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That's correct, yes. And much of that is in the low frequency realm. Mids and highs are already in half space, even for a totally free-standing speaker in empty air. It's the lows that soffits are good for.According to Floyd E. Toole's book 'Sound Reproduction', mounting a speaker inside a wall, flush with the surface, results in a 2Π half-space radiation.
More or less, yes. I wouldn't say "more focused"... Tighter and cleaner, yes, but the soffit doesn't really focus sound: it just prevents the low end from wrapping around, as you mentioned. There's an equation for calculating the "baffle step response", which basically tells you at what frequency that "warp around" starts happening for any give baffle dimension. Or rather, the equation tells you where the center frequency is: the 3db drop point. That can be useful sometimes, in figuring out how your speakers are doing now (at what point they start "wrapping"), and how that will change with your soffits in place. The concept of a perfect soffit is that it emulates an "infinite baffle": one for which the step response happens at zero Hz. Theoretically, of course: not possible in real life, The closes you can get (and some people actually do, for testing), is to bury your speaker in the ground, facing upwards, flush with the surface.... in the middle of a very large field! In real life, you are limited by the dimensions of the room, and other factors, so your soffit baffle won't be perfect. But the general rule is to make it as wide as you can, to get the step response frequency down as low as possible, and hopefully much lower than the "cut off" frequency for the speaker itself. If you can achieve that, then you have the basis of a good soffit design. There's some other tricks that you can use in small rooms that don't allow large baffles...As far as I understand it correctly, this means that the low end becomes more directional towards the front because the LF produced by the speaker can't 'fold around' the speaker because of the wall/baffle. This results in less/no SBIR and a +6dB output of LF. Hopefully I've got this right.
Yup! Quite a bit, actually. Even more so if your speaker is rear-ported, or has any type of rear radiation element on it (active or passive). But all of that is BEHIND the baffle, and there should be abundant trapping in there anyway, so it's not an issue. It won't make it back out into the room.My question is: Does a monitor-speaker 'leak' any LF through the back of the speakerhousing/cabinet?
Not an issue at all, even with quite large speakers. The baffle itself does a good job of keeping the stuff in front where it belongs: in front.... and also keeping the stuff in the rear where it belongs.... in the rear. Glenn's design is a good one, and works well. I use a similar concept in most of the rooms I design where the speakers are soffit mounted.And if so, are these amounts of leakage enough to be reflected of the shell-wall, back through the LF trap, back in to the room in case of Glenn's design with a relative large opening below and above the baffle?
I just read through that, and I didn't see any place where it implied that the soffit should be sealed. He only mentions seals around the hole where the speaker pokes through, but not that the entire soffit needs to be sealed.I was under the impression that the baffle should be an airtight/closed construction.
A cavity doesn't need to be sealed in order to resonate. It's more efficient if sealed, but not a necessary condition at all. If it were, then acoustic guitars wouldn't work very well.... nor violins... nor kick drums... Nor Helmholtz resonators... Yes, you can try to calculate the resonant frequency of the soffit interior if you want to, but frankly.... good luck with that!Thomas further mentioned calculating the resonance frequency of this space behind the baffle to make sure that frequency is very low further implicating a 'closed box'.
