Soffit-mounting a rear-ported D'Appolito array?

[mbene085]

I've been reading up as much as I can on this subject, but I'm hitting a wall (no pun intended).

I'm designing a room-within-a-room plan for my new home studio (having just moved houses), and since I'm building walls for isolation anyway, I'm exploring using the interior wall to shape a reflection-free zone at the mix position instead of relying on absorption and diffusion in my old cuboid room. This also presents the opportunity to flush-mount/soffit-mount my mains. The current room is roughly a 16'x16'x9' square.

I'll be primarily using the space for mastering (80/20 mastering/mixing), and I've got a pair of Tyler Acoustics D1x monitors as my current mains. They're rear-ported and his implementation of the D'Appolito array includes a contour in the depth dimension - they're not flat across all drivers like most soffit-mounted monitors I see. The woofers are a bit further forward than the MTM array (or the MTM array is inset, depending how you look at it).

Is it a lost cause to try to figure out how to make these work in this mounting style? I see more and more mastering suites these days opting for mounting their monitors to combat SBIR.

These are the monitors in question. It's probably easier to see the issue than to describe it:



I haven't been able to find any specific references online to flush/soffit-mounting monitors with varying depth. Would I set them in the wall such that the mids and tweeter are flush with the wall, and the woofers are projecting outward, or would I set them in such that the woofers are flush and the MTM array is set deeper in? The latter seems like it might introduce more problematic reflections, but if I was an expert, I wouldn't be asking these questions!

Another option is to stay with freestanding placement, close enough to the wall that the 1/4 wavelength works out to a range where absorption behind them could deal with much of the issue. A third option would be to sell/trade the D1Xs and get something flat and sealed or front-ported to eliminate this design challenge, but I really like the D1x's. Tyler has a model called the MM5X which would be much easier to soffit-mount, but they're a little less sensitive, have a tiny bit less low-frequency extension, and lose the advantages of the D'Appolito design (like the way it handles floor/ceiling reflections). Since I like the D1X, I'll probably like the MM5X, which is why I mentioned them, but I could of course look at other front-ported or infinite baffle monitors.

My thoughts are that my angle of my walls could create an RFZ in the horizontal plane, the soffit mount could eliminate SBIR, and the D'Appolito array would then make vertical reflections easier to manage.

Is it a pipe dream, though? Any thoughts or suggested resources are appreciated, thanks!

[gullfo]

for mastering - most use a rectangular room with good ratios which mimic (RT and freq balance) a very nice living room. so if mastering is your main day-to-day activity, i'd stick with the rectangular room. i'd also not soffit mount the speakers as they're rear ported and it will likely affect your mastering decisions even if you mainly get those speakers right. the d'appolito array would not be the concern there. that said, you have/get another set of speakers more suited to soffit mounting and mount those.

[DanDan]

Very deep treatment on the Front Wall would take care of SBIR. Add the same on the Back wall and the Square room problem is gone.

[mbene085]

Thanks for the input. I've really been running myself in circles, since everything I read and measure reinforces the idea that my square room is horrible, but I'm not sure if sacrificing room volume for improved ratios is worth it vs targeting the modes with absorption and then using diffusion to keep from over-deadening it.

This is the modal distribution of the current room.

Way out of the Bolt-area, the Bonello graph starts of by dipping, then only increasing in fits and spurts, and of course the square walls cause coincident modes in integer multiples.

Since I want to build a second set of interior walls for isolation purposes, I played around with the idea of bringing in a wall to improve the ratio. Here's what that yielded.

Just inside the Bolt-area, a much prettier upwards progression on the Bonello graph, and the elimination of those coincident modes.

This is a little layout I threw together in Sketchup, trying to splay some walls while I was at it.

You can see the square outline of the old room, and the smaller volume of the new room within it. Those are the monitors toward the center of the front wall - the outer ones were just me playing around with symmetrical locations that AVAA C20 active traps could be placed, since the smaller room has less room for LF absorption.

