflection of the back wall. The rear loudspeakers receive a delayed and opposite phase version of the signal presented to the front loudspeakers, thereby canceling the plane wave when it reaches the rear of the room. Unlike equalization techniques that emphasize a single sweet spot, this approach covers the entire room. The upper frequency limit of the system is higher for smaller rooms. Based on an evaluation in two rooms, the system works as intended. Using this method, listeners experience a clear bass sound image, and the rear loudspeakers are inaudible.

I see this as a complex and specific means of achieving the same thing as the use of multiple subs. In the end I expect the two approachs to sound about the same, but one is far easier to impliment.

John - how can you be serious about audio and NOT be an AES member?

[markus76]

Originally posted by gedlee
I see this as a complex and specific means of achieving the same thing as the use of multiple subs. In the end I expect the two approachs to sound about the same, but one is far easier to impliment.

You responded completely different when comparing the multiple subwoofer approach and the plane wave approach (CABS) lately!?

P.S. Please don't make me search your posts here to prove it...

[soongsc]

The second paper is exactly like what I had in mind!

I'm with John, not a member.

[gedlee]

Originally posted by markus76


You responded completely different when comparing the multiple subwoofer approach and the plane wave approach (CABS) lately!?

P.S. Please don't make me search your posts here to prove it...

Well you'll have to because I don't recal saying anything different. I know that the CABS thing will work, I proved this to myself years ago, its nothing more than an application of Active Noise Control. So I'm sure I never said that it won't work. I might have said that I'm not sure that a plane wave is what one wants, and I'm not, because thats like being outdoors and bass outdoors is never as good as it is indoors.

I can see that the CABS thing could yield comparable performance to multiple subs if outdoor bass is what you want, but not if the kind of bass reinforcement that you get from a room is what you are after, then I don't see CABS as achieving the goal. But if you see these points as being "completely different" then your are being very picky. I see them as perhaps two slightly different objectives, but both aiming at a smooth response that is not limited to a single point. In this later regard they are the same thing, but the multiple subs is a lot easier.

[markus76]

Thanks for the clarification. I can live with being labeled "picky". Isn't that one of the primary qualities of any audiophool?