Benefits of a cardioid subwoofer?

Thanks RWW, Marty, and Duke. I have my suspicions about how well these subwoofers would work in a listening room. They are designed as professional audio subs, so the designers probably had a much larger room in mind than my poky little listening room. Maybe I will look at the larger JL's instead.

Duke,

Thanks for clarifying, but that raises another question:

Both waves will still be omnidirection. So, the wave from the forward woofer goes outward (not forward), there is a computed delay, and then the back wave goes outward (not forward). This does not look much like the behavior of a di-pole at higher frequencies, and doesn't compute to a cardiod pattern, either.

What am I missing?

Marty

"...What am I missing?"

The time delay on the rear woofer corresponds to the time it takes for the output of the front woofer to wrap around the cabinet and reach the rear woofer. So the reversed-polarity rear woofer "fires" just as the output from the front woofer reaches it, and this way cancellation occurs behind the sub, rather than off to the sides, as with a dipole. The rear woofer's output might also be attenuated a bit relative to the front woofer, I dunno... never actually done it.

Duke

Duke,

So the null is behind, rather than beside the cabinet (normal dipole) - I'm not sure how that plays out. If the sub is in free space, there will be less/no subsequent reflection off the wall behind the woofer which should reduce random destructive interference from that reflection - yes? But can't you drive that cancellation frequency up in pitch and down in amplitude by placing the woofer flush to the rear wall, anyway?

Is the idea basically to allow placement away from the rear wall, or is there another benefit here?

This is kind of a brain teaser for me - I can't visualize the impact that the designer is attempting to generate.
Sorry if I'm a bit slow on the uptake here, but any explanation is - as always - appreciated.

Marty

Cardioids and dipole sub-woofers couple differently to room modes than monopoles in the same location. This works very well with dipoles and the height mode.

Cardioids implemented with an acoustic delay and dipoles don't benefit or suffer from room gain (a 12dB/octave boost below the room's fundamental resonance assuming an infinitely rigid room) which is one source of boomy bass in small rooms.

The disadvantage to cardioids implemented with an acoustic delay (this is the case for the BASIS 11K sub-woofer, which has one driver plus an output slot on one side) and dipoles is that maximum output decreases 6dB/octave compared to a monopole which is already decreasing 12dB/octave. In a domestically friendly enclosure you get 18dB less output at 20Hz which is the difference between sufficient and inaudible.

You can also tame room modes with multiple conventional sub-woofers without this drawback. You can even eliminate eliminate room gain and fix the height mode problem with a pair of sub-woofers at each room corner wired with opposite polarity.