Designing My Soffit Mounts

#46

So I thought some more about it & I think its fine?

I realized that the graph I linked above compares double-leaf & triple-leaf systems that use the same overall thickness & mass.

They're essentially doing the tests with the same number of drywall sheets & wall space. like so:

So it makes sense that the triple-leaf system would perform poorly in the lower frequencies, because the resonance frequencies of the air gaps have increased because they're smaller. The larger the air-gap, the lower the resonance frequency.

(Would recommend reading Stuart's thread on MSM (double-leaf) systems in walls, if you haven't already. It explains all of this v well: https://www.digistar.cl/Forum/viewtopic.php?t=413)

MY situation differs in that I'm not sacrificing any amount of spring gap (my spring being rubber instead of air) for the sake of making it triple-leaf. The gap inside the speaker box is unchanged.
The wall equivalent would be keeping the original, large air gap & adding another air gap+drywall combo on one side. The resonance frequency would stay the same because the original air-gap hasn't been changed and the isolation above the resonance frequency would be improved by the new 'layer' (of course, you might not want to do this as you're spending more money on materials & sacrificing room space for a thicker wall).
THAT SAID, in a wall situation it would still be better to use that space for one large air-gap, as you can drive the resonance frequency even lower & get much better isolation of the bass frequencies.

But in my case I can't make my Sorbothane any thicker than 1", so my best option IS to go triple-leaf.

One thing that DOES carry over to my situation is that by separating the housing box from the overall soffit structure I'm effectively 'splitting the mass', which can result in a higher resonance frequency. However the weight of the housing box pales in comparison to the weight of the entire soffit structure so really I'd only be splitting the mass a little bit. Theoretically the benefits should massively outweigh this very slight pitfall.

What should result from my triple-leaf system is a low-end performance roughly equal to that of a double-leaf system, but a much better performance in the mids & highs. Sort of like this:

#47

I feel more often than not with these things that simple is better. I wonder by having so many layers/leaves each floated using Sorbothane is making things complicated in a way that could be detrimental. Maybe start with a design like mine and see how you get on? If that isn’t as good as hoped, taking off the baffle to add additional Sorbothane damping/isolation won’t be that difficult? Better to start with solid framing and see how you get on, I wonder?

I won't lie I was right on the cusp of abandoning the triple-leaf system but then I realized I still needed 2 6x6 sheets (Gelmec only sell 1" sheets in 6x6 and 12x12 and i need around one and a quarter sheets of 6x6 for the inside of the housing boxes). I could've cancelled the washers but I figured I may as well try & make sense of the triple-leaf pitfalls and I could always sell the washers if I decide too late.
Feeling a lot more confident about the triple-leaf system now

I should imagine there will be other elements of the room/design that have a greater impact on the sound than this?

You are almost certainly correct about that lol

#48

So I finally got around to doing my first REW test!

First I sealed all the gaps & holes with Sikaflex PRO 11-FC (the sealant that Stuart recommends). I got a really good deal on a box of sausage refills (£35 for 20 300ml sausages!!!) but they only came in brown so now my soffit structure looks like a big cake

(Tip for fellow idiots: ALWAYS wear disposable gloves when handling Sikaflex. It gets everywhere and is VERY hard to get off your skin. Also, wear crappy clothes cuz if you get any Sika on that, it is never, ever coming off, which isn't ideal if your sealant is poo coloured)

For the mic position, instead of hanging a plumb bob all the way to the floor, I used it to align the tip of the microphone with the stem of the stand. This way the mic is always at the same angle (around 70°)

Then I simply align the base of the stem with the mark I made on the floor

For the height, I put the stand next to a speaker and adjusted the height until the tip was aligned with the acoustic axis of the speaker. I then wrapped the stem in thick tape so the height was locked

So now the mic position should be roughly the same for every REW test I do :yahoo:

Here are the results
SPL (both speakers playing together):

SPL (each speaker):

Impulse (both speakers):

RT60 (both speakers):

: RT60.jpg (46.9 KiB) Viewed 28348 times

: RT60.jpg (46.9 KiB) Viewed 28348 times

Waterfall (both speakers):

Spectrogram (both speakers):

LR Difference:

This last one is for determining if my speakers are sufficiently balanced & if my mix position is optimal, but I feel like I've done something wrong...
For some reason my difference graph shows my L speaker as getting gradually louder as you go higher in frequency? From 1KHz upwards it fluctuates around the 5dB line, indicating that the L speaker is 5dB louder for most of that freq range, but look at the 'SPL (each speaker)' graph - that's clearly not the case. In fact, from around 2KHz upwards, the R speaker is louder.

