Double Bass Array (DBA)

[Gadgety]

Very impressive results. How did you select which speakers to measure? What was the tweaking procedure, what adjustments did you have to make, and how long did it take?

 

[olieb]

The large dip around 33 Hz is caused by the 1st standing wave mode in the width axis of the room, and the listening position is of course in the null of this mode.

Hmm. Why is there no null in the FR from the rear array then? I do not see how a null in a single mode can produce a dip. That is only possible if there are no other modes with response either.

But very cool and impressive. How do you cope with the vibration from the heavy diaphragms?

 

[StigErik]

Very impressive results. How did you select which speakers to measure? What was the tweaking procedure, what adjustments did you have to make, and how long did it take?

Thank you!

I'm not sure what you mean with you first question?

Anyway - the setup was done like this:
1. Calculate the delay for the rear array - which is equal to the distance between the front and rear wall. Set this delay in the DSP for the rear array. In this case 7 meters = 24.5 ms.
2. Calculate the transmission loss for the distance between the rear and front array in dB, and set this level for the rear array. 7 meters = ~8.5 dB (3 dB/doubling of distance)
3. Invert the rear array
4. Measure, set EQ global EQ (on MiniDSP input) to flatten the overall response.
5. Tweak rear array distance, level and EQ to improve the decay
6. Readjustment of the global EQ.

I did it from scratch today just for fun. Took me less than 1 hour.

 

[StigErik]

Hmm. Why is there no null in the FR from the rear array then? I do not see how a null in a single mode can produce a dip. That is only possible if there are no other modes with response either.

Good question. I am thinking it may be because the main listening position is quite close to the rear array, and that we still have a plane wave from the rear array.

Anyway - my room is 5 meters wide. That gives us a standing wave mode at 343/5/2 = 34.2 Hz. The other fundamental modes are at 24.5 Hz and 57.2 Hz.

 

[Gadgety]

Thank you!

I'm not sure what you mean with you first question?

Anyway - the setup was done like this:
1. Calculate the delay for the rear array - which is equal to the distance between the front and rear wall. Set this delay in the DSP for the rear array. In this case 7 meters = 24.5 ms.
2. Calculate the transmission loss for the distance between the rear and front array in dB, and set this level for the rear array. 7 meters = ~8.5 dB (3 dB/doubling of distance)
3. Invert the rear array
4. Measure, set EQ global EQ (on MiniDSP input) to flatten the overall response.
5. Tweak rear array distance, level and EQ to improve the decay
6. Readjustment of the global EQ.

I did it from scratch today just for fun. Took me less than 1 hour.

I was referring to this

I have measured six of the subwoofers in the front array individually. These are labeled A1, A2, B1, B2, C1, C2 as shown on the diagram in the first post here. ... As the measurements show, there are very large differences in the response from these six different subwoofers.

I thought you might have applied EQ on an individual sub level. I remember reading a piece where they had a very small room, and a very dense DBA, where each sub was individually controlled, and I guess I had that in mind when I read your description. That said I am familiar with DBA but thought that perhaps the height of your room combined with the saddle shaped ceiling influenced the array, hence requiring variation in settings of individual subs.

 

[StigErik]

How do you cope with the vibration from the heavy diaphragms?

They subwoofers are suspended in rubber ropes and hung from the ceiling.

 

[Keith_W]

If I had a DBA I would set them to output the same signal. Then put a sealed empty plastic bottle in the middle of the living room and watch it get crushed. That would be pretty cool

 

[olieb]

Good question. I am thinking it may be because the main listening position is quite close to the rear array, and that we still have a plane wave from the rear array.

Anyway - my room is 5 meters wide. That gives us a standing wave mode at 343/5/2 = 34.2 Hz. The other fundamental modes are at 24.5 Hz and 57.2 Hz.

Sure, but modes create peaks. Dips are the result if there are no modes or the ones that are there are not coupled to speakers/LP ("null").
One can simulate the front (and rear) array quite nicely in REW if one uses the symmetry and models only half a room.

EDIT:
For the (front) array FR I placed the LP 3.1 m from the back.
With LP at 3.1 m from the front one gets the respective FR for the modeled rear array. Dip at 33 Hz is gone then.

 

[StigErik]

Modes also create dips at their nulls. First order modes will have nulls in the middle of the room.

 

[Sokel]

Yep.that does it.
Really-really good effort!

 

[Gadgety]

If I had a DBA I would set them to output the same signal. Then put a sealed empty plastic bottle in the middle of the living room and watch it get crushed. That would be pretty cool

You can combine the bottle crushing effect with the DBA through an inversion filter of the inversion to the rear wall below a certain frequency. Pressurize the room at the lowest frequencies for movie effects, while maintaining the DBA still in effect in the audible range.

 

[StigErik]

I am planning to do just that, by running the rear array in phase with the front below 20 Hz.

 

[ChrisG]

Cool project, and impressive results.

2. Calculate the transmission loss for the distance between the rear and front array in dB, and set this level for the rear array. 7 meters = ~8.5 dB (3 dB/doubling of distance)

Just a quick note that there is no transmission loss for a plane array, since the wavefront does not expand.
Both arrays should be at equal levels so that the wavefront from the front subwoofers can be 100% absorbed by the rear subwoofers.

Chris

 

[olieb]

Modes also create dips at their nulls. First order modes will have nulls in the middle of the room.

Then there would have to be a null for the rear array too.
A null in one mode creates a general null only when there are (almost) no other modes (that couple to positions). In a tube (1D) that is quite common, but in a room not so much.
And with the array the (lower) width modes are not excited at all because of the symmetry (that is the point of the array). So the null in width mode is filling the whole room. Same for the height modes.
The 33Hz dip is a result of the placement of the LP in length direction.

 

[StigErik]

The 33Hz dip is a result of the placement of the LP in length direction.

Yes, you are correct. The plane wave from the arrays will cancel the width and height modes, so the 34 Hz width mode would not be there anyway.

 

[Gadgety]

I read somewhere you use MG dipolar speakers, so side walls have less of an influence. I can imagine you have spectacular clarity. How well behaved is your room-speaker combination above 200 Hz? Have you measured IACC? Any plot for the complete spectrum?

 

[StigErik]

Cool project, and impressive results.



Just a quick note that there is no transmission loss for a plane array, since the wavefront does not expand.
Both arrays should be at equal levels so that the wavefront from the front subwoofers can be 100% absorbed by the rear subwoofers.

Chris

If that was true - we just invented a perpetuum mobile.

 

[StigErik]

I read somewhere you use MG dipolar speakers, so side walls have less of an influence. I can imagine you have spectacular clarity. How well behaved is your room-speaker combination above 200 Hz? Have you measured IACC? Any plot for the complete spectrum?

Dipole vs Box speakers

Ok—here is my first in room response curve sweep for the Enterprise Omnis. I used the mic in my iPhone for the test. I took the measurement from the centered listening position: Distances: Front wall: 6 feet Side wall: 4.5 feet Speaker separation: 6.5 feet Speaker To listener: 6.5 feet...

audiosciencereview.com

 

[Gadgety]

Wow, fabulous results.

 

[Flaesh]

(3 dB/doubling of distance)

This is true for a cylindrical wave, not for plane.