Stuart's Rules for Soffits / Flush mounting speakers

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Soundman2020
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Stuart's Rules for Soffits / Flush mounting speakers

#16

Postby Soundman2020 » Sat, 2023-Sep-23, 00:03

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.
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.

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.
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...

My question is: Does a monitor-speaker 'leak' any LF through the back of the speakerhousing/cabinet?
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.

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?
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.

I was under the impression that the baffle should be an airtight/closed construction.
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.

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'.
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! :) 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!

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.
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.

Obviously the cooling of the amp of the monitor becomes a problem with a closed box
... which is why I don't use a closed soffit! :) 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 that's why some sources claim that only monitor-speakers with a detachable/remote amplifier section are suitable for flush-mounting.
... 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. :)

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:
genelec-flush-mount-soffit-kit.png
As you can see, almost half of the speaker ends up poking out through the wall! So they don't take their own advice sometimes... because they know what they are doing!

Here's another room I did with vertically soffit mounted speakers that break several other "rules":
FKCA-Focal-6-Be-soffits-finished.jpg
For example, the soffit baffles are tiny! Only a bit wider than the speakers themselves. Yet that room has nearly flat response down to about 17 Hz.... (+/- 4 dB, if I recall correctly).

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! :)

Sorry if I come across trying to debate the positions that have come forward in this thread, that's not my intention.
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.

So please do keep in asking! :thu:


- Stuart -



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Stuart's Rules for Soffits / Flush mounting speakers

#17

Postby PLG-88 » Sun, 2023-Sep-24, 11:18

Thanks Glenn and Stuart for your extensive replies!

Things become a lot more clear on how the flush-mounting of monitors works from a practical point of view. Since last year I'm the owner of a pair of Focal Shape Twin's which I really like. Unfortunately those are not suitable for flush-mounting because of the two passive radiators on the sides of the monitor. But if the time (and money) is there and I feel it's absolute a must to flush-mount my speakers, I am open to possibly upgrade to the Focal Trio6 BE's. With the extra benefit they go 5Hz lower down to 35Hz (+/- 3dB) in comparison to the Shape Twin's. I'm really curious how that will sound flush-mounted in a well treated room.

Yup! Quite a bit, actually.

That's not the way I do it! And I don't think Glenn does either.

Well, thinking about this now, if the back of the opening would be closed, all that LF energy from the rear of the monitor-speaker will go partially through that closed back but probably is also partially reflected back where it will interfere with the original output of the monitor-speaker?

Soundman2020 wrote:Source of the post I just read through that, and I didn't see any place where it implied that the soffit should be sealed.


I was under the assumption that he implied it with this sentence:
Also make sure the wall cavity remains air tight.


Here's one of those cases: A properly treated and tuned control room: S3P

Very nice project with beautiful results. Those REW measurements look amazing. It's a shame that the owner went out of business.



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Stuart's Rules for Soffits / Flush mounting speakers

#18

Postby gullfo » Sun, 2023-Sep-24, 13:43

i'd check to see if the amp on the Focals can be moved or not and if not, consider some passive JBL, ATC or PMC monitors (or a passive 1032 sized monitor) and w/ minimum 8" LF or better 12" for soffit mounting.



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Stuart's Rules for Soffits / Flush mounting speakers

#19

Postby Soundman2020 » Sun, 2023-Sep-24, 14:29

I am open to possibly upgrade to the Focal Trio6 BE's. With the extra benefit they go 5Hz lower down to 35Hz (+/- 3dB) in comparison to the Shape Twin's. I'm really curious how that will sound flush-mounted in a well treated room.
It would probably work out something like this...
FrKCAUS--complete-soffits-Focal-trios.jpg

FrKCAUS-REW-WF-12-23k-FINAL.png

FrKCAUS-REW-FR-12-22k--1..3-FINAL.png

FrKCAUS--REW--FR--1k-21k--1..12--Box-2.png


Take a close look: your eyes are not deceiving you.... Yes, it really is flat down to 17Hz, and -6db at 12 Hz. That's pretty darn impressive, even if I do say so myself!

That's about as good as it gets!

That's a very small room, and it is a "corner control room" design too.

So if you design it well, and build it well, and treat it well, and tune it properly, then you can probably get something similar.