I did the right thing, didn't I? I went to All SPL>Controls>Trace Arithmetic, selected L and R as A & B respectively, then generated an 'A / B' graph

I figured IF the graph is correct, then doing B / A would give me an exact mirror flip around the 0dB line. Here's what I got:

The curves starts off perfectly inverted around the 0dB line, until around 200Hz where the 'reflection line' starts to rise, with both graphs fluctuating around it?
Is this a calibration problem or something? Maybe an issue with the smoothing? If you look closely you can see that (B/A) isn't even an exact inverse of (A/B) at certain points above 200Hz, so I think the trace arithmetic function might be broken

I tried generating A / B difference graphs using other people's .mdat files to see if they match the ones posted on the thread & I get the same weird result - they're identical to begin till around 200Hz, then my one slowly starts to rise. Here's one from a 2017 JS thread:

And here's my re-creation:

Seems identical at first, but look at mine closely. It's rising! From 1KHz - 5KHz it's barely touching the 0dB line!

My mixing position needs to be optimal before I move onto the next step. I may well have to take several REW tests with the mic at slightly different positions to find the best one, but I want to make sure I'm generating the LR Difference graph correctly. Can anyone help me out with this?

Here's the MDAT:

snailboyawayy baseline test w soffit frame.mdat: (8.29 MiB) Downloaded 955 times

snailboyawayy baseline test w soffit frame.mdat: (8.29 MiB) Downloaded 955 times

#49

So I did an A*B on my LR Difference graph and its inverse and it pretty much confirmed my suspicions:

If REW were working properly (A/B) would cancel out (B/A) and this resulting line would be completely flat at 0dB, but from around 200Hz upwards it starts spiking.

Here it is with 1/6 smoothing:

Not really a gradual rise like I initially suspected, but still

#50

So I sought help from John Mulcahy (the author of REW) and he told me the apparent slow rise is an artifact of the way smoothing works on difference curves that include room reflections.

Basically it is possible to generate a usable difference graph, but only for the direct sound from the speakers. You do this by reducing the window. Here it is with a 1ms window:

As you can see the product of the curve & its inverse has resulted in a completely flat line at 0dB, like we wanted.

Problem with this is the traces cut off at 1KHz since the window width sets a lower bound on the frequency response.
To generate a difference graph for the entire frequency range you need to select the frequency dependent window window option (the default 15 cycle setting is probably fine). That has the effect of a variable window which gets narrower as frequency increases.
Here is how that looks, with the default 500 ms RH window:

THIS is what I wanted, pretty much
(FYI the (A/B)*(B/A) isn't flat for 80Hz and below because noise is interfering)

So basically I can still use difference curves for figuring out the mix position in relation to the speakers. The difference graph only becomes problematic once you start including room reflections, so as of yet I'm unaware of a good way to ensure the speakers are balanced in relation to the room beyond making sure all your measurements are correct and just eyeballing the acoustic responses.

#51

That is interesting! Something seems strange there. I'll send a message to John Mulcahy (the developer) and see if he can shed some light on this. Maybe it is phase-related, or simply due to the two measurements not being taken exactly identically... or gremlins in the software!

- Stuart -

#52

Soundman2020 wrote:Source of the post That is interesting! Something seems strange there. I'll send a message to John Mulcahy (the developer) and see if he can shed some light on this. Maybe it is phase-related, or simply due to the two measurements not being taken exactly identically... or gremlins in the software!

Read my last post Stuart (post #50), I already contacted John and he explained the issue. I can still use difference graphs for direct sound from the speakers but it becomes problematic when you start to include room reflections (it is minimum phase related).