I mocked up the layout with the monitors placed just 4" from the back wall - the diameter of the rear port. I know there would be a LF buildup from doing so, but figured that could be compensated for more easily than SBIR. 4" at 1/4 wavelength would raise the lowest SBIR frequency to around 850 Hz, which would be easily absorbed by a fiberglass or rockwool panel behind the monitor.

If I don't bring that wall forward, I have lots more room for big, soffit-style bass traps all along the front wall, and I could go for a LEDE design, hoping to smooth out the massive peak and dips from the coincident modes of the square room.

Seeing the two modal distributions and the room itself, what do you think the right move would be? Bring the wall in +/- free up floor space by switching to a monitor that could be mounted in-wall, keep the full room volume and load up the front half of the room with thick absorbers?

[gullfo]

square rooms are generally problematic because at least 2 sides of it are doing the same reinforcement. a rectangular room has predictable model distribution and are generally well understood. angled boundary surface rooms are trickier to make predictions but if generalized (in terms of response) can be quite nice to build and use. so, avoid square rooms whenever possible, and depending on your needs and space, rectangular or angled rooms are better choices.

some of the things which will guide your selection - SAF (spousal approval factor), re-use/re-sale, dedicated room, space and must-haves (e.g. a small control room and a booth in a single 15x15 room - an angled wall approach here would work nicely. etc.

for mastering, the majority of mastering rooms i've been in or seen are rectangular. even Galaxy in Belgium has a rectangular mastering room. most have free standing monitors, and many those monitors are some ways off of the front wall.

[DanDan]

Boggy was asked about Modal issues....... What modes? he replied. His rooms typically had 60cm of light cheap insulation in light cheap framing.

[mbene085]

square rooms are generally problematic because at least 2 sides of it are doing the same reinforcement. a rectangular room has predictable model distribution and are generally well understood. angled boundary surface rooms are trickier to make predictions but if generalized (in terms of response) can be quite nice to build and use. so, avoid square rooms whenever possible, and depending on your needs and space, rectangular or angled rooms are better choices.

some of the things which will guide your selection - SAF (spousal approval factor), re-use/re-sale, dedicated room, space and must-haves (e.g. a small control room and a booth in a single 15x15 room - an angled wall approach here would work nicely. etc.

for mastering, the majority of mastering rooms i've been in or seen are rectangular. even Galaxy in Belgium has a rectangular mastering room. most have free standing monitors, and many those monitors are some ways off of the front wall.

Thanks again for your reply. The challenge in predicting modes in a non-cuboid room is exactly why I tried only minimally angling the walls in that layout I posted - in the hopes of better predicting a favourable modal distribution.

I am fortunate to have no SAF concerns - this room and the nearby loft that will be becoming a live room have full approval for rebuilding however I see fit for my purposes. Likewise, re-use/re-sale is not a concern. These are dedicated spaces in a home we anticipate owning longterm.

But you mentioned something that has been confusing me a bit - I definitely see a lot of rectangular mastering rooms, but then I see mastering engineers talking about how the most important thing is having a neutral-sounding room. I thought these two things were mutually exclusive? I thought an angled wall, RFZ design was regarded as superior for critical listening tasks.

But now I'm wondering if I've been misunderstanding. Are RFZ rooms more favourable for mixing, rather than mastering? Are cuboid rooms used to master because they more closely resemble the rooms people will be listening in? Or is the issue simply that cuboid rooms can be adequately treated with absorption and diffusion, and complex RFZ designs are incorporated into control rooms of studios with tracking/live rooms because of other design constraints like sight lines, compensating for the reflections off of massive flat consoles, etc?

I'm taking a Berklee studio design course taught by John Storyk and he seems to be a very big fan of RFZ designs, which is one of the reasons I felt like I should be finding ways to splay more of my walls. But he's also an architect, and seems to primarily design rooms with hard surfaces and uses tuned membranes and helmholtz resonators for control of room modes, so building a square room with 2-3 feet deep of absorption behind fabric walls is something he'd be allergic to philosophically and aesthetically.

I'm wondering if maybe I'm trying to apply design principles from a design philosophy that might not serve my application as well as simply building my second layer of walls (for isolation) and then using a large amount of absorption for modal control.

