Designing My Soffit Mounts

#31

I’m very interested to see the results of these - hopefully you can do some REW measurements before and after.

Oo good shout, I'd forgotten about doing REW tests. I'll do one one immediately before & after each stage of the treatment (floating the housing box, hangers, loose insulation etc)

#32

Hi Snailboyawayyy.
Congrats on your awesome soffit design! Seems like you're really giving it the necessary attention to detail. This is my first post on this forum. I'm gearing up for a thread of my own, but still trying to absorb as much as I can from other people's projects that are already out there. Your thread, along with so many others on this forum, has been really helpful. I'm looking forward to seeing more posts and pics of how your project progresses.

I just wanted to ask, how did you do the awesome ray tracing for your RFZ on the first page? Was it directly in Sketchup? Are you using the full desktop version of Sketchup? So far I've been using the free web version, which is obviously limited in its features.

Thanks!
Mason

#33

Welcome to the forum MaseyRoy!

I already answered that Q on the first page of this thread:

So I used amray (https://amcoustics.com/tools/amray) for a general view of all the 1st reflections (the pics w/ all the blue lines - very rough as you have to trace lines over a top-down 2d export of your design), made a note of all the ones closest to the head then went into Sketchup and calculated the closest reflections w/ the protractor tool

And I'm using Sketchup Make 2017, which you can download here (it's free and apparently far superior to the online version you're using):
https://help.sketchup.com/en/downloading-older-versions

#34

Thanks Snailboyawayyy! Sorry, I must have missed that when I returned to the thread.
Cheers!

#35

So after many a cock-up, I finally finished my soffit framing. Phew!

I actually thought I'd finished a few weeks ago but to my horror I noticed that the horizontal studs weren't parallel to the plywood platforms (which are cut at the correct angle), so I knew I'd messed up the angle somehow. I realized that in the process of cutting & fitting the front horizontal studs I was unknowingly bending the whole middle section slightly to one side.

So I had to take out all the timber in the front, remove the horizontal studs, buy new timber, cut new studs, fit them again (this time making sure they were parallel with the plywood) and cramming it back in place.

Anyways, here's where I'm at:

I started working on the housing box design & I'm having some trouble figuring out the Sorbothane, specifically which duro to go for.

I've adapted garethmetcalf's design (he has Mackie HR824 speakers as well) to include Sorbothane isolators on top, bottom & sides of the speaker (i.e. inside the box). There will also be a 2nd leaf of isolators underneath the box, which will be pushed down with turnbuckles.

So I decided to start with the bottom isolators & input the following into the online Sorbothane calculator:

Assumed % Deflection: 20% (apparently the ideal amount of compression for Sorbothane)
Load per isolator: 13lbs (5 isolators underneath the 35lb speaker, with 30lbs of added pressure from above)
Excitation Frequency: 35Hz (my speaker's cutoff is -3dB at 37Hz so I figured this was fine (most ppl use 35Hz so its easier to compare results))

And here are the results I get for each duro (adjusting the length & width of the isolators by 1/8" increments until the deflection is close to 20%)

(Full size: https://i.ibb.co/Kyk9HTS/13lbs-2.jpg)
As you can see, the 2 duros that yield the worst % isolation are 30 and 50 (you'd expect the isolation to increase with each duro upgrade but no, it dips at 50 duro for some reason)
Here are the results when I use 6lbs (for the isolators on top of the speaker):

(Full size: https://i.ibb.co/SBLSpyZ/6lbs-2.jpg)
Similar results.

So why are 30 & 50 duro the most popular choice for people trying to float their speakers? Why not use 70 duro, as it clearly has the best % isolation? Is this because the isolators would be too thin and thus more likely to collapse to one side? Well then surely 60 duro is the next best option? Even 40 yields pretty decent results.

What's even more perplexing is that some people on JS contacted Sorbothane engineers and were recommended 30 duro for floating speakers???

Which one do you guys think I should go for?

#36

Hi Snailboyaway

Congrats on spotting the problem and dealing with it. Sometimes it would be all to easy to say f*** it, but it is better to correct those mistakes as you have.

Regarding Sorbothane, I have just used hemispheres to isolate my new Neumann monitors from the soffit wall, and I contacted Sorbothane, explained the issue and they recommended what to get. They said:
"You determine diameter based on the gap space between the speaker and enclosure. Hemispheres work best with a 30% compression so you determine height of the hemisphere by determining what would give it about a 30% compression in that gap."

Sorbothane recommended I contact www.gelmec.co.uk to buy them in the UK, and their advice was:
"By compressing the hemisphere by 20-30% allows the units to give optimum protection from shock and vibration. As you mention compressing the side units by this amount will optimise the hemispheres accordingly. As there will not be any direct load on the side and top units I do not think it is so critical which Duro, but would suggest 30 duro as these are the softest and easiest to compress."