Here's the full thread: https://www.avnirvana.com/threads/probl ... etic.8993/

#53

there are the two domains - frequency and time. symmetry is both physical boundary and the balance of absorption (generally some way of converting the acoustic energy into heat) and reflection control (scattering, diffusion, reflection). so once you have adjusted the design (or existing room) to balance the frequency response, you need to control the timing of the responses from parts of the room. anechoic is generally not good (but where you like to see speaker tests and their frequency response), reflective is also bad because... (too many reasons why). as noted by the window settings to include or exclude reflections, you learned the impact of them from a frequency perspective, but your hearing integration of the timing signals is what will be equally if not more important. if the sound arrives late enough to cause your brain to recognize them as separate sounds, you won't have a strong comb filtering effect in your brain because it can reject those, opposite, if the sounds are perceived as a single source, then how does your brain then differentiate (and thus not integrates) the comb filtering and timing impacts which skews the interpretation of the sound... so controlling the "windows" inside the room become the challenge to a well balanced room, and thusly why people have tried all kinds of room models to address it: controlled response, LEDE, RFZ, etc etc in an effort to find the magic solution.
all to allow someone to make decisions on a mix which ultimately will be listened to as MP3 on ear buds :horse:

#54

Hi gullfo! Fancy seeing u here

symmetry is both physical boundary and the balance of absorption (generally some way of converting the acoustic energy into heat) and reflection control (scattering, diffusion, reflection).

It's funny, if you look at the AV Nirvana thread I linked in post #52, John seemed to imply that attempting to balance the sound from the speakers post-reflections/absorption would be detrimental (though it's likely I was and still am misunderstanding him - you'll see he gets a little impatient with me at one point). That's what you mean when you say 'balance of absorption' right?

#55

not sure of the discussion, but you do want symmetry throughout the critical listening space - front back up down left right. how you achieve that balance and how big the listening location is, has been the challenge for the past 100 years... :ugeek:

so yes, a balance of the absorption/diffusion/scattering in terms of frequency response and time responses. as a guess - using digital or EQ etc to adjust the sweet spot is a nice bit to add after all the other work is done so as to tweak things which can be tweaked.

#56

(though it's likely I was and still am misunderstanding him - you'll see he gets a little impatient with me at one point).

He's like that! The guy is a software and math genius, and understands his field amazingly. But like most geniuses, he can be a bit short with the rest of us average Joe's! He tends to explain things in rather brief, terse and sometimes technical jargon, and assumes that everyone else should just get it, without question.

John seemed to imply that attempting to balance the sound from the speakers post-reflections/absorption would be detrimental

As Glenn mentioned, the issue is to FIRST balance the room acoustically as well as possible, eliminating all early reflections, or at least reducing them greatly in amplitude. Then, once the room is properly treated acoustically, that's the point where you can think about digital tuning.

I suspect that the point John was trying to make is that you cannot EQ a reflection, and trying to do so will create other issues that you might not even be aware of, except to note that the room just sounds "off" in some sense that you can't quite put your finger on. Reflections create phase issues, such as nulls or peaks in the response. You might be able to reduce the peak, but you cannot "fill in" a null. If you attempt to boost the frequency where the null is by applying an EQ filter boost to it, you probably won't actually raise it at all (since you are pumping more energy into the direct wave, but also the same amount of "more energy" into the reflected wave, so they still cancel each other out). Even worse, you run the risk of causing the unstable combination of reflection plus filter to "ring": the extra energy you are applying in the frequency domain, also has consequences in the time domain, as Glenn pointed out. The same applies to applying a cut to the frequency where a reflection null occurs: the combination of reflection plus filter can be unstable, and cause "ringing".

Here's a practical case, that explains it in simple REW graphs. I'm in the process of doing some digital tuning on a client's room right now, so I did just that (applying a filter where it shouldn't be), and here is the result, in the time domain:

There's a peak there at around 64 Hz, and one might think that a small tight PEQ filter right there would get rid of it, and in the frequency domain, that is true! You can actually "knock down" the peak, as you can see below:

If that's all you look at, the frequency response curve, then you think you did a great job, because you flattened out the peak! But that's only the frequency domain. Now look what happened in the time domain: Before applying the filter...

And after applying the filter...

