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Trying to work out how speakers which are silent from the rear work -- I.e. they have no bass spill out the back. I have recalled someone saying something about some clever technology, but I'm unable to find it.

  • Is this regarding "cardioid" subwoofers? – user9881 Dec 4 '14 at 1:39
  • I've been trying to find the correct term and failed at that, hence the question. A subwoofer with a cardiod pattern would do (how does that work?) but I've also heard of speakers that cancel bass behind them -- obviously treble isn't an issue since that's quite directional. – David Boshton Dec 4 '14 at 9:15
  • Never heard of, I too wonder if you can give an example? Quite frankly, to my ears it sounds impossible - bass travel like water through everything it can squeeze itself through, but if there actually is a way to direct it, it could be extremely useful in recording-sessions! – Christian van Caine Dec 4 '14 at 9:33
  • check out meyersound.com/products/mseries/m3d-sub -- they have a cardiod pattern and are marketed as directional. – David Boshton Dec 4 '14 at 14:01
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The M3D sub you mentioned includes a technical description of the function directly on the product page.

A pair of 18-inch drivers is mounted at the front of the cabinet, and works in conjunction with a pair of 15-inch drivers facing the rear and is driven by a sophisticated phase manipulation circuit. The resulting directional pattern assures that very low-frequency energy does not spill onto the stage or cause excessive reverberation.

DoritoStyle's answer hit this squarely on the head, but I wanted to offer some more expansion on the concepts. Basically you put a set of speakers behind the main speakers. Since you know the distance from the front speaker to the back speaker, you can calculate the delay needed for sound to come out the second speaker such that it is the inverse of the sound just reaching it from the main speaker.

This inversion will effectively cancel out the sound heading directly backwards and destructively interfere with the sound heading out in other directions. By setting the speaker distance carefully, they can make it so that the amount of destruction heading backwards is not as bad as the destruction going forwards (when the sound from the second speaker reaches the main speaker with a constant frequency being produced.)

It is worth noting that outside of a relatively small frequency band, this technique starts behaving quite erratically as you move through frequencies that don't align as well from to the distance between the main and secondary speakers. You could probably process it such that it still cancels going backwards, but at certain frequencies, that will also produce varying levels of cancellation going forwards as well. You could counteract that variation by providing less cancellation at certain frequencies, but then you will get a really weird sound out of the back of the speaker, with some frequencies almost unaltered.

  • So the "cardiod" pattern really only applies at a certain set of frequencies? – David Boshton Dec 5 '14 at 16:07
  • Well, you can make the proper sound to play going backwards to cancel out any frequency if you want, but some frequencies will also destructively interfere going forward unless you reduce the level of cancellation. – AJ Henderson Dec 5 '14 at 16:12
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"Cardioid" low end speakers (usually subwoofers in my experience) are usually 2 (or more) drivers in a single enclosure which are intentionally fed signals that are delayed and out of phase with each other in a certain way to cancel out sound in the direction of the back of the cabinet.

This approach sacrifices power for directionality in order to reduce sound bleed to unwanted areas.

You can achieve similar results with individual enclosures with some processing.

Math to follow soon.

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    "A pair of 18-inch drivers is mounted at the front of the cabinet, and works in conjunction with a pair of 15-inch drivers facing the rear and is driven by a sophisticated phase manipulation circuit. The resulting directional pattern assures that very low-frequency energy does not spill onto the stage or cause excessive reverberation." - from the product description of the M3Ds linked on the question. Sounds like you hit the nail on the head. +1 – AJ Henderson Dec 5 '14 at 15:55

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