markus76 said:
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.
john k... said:
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.
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/
http://aes.org/journal/toc/
john k... said:
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:
Here is another interesting paper from November:
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 said:
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.
john k... said:
Not to wax on endlessly on this but it's not something that can be subjected to proof. However, in any specific case, like your listening environment, it is a simple matter to set up a sub, or multiple subs, measure the response and decompose it into minimum phase and an all pass components and see if the allpass is a pure time delay.
Here is such a result for a single woofer.
An externally hosted image should be here but it was not working when we last tested it.
I have presented the data in two equivalent formats. The upper plot shows the the measured phase data in green and amplitude in fuchsia. The thin blue line overlaying the phase data is the MP computed from the measured amplitude data with the addition of a delay of about 14 msec. There is good agreement up to about 35 Hz but above that the deviation from MP plus delay indicated that the measurement is not just MP with a delay, but some non-MP response.
In the lower figure I removed a 14 msec delay from the measured data rather than add it to the MP result. The thin blue phase line is therefore just the MP computed from the measured amplitude. Again, there is agreement to only about 35 Hz. Here we see a phase wrap associated with the notch at 40 Hz where as the MP phase shows a wiggle. above 40 Hz it looks like the MP response follows the measured data for a while but remember to compare the data the 360 degree phase rap must be accounted for. This is a clear indication of non-MP response.
For what it's worth, here is a simulation of the response of the woofer system to a 1. msec pulse:
An externally hosted image should be here but it was not working when we last tested it.
Green is the pulse, brownish red the response when the woofer is MP and blue the response of the woofer based on the measured phase. No amplitude eq has been applied in either case.
And while I'm at my desk, here is a simulation of the impulse response when the amplitude is eq'ed to the smooth response shown in red in the upper plot of the figure below. The red impulse trace is what would happen is the woofer response was MP and the blue response is what happens when the actual woofer response is eq'ed to the smooth response. As can be seen, due to the non-MP nature of the real woofer response, when the amplitude is eq'ed there is still a lot of trailing "noise in the impulse.
An externally hosted image should be here but it was not working when we last tested it.
Nothin' say lovin' like something from the oven.
Dr. Kreskovsky ,
Please can you , in simple words, explain the difference between
a minimum phase system and a linear time invariant system?
Second thing, if the system is MP only up to 35Hz, does it means
that digital room EQ is pratically "wrong" approch ?
Cheers,
Paolo
gedlee said:
I know it's probably buried in one of these threads, but how low can one expect the practical in-room extension to be (more or less, and including reasonable EQ) with, say three subs like this? Most of the subs in this category have similar specs. - and extend down to about 35Hz (before room gain, I assume).
Sheldon
MP is a little difficult. Mathematically it means the system is stable and causal. In simple works it means just what it says. The phase shift is the least amount of phase shift such a system can have.
Linear, time invariant means exactly what it says. The system is linear means the output is a related to the input by a scale factor (which may be a function of frequency). Time invariant means that scale factor is not a function of time.
The thing about room eq is that the source to listener transfer function in a room is different for every listening position. Any type of eq is only capable of correcting the response at one position. If the correction is minimum phase then typically all that can be corrected is the amplitude response at some position. If you are not aiming at "perfect" amplitude response then you can probable get a smoother spatially averaged response over a wider listening area.
Linear, time invariant means exactly what it says. The system is linear means the output is a related to the input by a scale factor (which may be a function of frequency). Time invariant means that scale factor is not a function of time.
The thing about room eq is that the source to listener transfer function in a room is different for every listening position. Any type of eq is only capable of correcting the response at one position. If the correction is minimum phase then typically all that can be corrected is the amplitude response at some position. If you are not aiming at "perfect" amplitude response then you can probable get a smoother spatially averaged response over a wider listening area.
Sheldon said:
LF extension is seperate from the concept of multiple subs. Multiple subs won't necessarily extend the response lower, it is just smoother where it is applied. Since those Polk subs are ported they won't produce any usable output below the port tuning - unlike monopoles. A monpole could be EQ'd to extend the response, but not a ported design. So with those subs 35 Hz. is as low as you could go. In my theater I use one very large sub tuned to go 25 to 50 Hz to get this very lowest octave. The smaller subs and the mains then picks up after that.
A bassreflex speaker is commonly called monopole just as is a closed boxed.
A bassreflex speaker can be EQ'ed for extended response and increased LF output (within limits).
A multipple set up of subs will obviously not change the tuning of the boxes but in practice the response will be different than with a single speaker, not just by avoiding peaks and dips but since low frequencies will sum more in phase than higher frequencies.
The extension will therefore be better with multiple subs.
/Peter
A bassreflex speaker can be EQ'ed for extended response and increased LF output (within limits).
A multipple set up of subs will obviously not change the tuning of the boxes but in practice the response will be different than with a single speaker, not just by avoiding peaks and dips but since low frequencies will sum more in phase than higher frequencies.
The extension will therefore be better with multiple subs.
/Peter
Sheldon said:
This was not the case in my room, there was a strong mode at about 30 Hz, but nothing made any difference, not phase or location (within what was possible) etc. I am begining to think that the first mode is going to be dictated by the room and its construction and nothing that you do with the subs (within practical limits) will make any real difference. Now just above this mode things were very sensitive to the various subs phases and gains etc. This is a classic example where EQ is necessary, to bring down this mode that nothing else can deal with.
john k... said:
Thanks Dr Kreskovsky,
My math is worst of my english!
So a LTI system can be MP or not. But if a system is MP , is it automatically LTI ?
one example: a good amplifier ( to say a Spectral) is a MP device, and it is LTI also, right?
- a ideal speaker, to say a Manger , we can considerate near MP and LTI also, at least in axis, right?
If the system is LTI, we can pass from time domain to amplitude domain. If we correct the amplitude automatically we correct also time, am I right?
- a listening room, you have showned it is not MP. Is it at least LTI?
When we apply digital room correction ( for one point) are we correcting only the amplitude or also the time?
Perdone my confused exposition
Paolo