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Old 20th February 2012, 06:59 PM   #1481
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Quote:
Originally Posted by gedlee View Post
By your deffinition of room gain, all modes are "room gain". That is not the usual deffinition - its more the [pressurization mode that is considered room gain, and it is pretty much a falicy. But if the modes are the "room gain" then you are just talking about LF modal response in a small room. I don;t think that I agree with your characterization of it however.
So you've never heard "room gain" being applied to anything other that pressurisation gain ? So you would consider a room that has no pressurisation gain (or is driven by a dipole for example) to have no room gain ?

Perhaps just a case of semantics. To me room gain encompasses all the effects that cause an increase in bass response compared to the free field response of the speaker at the same measurement distance. The cause doesn't really matter, the result does.
Quote:
- this is simply a description of the "zeroith mode" in a closed room - this is the pressurization mode. In any real room however, there are substantial leaks and this mode is diminished in level and moved away from zero by these leaks. In any space bigger than a car it is not really a factor.
There is more than just pressurisation and standing wave modes that increase the bass response in a room over free field though - the room boundaries constrain the solid angle of radiation thus increasing the SPL at the listening point. All three effects combine together.

Imagine a shoebox shaped room with the speakers at one end and the far end wall near the listener is missing but all other walls are present. Is there any pressurisation gain in this room ? Nope, because an entire wall is missing.

Are there any modes ? Yes, although the length mode wont be present because there are no parallel boundaries in that axis for the wave to bounce back and forth repeatedly from. Width and height modes will still be present though.

Do the remaining modes explain all the increase in bass response over free field conditions ? No, because from the perspective of the listener within the room boundary the wave from the speaker can't expand into full space until it exits the open wall at the back of the room - within the "room" the SPL will be increased due to the constraining effect of the walls, and this will apply even below the lowest (remaining) room mode.

Below the lowest mode all the room reflections will arrive constructively and give maximum gain due to their long wavelength, but once you get into the modal region some of the reflections start to arrive out of phase and start becoming destructive.

The net result is that you get maximum coherent "gain" from the wall reflections below the modal region (independent of pressurisation) and on average less gain within the modal region as the reflections become incoherent - thus you inherently get a tilted down bass response which falls with increasing frequency in a room, which was my point.

(Another analogy is the transition between coherent addition and power addition of two widely spaced speakers - at low frequencies they will add to 6dB as they are in phase but at high frequencies they will tend to add to 3dB at anything other than perfectly matched distances due to dense comb filtering)

It's easy enough to prove with a reflection/image based room response simulator such as the FRD consortium spreadsheet - you can set the reflectivity of an entire wall to 0 and whilst the bass response certainly changes a lot (and one of the modes is eliminated) the general downwards trend in bass with increasing frequency remains, albeit with less slope. In fact even with two walls eliminated it does, depending on their orientation to each other.

One also only has to measure the response in typical rooms to empirically show that rooms almost universally cause a tilted down bass response relative to the actual nearfield response of the speaker.
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Last edited by DBMandrake; 20th February 2012 at 07:08 PM.
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Old 20th February 2012, 08:11 PM   #1482
gedlee is offline gedlee  United States
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Quote:
Originally Posted by DBMandrake View Post
So you've never heard "room gain" being applied to anything other that pressurisation gain ? So you would consider a room that has no pressurisation gain (or is driven by a dipole for example) to have no room gain ?
That is correct

Quote:
There is more than just pressurisation and standing wave modes that increase the bass response in a room over free field though - the room boundaries constrain the solid angle of radiation thus increasing the SPL at the listening point. All three effects combine together.
These effects are all already encompased in the calculation of the room modes and should not be considered as a seperate effects. There have been lots of mistakes made in this regard in the literature. Any and all effects of the room boundaries are already accounted for in the normal modes solution. Many claim that the sources "free field" response needs to be added to the room mode response, but this is incorrect. This "free-field" or direct field response is already present in the modal series solution, but convergence is slow. This effect is well described by Morse in Vibration and Sound in his discussion of Eq 20.4. He shows how the direct field term is contained in the normal modes solution and how it can actually be seperated off and defined independently, which, of course, requires a new normal modes series solution. This new solution now converges much faster than the normal one. A very neat trick.
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Old 25th February 2012, 10:55 AM   #1483
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It pretty exciting to be in this hobby for 30 or 35 years and come across such great info like multiple sub placement threads.

I'm in the middle of a project that will take my subs to the next level. I have four colocated 15's in the front center of my room that are way to heavy and way too large to relocate them. My only way to fill in the nulls at my LP is locating what i call "tuning" subs mid wall on the other three walls.

I built one already and tested it. It works perfect. I dragged in two other subs from other systems in my home to verify their effect on the listening position, and dont you know it, i now have a workable solution to the huge depression at the LP centered around 55hz.

