# Double Bass Array

#### 63781

I believe this forum doesn't have a thread dedicated to Double Bass Array (DBA), so I'm creating one.

Wikipedia article about DBA :
https://en.wikipedia.org/wiki/Double_bass_array

(Images below are taken from the Wiki article)

To create a DBA we must set up an array of at least four loudspeakers on the front wall, and a similar one on the rear wall. The rear array must play in reverse phase to actively absorb the plane wave from the front wall.

A simple way to understand what a DBA does can be described like this:

- The front array creates a plane wave which should cancel standing waves in the height- and width plane of the room (up to a certain frequency)

- The rear array shall actively absorb standing waves in the longitudinal plane

The result of this is that no standing waves should be exited, and you get a frequency response that is completely flat, and a decay without resonances. In theory at least. With some tweaking, it's possible in practice as well.

There are requirements and limitations of course:
• The room should be rectangular with a flat ceiling (relatively easy to accomplish)
• The room should have very stiff surfaces. Concrete is good. Japanese paper walls are not.
• Apart from the loudspeaker arrays, the room should ideally be empty. Any objects disturbing the wave fronts will alter the result.
• Digital delay and EQ is required
• Measurements during setup is required

My own room doesn't meet any of these requirements. Even so, I was able to create this result with a lot of tweaking. The decay is very short down to about 30 Hz, where the neighboring room starts to contribute to the decay we see on the measurements here.

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#### wafflesomd

That is an very lovely low end response there.

I wasn't aware of this sort of array years and years ago when my friend and I would throw house parties with silly amounts of pa bass bins in small rooms. We always had trouble getting low end coverage in small spaces, I mean come on if people see a big subs they expect to hear and feel them.
Thought some testing we found putting subs on both ends of a room and inverting the phase of one group pretty much solved all our problems. I ended up taking things further and would hire out pa rig to select electronic events and I employed that sort of technique in spaces where I was able to with great success. Kinda cool to see that there some rhyme to our reason.

#### 63781

There are a few steps to go through to set up a DBA properly :
1. Physical layout - how to place the loudspeakers in the arrays. The wikipedia article has all the answers here.
2. Set the correct delay for the rear array
3. Set det correct level for the rear array
4. EQ everything

#### AllenB

Paid Member
Considering the realities of a room the inverse delayed signal may not always be optimum, hence your suggestion to keep things in the room neat.

Could active absorbers, ie a noise cancelling setup on the far wall sometimes be the right choice?

#### 63781

The real trick is to EQ the rear array to get the waterfall as clean as possible.

#### 63781

Placing your listening position in the middle of the room is usually a bad idea. This is where you will get the worst bass response. With a DBA however, it does not matter. In fact, the middle of the room may be the best location.

Here's my room layout. As you can see, the room is rectangular, but the ceiling is not flat. I also have a lot of acoustic tuning and large dipole panel speakers in the room.

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#### 63781

As you probably can see from the room layout drawings above, I have twelve subwoofers in each array. They are SEAS L26ROY in small closed boxes, about 35 liters each. This gives us this response:

Since the subwoofers are placed close to the front and rear walls there is a lot of room gain, so the resulting response do look a lot different. I will show some measurements a bit later.

#### 63781

The amps powering all this are three ICEpower 2000 AS2, one for the rear array and two for the front array. The rear array is lower in level than the front, and needs less power.

Delay, level and EQ is done with a MiniDSP Flex. I will show all the DSP settings later.

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#### 63781

The first thing we need to do when configuring the DSP, is to set the correct level and delay for the rear array. This can be a bit tricky, and may require some adjustment under the way. A good rule of thumb is to set the delay equal to the path distance between the front array and rear, including the reflection off the rear wall. Like so:

In my case, I set it to 21.5 ms, which is equal to 7.18 meters. My room is 7 meters long.

The level must also be experimented with, but if you calculate the SPL loss for your distance, you won't be far off. I ended up with -7 dB on the rear array.

Also, the rear array must have the phase inverted. This can be done in the DSP or simply by switching + and - on the speaker terminals.

#### 63781

Measurements !

Here's the raw response of my arrays after setting level and distance. As you can see, EQ is required. But there is hope... and no large dips.

And here's with (a lot of) EQ. No smoothing on this measurement.

1 user

#### kipman725

Paid Member
Great results, what do you think is causing the peak in the un-equalised response at 45Hz?

#### 63781

The second longitudinal room mode I think.

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#### Frank207be

Very impressive DBA It's probably built into a dedicated home cinema room without annoying obstacles like windows and furniture?

#### 63781

It seems like the home cinema crowd is more concerned with their low end than those bound to 2-channel music playback?

I’m a bit of both - so my room and setup was built to offer the best possible sound, regardless of source type, and with high-end picture as well (4K projector and 130” screen).

To answer @Frank207be ’s question : there is no other furniture in the room than a 3-seater sofa and two small tables. However, there are som very large speakers in the room that will obstruct the wavefront from the DBA. No windows, just a door.

#### mr ed

Thanks for sharing this information.
I just finished two subs for my system (music only no HT).
My intention to build one or two more for multiple sub array.
With this info I can try this method. 🙂

1 user

#### mvs0

Placing your listening position in the middle of the room is usually a bad idea. This is where you will get the worst bass response. With a DBA however, it does not matter. In fact, the middle of the room may be the best location.

Here's my room layout. As you can see, the room is rectangular, but the ceiling is not flat. I also have a lot of acoustic tuning and large dipole panel speakers in the room.

View attachment 1189967

View attachment 1189966
Wow! So if I count right that's 24 subs? (each sub position in the top view has 3 subs on different heights?)
I assume you run bass in mono?

#### 63781

Yes, there are 24 subwoofers.

A DBA will always be mono, so it's some kind of compromise and it needs to be crossed over not much higher than 100 Hz.

#### planet10

Paid Member
To create a DBA we must set up an array of at least four loudspeakers on the front wall, and a similar one on the rear wall. The rear array must play in reverse phase to actively absorb the plane wave from the front wall.

So it only works at frequencies where the spearation of the walls is less than the quarter wavelength?

dave