Snickers-is
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I think this is a pretty intersting discussion. It is probably a good time to refer to common practices in professional audio. Even if cardioid systems are not that popular due to added cost per SPL, they are commonly known for improving the overall performance of bass, even indoors. I see Bjorn is refering to cardioid as being inferior, but at the time, we do not know of any other way of controlling dispersion at low frequencies. There are horns, commonly referred to as horns, but realisticly being transmission lines as sub frequencies will require the entire wall (or more) to be the horn opening, but they tend to not do much good in smaller rooms as they simply outsize the room itself.
One way to look at this is when we study the room modes, in particular the location of the nodes. Lengthwise in the room they are everywhere but near the walls. Near the walls we only have peak pressure zones for all modes. If we place a dipole in a room, it will stimulate all nodes that partially or fully falls in the same location (and direction) as the dipole, so the dipole will contribute heavily to these nodes and amplify their respective modes. On the other hand, if we place a closed cabinet subwoofer at the wall, it will stimulate this peak pressure zone for all modes. A closed box located in a node may sound like a good idea for that frequency, but really, it just sends sound both forward and backwards at the same time, stimulating the same modes, just a bit differently than the dipole, and sometimes not fully as pronounced.
But what if we place a dipole very close to the wall? For example, lets imagine we have a dipole within 0,5 meter from the wall. If we allow for a node to be at 1 meter from the wall, we could let it play up to 85Hz and it should contribute very little to the modes in its working range. But having a dipole at below 85Hz this close to the wall will make the back side completely over power the front side. Also it will not be very efficient unless it is pretty large. However, if we calculate an active cardioid subwoofer with the correct distance between the drivers, we should be able to maintain about the same distortion properties as the same driver just placed in a closed cabinet, with the addition of significantly lowered distortion at higher frequencies.
Due to the immediate wall reflection we would only need very little power for the rear driver(s). At the same time, the rear driver would really be the one thing that makes this different from "just another subwoofer" as it will be placed right at the pressure max of all longitudinal modes in the room and will combat every single one of them.
Practical tests show that using cardioids at low frequencies do change the frequency response of the room, but it also cleans up the tonal clarity. One can say that this is also possible to achieve with a closed or bass reflex sub as well, and I totally agree, one just needs a bucked load of good luck, which is probably why some people try to minimize the pain by purchasing expensive cables instead. But just placing a cardioid speaker randomly in the room will not give any guaranteed results either, it is just that you need so much less luck to hit the target.
And while horns and cardioids are comparable at higher frequencies, this is certainly not the case at sub frequencies in smaller rooms.
Lastly I would like to add that this is not a solution to make room acoustics obsolete, but then again, corner traps, pressure based bass panels, helmholz resonators with 1000sqcm opening etc, they are really not very effective for sub frequencies. Bass traps are resistive and while they can stop flow, they do very little to remove the pressure from the corner or wall. Pressure based panels can be golden, if you have another room on the other side, but typically they are placed on a wall, and the pressure has nowhere to go on the other side. A typical wall is maybe 7,5 square meters, and a pair of pretty beefy helmholz resonators with 1000 square centimeter openings will partyally dampen just a fraction of that pressure wave, and only at one single frequency.
One way to look at this is when we study the room modes, in particular the location of the nodes. Lengthwise in the room they are everywhere but near the walls. Near the walls we only have peak pressure zones for all modes. If we place a dipole in a room, it will stimulate all nodes that partially or fully falls in the same location (and direction) as the dipole, so the dipole will contribute heavily to these nodes and amplify their respective modes. On the other hand, if we place a closed cabinet subwoofer at the wall, it will stimulate this peak pressure zone for all modes. A closed box located in a node may sound like a good idea for that frequency, but really, it just sends sound both forward and backwards at the same time, stimulating the same modes, just a bit differently than the dipole, and sometimes not fully as pronounced.
But what if we place a dipole very close to the wall? For example, lets imagine we have a dipole within 0,5 meter from the wall. If we allow for a node to be at 1 meter from the wall, we could let it play up to 85Hz and it should contribute very little to the modes in its working range. But having a dipole at below 85Hz this close to the wall will make the back side completely over power the front side. Also it will not be very efficient unless it is pretty large. However, if we calculate an active cardioid subwoofer with the correct distance between the drivers, we should be able to maintain about the same distortion properties as the same driver just placed in a closed cabinet, with the addition of significantly lowered distortion at higher frequencies.
Due to the immediate wall reflection we would only need very little power for the rear driver(s). At the same time, the rear driver would really be the one thing that makes this different from "just another subwoofer" as it will be placed right at the pressure max of all longitudinal modes in the room and will combat every single one of them.
Practical tests show that using cardioids at low frequencies do change the frequency response of the room, but it also cleans up the tonal clarity. One can say that this is also possible to achieve with a closed or bass reflex sub as well, and I totally agree, one just needs a bucked load of good luck, which is probably why some people try to minimize the pain by purchasing expensive cables instead. But just placing a cardioid speaker randomly in the room will not give any guaranteed results either, it is just that you need so much less luck to hit the target.
And while horns and cardioids are comparable at higher frequencies, this is certainly not the case at sub frequencies in smaller rooms.
Lastly I would like to add that this is not a solution to make room acoustics obsolete, but then again, corner traps, pressure based bass panels, helmholz resonators with 1000sqcm opening etc, they are really not very effective for sub frequencies. Bass traps are resistive and while they can stop flow, they do very little to remove the pressure from the corner or wall. Pressure based panels can be golden, if you have another room on the other side, but typically they are placed on a wall, and the pressure has nowhere to go on the other side. A typical wall is maybe 7,5 square meters, and a pair of pretty beefy helmholz resonators with 1000 square centimeter openings will partyally dampen just a fraction of that pressure wave, and only at one single frequency.
