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Multiple Small Subs - Geddes Approach
Multiple Small Subs - Geddes Approach
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Old 29th December 2008, 08:35 PM   #431
gedlee is offline gedlee  United States
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For those looking for a cheap sub for use in multi-subs check out
Polk Audio PSW10 10-Inch Monitor Series Powered Subwoofer on Amazon - $124, amp and all.
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Old 2nd January 2009, 12:04 PM   #432
inertial is offline inertial  Italy
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Quote:
Originally posted by john k...


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.

Click the image to open in full size.

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:

Click the image to open in full size.

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.

Click the image to open in full size.

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
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Old 2nd January 2009, 04:25 PM   #433
Sheldon is offline Sheldon  United States
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Quote:
Originally posted by gedlee
For those looking for a cheap sub for use in multi-subs check out
Polk Audio PSW10 10-Inch Monitor Series Powered Subwoofer on Amazon - $124, amp and all.
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).

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Old 2nd January 2009, 04:34 PM   #434
john k... is offline john k...  United States
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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.
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Old 2nd January 2009, 04:46 PM   #435
gedlee is offline gedlee  United States
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Quote:
Originally posted by Sheldon


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

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.
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Old 2nd January 2009, 05:09 PM   #436
Sheldon is offline Sheldon  United States
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Thanks, makes perfect sense. Even with limited measurements in my own room, that lowest octave is not where the dips and bumps in the response are, so a single sub there should give reasonably smooth response there.

Sheldon
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Old 2nd January 2009, 05:47 PM   #437
Pan is offline Pan  Sweden
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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
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Old 2nd January 2009, 06:13 PM   #438
gedlee is offline gedlee  United States
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Quote:
Originally posted by Sheldon
Thanks, makes perfect sense. Even with limited measurements in my own room, that lowest octave is not where the dips and bumps in the response are, so a single sub there should give reasonably smooth response there.

Sheldon

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.
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Old 2nd January 2009, 06:21 PM   #439
markus76 is offline markus76  Germany
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Quote:
Originally posted by gedlee
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.
By what else should modes be dictated other than the room?? CABS or DBA or whatever you would like to call it works perfectly (when the room is rectangular and has sufficient rigid walls).

Best, Markus
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Old 2nd January 2009, 06:25 PM   #440
inertial is offline inertial  Italy
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Quote:
Originally posted by john k...
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.

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
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