I thought the idea of the sorbothane is to isolate speaker vibration from the mounting frame, which would flank into the ceiling, etc.
That's the basic concept, yes. Actually, it is one of two basic concepts. The other one (which also works well) is to clamp the speaker in place so rigidly, and with so much mass, that it also has a hard time transferring sound into the structure. To do that method, you don't use rubber at all: you make your enclosure box very tight fitting, and you build a massive, rigid, heavy frame to hold that all in place, very tightly, as though it were held in a large vice. That can work too, and John prefers to do his rooms that way, so it CERTAINLY is a good option if you want to try that. Personally, I prefer to "float" the speaker the way you describe: fully isolated and decoupled from the soffit. It just sits there surrounded by its rubber pads, and it is free to vibrate any way it wants, but it cannot transmit any of that vibration to the structure, because it is decoupled, floated. The floating system is tuned so that it's highest resonant frequency is still an octave lower than the lowest frequency of concern. You need that much to ensure that there won't be any significant transmission, even at low frequencies. Thus, if your lowest frequency of concern is a four-string bass, which goes down to about 40 Hz, then you could get away with tuning the system to 20 Hz. But if you plan on using a six string bass, which goes down to about 31 Hz, then you should tune it below 16 Hz. Of course, that assumes your speaker can actually get down to 31 Hz! If your speaker only gets down to 50 Hz at -6dBm then it won0t be putting out a lot of energy at 31 Hz. If you needed to mix like that, you'd probably add a sub with a cross-over then you wouldn't need to worry about tuning the floating system so low. So a lot depends on YOUR needs in YOUR room: the spectrum you expect to mixing, your speaker make and model, whether or not you use a sub, etc.
However, since the speaker can move, very slightly, with the floating system as it vibrates, you do need to leave a small gap around the front of the speaker, between the edges of the baffle and the speaker cabinet itself. That allows the speaker to vibrate if it wants to, without touching the soffit baffle, which would transfer the vibration into the baffle. Some people seal that gap with very flexible caulk, but I have a different method. In truth, unless you are going for extreme precision, you don't really need to seal it at all, as long as it isn't very big. A couple of mm is fine, but if you make it larger than that, then you can get air movement through that gap, due to the pressure changes around the speaker itself inside the soffit, as compared to the pressure changes outside the baffle, due to the woofer cone moving. If the gap is big, you can get a lot of air flow "luffing" through that gap, which can create an audible sound, and also mess with the acoustics in other ways. If the gap is small, it's not much of an issue. And if it IS an issue, then you can seal the gap and kill the problem.
I have been seeing the whole surrounding cavity (Front and sides - insulation, bass trap hangers, ventilation) as the method of enclosing and damping the sound from the back hemisphere coming out from the speaker.
That's about right, yes. There will ALWAYS be potential resonances inside ANY cavity, so you need damping in there to deal with that, definitely. It's a large box, so the resonances could potentially go down to lowish frequencies, thus the need for shiploads of insulation inside. But you also need to cool the speaker, so you do need to allow a suitable airflow path up the back and around the speaker body. That's another reason I prefer floating: if the speaker is clamped rigidly, there's no airflow around the sides, top and bottom, but with the floating system, there is. There's space between the pads for air to move, in addition to a much deeper space at the rear.
Can you clarify better what the goal is and how it works? I don't think it has quite sunk into my brain completely just yet.
To me, it sounds like you have it about right. Did I say something that confused you, and led you to think that there's something else going on here?
Maybe what you are missing here is that the speaker WANTS to move, and WILL move, due to the vibrations. The woofer is a rather large mass that is chugging in and out over a large distance: it has inertia, and "for every action there is an equal and opposite reaction", so the speakers cabinet wants to move as well, in the opposite way (180° out of phase). The cabinet itself is also vibrating in many different ways, and the rubber ALLOWS that movement to happen... it just prevents it from getting into the soffit structure or the baffle. The movement is not large, of course, if you do the mounting correctly, but it is still there, and you do need to consider it. If you do not restrain the TOP of the speaker, only the bottom, then the top will move MORE than the bottom, and that's a problem. So you need to restrain the speaker with Sorbothane pads all around, not just underneath. For a speaker sitting out in the room on a speaker stand, or the meter bridge, it doesn't matter, since there's no baffle around the speaker for it to "bump into", so it's fine to just have rubber feet for decoupling in that case. But soffit mounting is different, because there IS a baffle, and you have to prevent the speaker from ever coming into contact with it.
- Stuart -
