#3
Postby Soundman2020 » Fri, 2025-Feb-21, 16:19
You are very right that soffit-mounting (or "flush mounting", if you prefer) changes the sound of the speaker, but it does so in a mostly beneficial way.
First, it extends the dimensions of the front baffle considerably in all directions, thus driving down the "baffle step response" frequency to a region where it isn't so problematic: that's the frequency where, due to the dimensions of the front baffle of the speaker, the sound field changes from being mostly projected forward (from the speaker's point of view) into the room, to the situation where longer waves "wrap around" behind the speaker, and go in the opposite direction. That, of course, reduces the power in the low frequencies: Hence, the control on the back of many speakers for compensating for this issue, by providing a boost or cut of (usually) up to 6 dB in those low frequencies. That takes care of the power imbalance, but it still doesn't deal with the issue of waves going the wrong way. The baffle of the soffit does deal with that, ... big time. So that's a good thing. Basically, you get better low-end response from your speaker, and all of the sound goes the right way; into the room, not wrapping around behind.
Next is the "edge diffraction" issue. The waves that do wrap around behind an free-standing speaker do not do so cleanly. There's a range of frequencies (about an octave or two wide) where the waves interact with the edges of the speaker, causing all kinds of strange interference patterns. Not a major issue, but still there. Hence, the shape of some speakers with rounded edges, or faceted edges, which reduce this problem. A soffit eliminates it completely.
There's also the issue of the sound being emitted "backwards" (behind the speaker, from it's own point of view, or towards the front wall of the room from the listening position perspective). That sound is emitted by the vibration of the speaker body itself, and perhaps also by bass reflex ports on the rear. Some of that sound can bounce off the front wall of the room, then head in the same direction as the main sound output, but out of phase at some frequencies (due to the distance between the wall and the speaker), thus causing interference patterns ("comb filtering") as it interacts with the main sound field. There's not much that the speaker designer can do about that. But the room designer can take it into account with careful placement of the speakers, if they are free standing. Or much better: he can mount the speaker in a soffit, which completely eliminates that.
There's a few other issues, but those are the three biggies.
So from that point of view, the soffit is a wonderful thing! It either totally eliminates or greatly reduces these three problems, and gives you a low end boost at the same time.
But as you point out, there's also some bad news. If the speaker has ports or active elements on the sides, top, or bottom, then it probably isn't a candidate for soffit mounting, as dealing with those ports is a nightmare. Ports on the back are not so much of an issue: both passive bass-reflex ports and active elements facing backwards can be dealt with. Since the sound coming out of those is confined inside the soffit, the usual way of dealing with that is a truck-load of absorption inside the soffit itself, as well as some form of light absorption inside the port. Done deal.
But do take into account that those ports or active elements were part of the design of the speaker! They were there to improve the sound of the speaker in some way. But if they are inside the soffit, they can't do that any more. On the other hand, what they were doing is usually trying to deal with the issues that the soffit itself is doing, and much better, so the change in sounds isn't really a problem. The difference can be compensated by digital tuning applied to the speaker, which is needed anyway for acoustic reasons in the room. The remaining issue here is loading: any bass reflex ports on the rear of the speaker provided some type of acoustic loading on the bass driver. Interfering with that can create issues, due to the change in loading. You CANNOT completely block off those rear-facing ports! That would not be good. What I normally do is to build the "enclosure" box that holds the speaker inside the soffit, with large holes in the rear panel, exactly where those bass ports are, so that they don't "see" a major change in loading. I also usually put a bit of very light absorption in the port itself, or just behind the port, to damp the action somewhat.
That usually takes care of the speaker acoustics. But there's still the issue of heat. Speakers get hot if you drive them hard, and they get even hotter when they cannot dissipate that heat, due to being enclosed in a massive wooden box, inside a heavily insulated larger wooden box. The solution here is simple: passive ventilation ducts. Have a large duct that is most of the width of the speaker, and at least an inch or two deep, coming up through the soffit under the rear of the speaker, and extending all the way up past the entire rear face of the speaker, up into the top of the soffit, and out into the free air of the studio. You don't normally need fans in there: convection will move enough air to keep the speakers cool.
This is one of the major reasons I prefer the Sorbothane mount system: you don't need a lot of surface area of Sorbothane to fully decouple it from the enclosure box, so you can leave a very large air gap all around the speaker, maybe half an inch wide all around it, with just a few Sorbothane pads in a few key places. That allows for even better air circulation all around the speaker. However, it does also create a minor issue of possible resonances inside, between the speaker itself and its enclosure box. Probably not a big deal, but a little light-weight insulation partially filling that air gap is sufficient to damp any resonance without blocking air flow. I often suggest the type of filter material is used on kitchen stove hoods, or furnace filters very light and fluff. Enough to do the job of damping some resonance, but sill allow air flow.
So those are the main considerations in soffit design. Hold the speaker firmly, put a very massive, very large baffle on the front of it, and have it all attached to a very massive,, strong frame of some kind, to keep it all rock steady. With the Sorbothane mount system, the speaker is free to move and vibrate on its pads (just as it would be if it were free-standing in the room), and the pads are tuned to isolate that vibration from the soffit itself. The air "chimney" provides cooling, and the insulation filling most of the internal soffit cavity provides the needed acosutic damping. The holes on the rear panel of the enclosure box allow the reflex ports to still do their thing, to a certain extent, and the digital tuning is the icing on the cake.
There are other ways of doing it, sure, but that's the method I prefer.
- Stuart -
Translate This Page
I'm also making some assumptions on books and threads I've read so far and am ready to be incorrect.
and test it. And document it. And tweak it. I've found that speaker designers - there's a whole community of them just like here - like to test and measure things very similarly to the way rooms are measured and tested and there's even crossover between some of the tools and software used.