Boggy was asked about Modal issues....... What modes? he replied. His rooms typically had 60cm of light cheap insulation in light cheap framing.

I can see what you're saying - thin walls are permeable to low frequencies and a good depth of cheap fiberglass makes for a good absorptive bass trap - but how "light" is "light"? I'm not building my walls out of concrete, but I was planning on using a double framed wall with two layers of QuietRock with green glue between on 2x6" studs with fiberglass batting in between for isolation. Do you think this is what Boggy meant by "light cheap framing"? Relative to commercial studio construction using masonry and interior walls of QuietRock sandwiching plywood or MLV, they're definitely light and definitely cheap. Because I'm bringing my front wall in for modal reasons, I can fit a good couple of feet of fiberglass batting between the two walls back there. There's also a 2' deep closet along the rear 2/3rds of a side wall, which could be filled with fiberglass to form a large bass trap.

[gullfo]

RFZ doesn't necessarily require angled boundary walls. many times an RFZ (or really just about any acoustic model) is built inside of a rectangular room. this allows for high predictability of the room modes and the interior angled treatments can perform a variety of functions including resonant traps, pure absorption, pressure traps, scattering, etc. so it's possible to define a rectangular room boundaries and re-shape the acoustic response. you can also do a lot with angled walls, and in a number of cases, this is because of space utilization, ergonomics and workflow, and certain dedicated functions where this is more ideal than a flat wall approach.

which room is RFZ?

[DanDan]

Mass Air Mass, MAM, is for Isolation. Soundproofing. It is very beneficial to fully fill the A Air Gap with insulation, lightly touching both Masses to damp them. Low density Partition Rolls and such are perfect for this. Higher Density Batts do not perform better. Boggy's Rooms typically had 60cm of light fibre held in frames INSIDE the acoustic containment shell. I guess 60cm on every wall and ceiling will control modes to a large extent. The physical dimensions of the now buried containment layer no longer of any consequence. Worth noting that BBC Modular treatments were a foot or less in depth. For clarity and inspiration I strong recommend taking a look at the studio build pictures on Bogic Petrovic Instagram and FB pages. His finished designs typically have a slatted diffusive layer on the surface. But I am talking about LF only for now.

[Paulus87]

square rooms are generally problematic because at least 2 sides of it are doing the same reinforcement. a rectangular room has predictable model distribution and are generally well understood. angled boundary surface rooms are trickier to make predictions but if generalized (in terms of response) can be quite nice to build and use. so, avoid square rooms whenever possible, and depending on your needs and space, rectangular or angled rooms are better choices.

some of the things which will guide your selection - SAF (spousal approval factor), re-use/re-sale, dedicated room, space and must-haves (e.g. a small control room and a booth in a single 15x15 room - an angled wall approach here would work nicely. etc.

for mastering, the majority of mastering rooms i've been in or seen are rectangular. even Galaxy in Belgium has a rectangular mastering room. most have free standing monitors, and many those monitors are some ways off of the front wall.

Thanks again for your reply. The challenge in predicting modes in a non-cuboid room is exactly why I tried only minimally angling the walls in that layout I posted - in the hopes of better predicting a favourable modal distribution.

I am fortunate to have no SAF concerns - this room and the nearby loft that will be becoming a live room have full approval for rebuilding however I see fit for my purposes. Likewise, re-use/re-sale is not a concern. These are dedicated spaces in a home we anticipate owning longterm.

But you mentioned something that has been confusing me a bit - I definitely see a lot of rectangular mastering rooms, but then I see mastering engineers talking about how the most important thing is having a neutral-sounding room. I thought these two things were mutually exclusive? I thought an angled wall, RFZ design was regarded as superior for critical listening tasks.

But now I'm wondering if I've been misunderstanding. Are RFZ rooms more favourable for mixing, rather than mastering? Are cuboid rooms used to master because they more closely resemble the rooms people will be listening in? Or is the issue simply that cuboid rooms can be adequately treated with absorption and diffusion, and complex RFZ designs are incorporated into control rooms of studios with tracking/live rooms because of other design constraints like sight lines, compensating for the reflections off of massive flat consoles, etc?