I ended up with 50 duro for underneath and 30 duro for the sides and top:
bottom 0510120-50-10
sides and top 0510116-30-10

I can confirm that the 50 duro ones underneath, specified against the weight of my monitors, compressed perfectly under the weight of the speakers, and the sides and top ones the same once I'd got the box dimensions correct.

Hope this is helpful

Thanks
Gareth

#37

Dude I completely forgot that there's an entire section of this forum for construction threads! You hadn't updated your design thread in a while so I thought you'd frozen or even abandoned your build or were just building in secret :lol:

I'm legit floored at how much work you've done in so little time, well done! Puts my progress to shame :oops:

but then I assume you have a lot more building experience than me. Will defo be reading through your build thread later today :yahoo:

Regarding Sorbothane, I have just used hemispheres to isolate my new Neumann monitors from the soffit wall

So my concern with hemispheres is that I won't have the option to fine-tune the isolator size if need be AND I can't use the online vibration calculator to calculate the best possible isolation

I can confirm that the 50 duro ones underneath, specified against the weight of my monitors, compressed perfectly under the weight of the speakers, and the sides and top ones the same once I'd got the box dimensions correct.

But whilst you might be getting the right amount of deflection, it seems the natural frequency and % isolation are heavily influenced by the duro you use. In the results I linked earlier I tweaked the isolator size for each duro so they're all compressed at around 20%, and as you can see the 30 duro has 19% isolation (v poor) and the 70 duro has 89% (v good)

I also contacted someone from Sorbothane and tbh I was struggling to get straight answers. Kinda felt like they were just reading off a script. By the end of the conversation it felt like I was convincing them that 60 or 70 duro was the best option

The isolators for 70 duro would have to be really thin (likely to collapse to one side) though so I think I'll probably go for 60 duro, or maybe 40 if the isolators are still too skinny

#38

Hmm very good points and thorough research. I guess I can’t confirm or deny how much vibration transmission I’m getting and I guess it will be hard for me to ever know now!

You’re defo right to try and figure it out now before it’s too late and they’re buried in the wall like mine are!

Thanks for the nice comments. It’s been a ridiculous few months as I’ve been working at my day job four days a week, although I’ve taken maybe three weeks off work to build in that time.

Cheers
Gareth

#39

So I went with 40 duro in the end as I wanted a good compromise between high % isolation and isolator stability (e.g. if I went for 70 duro the % isolation would be v good but the isolators would be v thin & unstable, causing the speaker to shift over time. If I had really heavy speakers then I could go for larger isolators with higher duros, but alas)
Still not sure why 50 duro wouldn't yield better results than 40 but I'll just assume the calculator is accurate and hope for the best.

I also got some 50 duro washers for the turnbuckles (they only had 30/50/70)
Here are the Gelmec product codes for those who are interested: 0266100-40-10 (sheet) & 0510050-50-10 (washers)

garethmetcalf wrote:Source of the post I guess I can’t confirm or deny how much vibration transmission I’m getting and I guess it will be hard for me to ever know now!

Well theoretically your hemispheres should yield at least fairly decent results (at least 50% isolation at 35Hz for the 50 duro ones) plus you're combining the sorbothane approach for inside the box with John S's tight framing approach for the outside (i.e. a double-leaf system) PLUS your Neumann speakers probably vibrate a lot less than my Mackies so I wouldn't sweat it

#40

Also, here are some more framing tips for those of you who are inexperienced builders like me and want to avoid wasting time:

1.) ATTACHING TO HOLLOW WALLS
If you need to attach something to a hollow wall you should locate the studs already inside the wall (I knocked the wall & listened for changes in sound but you can also use a stud finder) and screw into them. I didn't bother with this at first because I thought it was very unlikely that my vertical stud would line up exactly with a vertical stud inside the wall. This was a mistake because most walls have horizontal studs that run along the entire length of the wall.
If for some reason you need to attach to the hollow wall alone then I highly recommend using metal hollow wall anchors WITH A FITTING TOOL. If you don't use a fitting tool the T-nut fangs on the anchor will eat into the plasterboard if you try to tighten it (if this happens, you can use a spring toggle fixing in the larger hole so it won't be wasted).
I wouldn't recommend using any other type of wall fixing (e.g. wall plugs, self-driving, plastic spring etc.) if you need your attachment to be very tight. I tried nearly all the different types before settling on the anchors and it was an extremely frustrating ordeal :cen:

EDIT: gullfo on JS suggested attaching a layer of plywood or OSB to the wall (screw it to the studs inside the wall) or running a 1x4 across the studs, 1/4 of the way from the top or bottom, and fastening to that

2.) UNWARPING TIMBER
So it turns out you can unwarp timber but its a bit more of a faff than simply weighing/clamping it down for a few days like I tried to do at first. Follow this here guide: https://www.diydoctor.org.uk/projects/warped-timber.htm (specifically the 'Using Pressure to Fixed Warped Timber' section). You'll need clamps, a large flat table, a bunch of hand towels, cling film & a heat source (I used a heat blanket). It took 7+ days but I can confirm that it does actually work.
I'd only recommend doing this if you're able to keep the timber straight by screwing it to a neighbouring wall/stud at several points, otherwise it might warp back over time (in which case I'd recommend just buying new timber).

