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Old 2nd December 2008, 08:33 AM   #1
thadman is offline thadman  United States
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Default Placement of subwoofers along standing waves, exciting modal resonances

I am under the impression that if two drivers are placed within 1/4 WL of each other, that a mode corresponding to that distance would not exist. For example, a subwoofer placed at the center of the first room mode (Wall-->Driver-->Wall) The walls would act as reflecting surfaces thereby placing the distance between all sources at 1/4 WL disrupting the aforementioned mode.

If a driver is placed at the node of a higher order mode (spaced at 1/3 of ones of the rooms dimensions for the 2nd axial mode of that dimension, Wall-->Driver-->Empty Space-->Wall) would that disrupt the aforementioned mode?

Thanks, Thad
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Old 2nd December 2008, 08:42 AM   #2
Pan is offline Pan  Sweden
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Yes you can avoid exciting standing waves/room modes to a great extent by clever poistioning of the source. Preferably four subwoofers or more for good results.


/Peter
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Old 2nd December 2008, 01:53 PM   #3
thadman is offline thadman  United States
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As long as the drivers are placed within 1/4WL of the desired frequency, no modes relating to that dimension can exist. Correct?

For example, lets say we have a vertical linesource. Drivers are placed along every 22" (1/4WL of 150hz) would that mitigate all modes up to 150hz in the vertical dimension?
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Old 5th December 2008, 11:08 PM   #4
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Just for provocation:

Without exciting room modes there is no bass response ...


The question is how to excite them in a balanced manner.

To excite the room at more than one point is helpful, indeed.

I prefer dipole subwoofers, two of them in different orientation
can give balanced behaviour und good "envelopment".

John Kreskovsky says something about that on his website
i think.

Maybe this is interesting:

http://www.musicanddesign.com/RM_Simulator.html

Kind regards
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Old 5th December 2008, 11:29 PM   #5
thadman is offline thadman  United States
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Quote:
Originally posted by LineArray
Just for provocation:

Without exciting room modes there is no bass response ...

Assuming a subwoofer was placed at the center of a cubic room (position of the antinode of the first standing wave), would there be no output within the room corresponding to the frequency of the first standing wave?
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Old 6th December 2008, 12:01 AM   #6
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Assuming a pressure source (say closed box subwoofer)
you will get room pressurization (room gain) for those
wavelengths large compared to the room's dimensions.

For higher frequencies when placed in the antinode of a
room mode there will occur a dip in the response.

A dipole subwoofer will cause a peak at the same place and
the same frequency ...

What is interesting with dipole subs, you can influence the
excitation of room modes by rotating it, while keeping the
position ...
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Old 6th December 2008, 02:52 AM   #7
MaVo is offline MaVo  Germany
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Quote:
Originally posted by LineArray
Without exciting room modes there is no bass response ...
so, this bass i heared outdoors at concerts... it must be something different, but not bass, because there were no room modes to be excited

for a more pragmatic view on this matter, read the harman papers, they tell everything one needs to get perfect response inroom.
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Old 6th December 2008, 03:51 AM   #8
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Quote:
Originally posted by MaVo

...
so, this bass i heared outdoors at concerts... it must be something different, but not bass, because there were no room modes to be excited
...
... or at least that bass you heard outdoors did not rely on
room pressurization at very low frequencies, which is why the
equipment used outdoors may sometimes look a bit different
in size and shape than the equipment most of us use indoors ...

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Old 6th December 2008, 05:17 AM   #9
thadman is offline thadman  United States
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Quote:
Originally posted by LineArray


... or at least that bass you heard outdoors did not rely on
room pressurization at very low frequencies, which is why the
equipment used outdoors may sometimes look a bit different
in size and shape than the equipment most of us use indoors ...

You are correct, a null will result if a subwoofer is placed on a node. An antinode does not seem to be a desirable location either as that will result in maximum excitation and resulting resonance from that mode. For one particular room mode (disregarding influence of other modes), is there an optimal location along the standing wave?

Theoretically if stimulation is distributed evenly along the length of a standing wave, will any modal resonances be present?
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Old 7th December 2008, 03:13 AM   #10
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Quote:
Originally posted by thadman

...
Theoretically if stimulation is distributed evenly along the length of a standing wave, will any modal resonances be present?
...
Hi thadman,

i am a bit on thin ice here, since i am no expert in room
acoustics.

If we look at an organ pipe shaped room, which is closed at
both ends just for simplification, it should also be sufficient
to place two woofers at the opposite ends (=walls).

Since the pressure maxima at the closed ends (=walls) of
the standing wave have 180 degrees phase difference.

If you excite those opposite ends with two woofers in phase,
the energy which is transferred to the standing wave at one
side should be detracted by the woofer at the opposite side of
the room (and vice versa).

Placing (monopolar) woofers near opposite walls and drive
them in phase is also suggested in 3).

My thought:
If we consider, that our rooms are commonly not pipe shaped,
we would have to find an arrangement which is a 3 dimensional
analogy to that 1 dimensional simplification, to get optimum
cancelation of modes with a small number of woofers ...
(right ?)

Placing 8 woofers in 4 edges of the room near the 8 corners
maybe ??

I would tend to take something like that as a base arrangement,
and then breaking the symetry to some extent if necessary
by experimenting.

Some papers:

1) http://www.davidgriesinger.com/vancouver_asa.ppt

2) http://www.sonicdesign.se/subplace.html

3) http://www.audioholics.com/tweaks/sp...ent-guidelines

4) http://www.interdomain.net.au/~bodzio/Article_7.pdf

1) Is a favourite paper of mine, its the one dealing with envelopment.


---
More thoughts concerning that, anyone ?

thadman, you asked a simple question. But it is a very good
and important question indeed to think about ...

P.S. I am a fan of dipole subs. To me the absence of room
pressurization at low frequencies is more enjoyable and
feels more like the bass experience you can have outdoors or
in a large concert hall ...


Kind regards
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