I'm taking a Berklee studio design course taught by John Storyk and he seems to be a very big fan of RFZ designs, which is one of the reasons I felt like I should be finding ways to splay more of my walls. But he's also an architect, and seems to primarily design rooms with hard surfaces and uses tuned membranes and helmholtz resonators for control of room modes, so building a square room with 2-3 feet deep of absorption behind fabric walls is something he'd be allergic to philosophically and aesthetically.

I'm wondering if maybe I'm trying to apply design principles from a design philosophy that might not serve my application as well as simply building my second layer of walls (for isolation) and then using a large amount of absorption for modal control.

Boggy was asked about Modal issues....... What modes? he replied. His rooms typically had 60cm of light cheap insulation in light cheap framing.

I can see what you're saying - thin walls are permeable to low frequencies and a good depth of cheap fiberglass makes for a good absorptive bass trap - but how "light" is "light"? I'm not building my walls out of concrete, but I was planning on using a double framed wall with two layers of QuietRock with green glue between on 2x6" studs with fiberglass batting in between for isolation. Do you think this is what Boggy meant by "light cheap framing"? Relative to commercial studio construction using masonry and interior walls of QuietRock sandwiching plywood or MLV, they're definitely light and definitely cheap. Because I'm bringing my front wall in for modal reasons, I can fit a good couple of feet of fiberglass batting between the two walls back there. There's also a 2' deep closet along the rear 2/3rds of a side wall, which could be filled with fiberglass to form a large bass trap.

Dan is talking about the framing around the acoustic treatment within the isolation shell (in your case I believe you're planning on doing a double leaf assembly? - this is your isolation shell).

As an example, 2x2 could be used for the interior framing around the acoustic treatment.

In a square/cuboid room I would either go for a geometric RFZ design like the ones you're leaning towards or just go for a full non environment room. I am more of a fan of the latter for mixing and mastering since if done correctly these rooms allow you to hear only what is coming out of the speakers and nothing else - no influence from the room.

With geometric RFZ rooms (an authentic RFZ design) the idea is to angle the boundaries to deflect energy to the rear where it can be diffused in the hope of preserving some MF and HF energy in the room at a lower level than the direct sound, so that it produces a supposedly more comfortable environment to work in. However, I don't see any benefit to ANY return reflections (as a result of the direct sound bouncing off of surface)... it's the acoustic equivalent of an instagram filter in my mind.

But, I'd rather have a geometric RFZ environment than a completely square cuboid room IF I am not willing to give up the necessary floor space that the treatments of a non environment room requires.

[mbene085]

RFZ doesn't necessarily require angled boundary walls. many times an RFZ (or really just about any acoustic model) is built inside of a rectangular room. this allows for high predictability of the room modes and the interior angled treatments can perform a variety of functions including resonant traps, pure absorption, pressure traps, scattering, etc. so it's possible to define a rectangular room boundaries and re-shape the acoustic response. you can also do a lot with angled walls, and in a number of cases, this is because of space utilization, ergonomics and workflow, and certain dedicated functions where this is more ideal than a flat wall approach.

which room is RFZ?

Wow. I have been completely misunderstanding several things, and this post helped them click into place. Thank you! I was completely conflating boundary walls with interior walls. I thought the boundary had to be angled, and was worried about how unpredictable that would make the room modes. But I now realize that you can use a cuboid shell, and then use walls of whatever desired thickness/properties to shape your reflections, and they wouldn't require nearly as much mass if they're not part of your double-leaf isolation.

But, a quick question - if you build a double-leaf cuboid shell for the room for isolation purposes, then introduce angled walls for interior acoustics purposes, would that not lead to a triple-leaf effect and undermine the isolation properties of the outer structure? Or is the thinner, lighter construction of the interior wall and its distance from the outer double-leaf sufficient to prevent the triple-leaf effect?

