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Some Flush Mounting questions
Old 28th February 2014
  #1
Gear Maniac
 
beau's Avatar
 
🎧 15 years
Some Flush Mounting questions

Hi, This would be a question for some of the designers on the forum.

Assuming the design is a smaller NE type of room with flush monitors.


1. There is usually major trapping on the ceiling to absorb ceiling bounce. But would it be crazy to put a hemholtz resonator box under the console to target and absorb the 118hz null caused by the floor bounce. Maybe this has been done, I just haven't seen it.

2. Also, is there any advantage to angling the portion of the flush mount wall above the speakers, to shape low frequency radiation towards the rear of the room? Would this help with ceiling bounce? The isolation ceiling is only 9'5"

I'm attaching a sketch up file and some screen shots, as I am reworking my control room.

Thank you in advance for your help.

Beau
Attached Thumbnails
Some Flush Mounting questions-screen-shot-2014-02-28-9.01.54-am.jpg   Some Flush Mounting questions-screen-shot-2014-02-28-9.01.21-am.jpg   Some Flush Mounting questions-screen-shot-2014-02-28-9.00.52-am.jpg   Some Flush Mounting questions-screen-shot-2014-02-28-9.35.48-am.jpg  
Attached Files
File Type: skp CR Revamp 2 Flush Wall.skp (1.38 MB, 126 views)
Old 28th February 2014
  #2
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John White's Avatar
 
🎧 10 years
Beau,

1. If an NE type room, the ceiling, et al is entirely absorptive so ceiling or any other "bounce" is a non issue especially as high as 118Hz.

2. Since the ceiling is entirely absorptive, there seems to be no reason to direct any frequency to anywhere. Regardless, the frequency would also be limited to that above the dimensions of your reflective area assuming the directivity of the speakers even radiate in that direction.

Since your ceiling is only 9.5', this may limit "complete" absorption in the lower frequencies. Still, the frequency in question is plenty high in order that complete absorption should not be the remotest of issues.
Old 28th February 2014
  #3
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🎧 5 years
You can read more about NE rooms here: A Proposal for a More Perceptually Uniform Control Stereophonic Music Recording Studios
Old 28th February 2014 | Show parent
  #4
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beau's Avatar
 
🎧 15 years
Hi John,

Thank you for your help here. A few more things.

I am aware the concept of NE rooms is to be entirely absorptive minus the flush mount wall and floor. I guess what I am asking, is would changing the shape of the flush mount wall above the speakers (basically changing the shape of the isolation shell, as the speakers see the room) help redirect any low frequency bounce from the ceiling through the ceiling absorption, (approximately 20" broadband absorption) back down to the listening position. Other frequencies will obviously be absorbed, but since low end radiates in a spherical pattern, it concerns me.

Would the above idea be better than having the area above the speakers remain vertical and than the usual trapping?


Also, regarding question 1. I think I will build it and try it, but have there been success stories of putting some sort of trap underneath the console to trap the Low Frequency floor reflection?

Thanks again.

Beau

Quote:
Originally Posted by John White ➡️
Beau,

1. If an NE type room, the ceiling, et al is entirely absorptive so ceiling or any other "bounce" is a non issue especially as high as 118Hz.

2. Since the ceiling is entirely absorptive, there seems to be no reason to direct any frequency to anywhere. Regardless, the frequency would also be limited to that above the dimensions of your reflective area assuming the directivity of the speakers even radiate in that direction.

Since your ceiling is only 9.5', this may limit "complete" absorption in the lower frequencies. Still, the frequency in question is plenty high in order that complete absorption should not be the remotest of issues.
Old 28th February 2014 | Show parent
  #5
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beau's Avatar
 
🎧 15 years
Thank you Adrumdrum. I own 2 books by Newell. I have read them and read them again. Thank you for your help.

I realize I am asking a very specific question that the answer very well could be. "that's a dumb idea" or "I've tried it, it doesn't work" or even "just build it and find out"

We all know how much attention to detail it takes to get flush mounting right, and I would like to build it only once

All this being said, I am going to build a hemholtz box and place it under the console and measure what happens to the dip at listening position caused by the floor bounce.

Beau


Quote:
Originally Posted by adrumdrum ➡️
Old 1st March 2014
  #6
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John White's Avatar
 
🎧 10 years
Beau,

It appears to me as if you are asking the same question. If so, I have the same answer. Please clarify if this is not the case.
Old 2nd March 2014
  #7
Gear Guru
 
1 Review written
🎧 15 years
Redirect

I do not believe an angled Front Wall will direct LF energy downward from the ceiling.

