OK, back to "soffit-mounting" (a.k.a "flush mounting").
First, here's something I wrote a long time ago, and have been meaning to update. Most of it is still valid, but as the disclaimer says: "Sometimes you need to bend the rules to make it work for your room".
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So, soffit-mounting. There's not really a simple way of doing this. Floyd Toole has written a lot about it, and so have others, but all of the "rules" have to be bent a bit to fit the room.
Some of these only apply to speakers on pedestals (not in soffits), other parts are specifically about flush-mounting, but most parts are about both:
Anyway, A few "rules" and guidelines for speakers in general and soffits in particular, in no specific order:
1) The speakers should not go in the corner, nor on the line that divides the corner. In other words, if your walls intersect at 90°, then draw a line out from each corner at 45°, and stay away from that line: don't put your speaker exactly on that line, since it implies that you'll be getting the same artifacts from the side walls as from the front wall. Put your speakers either outside or inside of those lines. More commonly you'll want your speakers "inside" those lines (more towards the center line of the room), unless your room is very narrow, in which case it might be better to have them "outside" the lines (more towards the side walls), and angled in more steeply.
2) The "38% of room depth rule" is not a rule, but it is a useful guideline for a starting point. You'll generally want to have your listening position a bit closer to the front wall than that location, but do be aware that you might be getting into problematic SBIR territory there. (You can treat that, to a certain extent).
3) Keep the mix position away from 25% and 50% of room depth, and try to stay between about 32% and 44%
4) You can angle your speakers differently than the "textbook" 30° angle: Anything in the range 25° to about 35° will work well under most circumstances.
5) Keep the speakers as far apart as possible, while not violating rules 1 and 4.
6) Keep the mix position at a good distance from the speakers, within the range of about 1m to about 5m. Further away is usually better.
7) Don't put the speakers at 25% of the room width: that's a modal null for some frequencies, and a peak for others. Try something more like 28% to 34%. (Note: You can break this rule if you know what you are doing, and want to avoid triggerieng a specific mode: then you CAN put it at 25%... sometimes...)
8 ) Make the front baffle of your soffit as wide and tall as you can, within reason. The width should be at least three times the diameter of your low frequency driver. In other words, if you have a speaker with an 8" woofer, then you want the soffit baffle to be at least 24" (60cm) wide. Wider is better. If you have a three-way speaker where the tweeter is in the middle, between two identical woofers, then the "diameter" is the distance from center to center of the woofers.
9) Do not put your speaker in the middle of the soffit baffle: Offset in both directions. In other words, the distances from the acoustic center of the speaker to each edge of the baffle should be very different, by at least 20%. So for example if your speaker axis is 30cm from one side of the baffle, it should be more than 36cm from the other side, less than 24cm from the bottom edge, and more than 44cm from the top edge. (Rough distances, for illustration only...). Larger differences are generally better. Try to get it at the 2/5th location side-to-side, if possible.
10) Make the baffle as massively heavy as you can, and as immensely rigid as you can.
11) Make the structure inside the soffit (the framing that holds the baffle and speaker in place) as rigid and massive as you can.
12) Mount the speaker inside an enclosure box that is either a very tight fit, in order to keep the speaker rigidly fixed in place, or mount it on suitable resilient mountings, to completely decouple it from the the box. Carefully choose the properties and dimensions of that resilient material, to make sure the speaker is still decoupled down to at least one octave below the speaker's low cut-off frequency. Sorbothane is the best resilient material for this. Do the calculations carefully to get the frequency right, with the correct static displacement.
13) Take into account that speakers need a lot of space behind them for cooling, and a path through the soffit for cooling air to flow. You also need space at hte back for acoustic reasons.
14) Rear-ported speakers need special attention: Do not overload the rear port, acoustically, with an enclosure box that is too small, or un-ventilated, or un-damped. Leave more space at the back of a rear ported speaker, and damp the port itself very lightly.
15) Damp the hell out of the soffit interior! Fill it entirely with suitable damping if you want, except for the cooling path.
There's more to it than that, but it's a start!
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Additional to the above:
Regarding the famous "speaker triangle" setup advice, see this:
Speaker setup, and the equilateral triangleHere's some more on the concept behind soffits, also from a few years ago:
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Here's how it works technically:
If you have a speaker inside the room (not soffit mounted) then some of the sound is focused like a cone coming straight at the mix position, and some of the sound expands around the speaker in all directions in a sphere shape. The reason for that is very simple: at high frequencies, the wavelengths are short, and the front baffle of the speaker (the front panel) is much larger than the wavelength, so the baffle forces all of the sound to go straight towards you. But at low frequencies, the wavelengths are large, much bigger than the width of the baffle (front face of the speaker), so they "wrap around" the speaker and go backwards behind it as well. This is called the "baffle step" problem, and it happens with ALL speakers, simply because of the way sound works: a sound wave is not affected much by objects that are a lot smaller than the wavelength, but they ARE affected by objects that are a lot larger than the wavelength. So a wave that is 10 feet long is not affected by much at all by a speaker cabinet (or even a desk or chair), but a wave that is 2 inches long is affected by pretty much everything in the room.