Ooops! There's some nasty ringing going on now, because the unstable combination was triggered! Now, by carefully adjusting that filter in ways I'm not going into here, you can actually get rid of both the peak AND the ringing, like this:

That was possible in this specific case, because the room is already well treated, but that isn't usually the case. And here we are talking about bringing down a peak, which sometimes is possible, but trying to "fill in a dip" is an entirely different matter. Dips are phase problems, and should be dealt with using acoustic treatment, not EQ.

So, I'm guessing that's what John was talking about, in his cryptic way: trying to use EQ to fix reflections is a fool's errand. They should be fixed with treatment. EQ is the wrong tool for the job... sort of like trying to put in a nail using a screwdriver, or put in a screw using a hammer!

It's important to use the right tool for the right job. Deal with reflections in the initial room design an in the treatment. Deal with pure SPL issues using EQ.

As Glenn so rightly pointed out, EQ can ONLY be used successfully on a room that has FIRST been properly treated, acoustically, leaving only problems that can be fixed electronically.

So, the way you get symmetry in a room, which is critical for a control room, is firstly by balancing the layout of the room, and the treatment of the room, such that the speaker response is also already balanced (in other words, acoustic loading on the speakers is about the same, and the path taken by sound coming out of one speaker is a mirror image of the path taken by sound from the other speaker). And when that is achieved, with good overall acoustic response n the room, then you can use digital tuning as the "cherry on the cake".

That's the general process, but in practice, tuning a room is nowhere near as easy as it looks! And certainly is nothing like the marketing hype you seen on the glossy advertising for "room correction" hardware and software.

- Stuart

#57

I'm back!

Resumed work on the soffits after Xmas and I finally made it to the hanger stage!

That's 12mm ivory insulation board with 50mm RWA45 rockwool on either side.
I intended for there to be small gaps between the hangers but the cavities ended up being tighter than anticipated and rockwool's thickness can vary, so the hangers are touching (only just though).
Is that fine or is worth fixing? I could shave off the rockwool a bit on all the hangers so the gaps are larger?

I figured I'd hold off on the bottom hangers until after I'd completed the housing box stage as I may well have to remove the lower plywood platforms (hence why I haven't lined the walls of the speaker cavities yet either)

But before any of that, it's time for an REW test:

What the hell??
What's with those massive dips at 3.5K and 10K? And that massive boost at 14K?
Is my mic or interface broken or is this the kind of thing I can expect after adding the lining/hangers? I swear that's the only thing that's changed in the room!

Here it is next to the Baseline test I made a few months back:

If this is the hangers doing then surely this is a massive step back, no? Even the bass peak looks like its gotten worse! Should I be worried? Can someone tell me if I've done anything wrong or if it looks like something is broken?

Here's the mdat:

snailboyawayyy lining + top hangers (possibly faulty).mdat: (8.36 MiB) Downloaded 643 times

snailboyawayyy lining + top hangers (possibly faulty).mdat: (8.36 MiB) Downloaded 643 times

#58

you've added something to the overall box-i-ness in the corners as well as highly absorptive space above. you're testing an incomplete system where you altered the LF cavity volume (less). and now you also have exposed a massive deep broadband absorber in an otherwise untreated room :-)

i'd say add the lower hangers and mock up some baffles for those speakers and then test...

#59

I intended for there to be small gaps between the hangers but the cavities ended up being tighter than anticipated and rockwool's thickness can vary, so the hangers are touching (only just though).
Is that fine or is worth fixing? I could shave off the rockwool a bit on all the hangers so the gaps are larger?

Ideally, you do want small gaps there, and it's not just for acoustic reasons: that area is also part of the cooling path for your speakers. The ventilation air rises up through that section, then exits out the inner sides of the soffits, into the central space between them, so it's good to have a clear air path there.

One thing you could do, is to just take off the insulation on the inner side frames and replace it with something much thinner. That would give you a little extra space to spread the hangers slightly further apart, so there could be a bit of a gap between them.