It's a non-trivial problem: the cavity is not a simple rectangle, so you can't use simple math to figure it out. You can use the volume, yes, but then you still have to take into account that it is an angled cavity, and there are multiple resonances going, in different directions. To be honest, I really don't try too hard, because it isn't necessary. No matter what number you come up with at the end, the solution is always the same: damp the hell out of it! Fill the entire cavity with insulation, as much as possible, except for the ventilation "chimney" that goes up past the rear of your speaker, and around it to some extent usually. There will be a resonant frequency yes. Several, in fact. But calculating them won't change they way you treat them. Just make the cavity as big as possible, in all dimensions, damp it to eternity with abundant insulation (vary the types and densities, if you want), and that's about it!That's not the way I do it! And I don't think Glenn does either. I normally have a very rigid, heavy enclosure box around the speaker, with rubber pads in between the speaker and the box, but the rear of that box has large holes in it, for acoustic reasons, as do the top and bottom panels, for ventilation reasons. The box is tough, rigid, and full of holes. It's only job is to keep the speaker in place while NOT interfering with air flow up the back, and NOT interfering too much with the acoustics at the rear of the speaker. That enclosure box is then mounted to the soffit itself, also using resilient mounts all around. The trick is to tune both sets of mounts together, carefully, to keep the overall resonant frequency of the mounting system, as low as possible. This creates a 3-leaf system, so it's tricky to tune right, but the results are really good. Unlike the resonances inside the soffit cavity, it is important to figure out the resonant frequency o the mounting system, if you chose to go this path.I was under the assumption, ... that the box where the monitor-speaker is positioned should have a closed back. So that it doesn't have any opening towards the space behind the baffle.
... which is why I don't use a closed soffit!Obviously the cooling of the amp of the monitor becomes a problem with a closed box
Well, one of the reasons. I would never mount a speaker in an sealed, enclose space. You'll end up killing the speaker eventually, and you'll totally screw with the acoustic performance of the speaker if it is rear ported. Not a good idea.... and other sources claim that you cannot soffit-mount rear-ported speakers, or speakers where the front baffle is not flat! On the other hand, I've done all three of those, successfully, many times over.and that's why some sources claim that only monitor-speakers with a detachable/remote amplifier section are suitable for flush-mounting.
Here's one of those cases: A properly treated and tuned control room: S3P I designed that several years ago (the full acoustic report is in that thread somewhere). Take a close look at those speaker. They are Eve Audio SC-4070's. They are rear-ported (there's a huge bass reflex port that runs practically the full height of the speaker). According to the manual, they should not be mounted vertically, should not be soffit mounted, and the amps require good cooling all around. I didn't like those answers, so I spoke to the chief engineer at Eve Audio about what I wanted to do with them, explained what I had in mind, and he agreed with me and gave that his blessing. He even offered some helpful additional suggestions. Great guy, by the way. Very helpful people at Eve. I did not remove the amps from the speakers (a bad idea, unless the manufacturers offers a kit to do that), I did soffit-mount them in the way I usually do, vertically in this case, and you can see for yourself how that worked out.
Manufacturers put things in the manuals for people who don't really know what they are doing (such as the famous "equilateral triangle"). The positioning and mounting advice you find in the average speaker manual is what will work for pretty much all cases, no matter who does it. For people who have a better understanding of speakers and acoustics, some of those rules can be ignored, and others can be bent a bit. If you follow the advice in the manual, which is meant for average situations, installed by average people, it will work, and you'll get average results. If you are not happy with "average", and want your speakers and room to perform as best the possibly can, then you can go beyond what the manual says, as long as you know what you are doing.
Case in point: You posted a general guideline from Genelec that says you must NEVER have the speaker recessed, or poking out, from the soffit face: it must ALWAYS be flush. On the other hand, Genelec sells this kit for their very own 8040A speaker:
Here's another room I did with vertically soffit mounted speakers that break several other "rules":
As the terrible saying goes: there's more than one way to skin a cat! There are different ways to soffit mount speakers, but the final goal is the same: a broad, clean sweet spot with zero early reflections, a diffuse low-level reverberant field, and good, tight bass, with the response as flat as you can get it, and a good room curve (I recommend the old B&K curve: it always works...). Glenn has his ways of getting there, my ways are pretty similar, and other designers have their own methods, but we are all heading for the same end result. Some manage to achieve it better than others!
Not a problem! Debate is a good thing. Learning as much as possible is even better, when you start out designing your room. There's so very much that needs to be taken into account with studio design, that you really should debate and ask and read as much as you possibly can.Sorry if I come across trying to debate the positions that have come forward in this thread, that's not my intention.
So please do keep in asking!
- Stuart -