Mass Air Mass, MAM, is for Isolation. Soundproofing. It is very beneficial to fully fill the A Air Gap with insulation, lightly touching both Masses to damp them. Low density Partition Rolls and such are perfect for this. Higher Density Batts do not perform better. Boggy's Rooms typically had 60cm of light fibre held in frames INSIDE the acoustic containment shell. I guess 60cm on every wall and ceiling will control modes to a large extent. The physical dimensions of the now buried containment layer no longer of any consequence. Worth noting that BBC Modular treatments were a foot or less in depth. For clarity and inspiration I strong recommend taking a look at the studio build pictures on Bogic Petrovic Instagram and FB pages. His finished designs typically have a slatted diffusive layer on the surface. But I am talking about LF only for now.

Thank you, this along with the two other recent replies have helped a lot of thinks click into place for me. I've been completely mistaken in my previous assumption that the angled walls needed to be massive and part of the double-leaf/MAM isolation solution. This is making so much more sense now. A high thickness of low density fiberglass behind thin or slatted interior walls makes perfect sense now as a solution to all but perhaps the lowest-frequency modal issues (which themselves may not be an issue at all, if the outer wall construction is insufficiently rigid and massive so as to become impermeable at 20-30 Hz). I have taken a look at some of those photos and will be scrutinizing them for quite some time now!

Dan is talking about the framing around the acoustic treatment within the isolation shell (in your case I believe you're planning on doing a double leaf assembly? - this is your isolation shell).

As an example, 2x2 could be used for the interior framing around the acoustic treatment.

In a square/cuboid room I would either go for a geometric RFZ design like the ones you're leaning towards or just go for a full non environment room. I am more of a fan of the latter for mixing and mastering since if done correctly these rooms allow you to hear only what is coming out of the speakers and nothing else - no influence from the room.

With geometric RFZ rooms (an authentic RFZ design) the idea is to angle the boundaries to deflect energy to the rear where it can be diffused in the hope of preserving some MF and HF energy in the room at a lower level than the direct sound, so that it produces a supposedly more comfortable environment to work in. However, I don't see any benefit to ANY return reflections (as a result of the direct sound bouncing off of surface)... it's the acoustic equivalent of an instagram filter in my mind.

But, I'd rather have a geometric RFZ environment than a completely square cuboid room IF I am not willing to give up the necessary floor space that the treatments of a non environment room requires.

Once again, thank you! These three comments together really drove home some of the errors in my previous understanding and approach.

Non-environment rooms intrigue but intimidate me. I have no access to any to test them out, and in the absence of experience, have read complaints that they can make translation a challenge since the recording will never again be heard in a non-environment. I understand that concern, but I also wonder why that would cause translation issues, since the actual mastering is being done in the absence of any reflections or modes that would cloud judgement of the recording, so I can see why you might find it preferable. I wouldn't trust myself to design such a room, though - I feel like an error in my approach could lead to a room that is anechoic at most but not all of the desired frequencies, leading to a greater frequency imbalance than a geometric RFZ room where a greater number of reflections are present, leaving a smaller gap between any overabundant frequencies and the desired baseline balance.

All of these responses have given me so much to think about that I am now heading straight back to the drawing board and discarding everything I had drawn up previously. With this new revelation that I can keep a cuboid outer leaf and then restructure internally as desired for acoustic purposes, I've realized that I have another space in the home that could be better utilized than I previously believed. Not only will I have the chance to try my hand at a geometric RFZ design, but I realize that I have the opportunity to create a room with a 23' depth that will make for a much better mastering suite overall.

Many thanks, all! I'll probably start a different thread as I work on the new design, since my question has been definitively answered - and I will not be attempting to soffit-mount these monitors, nor do I believe it will be necessary anymore. Thank you so much for your insights and for taking the time to reply. It has saved me many hours and many headaches as I've been wading through the many acoustics topics relevant to designing a studio.

[gullfo]

there is no free lunch. and with another solid intermediate boundary there could be triple leaf effects. however if the absorption is effective then this is a minimal effect at best outweighed by the attenuation.

