Questions about my flush mount plans

[beckyrec]

Hi Folks, here are my plans for the flush mount for my JBL 4425.

Maybe some experienced user has some comment.
I want to cover the front with 5cm of Fermacell to have enough mass, and a thin layer of nice wood, for optical reasons.

I took the center of the speaker, put it in 1,50m height and angled the whole thing about 10 degrees.
Then I will hit exactly the sweet spot in 2,50. Listeners ear position at 1,10m

The box for the speaker stands on a pole filled with sand to make it solid.
The space behind the speakers I want to fill with Sonorock, to have a superchunk.

My questions:
Superchunks or Hangers?
The area below the speaker: Should I make it refelctive (drywall) with a port for bass absorption, or should I cover it with fabric for better working superchunks`?

Same post at John Sayers...

[spice house]

Forgive my ignorance, but whats a superchunk? I've seen it thrown around Sayer's forum but in not sure exactly what it is.

[SAC]

Superchunk

[audiothings]

I'm no expert, so please do your double checking:

Superchunks or Hangers?

If you are not venting the front wall, I don't think there is any major advantage in putting any absorption behind the speaker, other than marginally increasing the efficiency of the wall as a membrane absorber. If you are using the wall as a membrane absorber, you'd want to put your insulation very close to, but not touching the front wall. Once again, an absorber like this will be effective over a very narrow bandwidth in a range that might not be useful to you. If, on the other hand the insulation is touching the front wall, the bandwidth of absorption is increased at the cost of reducing its effectiveness. Experts please clarify.

The area below the speaker: Should I make it refelctive (drywall) with a port for bass absorption, or should I cover it with fabric for better working superchunks`?

Adding a random port would make it a helmholtz absorber at some random frequency. You'd have to get lucky for this frequency to be in the range that you want absorption. Covering it in fabric results means that you are not mounting it in-wall at all. If it were me, I'll make the wall massive and sealed.

Watching this thread.

[beckyrec]

The upper wall (where the speakers sit in) I will make solid as I can and non membran, not absorbing.

The bigger questions I have on the lower part of the wall.
The bass port I saw on John Sayers Forum:
If the size of the port is big enough, the helmholtz effect will be not important anymore.
Filling the space behind with Sonorock is easier than building hangers.
But which version is better?

[SAC]

The edge in the baffle mount in the design will defeat the purpose - it is not flush.

This will result in diffractive sourced superposition.

The purpose is not to extend the baffle a few inches, but to make the mounting surface flat and contiguous and as large as possible.


Also, you ask about the performance of some assembled idea from another forum. Did they measure the performance? And the HH placement corresponds to to the modal distribution? It is not quite as easy as saying you will include a laundry list of lots of technologies. How are they appropriate and optimized? With all due respect, I suspect we could ask you more questions about the proposed performance than you can ask us.

For instance, hanging baffles can be quite effective as velocity based absorbers. But the use of velocity based techniques employed perpendicularly to any reflective surface (thus being perpendicular and negating much of any velocity based incidence) inside of the pressure zone of a pressure based HH resonator...is this an optimal application? Sure, it seems to include LOTS of potentially useful techniques...but... (IOW, what is the velocity of the energy inside a partially sealed pressure based device in an area of maximal pressure and minimum velocity???)

And is the "cavity packed with insulation" supposed to function as a membrane resonator????

And have you considered the impact of making the wall non-reflective?

Too many factors are dealt with as if they function independently of the system in which they have been placed rather than making sure all of the components function synergistically in the manner assumed.

And dare we ask, why are the monitors not positioned on a horizontal plane, eliminating an unusual axial polar distribution from which you move in and out as you naturally move back and forth? If you are designing the 'wall', I would definitely mount them flush in a horizontal plane and avoid unnecessary and undesirable angles and additional reflections - building in additional complexities rather than avoiding them altogether.

Also, the speakers must be mechanically decoupled from the mount and the surrounding wall surface using the appropriate elastomeric or spring/elastomeric shock mounts!!! Otherwise the wall simply becomes a secondary transmission source featuring absolutely horrendous transients and group delay!

[beckyrec]

No, theres no edge in the baffle.

The surface I will put on is exactly 5cm so it will exactly fit to the speakers front.

[audiothings]

What material is the "acoustic hanger" in the pic made of?

[beckyrec]

In my pictures there is no hanger.

In John Sayers drawing its (I think) a panel from HDF oder fibre, wrapped with mineral wool.

[audiothings]

In John Sayers drawing its (I think) a panel from HDF oder fibre, wrapped with mineral wool.

That kinda makes sense... Its the kind of thing that Philip Newell uses around his NE rooms. If they were simply porous insulation, as SAC points out, it would not make sense.

[johndykstra]

No expert here, but I've read a bit on soffit mounts so:

The absorption on the bottom of the soffit walls. There's two elements at play in that Sayer's soffit drawing.

1. The hard soffit wall is recessed under the speakers. This is to allow a few inches of absorbent in front of the baffle to minimize reflections bouncing from the back of a console. From what I have read, the minimal amount of baffle inconsistency is battled by this absorbent, and diffraction is kept to a minimum... A necessary evil to eliminate those pesky console reflections. The face of the absorbent should be flush with the hard surface above it... again, minimize diffraction. If you are employing a minimal work surface, with no console, I would extend the hard face baffle down as far as possible.

2. That particular design is for powered monitors. Thus there is an air flow running at the floor behind the soffits and through to the top. It's my understanding that because of this air channel, the space is filled with absorbent... an effort to maximize the space as best possible. The hangers at the bottom, in theory, should allow more air flow than pure absorbent. Thus, better speaker cooling, and also, adding to the effectiveness of the rest of the absorbent at the top of the soffit wall for bass absorption.