That's the basis. ALL speakers suffer from the baffle step problem. The only difference is the frequency where things change over from "focused forwards" to "wrap around". For physically small speakers, that happens well up in the mid range, while for very large speakers it can be down a lot lower. The ONLY factor that influences this, is the size of the front baffle. In the ideal situation, the front baffle would be larger than even the lowest frequency wavelength, so there would not be any problem! In other words, if the baffle could be "infinitely large", then there would be no baffle step problem, and ALL of the sound would be focused directly towards you.
And that's exactly what a soffit does! It emulates an "infinite baffle". It gets rid of this problem by putting a much larger baffle around the speaker, so that the baffle step problem is forced down to a much lower frequency, ideally below the bottom end of the speaker's range. That's why small speakers are really good candidates for soffit mounting, precisely because the "baffle step problem" with a small speaker occurs at a rather high frequency, somewhere in the mid range!
OK, so you might wonder: "What's so bad about having some sound focused forwards and some wrapping around?". Why is that a problem at all?
Actually it is not just one problem, but SEVERAL problems.
First, is power imbalance. Think of this: If ALL of the power put out by the tweeter is heading towards you, but HALF of the power put out by the woofer is NOT heading towards you (it's going the other way: away from you, out to the sides and also backwards, behind the speaker), then you have a power imbalance problem! The highs are 6 dB louder than the lows, because half of the lows wrapped around behind the speaker and went away from you. So there's a major issue here! Therefore, the speaker manufacturer has to pump TWICE as much power into the woofer, so that it puts out the same amount of sound heading YOUR way, as the tweeter, and you hear them at the same level. Have you ever wondered why speakers always have much more powerful amps for the woofers, compared to the tweeters? Part of that is due to this "power imbalance" problem. For a typical studio monitor, the tweeter amp might be 150 watts, but the mid range and woofer amps together are 500 watts! (Just the woofer alone might be 300 watts....).
So there's the first problem: power imbalance. But if you put the speaker in a large soffit, then ALL of the energy is now going straight to you, NONE is going backwards, and you don't need to waste all that power any more. So you can roll-off the bass, because the built-in compensation is not needed any more, and the entire speaker is acting the way it should: sending all of it's power to you.
For this reason, most high-quality studio monitors have a bass roll-off switch on the rear panel, so that you can turn off the compensation if your speaker is flush-mounted, or mounted very close to the front wall.
Next problem: If the speaker is not in a soffit, then think about what happens to that "half of the low end" that went behind the speaker: it hit the front wall, and bounced back! And now it comes towards you again, but out of phase and delayed a little (because it took a little bit of time to reach the front wall and come back again).... This is clearly NOT a good thing! It will interfere with the direct sound, causing phase cancellations, comb filtering, and other nasty problems. This is called the "SBIR problem", where SBIR means "Speaker Boundary Interference Response". It means what it says: the frequency response of the speaker is now all messed up, because the sound that bounced off the "boundary" (front wall) is "interfering" with the sound that came straight out the speaker. Not a good thing at all. It totally screws up the frequency response, and causes a very large dip in the spectrum at a frequency that is related to only one thing: the distance between the speaker and the front wall.
You cannot get rid of SBIR from the front wall by using treatment there, since the frequency is way too low for that. There's basically nothing you can do about it.... except mount your speaker in a soffit! If your speaker is soffit-mounted, then there is no SBIR problem with the front wall! Because NO sound gets around behind the speaker anyway, and even if some did, it can't come back into the room again, because the soffit baffle stops it.
So there's another problem killed. SBIR from the front wall is gone. Impossible.
Then there's "edge diffraction", which is a bit more complex. It goes like this: remember that waves that are a lot shorter than the width of the speaker (higher frequencies) all go directly towards you, while ones that are a lot longer wrap around behind? OK, so what happens to the ones that are roughly the SAME width as the speaker? Well, they sort of try to wrap around, but they can't quite make it... and they also can't quite go directly forwards either! The problem happens at the front edges of the speaker cabinet, just where they waves are reaching the edges of the front baffle. They sort of want to go around, but they sort of can't, so there's a kind of "push pull" fight going on right at the edge, with part of the wave almost getting around the corner, and part not quite making it, and that creates all manner of ugly stuff, right at the edge of the box. (For those who really understand the technical issue here: Yeah, I know that explanation isn't accurate, but it's a good way of thinking about it to get the general idea.) So that's why it is called "edge diffraction". It's a problem, because it messes up the frequency repose, and the power response, and the phase response, for all frequencies where the wavelength is similar in size to the dimensions of the box. It creates interference patterns that mess things up. That's why many studio monitor speakers have beveled or rounded corners and edges: to reduce edge diffraction, and some speakers (such as many Genelec's) are even egg-shaped, totally curved, to reduce it even more. But you can't get rid of the problem completely. ... Unless you mount the speaker in a soffit! Since the front baffle of the soffit is basically an extension of the front of the speaker box, there is no edge! So there is no edge diffraction!
