Cardioid Subs?

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I'm surprised that a search on this topic turned up nothing.

I always thought that you needed three subs to do this, but I guess I was wrong. After reading some info from Presonus on how they do it with only 2 subs, it's starting to sound like something to consider. It seems simple enough, using plate amps with DSP. (Or a processor with enough outputs)

Supposedly, all you have to do is put 2 of their powered subs side-by-side, face one backwards, then set one for out of phase, plus adding their pre-set delay to it. (Delay equal to the depth of the cabinet.)

(There's another method also, which supposedly is more effective, but it requires twice the stage depth in front of the band / DJ, so not something I'm interested in.)

So how come this hasn't been discussed to death on this forum?
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I have one primary concern with doing this, other than the obvious need to invest in new amps or whatever:

* Does it sacrifice any dB out front?
- Or more importantly, does it require more amperage to maintain the SAME dB out front that you had when both subs were omni?

It seems to me that all that phase-cancellation could possibly do this.
Or not:
If the direct-radiating power stays the same, but all you lose out front is the stuff bouncing off the back-of-stage wall, with all it's phase smear, then I could live with that.

Does anyone have a definitive answer?

Thoughts in general?

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FWIW, there is one other possible downside, for me at least: I always use earbuds for monitoring, as does my entire band. Most affordable earbuds don't have a lot of bottom, and even if they do, it's not bottom that you feel in your body, viscerally. I really LIKE having a lot of LF on stage!

So... I dunno if this would be a good thing or not.
 
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Cardioid subwoofers are not useful below the Schröder frequency as you are listening to room modes, so directivity loses its meaning. Neither can you speak of sound bouncing off the back-of-stage wall. Think of a room of a box that supports standing waves. What you hear depends on to which modes your ears couple and by how much these modes are energized by the subwoofer.


Yes it sacrifices output. You can calculate the loss from the spacing between the drivers and the frequency. You lose 6 dB / octave below some frequency that depends on the spacing. At home audio you easily lose 10 dB. At pro audio the frequencies are higher and spacing is larger, so they do not lose that much. Pro audio 'rooms' also are larger (or the event is outdoor) so the subwoofers play above the Schröder frequency, where directivity matters.
 
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When you start trying to cancel out sound, you automatically lose SPL. Think about it for a moment, and it should be perfectly obvious.

Cardioid sub setups usually have lots of compromises. Most will only give a good cardioid shape over a fairly narrow bandwidth. The ones that do well are things like end-fire arrays, which use lots of cabinets to do it well.

Chris
 
intrested on how this is achieved.
imho not possible ,yet here whe are.

whe always use endfire setup on the side of the stage centered around 40 hz 2 mtr spacing.
no loss in db in forward dirction ,and max lf rejection to the back.
whe us the monitors for lf on stage.
 
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Gradient-type cardioid is substractive method. It tries to remove radiation energy from the space around it. Leaving acoustic energy at the front is the side effect.
+ optimum pattern over a range of frequency
+ small physical distance over radiating components
- 6dB/oct efficiency drop below 'tuning' frequency
- requires large displacement drivers with high level of linearity
- bad time domain response, bad wavefront, step response is flipped over

Endfire-type cardioid is additive method. It tries to add radiation energy to the space around it. Removing acoustic energy from the rear is the side effect.
+ constant efficiency at the front which is the sum of efficiences of radiating components as monopoles
+ right time domain response, preserves wavefront coherency, unaltered step response
+ works well with any kind of driver
- optimum pattern over single frequency
- requires more distance between radiating components

Cardioid are only one discrete form of unipolar radiation. There more such a forms, supercardioid, hypercardioid, etc.
 
When you start trying to cancel out sound, you automatically lose SPL. Think about it for a moment, and it should be perfectly obvious.....

Chris

Yeah that's what I was suggesting, above. But the question is:

Are you losing USEFUL energy, or energy that isn't getting to the audience anyway (outdoors) or energy that is bouncing off the stage wall then back to the audience at diminished volume and with massive phase smear?

Re the third option, I don't get why @TBTL wrote, "Neither can you speak of sound bouncing off the back-of-stage wall." I'd love a clarification on that.

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It might be a bit like PPSL: Some folks like the diminished distortion, why others dislike it because the muddy distortion feels like volume out in the room.
 
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TBTL,

Any chance you could clarify that statement, above?
It would really help.

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Well, regardless, I guess this get's filed squarely under "no free lunch." :(

Even that Fullcrum sub that does it with a single cabinet (time-delay, resistive rear ports?) has pretty bad dB output.
 