[DanDan]

Our final customers typically listen in quite lively rooms. So the perceived tonality, spectrum, is quite tilted down towards HF. The B&K Curve is about +3dB at 100Hz dropping to -3dB at 10K, a 6dB tilt ending up similar to the Harman 1dB per octave. In my experience as a Free Lance Engineer, many studios are voiced Flat or close to it. This can sound spectacular due to the treatment and superb speakers, but ultimately it is tedious, and leads to repeated failures to translate to the domestic listening scenario. Might work OK for Headphone listeners though!
Many of us end up Anechoic in the early sound field at least, but we adjust the nett tonality to be similar to B&K/Harmon using Eq.
It is somewhat similar to 'Very Big Headphones'. My current scenario is like that, and I experience great translation with little effort. Masters and Mixes sound remarkably the same when piped to different systems in different domestic rooms in the building.

[Paulus87]

The angled walls CAN be part of the isolation shell, and that is often the approach taken. But, you can also build a rectangular cuboid with an even modal spread, then build angled sections (such as the front wall with flush mounted speakers). Both approaches work and have their compromises.

I will just say this: A rectangular cuboid (The inner leaf I mean) is easier both practically and acoustically. Assuming the hard walls are fairly rigid and dense then the modal spread can be predicted from the get go. This means you will know what the axial modes (and therefore their harmonics) will be for the length, width and height. This means you can also design suitable treatment for those 3 modes from the get go. Treat those sufficiently and the room will be accurate. Usually by treating those, first reflections are also treated automatically (if pressure traps need to be deployed to treat the modal resonances then these can be fronted with velocity treatments to deal with first reflections). Once axial modal decay rates and first reflections are obliterated then what more is there to do? If one finds the resulting environment a little unnatural then bringing back some "life" with carefully placed slats or binary amplitude diffusers, among other things, will solve that. It really can be that simple when you are designing from the ground up.

Designing a room with fancy geometry to provide reflection control is far more complicated and yields less accurate results in my opinion. These rooms will almost always colour the direct sound and modal spread is unpredictable. Depending on how angled each surface is it is true that strong axial modes will no longer be a problem, but now you have the task of hunting down where the problem resonances are located in your multi-splayed environment. And you cannot deflect LF reflections with geometry unless the angles are exceedingly large relative to their wave length. Even HF needs a large surface and angle in order to be deflected effectively due to the conical nature of reflections. In the BBC CID White paper it is recommended that a reflector as wide as at least 1 meter is required in order to deflect 1khz... that is a surface roughly 3 times the size of the wave length. In short it is my opinion that the only way to know if your geometrically controlled RFZ room is accurate is to build it and test it and if it doesn't work then you need to tear it down and try again.

It is a misconception and rumour that non-environment rooms are difficult to work in. One reason for this is because so many old skool engineers have gotten used to being in old skool inaccurate LEDE rooms which colour the direct sound. The argument that no consumer is going to listen to music in a non environment room or an anechoic chamber and therefore a mix room should emulate a consumer environment is flawed... first of all, every consumer's room is different! So which version of the consumer's room is correct? If my living room is 24' long and my friend's room is an L shape with a built in kitchen then which one should we use to judge how well a mix translates? Surely it is better to hear only what comes out of the most accurate speakers one can afford in a room void of any influence on the actual sound so that you can make decisions based on that alone? At least that gives the music a fighting chance to be as accurately represented as possible in any environment? Have you ever been to a concert outside in a big field void of any boundaries? Remember how amazing it sounds? Contrast that with a large circular stadium or a small club...

Lastly, non environment rooms have plenty of natural reflective surfaces from which we can gain self noise cues. The entire front wall is reflective which is the boundary you will be facing 90% of the time, as well as the floor, windows, doors, mixing desk and any other hard furniture. It is not as unnatural as many people assume. It is like saying being out in nature feels unnatural due to the lack of walls surrounding you...

[DanDan]

Just some historical tweaking...... Hidley made his first Non Environment Room in 83. I suspect there were examples before that, but perhaps he coined the name.
Newell and many others adopted the notion and continue to do so. Some apply their own acronyms to it, which to me seems a tad needy....
The Direct Paths of LEDE and RFZ should be devoid of early reflections and thus colourations. Working in very varied studios prior to my study of acoustics, I did not discern differences caused by the 'type'. They are all anechoic and pure. The abiding problem has always been that quite a few designers of studios and speakers or of the opinion that Flat response is desirable.