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Double bass array. Low Q subs. [Quasi]plain wave radiation?

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#1
Hi there!
Briefly:
Here is the room 14'W*30'L*10H. Speakers not shown as their location may vary. Subs (2F+2R) placed as expected at half of room height.
1624219431887.png


Here is measured LF response approximately at "sweet spot". As always (unless otherwise specified) no smooting, without eq\correction, with long default window.
1624220252106.png


Subs are closed 200 l with conventional old fashioned 18" (Fs 29. Qts .29 Vas ~400+ l):
1624220943477.png

One can say it's not sub at all and yes, for open space this is true. But I usually live at home.
Let's enter the value "0.0 x Pi" for Ang:
1624221918459.png

1624222299084.png

and funny acoustical impedance
1624222403043.png

I'd love to hear @René - Acculution.com 's comments on the above.
 
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René - Acculution.com

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#2
@Flaesh The DBA concept is interesting. I have listened to it in action when working with Adrien years back (https://vbn.aau.dk/en/publications/controlled-acoustic-bass-system-cabs-a-method-to-achieve-uniform-), and you could certainly hear a difference when moving around in the room. I think it is more ideal to have 4F+4R, but placement is not easy then. I imagine you have all 4 on the floor (?). So in a true plane wave you have a specific acoustic impendance of density times sound speed, but the question is of course to which degree you can have a plane wave in room like this. One tries with the DBA/CABS to turn the room into a tube in some sense, and if you excite a plane wave in a tube, then that is the only wave you will have at all frequencies (even above any cut-on frequency for the transverse modes). From you measurement is seems that you have at least a one point in the room where the (specific) acoustic impedance is constant at all frequencies (?), but above that a figure (where I would need a color legend to see the values in the room) where the impedance varies with position. As you have a plane wave in a "tube", you will not experience any room gain, or modes in general for an ideal plane wave, and so your speakers will dictate the roll-off for the frequency response in a point. Leakage in the room will however affect the sound pressure as your 'tube' needs hard walls for all of this to work ideally.
 
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OP
Flaesh
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Thread Starter #3
Good ol' nice sweet pleasant :Ddiscussion about *BA vs multisubs with Nils and Earl https://www.diyaudio.com/forums/multi-way/280249-controlled-directivity-diys-5.html#post4466544
I have not experimented with sub's configurations in the current room, only DBA; in the previous room 14'W*16'L*10H (rudiments of the transverse wall are visible on the room plan) DBA was subjectivly preferred by uninformed listeners in comparations with another eqalized\delayed\inverted subs. While I don't try four channels, random sub positions and using MSO. Some sence of "front bass" with DBA is good for me, also as nothing like bass in head\separate bass and so on.
I imagine you have all 4 on the floor (?).
No,
Subs (2F+2R) placed as expected at half of room height.
5' from floor, 5' from ceiling. These are not things that are easy to move around. When deviating from 1/4 of the width, the result is the same as the simple model - second width mode appears. Here is REW simple model for empty room without doors\windows:
1624285245254.png
 

René - Acculution.com

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#4
Good ol' nice sweet pleasant :Ddiscussion about *BA vs multisubs with Nils and Earl https://www.diyaudio.com/forums/multi-way/280249-controlled-directivity-diys-5.html#post4466544
I have not experimented with sub's configurations in the current room, only DBA; in the previous room 14'W*16'L*10H (rudiments of the transverse wall are visible on the room plan) DBA was subjectivly preferred by uninformed listeners in comparations with another eqalized\delayed\inverted subs. While I don't try four channels, random sub positions and using MSO. Some sence of "front bass" with DBA is good for me, also as nothing like bass in head\separate bass and so on.

No,

5' from floor, 5' from ceiling. These are not things that are easy to move around. When deviating from 1/4 of the width, the result is the same as the simple model - second width mode appears. Here is REW simple model for empty room without doors\windows:
View attachment 136779
Ah, okay, they are high up. Nice. Yes, I read the post you linked to quite recently. I enjoy these post not just for technical points, but also to see how different experts discuss things; do they get heated and insist on their way being the best, are they open for suggestions, things like that, the psykology of it all ;-) When it comes to "does DBA inherently sound good/bad, natural/unnatual, ..." I really have no insight. I coooould be onboard with the fact that the bass being more 'outdoors-y"/propagating and the rest being "indoors-y" could sound somewhat unnatural to some ears, but I have no idea. I am more into the physics and the mathematics ;-)
 
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Flaesh
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Thread Starter #5
post you linked to quite recently. I enjoy these post not just for technical points, but also to see how different experts discuss things
There are 3 or more pages after in similar manner))
that the bass being more 'outdoors-y"
I agree. It's funny and unexpected that so far none of the guests said: "there is not enough bass" or something like that.
I am more into the physics and the mathematics ;-)
I would like to see a correlation between physics and audible effect :). At one time I believed that a wavelet without irregularities characterizes such a bass, but this was a mistake.
 