I also realized, from going back over your thread, that I never did respond to the whole Sorbothane / 3-leaf discussion: Yes, you are right in your conclusion. There's plenty of isolation, going down plenty low enough. There's also the issue that the two sets of pads are actually acting on different vibrations, going in different directions. The ones inside the enclosure box are mostly acting on the relatively large front-to-back movement of the box (having that sub driver and the air that it moves at high power levels, all slamming around forwards-and-backwards, is a big deal). The pads under the box, are dealing mainly with up-and-down vibrations. So the axes of the two systems do not really coincide very much (only a small amount) About the mass issues: As you surmised, there are three rather different masses involved here. 1) The relatively small mass of the speaker alone inside the enclosure box. 2) The larger mass of the speaker PLUS the enclosure box, resting on the base pads. 3) The mass of the entire soffit structure, along with the room, building and planet! So essentially, you can set the mass to "infinite" (or just a very large number) for that third calculation. Thus, in short: Yes, it is a 3-leaf system, but no, you don't lose anything with this design. Good isolation, going down low. It took me many years to develop and perfect this proprietary design. It's more complex than it seems at first glance!

- Stuart -

#60

you've added something to the overall box-i-ness in the corners as well as highly absorptive space above. you're testing an incomplete system where you altered the LF cavity volume (less). and now you also have exposed a massive deep broadband absorber in an otherwise untreated room i'd say add the lower hangers and mock up some baffles for those speakers and then test...

Hmm okay. I figured that adding 2 or even just 1 bass trap would yield nothing but improvements. It certainly has in other areas (e.g. waterfall) but how is there a boost at 14K now? What's exciting those high freqs? And there seems to be v little improvement in the bass (in fact the bass peak is now worse for some reason). It's likely I don't fully understand how these things actually work

I guess I just wanted to check if I should be worried or not. I'll press on and make the housing boxes, lower hangers & baffle before taking another test, if you think there's no cause for alarm (I'd originally planned to take an REW test after every step, but now I'm thinking that might not be necessary)

Ideally, you do want small gaps there, and it's not just for acoustic reasons: that area is also part of the cooling path for your speakers. The ventilation air rises up through that section, then exits out the inner sides of the soffits, into the central space between them, so it's good to have a clear air path there.

But there's plenty of space under the hangers, and the baffle won't be so high that it would block that gap:

As you can see, I designed it so that the upper hanger cavities are completely exposed the room, at the cost of having a shorter baffle (plenty of airflow space under the hangers)

Unless you think this is a bad move, acoustically? The alternative would involve cutting part of the hangers to facilitate the higher baffle, which seemed like it was more trouble than it was worth (and yes, this would also mean the air gap under the hangers is mostly blocked):

One thing you could do, is to just take off the insulation on the inner side frames and replace it with something much thinner. That would give you a little extra space to spread the hangers slightly further apart, so there could be a bit of a gap between them.

Yes, I wish I'd done this at the start but doing it now would mean re-positioning all the hooks in the ceiling, and good God was that ever a pain in the arse the first time round! I'll definitely make sure there are gaps between the lower hangers but I'm not particularly keen on re-doing the top ones.
I figured it wasn't a big deal if the hangers were slightly touching as I'd seen other studio designs on here/JS where this was the case (I recall seeing pics of hangers that were fully squashed together), but if you think it's worth going to the trouble of fixing then I guess I'll figure out a solution.

I also realized, from going back over your thread, that I never did respond to the whole Sorbothane / 3-leaf discussion: Yes, you are right in your conclusion. There's plenty of isolation, going down plenty low enough. There's also the issue that the two sets of pads are actually acting on different vibrations, going in different directions. The ones inside the enclosure box are mostly acting on the relatively large front-to-back movement of the box (having that sub driver and the air that it moves at high power levels, all slamming around forwards-and-backwards, is a big deal). The pads under the box, are dealing mainly with up-and-down vibrations. So the axes of the two systems do not really coincide very much (only a small amount) About the mass issues: As you surmised, there are three rather different masses involved here. 1) The relatively small mass of the speaker alone inside the enclosure box. 2) The larger mass of the speaker PLUS the enclosure box, resting on the base pads. 3) The mass of the entire soffit structure, along with the room, building and planet! So essentially, you can set the mass to "infinite" (or just a very large number) for that third calculation. Thus, in short: Yes, it is a 3-leaf system, but no, you don't lose anything with this design. Good isolation, going down low. It took me many years to develop and perfect this proprietary design. It's more complex than it seems at first glance!

That's great to hear! Very glad I went down the 3-leaf route

Soundman2020 - Studio Design Forum