Soffit Mackie HR824? Barefoot or John Sayer

[darwin]

After reading the information provided in the SAE site, I really think it would be good for me to wall-mount my Mackie HR824s. However, after searching this forum on the topic, I am confused about the following post by Barefoot. I really want to soffit these speakers, but I don't want to make a colossally bad acoustic construction. Please, could you clarify the following issues?

https://homerecording.com/bbs/showthread.php?postid=233186&highlight=flush#post233186

Specifically, I don't understand the following statement and how it relates to soffit-mounting Mackie HR824s.


"1. No rear cavity - The problem with this one is self evident. The baffles must be "free floating". You can either suspend them from the ceiling or from an open frame mount. "

-I will treat your assesment that a rear cavity is problematic as gospel. I just don't understand what I should do. Should the back of the speaker rest directly against a wall built into the rear of the frame? I think a huge problem for me is that I don't understand what you mean by a "baffle" in this type of scenario.

Also, on the HR824s, there are several different ways that you can set the monitors up. For example you can set them up to be in the very corners of a room, or you can set them up to be more in the middle of a room. The dimensions of this control room are 10.42 ft (wall that speakers would be mounted against) X 11.83 ft (length) X 6.8 ft (height-it has a 4 in drop ceiling)

I had planned on putting some acoustic panels per SAE site under the monitors.

 

[John Sayers]

Really Barefoot (or perhaps sjoko) should answer this one but here are my thoughts.

Firstly are the Mackies suitable for flush mounting?? I have found that with a lot of these nearfield monitors that they are bottom end heavy in their design .intentionally and flush mounting could make the low end worse.

For an example of what barefoot is referring to in the baffle design check out sjokos monitor mounting.

http://www.locall.aunz.com/~johnsay/Studio/Pages/Sjoko_2.htm

where the baffles are free mounted.

cheers
john

 

[darwin]

Thanks for the reply. I hadn't stumbled on to that web site before. All of the pictures of various studios under construction are very interesting.

Now, I think I kind of understand the baffle that barefoot is referring to. It is a very "loose-fitting" construction around the speaker. I still don't understand if the rear of the speaker sets flushly against a wall constructed within the framework.

Of course, I now have more unresolved issues. For financial reasons, I really need to make the Mackies work as my monitors for the next 3-5 years. John, you seem to cast some doubt on the effectiveness of soffit-mounting them because of the magnification of LF. So, let me tell you what I was thinking of doing to help absorb the LF within the construction. Within the framework, the speakers would be resting on .15 meter deep wooden boxes filled with sand, and acoustic panels per the SAE site would be hanging underneath. The tallest length of panel that I could get would be 1.05m. If I didn't use the boxes, I could get 1.2m.

Additionally, after entering my room length dimension into the fundamental frequency calculator on the SAE site, I find that I have a fundamental standing wave at 48 Hz.

I can also change many of the settings on the back of the monitor. There is a setting for steeply rolling the low frequency off at either 37 Hz, 47 Hz, or 80 Hz.

So, given these construction options and speaker setting options, do you think that I would be better off to not surface-mount the Mackies?

Once again thanks

 

[John Sayers]

My concern is nor so much the room as the speakers themselves. From what I've read of the Mackies it appears they are pretty well loaded in the low end when free standing. I remember flush mounting a pair of JBLs once and the increase in low end due to the flush mount made the speakers sound awful with way tooo much low end. When the speakers were replaced with the LSR series everything sounded great again.

I think the sand idea you mentioned is a bit of over kill - so long as the framework is glued and screwed it should be rigid enough.

cheers
John

 

[barefoot]

darwin,

The rear port, or rear passive radiator in the case of the HR824s, should be as far away from any walls as possible if you want to flush mount this type of speaker. I've updated the drawing from that old thread to hopefully make things clearer. The green structures are the flush mounting baffles. The doubled arrowed lines indicated that the baffle edges and the rears of the speakers should be away from any walls. You can suspend the baffles with wires or build some sort of open frame to mount them.