I'm sorry for not having answered yet. I find it difficult to put my thoughts about this into words, so I hoped someone else could explain it. I'll try:
Mehlau.net said:
At frequencies below the Schröder frequency, modes become a problem. Our hearing needs a couple of cycles before it can determine a sound's timbre but in acoustically small rooms reflections become part of the sound almost instantly because wavelengths are in the range of the room's dimensions (e.g. 40 Hz = 8.6 m wavelength).
Multiple Subwoofer after Geddes

At low frequencies the sound indeed bounces off the rear wall, but your hearing cannot distinguish between the direct and reflected sound. Before you can interpret the sound, the sound has bounced several times between walls. Therefore it makes no sense to treat the direct and reflected sound separately. The reflections and the direct sound combine at your ears. Depending on the frequency the waves add constructively or destructively. This description about waves adding is equal to the desciption that starts with room modes, so room modes are a suitable way to describe what you hear.

A cardioid subwoofer does not send sound to the rear wall. The sound that is sent to the opposing wall does however reflect, then hits the rear wall, reflects etc. so the rear wall is not out of the equation. What you hear is different that with a monopole subwoofer. The description starting with room modes still applies, because a cardioid source couples to the modes differently. So this description can still be applied.
 
Well, regardless, I guess this get's filed squarely under "no free lunch." :(

Even that Fullcrum sub that does it with a single cabinet (time-delay, resistive rear ports?) has pretty bad dB output.

Yep, the Fulcrum subs look rather neat, but they're using the rear output of the driver to cancel out some of the front.
Conventional subs would aim to use the rear output to reinforce the output from the front, so it's pretty clear why there'd be a gap in SPL capabilities.

Those subs do fill a niche, though, and from what I've heard they're pretty effective.

I'd consider a dual-sub cardioid in the same boat, though - instead of using the second sub for more output, you're using it to cancel some of the output of the first. That means that, at best, you've got the approximate output of one sub for the audience, while carrying twice as many boxes.
Compromise, compromise...

Chris
 
One could boost output on front of conventional subs by "v-plating" or using "barn doors". I don't know if anyone actually does this in a band setting. Not very effective comparing to cardioid system, but doesn't require additional amp power nor much additional transport space.
Here is another idea :D Another Hopeless Attempt At Reverse Engineering a Danley Design
 
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Quote from the thread by P. Bateman:

"The Danley boxes are cumbersome, they're heavy, but they're DIRECTIONAL. And the whole audio market is moving in that direction, the era of cardioids has begun. But when you make a cardioid with DSP, you're just throwing away efficiency. Yes, they work, but there's a price.

So here we have another Danley solution, where you can use fewer speakers. YES, those boxes are very large, but I'll bet a cardioid with four of the BC subs is easily 15 decibels louder than a cardioid with four vented subs.

Basically either solution will work. But to match the output and directivity of four of the BC subs, you'll likely need something like eighty(!) vented subs and much, much more power, plus DSP for the whole lot."
 
I'd consider a dual-sub cardioid in the same boat, though - instead of using the second sub for more output, you're using it to cancel some of the output of the first. That means that, at best, you've got the approximate output of one sub for the audience, while carrying twice as many boxes.
Compromise, compromise...
Chris,

Saying that "at best" the output of twice as many boxes in cardioid configuration would equal the approximate output of one sub would imply a 6 dB forward loss, while the loss is more like 1-2 dB.

The rear output is cancelled due to being (largely) out of phase, while the forward output is (largely) in phase, constructive.

Art
 
Hi Art,

From what I've seen, that can be rather frequency-dependent. Since we're working with a fixed delay, towards the bottom of the passband you'll get more cancellation out front.

I didn't realise the forward loss was that small, though. Thanks for catching that.

Chris
 
One could boost output on front of conventional subs by "v-plating" or using "barn doors". ......

Doesn't that only work with horns, by changing the mouth impedance, or something?

I know some folks around here have, in the past referred to those as "wave guides," but AFAIK (which admittedly is not much :eek: ) they are not really physically guiding anything, just making the horn effectively longer, so more even throw at distance.

No? (I'm sure Art can answer this one.)
 
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I know some folks around here have, in the past referred to those as "wave guides," but AFAIK (which admittedly is not much :eek: ) they are not really physically guiding anything, just making the horn effectively longer, so more even throw at distance.

No? (I'm sure Art can answer this one.)

Whether you call a larger frontal area extension a "barn door" or "wave guide", it will increase forward gain in the same manner as any boundary does.
The gain is surprising (to me..) for the amount of boundary area increased, and not dependent on increasing horn length.
The frontal area of the stack makes a significant difference in response shape, adding “wings”, flat pieces of plywood (AKA “barn doors”) to the stacks add 2 to 3 dB to the LF response when the frontal area is tripled.

Multiple Cabinet Combined Response
 
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