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How To......Cardioid Sub Arrays
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jeddere View Drop Down
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Points: 75 		Post Options Post Options   Thanks (0) Thanks(0)   Quote jeddere Quote  Post ReplyReply Direct Link To This Post Posted: 09 May 2022 at 5:23pm
Originally posted by dymondaudio dymondaudio wrote:

So… flip the centre sub, reverse its polarity and delay it by the distance between the rear driver and the drivers in front. 

Is this it or have I missed something? (Seems too simple!)

That is essentially it however the required delay will not be exactly equal to the measured direct path difference. Ideally you would setup up a mic behind the speakers (where you want the cancellation to occur) and measure the forward facing speakers to get a phase response. Then measure the rearward speaker (polarity reversed) and adjust the delay until they are 180 degrees out of phase. Alternatively stand some distance behind and adjust the delay until it is quietest. I've not tried this but I'd start with the measured path difference and increase from there.

Summation on the front side is not going to be perfect but you are sacrificing sound quality out front to get the reduction behind.

EDIT: to avoid confusion my reply relates to a inverted type of cardoid bass array which it seems you are proposing. Other posts earlier in this topic relate to different types of cardoid bass array.


Edited by jeddere - 09 May 2022 at 5:59pm
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dymondaudio View Drop Down
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Points: 1464 		Post Options Post Options   Thanks (0) Thanks(0)   Quote dymondaudio Quote  Post ReplyReply Direct Link To This Post Posted: 09 May 2022 at 7:16pm
Originally posted by jeddere jeddere wrote:

Originally posted by dymondaudio dymondaudio wrote:

So… flip the centre sub, reverse its polarity and delay it by the distance between the rear driver and the drivers in front. 

Is this it or have I missed something? (Seems too simple!)

That is essentially it however the required delay will not be exactly equal to the measured direct path difference. Ideally you would setup up a mic behind the speakers (where you want the cancellation to occur) and measure the forward facing speakers to get a phase response. Then measure the rearward speaker (polarity reversed) and adjust the delay until they are 180 degrees out of phase. Alternatively stand some distance behind and adjust the delay until it is quietest. I've not tried this but I'd start with the measured path difference and increase from there.

Summation on the front side is not going to be perfect but you are sacrificing sound quality out front to get the reduction behind.

EDIT: to avoid confusion my reply relates to a inverted type of cardoid bass array which it seems you are proposing. Other posts earlier in this topic relate to different types of cardoid bass array.

Thanks for this :)

To clarify regarding summation at the front, as long as I'm not going to loose output at the front then it's fine.  Main concern is rejection behind the system.
If you judge a fish by its ability to climb a tree, it will live its whole life believing that it is stupid
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jeddere View Drop Down
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Points: 75 		Post Options Post Options   Thanks (0) Thanks(0)   Quote jeddere Quote  Post ReplyReply Direct Link To This Post Posted: 10 May 2022 at 10:14am
Summation at the front will be roughly 1.5 dB below what you would have if you faced all 3 cabinets forward, with up to approx. 6 dB ripple in the response as some frequencies will be closer in phase than others when comparing the signal propagating forward from the rear facing and front facing cabinets.

The delay time that you set needs to be long enough so that you do not get cancellation out front at the lowest frequency your cabinet will play. This means making sure that the signal out front from the rear facing and forward facing cabinets has a phase difference of less than 120 degrees either side of 360 degrees. For example:
Cabinet low cut off: 30 Hz
Period (time taken for 1 wavelength of 30 Hz), T=1/f = 33 ms
120 degrees at 30 Hz = (120/360) x 33 = 11 ms
The rear signal is 180 degrees out of phase (polarity flipped) = (180/360) x 33 = 16.5 ms
When this combines with the signal from the front facing cabinets it needs to be at least 33-11 = 22 ms behind them. This will then sum with the signal from the front facing cabinets, but 1 phase wrap later.
There is already 16.5 ms of delay, or offset due to the polarity flip, so only another 22-16.5 = 5.5 ms is required.
The rear facing cabinet is delayed twice by the time its signal combines on the frontside, once by the delay setting in LMS and once by the physical path length. Therefore the minimum delay setting is 5.5/2 = 2.75 ms.

TLDR: If you end up with a delay setting for the rearward facing cabinets of less than about 3 ms then the bottom end of the bass frequencies are not going to sum as well as they could out front. Solution: move the rear facing cabinets backwards to increase the physical path length delay to about 3 ms.


Edited by jeddere - 10 May 2022 at 12:50pm
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