Kvalsvoll

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#6
In this room, I would try all 4x subwoofers on front wall, and absorption on rear wall.

Math/model for DBA is very simple - for the perfect situation - and you get perfect frequency response, and a sound field where direction (pv) is predictable and similar to free-space at very large distance, where the wavefront is a plane wave. The problem is, practical rooms do not have rigid wall surfaces and 2x units on one wall is not enough to produce a sufficiently plane wave. And as you approach 100hz and so, the deviation from the model becomes so large, it will be very difficult to tune the system so you avoid hearing the rear sub units.

With 4x and individual processing, you can tune the system using other approaches - experimenting with delay on the rear units are crucial here. the goal should be to achieve a decent frequency response AND timing and preserved sound field intensity/velocity. This can usually be achieved in most rooms with 4x, it does not require a DBA/SBA approach, but it it will require some work on calibration.
 
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Flaesh
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Thread Starter #7
In imperfect rooms, where the upper tens of hertz range is still damaged by openings and furniture, precise positioning of subwoofers may not be all that necessary. See "Subjective Preference of Modal Control Methods in Listening Rooms", Fazenda et al. The name speaks for itself. DBA (CABS) considered and their "SSS" is what called "DBA" by some people)). They also used two different music samples ;)..
Paper in open access https://www.researchgate.net/public...reference_of_modal_control_in_listening_rooms

In this room, I would try all 4x subwoofers on front wall, and absorption on rear wall.
Yes, I'm going to try sometime. I think you can imagine what effective absorber for say 30-40 hertz looks like :p. Adding 4 rear subs may be easyer.
 

Kvalsvoll

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#9
I think you can imagine what effective absorber for say 30-40 hertz looks like :p. Adding 4 rear subs may be easyer.
It can be done, build a helmholtz-wall covering the rear wall, use the online porous-acoustic-absorber calculator to find correct size of openings in the panel. Just doing a simple absorber wall is not so efficient at very low frequencies.
 
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Flaesh
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Thread Starter #10
Fazenda et al. are not hardworking enough. As I:) Undoubtedly it would be interesting to compare with Geddes'\MSO approach. But I think "1.6 Front-Back (FB) Configuration" is something similar in generally.
My friends and I definitely preferred the outdoorish bass in my system with samles like Fazenda's "sample A".
You can consider it as waveform reproduction or so(?))
a helmholtz-wall covering the rear wall
HR are band-limited. Wide range perforated\slotted absorbers aren't very helmholtz-ish. See Bogich Petrovich (RIP) papers.
Nils had rockwool rear wall with SBA and had good results.
One buddy (simple hiend stereo towers) consulted with some "acoustician" build huge helmholtz rear wall. Bad before and slightly less bad after :oops:. For hiend-ish price.
 

Kvalsvoll

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#11
Fazenda et al. are not hardworking enough. As I:) Undoubtedly it would be interesting to compare with Geddes'\MSO approach. But I think "1.6 Front-Back (FB) Configuration" is something similar in generally.
My friends and I definitely preferred the outdoorish bass in my system with samles like Fazenda's "sample A".
You can consider it as waveform reproduction or so(?))

HR are band-limited. Wide range perforated\slotted absorbers aren't very helmholtz-ish. See Bogich Petrovich (RIP) papers.
Nils had rockwool rear wall with SBA and had good results.
One buddy (simple hiend stereo towers) consulted with some "acoustician" build huge helmholtz rear wall. Bad before and slightly less bad after :oops:. For hiend-ish price.
Calling it Helmholtz-wall was not a good choice, I see it can be misleading.

The acoustic properties of such a rockwool wall can be adjusted using slats - more slats -> more reflective at higher frequencies. This is used to adjust the frequency balance of the room. When the slats cover most of the area, so that there are only small gaps left, the absorption efficiency increases a little at low frequencies.
 
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