The "A" Acoustic Space setting looks like it would be the most appropriate for flush mounting. But, you definitely should contact Mackie to make sure they even recommend flush mounting at all.

barefoot

 

[ausrock]

John and barefoot,

Some questions have arisen from your answers...........(apologies in advance if these are dumb questions);

1; Should the opening in the baffle be a neat fit around the speaker cab.

2; Is there anything to be gained or lost by covering the front or back of the baffle with acoustic absorbtion material.

3; I presume the density (resistance to vibration) of the baffle would be of some importance.

4; What are the advantages of soffit or baffle mounting over sitting monitors on stands, etc.

5; If I use this method for my Reveals (rear ported), what changes could I reasonably expect to hear.

 

[barefoot]

1; Should the opening in the baffle be a neat fit around the speaker cab.

The fit should normally be less than 3mm to push any diffraction issues above 20kHz. The Revels have a sculpted baffle, so you won't get an ideal mounting. You should mount the Revels sticking out a bit, so the deepest part of the speaker baffle at the top corners is flush with the mounting baffle. I've also suggested mounting the monitors to the wall separately from the baffles with a 3mm air gap on all sides of the opening. This would isolate the monitors from the baffle and prevent vibration transfer.

2; Is there anything to be gained or lost by covering the front or back of the baffle with acoustic absorbtion material.

It could be beneficial to cover the back in order to cut any standing waves that might form between the wall and the baffle. Don't cover the front.

3; I presume the density (resistance to vibration) of the baffle would be of some importance.

Yes, stiff, dense material with good internal damping is best. Medium density fiberboard (MDF) is an excellent material for this. It's often used for speaker cabinets and has a nice smooth paintable surface. You could make a very nice free floating baffle by sandwiching linoleum between MDF and void free plywood using all purpose flooring adhesive.

4; What are the advantages of soffit or baffle mounting over sitting monitors on stands, etc.

Read this post:
https://homerecording.com/bbs/showthread.php?postid=449183&highlight=flush+AND+monitors#post449183 It describes the biggest advantage, though there are other adavantages.

5; If I use this method for my Reveals (rear ported), what changes could I reasonably expect to hear.

The main sonic advantage is as I described in that post. The direct response and power response are matched. The Revels might have a low shelf boost like I described. Ask Tannoy about this. I wouldn't recommend flush mounting if they do have this type of filter.

If you do flush mount, the rear port is a problem. I would conservatively recommend that the path between the woofer and the port should be less than 1/4 of a wavelength of the highest wavelength at which the port works. The lower cutoff of the Revels is 65Hz, so the port is working to about 1 octave above this or 130Hz. A quarter wavelength at 130Hz is 0.65m. So, I would recommend a rectangular baffle about 100cm tall by 65cm wide, no larger. This will still flatten out the direct response well down into the upper bass, but it shouldn't have any adverse impact on the port response. Oh, and round the outside edges of the baffles with a large router bit or round molding.

darwin, you could safely make your shortest baffle dimension 1m (3 ft). Baffles 4ft tall by 3ft wide would be a good choice.

Of course, this all assumes the manufacturer has no objections to flush mounting.

barefoot

 

[Son of Mixerman]

Just as a quick note, I saw a thread over at the Mastering Engineers Webboard that the Mackies were not suppoosed to be Soffit mounted. If I remember correctly it had more to do the cooling of the amps on the back of the actives needing alot more circulation than most high end speakers. Someone could double check this to see whether its fact or fiction.
Another question it to whether nearfields should be flush mounted at all in a soffit? 3 ft equilateral triangle for optimum monitoring?? Sorry for the interruption

SoMm

 

[darwin]

Thanks for the very informative comments. I will contact Mackie and ask them if they recommend flush-mounting, however there are a ton of variables involved in getting a control room that is workable. So far, I haven't been able to get mixes that translate well outside of this room. Fortunately, I have read enough about acoustics and acoustic treatments to visualize several different variables that could effect "translateability". (i.e. portable acoustic treatments and the individual settings on the back of the Mackies)

This might sound kind of weird, but I have a process engineering background, and I am fairly comfortable with implementing "Design of Experiment" (along the lines of the Taguchi approach) studies to improve product quality. In this case, the product quality is "translateability". At this point in the game, I am not going to examine the effect of the flush-mounting variable because of the effort involved in making the construction. I have enough factors to look at as it is.