That's yet another issue that soffit mounting eliminates.
The list is growing:
- Power imbalance
- Front wall SBIR
- Comb filtering
- Edge diffraction
All of those are eliminated by putting your speakers in soffits. Gone. Not there. There are other things that soffit-mounting fixes too, but those are the main ones.
But what does that mean, in terms of how it sounds? Major, big-time improvements! First and foremost, the bass is much tighter and smoother, and extends down lower. None of those artifacts are there, so they cannot distort the bass response. All you hear is the pure, deep, focused, rich bass of your speakers. And since the low end is extended by the infinite baffle, the speakers actually sound bigger than they are. Then there's the mid range: since it is no longer muddied by the edge diffraction problem, that is cleaner, smoother, and better defined as well. And for the high end, the soffit helps with yet ANOTHER problem, which is impedance mismatch! I haven't mentioned that yet, but in brief simple terms, it works like this. The tweeter cone is a hard, rigid, massive solid object, and it is trying to transfer energy to a softy, mushy, springy, compressible, floppy fluid, called "air". That energy transfer is VERY inefficient, because the the two materials are so very different! Try flapping your hands up and down in air, and see how well you can move it around.... But if you flap your hands up and down underwater, you do a LOT better! You can swim in water, but you can't swim in air, because the "impedance" of the water is closer to the impedance of your hands, but the impedance of air is very different. So you can transfer your "flapping hand" energy to the water, much better than you can transfer it to the air, because the impedance is better matched. If you wanted to transfer "flapping hand" energy into air, then you could maybe attach ping-pong paddles to your hands, or better still, big pieces of cardboard or thin piece of plywood: That would improve the impedance of your hand a lot! It would be much more closely matched. The bigger the sheet of wood you used, the better it would be. You'd have to work harder, but you could move a LOT of air if you attached a really large piece of wood to your hand.... maybe the size of a soffit baffle, for example!
Speaker manufacturers go to a lot of trouble to carefully design the "wave-guides" around the tweeters of their speakers, to improve the impedance matching problem, so that the tweeter cone can transfer it's energy much more efficiently into the air. That's the reason for the curved shape plastic or metal "thingy" around the tweeter: to improve the impedance matching between the tweeter and the air... but that wave-guide is limited by the size of the box that the speaker is mounted in: it can't be bigger than the box! Unless you put it in a soffit....
Yup, you guessed: The soffit also improves the impedance matching of the tweeter to the air. Not by a lot, to be honest, but enough to be useful. So the high end is also cleaner, smoother, crispier, more "airy", better detailed, ... because the tweeter can do a better job of transferring all of its energy into the air..... (If you have ever heard what a tweeter sounds like when it is take out of its wave-guide , and just working in the open air, you have certainly noticed how tiny, thin, tinny and ridiculous it sounds! The same happens if you take your earbuds out of your ears, and hold them a couple of inches away from your ears: all the bass and most of the mid-range is gone, because your ear canal was providing the impedance matching).
In other words, the soffit does many, many things a once to greatly improve the full spectrum of sound coming out of your speakers.
In my opinion, tit's the best single thing you can do to a studio control room, and to your speakers. Everything else is "icing on the cake", but soffit-mounting your speakers is the cake itself. It makes a huge difference.
It's interesting that smaller speakers benefit from it more than large speakers. Most speakers will do great in soffits, and will pump out cleaner, deeper, smoother, tighter sound than you have ever heard them produce before. Because your soffits, if they are properly designed, will do exactly that.
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And one final thing, about orientation of your speakers. If the manufacturer gives you the option, then ALWAYS set them up vertically. FOr this reason:
As you can see, placing them horizontally can potentially produce interference artifacts that you will notice as you move your head around, side to side. That doesn't happen with vertical mounting, since you don't have ears on the top and bottom of your head! Your ears are on the sides of your head, so you are affected by this issue only for horizontal mounting. It doesn't affect you with vertical mounting.
There's more I could say about
how to actually design your soffits properly, but that's an entire different thing. This is just about the acoustic reasoning behind
why you should do it.
- Stuart -
