Placement of subwoofers along standing waves, exciting modal resonances

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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
 
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?
 
Just for provocation:

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

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
 
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 ...
 
LineArray said:
Without exciting room modes there is no bass response ...
:cool:

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.
 
MaVo said:

...
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 ...

:Popworm:
 
LineArray said:


... 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 ...

:Popworm:

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?
 
thadman said:

...
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/speaker-setup-guidelines/subwoofer-placement-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
 
LineArray said:
I prefer dipole subwoofers, two of them in different orientation
can give balanced behaviour und good "envelopment".

Interesting. I was planning dipole subs in multiples around the room to achieve a flatter response but by the sound of it just two might work well?

Could they even be stacked on top of each other? For instance, with W profile dipoles could one have the front opening facing the listener and the one atop have the front opening facing say 30 degrees from the listener? Would this aid a flatter response?

Saves floorspace if it does! Making the bases and tops of the subs round allows for any increment of rotation maintaining the same aesthetic (with round grilles too).
 
LineArray said:


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).

Lets take a look at your organ pipe example. For the first mode, where there are two nodes and one antinode, assuming drivers are placed at the positions of the nodes wherein the pressure maximas have 180 degrees phase difference, sound would propagate without modal influence thereby removing the unequal distribution of low frequency energy in the pipe at that particular frequency corresponding to the first mode. Is that what you are inferring?
 
Hi Dr. EM,

i have not tried an arrangengement like that until now.

I placed my dipole sub in the edge of the room using a side
wall and the bottom as borders. This increases the efficiency.
When doing this, is does not make sense to place a second one
rotated ontop of the other, because the radiation axis of the
(rotated) sub does not coincide with the edge.

I place a second one in a different edge of the room 90 degrees
rotated. This way i excite different room modes with the second
one, given the room has no square footprint.

Your proposed arrangement makes sense to me, if placed
far from walls. By rotating one against the other you will loose
efficiency at frequencies of maximum excitement but the overall
frequency resonse may indeed be more balanced.
Even angles from 30 up to 90 degrees may be tried ...
Could make sense, it is a matter of placement and
experimenting with the angle.

Please report your experiences, i am curious ...

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

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

I'm glad you made that point MaVo:

You can have boundary loading without being in an enclosed space.
Outdoors, or indoors in an open area with no barriers to adjacent space.
What if you are not in a "sealed" 6 sided rectilinear room?
In a larger area with a minimum, or no parallel surfaces there are less standing waves.
I'm not sure about the helpfulness of "excitation". If it's to augment a deficiency (hole) in a speaker response, it seems like it's an oblique strategy to fix a speaker problem, Or Is this a reference to cabin gain? Again IMO usually an attempt to add bass extension, because the listening environment is small enough and tight enough to be a resonator. The 6 sided room becomes part of a sort of bandpass box system, with the listeners inside a chamber.
I think I would rather "de-resonate" an area, and make it less of a box to reflect and color, so that the drivers are to the primary source of sound.
Less room effect, more speaker direct.
 
Hi Rybaudio,

i never thought about free space conditions before,
interesting.

Modal density increases with size of the room. So if we
blew up our living room to the size of a concert hall, modal
density would increase, if we blew up the concert hall ...
to infinite size in each (space) dimension,
we get infinite modal density and every frequency is a mode.

Understood :D :bigeyes: .

What i wanted to say with my - not as provocative as i thought
it was - statement, is that we cannot influence the modal
density of a given listening room, if we cannot change its size.

(Of cause we could change the medium, it is filled with :redhot: )

All we can do is making a concept how to excite the given modes
to a similar (ideally all modes to the same ) extent.

Right ?

What do you think about, how to achieve that practically ?

Kind regards
 
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