So, I think for now, I will see if this study will improve my situation. I will report back to you what Mackie says, and, if I do implement flush-mounting as a variable, I will, of course report those findings to the group.

 

[John Sayers]

just another question for barefoot.

What would happen to the speaker response if you blocked off the rear baffle in the Mackies and the Reveals and then flush mounted them ????

cheers
John

 

[ausrock]

Another possible option just occured to me which may give the appearance of soffit or wall mounting but should not obstruct either ventilation or sound...............

Bracket mount the speakers in the position they would be in if they were actually sitting in a soffit. But instead of using a rigid MDF baffle, construct a timber frame which would attach to the surrounding walls with an opening to fit around the speaker. Cover the frame in coloured hessian (burlap) leaving the speaker opening clear.........attach the frames to the walls with the speakers (bracket mounted of course) having the appearance of actually sitting in a purpose built soffit.

The rear walls could still have acoustic treatment, etc., mounted correctly, there would still be plenty of air around the speakers and you would have a neat looking monitor setup.

John, barefoot...........comments?

 

[John Sayers]

Cover the frame in coloured hessian (burlap) leaving the speaker opening clear

you wouldn't have a baffle then and it would be a free standing speaker

cheers
john

 

[barefoot]

John,

If you plug up the port in any speaker it turns into, well, a closed box speaker.

A given driver requires a larger ported box than a it does a closed box for "normal" alignments of each type. So, if you close off the port of a ported design the result will be a larger than normal closed box alignment. You'll loose about 1/2 octave or more of bass extension, but the response will be significantly improved.

The Mackie has a passive radiator (dummy speaker cone) instead of a port and it would be more difficult to disable, but the same principle hold principle holds.

If you did want to mount a rear ported speaker in normal soffit arrangement, you'd definitely want to disable the port. The interaction between the port and a small soffit cavity could cause significant response problems.

ausrock,

I don't see the advantage in this setup. It's basically like putting foam on the wall and hiding it with cloth. The open baffle I described would be better. Extending the baffles with cloth like you describe would be fine.

barefoot

 

[ausrock]

you wouldn't have a baffle then and it would be a free standing speaker

It was meant more as a asthetic or visual improvement as opposed to have monitors sitting on a shelf or stands, obviously there would be no acoustical improvement........well I doubt it

 

[ausrock]

The Revels have a sculpted baffle, so you won't get an ideal mounting. You should mount the Revels sticking out a bit, so the deepest part of the speaker baffle at the top corners is flush with the mounting baffle. I've also suggested mounting the monitors to the wall separately from the baffles with a 3mm air gap on all sides of the opening. This would isolate the monitors from the baffle and prevent vibration transfer.

Barefoot,

What if the front surface of the baffle was contoured to match the front of the Reveals..................would having a baffle with a sculptured surface create more problems. Again, I also see things from an asthetic point of view .

With a bit of imagination in design, it would still be possible with rear ported monitors to keep the baffle de-coupled from the speaker.

 

[barefoot]

What if the front surface of the baffle was contoured to match the front of the Reveals..................would having a baffle with a sculptured surface create more problems. Again, I also see things from an asthetic point of view

I like that idea! I would still make it all fade into a flat surface though. It would be the "baffle with the bulge"!

barefoot

 

[Dr X]

Reveal sculpted fronts

I have Reveals..does the sculpted front actually do anything or is it just for show?

 

[barefoot]

I guess it's an attempt to reduce diffraction. But, I think it's mostly just for looks.

barefoot