A force canceling subwoofer for implementing DBA's ( and Trinnov variant) around the periphery of a videowall based on the Purifi PTT10.0X04-NAB-01.

[Impossible]

Silly question probably but if the woofers are facing each other don't you get phase issues, cancelations or some comb filtering? Has the air speed coming out of the mouth been considered?

Paradigm sub2 subwoofer is also force cancelling and also using 10" custom designed for woofers but the woofers are facing out. It can do 7hz but not sure what spl. It uses 6 drivers. The internal volume is shared between the drivers.
Force cancelling defiantly works as they demoed it when it first came out with a coin staying balanced on it when playing loud.

 

[kemmler3D]

if the woofers are facing each other don't you get phase issues, cancelations or some comb filtering?

For sub frequencies this isn't usually an issue, because the waves are so long that they are still substantially in-phase when they meet, so you get constructive interference instead of destructive.

Consider the wavelengths and you'll see why it's not a problem for subs:
(copied from Fabfilter.com)

You will also see why it's a huge problem for mids and tweeters.

 

[Impossible]

For sub frequencies this isn't usually an issue, because the waves are so long that they are still substantially in-phase when they meet, so you get constructive interference instead of destructive.

Consider the wavelengths and you'll see why it's not a problem for subs:
(copied from Fabfilter.com)

View attachment 388529

You will also see why it's a huge problem for mids and tweeters.

Thank you, this is very helpful.
At what frequency do you think it starts to be a problem?
Is there any science/maths behind it?

 

[kemmler3D]

Thank you, this is very helpful.
At what frequency do you think it starts to be a problem?
Is there any science/maths behind it?

Definitely full science behind this, here's the relevant Wikipedia page: https://en.wikipedia.org/wiki/Wave_interference

The sound will cancel out when it converges and is half a wavelength out of phase... for 20hz you can put subs pretty far apart and it's not an issue.

For 200hz if you about 1m apart the waves start to cancel out fully, and you get some degree of cancellation before that.

 

[Impossible]

Ah OK for this design we can do 800hz and it's fine?

 

[NTK]

Ah OK for this design we can do 800hz and it's fine?

If you don't care that the sources may be localizable above ~80 Hz.

 

[Impossible]

If you don't care that the sources may be localizable above ~80 Hz.

Disregarding localisation for now. I am just trying to clarify in regards to cancellation and comb filtering.

 

[kemmler3D]

Disregarding localisation for now. I am just trying to clarify in regards to cancellation and comb filtering.

In this design the woofers are 12" apart or 0.34m. Full cancellation would happen at 1/2 wavelength which gets us about 2Khz. About 500hz. Thanks @SIY .

In this case since they're being used as subs (probably no higher than 150-200hz at most) they are pretty far from the frequency range where cancellation is a concern.

Comb filtering describes what happens with wider frequency ranges, you see periodic notches in the frequency response everywhere the wavelength is a multiple of the distance between the sources.

With subs like this, the frequency range just isn't wide enough for that to start happening, at least due to driver placements. From 1khz - 20khz it can happen a lot.

This is the reason that the distance between the woofer and tweeter on 2+ way speakers is so important.

 

[SIY]

Full cancellation would happen at 1/2 wavelength which gets us about 2Khz.

I confess that I don't follow that. 1 kHz is about 12", so a half wave foot would be a two foot wave or 500 Hz. Or am I getting things backwards?

 

[kemmler3D]

I confess that I don't follow that. 1 kHz is about 12", so a half wave foot would be a two foot wave or 500 Hz. Or am I getting things backwards?

this is what I get for trying to do mental math the day after a healthy dose of red wine. Thanks for the clarification, I got things backwards.

 

[Impossible]

In this design the woofers are 12" apart or 0.34m. Full cancellation would happen at 1/2 wavelength which gets us about 2Khz. About 500hz. Thanks @SIY .

In this case since they're being used as subs (probably no higher than 150-200hz at most) they are pretty far from the frequency range where cancellation is a concern.

Comb filtering describes what happens with wider frequency ranges, you see periodic notches in the frequency response everywhere the wavelength is a multiple of the distance between the sources.

With subs like this, the frequency range just isn't wide enough for that to start happening, at least due to driver placements. From 1khz - 20khz it can happen a lot.

This is the reason that the distance between the woofer and tweeter on 2+ way speakers is so important.

Thanks this clarifies it.

 

[NTK]

Here is how the radiation pattern of 2 point (compact) sources separated by 1 ft (δ=0.305 m) would evolve from 40 Hz to 2560 Hz in free space (anechoic condition).

 

[Impossible]

Here is how the radiation pattern of 2 point (compact) sources separated by 1 ft (δ=0.305 m) would evolve from 40 Hz to 2560 Hz in free space (anechoic condition).

View attachment 388559

Nice visual, even untill 350hz seems fine.

 

[howard416]

With this much space, I'm wondering if it you would be able to do a force-cancelling DBA using FLHs. Treating the horn mouths as the "drivers". You would have fewer DBA units but when you're talking about 7 Hz you don't need a particularly dense pattern.

Obviously there is significantly more propagation delay but if the video sync is reliable (and playing only pre-recorded media) then can't see why that would be an issue.

 

[CINERAMAX]

Looks like a neat setup plan.
Is there a specific reason why you would use this purifi driver contraption instead of using just bigger sub drivers? Scan Speak drivers for example are really popular for DBAs here.

And is the second center channel really necessary?
If you're using a cinemascope LED wall that's on eye level then the upper/lower border of the screen is only about 12 degree off.
The center image shouldn't be that much off and Trinnov also has this position remapping AFAIK. A second center could potentially be even more harmful to useful.

Both points are valid. Given that Perlisten is releasing a force-canceling subwoofer, I've halted my own development. A single center channel placed below a scope screen at sofa height performs superbly, as I was able to test at home after installing my first videowall scope screen. The REVOX center channel works great.

 

[Salt]

Here is how the radiation pattern of 2 point (compact) sources separated by 1 ft (δ=0.305 m) would evolve from 40 Hz to 2560 Hz in free space (anechoic condition).

View attachment 388559

Is it under security control which program is used for this simulation?

 

[NTK]

Is it under security control which program is used for this simulation?

I wrote a script in Mathematica for this particular one. It is not too complicated to port the script to Python or Octave.

 

[Salt]

I wrote a script in Mathematica for this particular one. It is not too complicated to port the script to Python or Octave.

"Sat down and wrote a script and after 5 min had the result" ... is an other universe for me.
Not that I would not/could not be able to learn some 'language', but my mind refuses to learn one or more non human language.
Sigh.

 

[NTK]

Not 5 minutes. May be 45 on a good day

 

[Keith_W]

I have "force cancelling" subwoofers that I designed myself.

The force cancelling effect does not work. Music with a lot of bass can make these subwoofers go walking. I am not sure what I did wrong!