A big Helmholtz under the console may have some effect on a height mode if tuned exactly to it, but I wouldn't be too hopeful about the floor bounce. A blocking wall under the console might help, if there is a floor bounce path under there.

DD
Old 3rd March 2014 | Show parent
  #8
Gear Maniac
 
beau's Avatar
 
🎧 15 years
Thanks DanDan,

Maybe "Floor Bounce" is the wrong term? I have a fairly beefy analog console that will be blocking any mid / high frequency floor reflections, so I am not worried about that. (comb filtering from the top of the console will be another issue for a later date) I'm concerned about any phase issues being caused at the listening position due to the difference in time it takes the reflected (low frequency) sound vs. direct path. I posted a screen shot of the Floor Ceiling Bounce Calculator in my original post.

Since there is usually one major peak and one major null in the low end, I was wondering about placing 2 hemholtz, or limp mass, etc. absorbers tuned to each frequency at the reflection point hoping to absorb said frequencies causing the phase issue at listening position to be minimized. So far, the plan is to have 18" porous absorption above front half of the room to help with ceiling reflection, with the understanding I may have to place some tuned traps in the ceiling for extra help.


Am I way off on this?

Thanks,

B


Quote:
Originally Posted by DanDan ➡️
I do not believe an angled Front Wall will direct LF energy downward from the ceiling.

A big Helmholtz under the console may have some effect on a height mode if tuned exactly to it, but I wouldn't be too hopeful about the floor bounce. A blocking wall under the console might help, if there is a floor bounce path under there.

DD
Old 9th March 2014 | Show parent
  #9
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akebrake's Avatar
 
🎧 5 years
Quote:
Originally Posted by beau ➡️
.... Maybe "Floor Bounce" is the wrong term?
Beau,
the "Floor Bounce" term is often used in this forum and most of us know what you mean.

Floor bounce isolated (theoretically) can easily be calculated.

In reality a mix of direct sound plus all primary reflections from floor, ceiling and all the walls plus (some milli seconds) later, all the modes start piling up

Quote:
......Since there is usually one major peak and one major null in the low end, I was wondering about placing 2 hemholtz, or limp mass, etc. absorbers tuned to each frequency at the reflection point hoping to absorb said frequencies causing the phase issue at listening position to be minimized....
(bolded mine)

The floor bounce is not a ”118Hz issue” because we have two strong+ more fairly ”broadband” LF- waves with Path Length Differences (delay) to the arrival of the direct sound.

As Helmholtz is a resonant Hi-Q absorber and the signal we like to attenuate is LF broadband say 30-200 Hz -(non resonant) bouncing.
IMHO it seems to be the wrong cure...

Best

Ake
Old 20th March 2014 | Show parent
  #10
Gear Maniac
 
beau's Avatar
 
🎧 15 years
Thanks Ake,

I understand how all the reflections from walls and ceilings will cause peaks and dips at mix position due to timing / phase. So I guess my simplest question would be, can I eliminate the null or dip caused by the floor bounce by simply trapping that frequency so that it never makes it to the listening position? It seems this is a common cure for other surfaces, including ceilings. I just never see it being used on a floor under a console.

B
Old 20th March 2014 | Show parent
  #11
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adrumdrum's Avatar
 
🎧 5 years
The console should be angled to create vertical RFZ.

unless you have built the room in your drawing I would recommend to follow the
geometrics and treatment seen in the drawing in the
the link I posted earlier.
Membrane trapping(thick black line), acoustic hanger traps(behind false walls/ceiling) and 30 degrees flush mount speakers.
Old 22nd March 2014 | Show parent
  #12
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akebrake's Avatar
 
🎧 5 years
Quote:
Originally Posted by beau ➡️
Thanks Ake,

.... So I guess my simplest question would be, can I eliminate the null or dip caused by the floor bounce by simply trapping that frequency so that it never makes it to the listening position?
You're welcome!

Unfortunately, I don't think so. If that would be a simple remedy, why aren't prof studios using the technique....


Quote:
It seems this is a common cure for other surfaces, including ceilings.
I just never see it being used on a floor under a console. B
But of coarse you can measure with / and without a large absorber on the floor.

Quote:
Since there is usually one major peak and one major null in the low end, I was wondering .....
(above quote from earlier post)

Good observation! And me too, just wonder why, …
This is a common issue, if that will give you some comfort…
(Genelec scientific papers
A Survey Study Of In-Situ Stereo And Multi-Channel Monitoring Conditions
http://www.genelec.se/documents/publ...s/aes111th.pdf


My guess is:
1. Most loud speakers and listeners are placed at "similar distance" from boundaries in medium and small control rooms.
E.g. typical 1.2 m (4 ft) above floor. The ceiling is usually (too) low and sidewalls are too close. More PLD:s adds together and gives cancelations in the 100 Hz area.