On the bottom, we have the luxury of the "console killing absorbent" on the front. This is enough to minimize resonate properties of the baffle wall. On top (and around the speakers), the absorbent within the wall is responsible for this... thus the cavity being packed in these areas.

Again, I believe the main purpose of the absorbent behind the wall is for resonance reduction. Because the wall is opened for air flow cooling considerations, the absorbent within should be seen as "bonus bass trapping"... and not a huge participant in your bass trapping scheme.


In a perfect world, (I think), one would have passive monitors, and a minimal work surface.

This way, there's no need for air flow for cooling, and no need to minimize solid console reflections. In this situation, I would extend a hard baffle to the floor... however, I would employ a perforated panel in place of the absorbent bottom half, and really get the most out of those "corner traps".


Corrections welcomed.

[johndykstra]

Also, I would minimize or eliminate the tilt of the monitors. I can't really see the rest of your plans, so this may be a necessary evil, but more times than not, you are increasing the likelihood of reflections off the top of your work surface. And at the same time, increasing diffraction effects from where the lower straight wall and it meet.


Again, this is with all things the same, and knowing nothing else of the design.

Not sure what HDF is... from what I understand, the board of a hanger is generally homasote. This may be the same thing.

[beckyrec]

Thanks!

Without angling its much easier to build.
My plan is a small as can mastering desk.

I want to use the space behind the speakers as a bass trap, of course.
So I think I will do the John Sayers Design but with superchunks.,
If there will be active monitors in future tim I ca change it.

[johndykstra]

superchunks with no recessed baffle would account for some amount of diffractive loss. How much? No idea. My guess? There's a reason it hasn't been a more documented plan. However, it would be quite simple to start with a super chunk, perform acoustic measurements, and temporarily cover the chunk with a hard baffle extension and perform the measurements again.

In fact, PLEASE do this!

[johndykstra]

Originally Posted by beckyrec

Without angling its much easier to build.

also, with level mounted monitors, you will experience less phase shifting when moving your head forward and back.

[audiothings]

also, with level mounted monitors, you will experience less phase shifting when moving your head forward and back.

good point makes perfect sense but never occurred to me previously.

Not sure what HDF is... from what I understand, the board of a hanger is generally homasote.

HDF is high density fiberboard / hardboard. It is denser than the MDF normally used to build loudspeaker cabinets. It is far more dense than homasote.

[jhbrandt]

The HDF between the insulation can be anything to facilitate hanging. 3mm plywood works good too.

One point that was missed and often overlooked by flush mount builders is the mass of the plane surface (the front of the flush mount). This should be as massive as practically possible. I usually use 2 layers of 3/4" (2cm) plywood & as said previously, should extend as far as possible.

Cheers,
John

[amishsixstringe]

I almost built my speakers tilting down in my soffits. I was talked out of it by people who knew what they were doing. I'm very grateful. They sound pretty awesome and the room/front wall isn't even finished yet. I positioned my speakers on heavy stands and kept changing heights and angles and things with a measurement mic. I found the best spots for them by making faux baffles of scraps of MDF I had laying around. Then I built the soffits. Don't just guess or use 'rules'. I found that moving the speakers 2 inches higher than originally designing for was best. I will set my chair up a little higher to compensate.

And do extend the baffle below into the absorbent area as per John Dykstra's suggestion. He knows what's up.

Neil

[beckyrec]

@John

Yes, my plan was:

First layer gypsum dryboard 2cm, then 2cm MDF, and then 1cm nice wood with glossy finish, nice veneer or similar.

[avare]

Originally Posted by beckyrec

First layer gypsum dryboard 2cm, then 2cm MDF, and then 1cm nice wood with glossy finish, nice veneer or similar.

There is no acoustic advantage to having 2 different materials under the veneer.

It should look fantastic!

Andre

[beckyrec]

Ah, ok. I thought its more solid with dryboard.

But its easier to use 4cm panel from wood and on top venner or maybe parquet, longside vertically.
I have to think about it...

Today we started building the frames. very exciting.

[beckyrec]

So today we started. We build the first frames, fixed the lower part to the wall. Its really solid. The upper part will be fixed to the wall also.

The broadband absorber for the side walls are ready also. I built them with perforated MDF panels. The front is angled, inside they are damped with rockwool.

[SAC]

How are you going to mechanically decouple the speakers from the mount surface and surrounding wall surface?

Have you sourced the appropriate elastomeric or spring/elastomeric shock mounts?

If you have not, STOP and do this before you proceed.

[beckyrec]

Thanks, mate.

Speakers will be decoupled by sylomer, (yellow one).
I will put it under the Box and to the sides.

[avare]

Originally Posted by SAC

How are you going to mechanically decouple the speakers from the mount surface and surrounding wall surface?

Have you sourced the appropriate elastomeric or spring/elastomeric shock mounts?

If you have not, STOP and do this before you proceed.

You have a reputiation for helping people by writing with information that people can gain from by doing some additional research, which is great.

Why are you asking the quoted questions a week after the OP started the thread and has started construction?

Andre

[beckyrec]

No problem.

I internalized your signature, Andre. :-)

[SAC]

Originally Posted by avare

Why are you asking the quoted questions a week after the OP started the thread and has started construction?

Andre

...cause I looked at the illustration and noticed that no accommodation was readily indicated...despite the issue being featured prominently in the original Genelec documentation to which we had referenced.

And it seemed prudent to mention it before they got too far into it where to accommodate the feature would have been prohibitive.

Next time when its my responsibility to design the project or to serve as project manager I will make sure it is addressed earlier...

[avare]

Originally Posted by SAC

...Next time when its my responsibility to design the project or to serve as project manager I will make sure it is addressed earlier...

No rudenes meant. You are wonderful.

Appreciatively,
Andre

[beckyrec]

work in progress...