- Stuart -



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Stuart's Rules for Soffits / Flush mounting speakers

#20

Postby Soundman2020 » Sun, 2023-Sep-24, 14:32

i'd check to see if the amp on the Focals can be moved or not
It can, and we did it in this case, because of the very limited space in the soffits and the need to access the rear panel. There was no place to put an access hatch in the soffit. So we did it here, but I wouldn't recommend it, unless really necessary.
FrKCAUS--B-Focal-amp-mount-ENH.jpg
It was a pain to make those extension cables and get them working reliably. You also have to replace the rear panel on the speaker with something similar, since it's part of what makes the speaker. I don't recall for sure, but I think we used an aluminium plate of the same thickness, cut to shape.


(PS. The Tannoy sub was not part of the final setup! That was replaced with somthing more substantial, that goes really low, and really smooth. The Tannoy was just there for initial testing.)

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Stuart's Rules for Soffits / Flush mounting speakers

#21

Postby PLG-88 » Thu, 2023-Sep-28, 18:27

Wow, inspiring project and an extreme flat room response. I'm not planning on adding a subwoofer, but still: this is as good as it gets. Thank you for sharing!



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Stuart's Rules for Soffits / Flush mounting speakers

#22

Postby Soundman2020 » Fri, 2023-Sep-29, 00:38

Subwoofers serve two purposes. One is to extend the bass range, and that's usually what people think of when talking about subs. But they can also be used to deal with problematic acoustic issues in the room itself. We did both here. You can use a sub to cancel out certain acoustic problems, as long as you realize that it will only work well for one region of the room; practically always, that is the mix position. Stubborn modal issues, and even some SBIR issues are prime targets for this, but you do have to be careful: don't try to fill in a null. That most likely will make things worse, not better. But you can bring down peaks, in both the frequency domain and time domain. So if you end up with problematic issues in your room after you have treated it as well as possible, then digital tuning with a sub is an option. Something to keep in mind when the time comes.

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Stuart's Rules for Soffits / Flush mounting speakers

#23

Postby ianlawrencemacdonald » Fri, 2024-Feb-09, 14:17

gullfo wrote:Source of the post the speaker itself is decoupled from the assembly, and sealed to the front. the baffle plate, while not "infinite" is large enough to require the baffle step compensation you would expect in the fully enclosed wall system. bear in mind, the entire back of the assembly is a LF trap so attenuation of LF on that wrapping top and bottom of the baffle plates will be significantly reduced and not cause SBIR.

you'll note in my drawing example the use of duct board to create ducts for convention flows, generally not enough for amp cooling without some forced air flow -- one option to tap off the cooling duct in the wall soffits to put cool air into the speaker box, or best - properly move the amplifiers out and into the center or center cooled cabinet better yet.

if the manufacturer doesn't support removing the amp, then increase the cooling / duct size to ensure sufficient flow. small Genelec etc aren't necessarily an issue and units like ATC and PMC will support remote amping as would fully passive speaker units requiring external amps. however even passive speakers should have venting as speakers can and do generate heat in the course of usage.


Hey gang! So happy to have stumbled across you're forum here. I'm in the process of studding my basement and wanted to construct a soffit on the back wall in order to flush mount my HS8s. In fact, I believe I will use the design you posted in the above comment to guide my build. With that in mind, and in regards to the issue of air flow, do you recommend installing a fan inline within the duct board venting for a pair of HS8s? Was thinking I could install silent cpu fans on the bottom side of both boxes to introduce some additional airflow.

Thanks! I'm so excited to have this resource to assist my build.

Ian



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Stuart's Rules for Soffits / Flush mounting speakers

#24

Postby gullfo » Fri, 2024-Feb-09, 18:51

the main issue isn't venting, and most times the passive air flow will suffice, it's the open port on the back of the speaker - being buried in the soffit, or plugged will change the overall speaker response - which may be better if plugged. another is remove the electronics and create a backing panel (if you were going to plug the port anyways) to seal it. then mount the amp sections into the center section of the soffit so you can access them and not have heat rising in front of the speakers. you'll still want to vent because passive speakers will still generate heat but it will be less potential for heat issues on the amps.




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