My plan is to build two more tuning subs for a total of three that will surround the LP, sides and back, with the front colocated subs to fill in the front wall.

The whole thing will be EQ'd with a single MiniDSP.

Here is one tuning sub, it an RS Dayton 12" and a Dayton 240 plate amp

Click the image to open in full size.

Here are my colocated 15's

Click the image to open in full size.

Last edited by kgveteran; 25th February 2012 at 11:16 AM.
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Old 5th January 2013, 02:20 PM   #1484
OllBoll is offline OllBoll  Sweden
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Dun dun dun... Thread revival!

I'm planning a multi-sub arrangement with dipole subs but I'm not sure of positioning: Should I still arrange them to excite the most amount of nodes as with monopoles or should I try to excite the least since they are dipoles?

If I've understood correctly the excite most if they are 45 degrees to the walls and least if I place them parallel?

Thanks in advance,
Olle
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Old 5th January 2013, 02:43 PM   #1485
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Quote:
Originally Posted by OllBoll View Post
I'm planning a multi-sub arrangement with dipole subs but I'm not sure of positioning:
Place them directly behind the listening position:
Comparison of different near field and far field subwoofer configurations
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Old 5th January 2013, 03:40 PM   #1486
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There are issues with bass reproduction in domestic settings which are all but unsolvable. Therefore, you need to address the problems in a statistical/decision approach.

It must strike terror into the hearts of the wannabee engineer kind of person, but the right approach is to be as heterogeneous as possible in your woofing, in terms of the kinds of housing, the locations, etc.

And read Nate Silver's best seller on statistics of prediction.

Ben
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Last edited by bentoronto; 5th January 2013 at 03:42 PM.
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Old 5th January 2013, 04:16 PM   #1487
gedlee is offline gedlee  United States
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Personally, I would not use dipoles. At these frequencies the power required will be tremendous. But if you have the power then I doubt that the location will make all that much difference if you follow the proper setup approach.
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Old 5th January 2013, 04:33 PM   #1488
OllBoll is offline OllBoll  Sweden
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Quote:
Originally Posted by gedlee View Post
Personally, I would not use dipoles. At these frequencies the power required will be tremendous. But if you have the power then I doubt that the location will make all that much difference if you follow the proper setup approach.
Ok, what makes me want to test dipoles is to see if there is less bass leaking to surrounding rooms. I want to have the setup apartment friendly so reduction of sound for the neighbours is more important than full volume down at 20 hz. From 30 hz and upwards there there will still be enough headroom to get complaints so the only sacrifice would be the really low bass but this is a music setup so should be fine.
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Old 5th January 2013, 05:07 PM   #1489
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Quote:
Originally Posted by OllBoll View Post
If I've understood correctly the excite most if they are 45 degrees to the walls and least if I place them parallel?
That's not accurate. The impact of the angle itself can't be stated without complete context of its location in the room.
To maximally excite a given mode, simply locate the dipole at the place of a modal "hump" in the room, with the dipole axis matching the axis of the mode. For a perfectly undamped rectangular room, this would require 3 dipoles, located at the midpoint of each dimension l, w, h. The one for the vertical dimension would have to point toward floor and ceiling.

. . . Or just park a single monopole in the corner.

The O'toole/devantier paper suggests mid-wall ocations, but for completely opposite reasons!-- placement of monopole/omni subs at these locations actually suppresses the resonance of a given axial mode. A dipole has exactly the opposite effect at THAT location.

I believe that dipoles are better used (in Geddes approach) as fill subs.
All-dipoles can be used, but some of the fundamental benefit of his method will be lost; the Geddes method is the most efficient because that first corner sub provides a high-output foundation, with subsequent subs primarily functioning to smooth out the response. Dipoles can be very effective for this as long as the user is aware that he/she has added a variable to the mix (polar pattern with phase flip). The way to address this is to optimize dipole orientation at a given location BEFORE making electrical phase and volume adjustments.

--Mark

Last edited by Tubamark; 5th January 2013 at 05:18 PM.
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Old 5th January 2013, 05:26 PM   #1490
gedlee is offline gedlee  United States
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Quote:
Originally Posted by OllBoll View Post
Ok, what makes me want to test dipoles is to see if there is less bass leaking to surrounding rooms.
Trust me, dipoles won't work any better in this regard than anything else. For a given SPL in the room, the leakage will be the same regardless of the source type. Dipoles just have less output than a monopole so there is less SPL and hence less leakage (apparantly, but not actually).

It is true that dipoles want to be closer to the center of the room and away from the walls (there is a lower modal velocity near the walls), while monopoles tend to want to be nearer the walls where there is always a pressure maximum.
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