2. Absorbers in the room not efficiant enough at low frequencies.

3. It might also be the technique we are using. Trying to measure floor bounce (e.g what happens within the first couple of meters after direct sound) with a 100 meter IR window (300 ms). Actually a ”steady state” measurement)

Everything that happens within that measuring-window will affect the measured result.
This means it is tricky to know what is the floor bounce alone.
Everything counts.

Someone suggested a deep (and large) pit filled with soft absorbent in front of the console. But...because ceiling height is often restricted this is probably no option of practical/ economical reasons.


You need VERY efficient wideband absorbers for the first bounces from all boundaries close to the loudspeaker. You only have ”one chance”!
Much later, the modes start piling up (which take several reflections)
Modes can be attenuated by less effective absorption material as the sound encounters these traps many times,

NE-style:
I think the Newell design with a lot of special material is complicated and expensive to build.
BTW Newell mentioned trap under the floor in the article linked in earlier post by adrumdrum

Take a look at Boggys design ideas. (Soffit speaker thread)
https://www.gearslutz.com/board/9322567-post3.html

Best

Ake
Old 24th March 2014
  #13
Gear Head
 
🎧 15 years
Beau,
I also tinkered with the idea of a NE style room, so have considered these this as well.

1) I have seen some earlier room designs (Hidley I think) with a large floor trap for this reason. The general advice is that a large format console will largely redirect this floor reflection, so extra trapping is overkill. If you don't have a large console this becomes more of a consideration.

It is worth noting that you won't have a single problem frequency, but rather a comb filter starting at 118Hz in your case. This filter will rise and fall as you move from the sweet spot. Lets assume that >250Hz will be reflected by the underside of your console. Helmholtz traps are known for being hard to tune and pretty narrow Q. I think it would be better to use a widely tuned panel absorber to cover 100-250Hz. Tim's limp mass or VPR come to mind.

2) Thomas (of Northward) has suggested that his isolation shell is shaped to control the bass propagation in the room. To do this effectively the isolation shell needs to be 100% reflective at LF (i.e concrete). Also, the actual behaviour is hard to predict without modelling software, and in a small room this volume may be more useful for absorption.
Old 27th March 2014
  #14
Gear Maniac
 
beau's Avatar
 
🎧 15 years
Thanks guys! I did go with a slightly different design. I will post pictures and measurements soon. upon first measurements, I am seeing a null at 103. which could be due to my 9.5' ceiling height. rear half of the room has 1' pink fluffy covering entire walls and ceiling. rear wall has 20" of pink fluffy. front half of the room has 3" rock wool safe and sound against the ceiling. below that is 9" pink fluffy in a 4'x7' cloud. The console is a harrison 3232c. Not "large" but still able to redirect low frequencies, I suppose.

Regarding the 103 null at mix position. I can hear an abundance of 103 ish down under the console. I built a Tim's limp mass absorber, and placed it under the console. even one box, 4" deep, 14" wide 20" long reduced the null by almost 3 db. It seems like they are worth experimenting with more. I would think that 6 of those would really make a dent.

B
Old 3rd April 2014 | Show parent
  #15
nms
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2 Reviews written
🎧 10 years
Quote:
Originally Posted by beau ➡️
Thanks guys! I did go with a slightly different design. I will post pictures and measurements soon. upon first measurements, I am seeing a null at 103. which could be due to my 9.5' ceiling height. rear half of the room has 1' pink fluffy covering entire walls and ceiling. rear wall has 20" of pink fluffy. front half of the room has 3" rock wool safe and sound against the ceiling. below that is 9" pink fluffy in a 4'x7' cloud. The console is a harrison 3232c. Not "large" but still able to redirect low frequencies, I suppose.

Regarding the 103 null at mix position. I can hear an abundance of 103 ish down under the console. I built a Tim's limp mass absorber, and placed it under the console. even one box, 4" deep, 14" wide 20" long reduced the null by almost 3 db. It seems like they are worth experimenting with more. I would think that 6 of those would really make a dent.

B
Can you post your room measurements? I'd like to check it out. What did you use for the limp mass absorber? 1lb MLV?

My (flush mounted) room has a 16 degree sloped ceiling for first half the room. by far the area that measures loudest in the sub freqs is the rear of my room and there are no ceiling areas which measure anywhere near that in the front end. Difficult to be certain if it's better or worse for LF though unless you build and test both ways.. but with it angled I have the option of cutting out holes for recessed trapping boxes on the other side such as Tim's or even broadband I suppose. I need to get my hands on some MLV and test I think. I'm currently in the process of doing the acoustic treatments.
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