3.) CLS vs. PAR TIMBER
I used PAR (planed all round) timber for my framing because I figured it would be better for studs that are attached to each other along their length (more tight & flush, less gaps), however you might want to go with the regular CLS stuff as it's cheaper and is usually stacked horizontally in timber yards (as opposed to PAR which is usually upright). This makes it easier to look at the end grains which will help you find timber that is less likely to warp (here's a good guide on how to do that: https://growitbuildit.com/how-to-buy-lu ... esnt-warp/)

4.) USE A NICE, WHOLE-NUMBERED ANGLE IN YOUR DESIGN
I used 29.4° and I really regret it. Wish I'd gone with 29° or 30°.

5.) DON'T USE GLUE IN YOUR FRAMING, ONLY SCREWS
If you make a mistake you don't want to be chipping away at your fully dried joining for hours trying to separate the timber, trust me. Thankfully I stopped using glue very early on & only used screws for the rest of the framing. Gareth suggested this on the 1st page of this thread and I wish I'd heeded his advice

#41

5.) DON'T USE GLUE IN YOUR FRAMING, ONLY SCREWS

Yes. Typically only the bottom plate is glued and screwed when framing.

That said, when I was growing up, my parents had a handyman who used construction adhesive and nails (he hardly ever used screws; guess that's why his prices were so good!) for EVERY project. Fortunately, he never did any framing, or who knows how it would have turned out.

#42

That said, when I was growing up, my parents had a handyman who used construction adhesive and nails (he hardly ever used screws; guess that's why his prices were so good!) for EVERY project. Fortunately, he never did any framing, or who knows how it would have turned out.

Interesting, it was my understanding that wall framing for houses etc. was usually done with nails

#43

Update: I'm beginning to reconsider my Sorbothane purchase...

As it currently stands my housing box design uses a triple-leaf decoupling system (I said it was double-leaf earlier, my mistake!) - there's a 'layer' of Sorbothane pads between the speaker & the housing box, followed by another layer of pads underneath the box (i.e. MSMSM). I figured that a 2nd wave of decoupling could only improve the overall isolation - turns out it's not that simple.

Here's a graph showing the isolation of a double-leaf vs. triple leaf system in walls (from Architectural Acoustics by Marshall Long):

As you can see, whilst a triple-leaf system has significantly better isolation than double-leaf above a certain frequency, the double-leaf performs better with the bass frequencies (until f0 where the mass law takes over & the performance is the same). The book itself says "In general, walls with double panels are more efficient for the isolation of music, where bass is the main concern, and triple panels for the isolation of speech", and this is apparently why studio walls are rarely triple-leaf (it's also cheaper & easier)

Apparently I'm meant to follow this here equation to figure out the two(?) resonant frequencies of the triple-leaf system, to make sure that fα & β are 'well outside the range where the system won't be isolating':

: 3-leaf-MSM-resonance-equation.jpg (32.39 KiB) Viewed 28510 times

: 3-leaf-MSM-resonance-equation.jpg (32.39 KiB) Viewed 28510 times

Where
fα & β = the 2 resonant frequencies in a triple-leaf system (because it's a 'two-degree of freedom system'?)
m1,2 & 3 = mass (kg/m^2) of the speaker, housing box & entire soffit respectively
M = m1 + m2 + m3
d1 & 2 = length of gap (0.8" in my case)
ρ0 (Rho) = density of rubber
c0 = speed of sound in rubber

This equation is for triple-leaf systems in walls, but Stuart has said that you can use it for soffits.
But I don't see how I could possibly know the mass of my entire soffit structure (in kg by m^2 no less), as well as the speed of sound in Sorbothane rubber.

I'm considering cancelling half of my order and just going with garethmetcalf's approach: keeping the 'inner layer' of Sorbothane pads but using framing outside the box to keep it rigid & tight (i.e. John Sayers' approach), or maybe I should stick with the triple-leaf system as it's much better in the mid-high frequencies?

Anyone here with any experience with this stuff want to weigh in?
(I'm guessing not but I figured I'd make this post anyway as it might be useful for some people)

#44

Sorry, I wasn't clear. I meant my parents' handyman would have NAILED the bottom plate, not anchored (screwed it). I'm functioning on too little sleep lately.

#45

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

Gareth

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