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anguswoodhead
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23rd January 2009
Old 23rd January 2009
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Unhappy Control Room Problem Dip between 70 and 120?

Hi guys.
I haven't searched around much, but....
I had an acoustics guy here today looking at my control room.
I have a big dip between 70 and 120 approx. (Seems this is quite common )
Everything is else is kinda sitting ok which is nice. (Not perfect but workable)

Short of re-building / re-designing the whole downstairs of our home (which is the control room, piano room which is open with the control room, bathroom and laundry he said there wasn't much I could do, without spending a lot of cash.
He suggested gluing 4mm ply on the back of my existing 6 wall mounted foam panels and mounting them 50mm off the walls.
The foam panels are 1.2m x 0.7m (75mm thick) - see photo.

Any other suggestions?
How does his suggestion sound?
Any help would be appreciated please.
I am going away for a weeks holiday but hopefully can find an internet connection on the island to check in for possible answers.
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Control Room Problem Dip between 70 and 120?-control-room.jpg   Control Room Problem Dip between 70 and 120?-studio-dimensions.jpg  
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#2
23rd January 2009
Old 23rd January 2009
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Quote:
He suggested gluing 4mm ply on the back of my existing 6 wall mounted foam panels and mounting them 50mm off the walls.
You must have misunderstood him or the guy does not really understand this stuff. Mounting a rigid piece of wood to the back of those panels and spacing them off the wall will do NOTHING to help. Actually I could see making things worse as you are closing in the early reflection points (now closer to you). I would agree that spacing them off the wall is correct BUT you do not want any kind of RIGID WOOD ON THE BACK.

For the specific problem at 70 and 120 the spacing will not help. What you REALLY need is rigid fiberglass or mineral wool based panels straddling corners. Foam is ok for the upper stuff but not so good on the low end.

Glenn
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23rd January 2009
Old 23rd January 2009
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Just to add to Glenn's correct comments, you'll never have a perfect bass response. It just doesn't happen in a room that size. But replacing or covering the corner foam with real bass traps will definitely help. Especially in the wall-ceiling corners. Adding yet more bass traps on the front and rear walls will help further.

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23rd January 2009
Old 23rd January 2009
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Thanks for your replies guys.
Are you saying that I need to replace the current foam corner bass traps with something different?
They are a Fonic product - I have the Low Mid Trap behind the Bass Trap - Cliff Electronics - Acoustic treatments by FONIC ACOUSTICS
Then do some bass trapping between the walls and the ceiling down the sides?
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23rd January 2009
Old 23rd January 2009
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Quote:
Originally Posted by Ethan Winer View Post
Just to add to Glenn's correct comments, you'll never have a perfect bass response. It just doesn't happen in a room that size. But replacing or covering the corner foam with real bass traps will definitely help. Especially in the wall-ceiling corners. Adding yet more bass traps on the front and rear walls will help further.

--Ethan
Just been checking out your website again Ethan (Acoustic Treatment and Design for Recording Studios and Listening Rooms).
Using the room ratios of 1.00 / 1.60 / 2.33 if I build the back of my control room in to those ratios that would make big difference right?
My height is 2.4, my width is 3.86 so just make the length 5.59
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23rd January 2009
Old 23rd January 2009
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Angus, thanks for the call today. As I said on the phone, generally I don't recommend reducing the size of a room to get better ratios. There are exceptions, but for the most part having a larger room volume is preferable, because any room anomalies can be dealt with via room treatment.

You've already gotten excellent advice above. Generally if you are still experiencing problems in your room response, it indicates that additional bass trapping is needed. It's difficult to tell what your existing panels are doing, since I couldn't find any specs on their site. So one solution is definitely to get additional bass traps into the room; the more you add the flatter the room will get.

Also, the placement options we spoke about could help, maybe help quite a bit. Experiment with that.

The rear wall is also sometimes the culprit.... put on some bass-heavy music (preferably complex bass, such as a master jazz bassist or something) and listen at the back of the room. Is there a large bass buildup there (there often is)? If so, the back wall is another great candidate for bass trapping.

Also, because your room is somewhat complex (ie, a non-rectangular room with several acoustically coupled spaces within the room), I'd suggest doing some experimentation as outlined on this page:
RealTraps - Filtered Noise
When you walk around the room with the test tones playing, you will be able to hear where the bass problem areas are in your room. Anywhere with heavy bass is a great place to put a bass trap.

Let us know how the testing goes....
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23rd January 2009
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Thanks for all your help mate.
When I get home in a weeks time I will try the things you have mentioned.
Cheers
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24th January 2009
Old 24th January 2009
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Quote:
Originally Posted by anguswoodhead View Post
Are you saying that I need to replace the current foam corner bass traps with something different?
YES!!! I'm going to reiterate to show importance... Foam is good for high frequency stuff but does not have the mass/density to have any effect on low frequency energy.

Not only do you need something dense like semi-rigid fiberglass, you need a decent amount of it. That is why it is always recommend to install a corner trap, and if you can, fill the air gap behind the face with fiberglass as well.

The other thing you can do is look into membrane absorbers like the ones Ethan makes/sells. Maybe that's what your "guy" was referring to when he told you to slap some plywood on the back of your panels? Although, membrane absorbers are not that simple by any means. If space is an issue, look into the membrane absorbers. Otherwise try to fill in your corners with fiberglass. I don't know the exact calculation to figure out how deep the corners need to be in order to flatten out 70Hz, but I'm sure someone else could point you in that direction. I'm guessing distance from the face of the trap to the corner needs to be around 3ft? Anyone?
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24th January 2009
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If you have a specific problem at frequency, let's say 80 Hz , you should use tuned resonant. Commercially the only brand I know that does this is RPG. There are other DYI solutions for this but must be measured in place

If the dip is something that covers a large number of frequencies from 70 to 120 Hz you can go broadband and place more bass traps in the place.

Another thing you should test is your position and speaker's location
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24th January 2009
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Quote:
Originally Posted by andrebrito View Post
If you have a specific problem at frequency, let's say 80 Hz , you should use tuned resonant. Commercially the only brand I know that does this is RPG. There are other DYI solutions for this but must be measured in place

If the dip is something that covers a large number of frequencies from 70 to 120 Hz you can go broadband and place more bass traps in the place.

Another thing you should test is your position and speaker's location
I wouldn't say you need to necessarily place MORE bass traps. You just need to widen the "Q" of the traps you are using.

You can create a somewhat tuned trap just by playing 4" of 703 or 705 across a corner in the room. The closer into the corner you place it the higher the frequency it will cut...the farther out from the corner you place it, the lower the frequency it will cut. If you fill in behind it with fiberglass (doesn't have to be 703/704, you can use R11 batt), it will widen the "Q" around that frequency and absorb a wider range of frequencies.

I could be wrong about this... but when I learned about acoustics and NRC/SAA ratings this was one of the things we learned. Granted that was a long time ago!!! hahahaha!!! But in my experiences this has held true.
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25th January 2009
Old 25th January 2009
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Thanks guys.
Could I put the fibreglass panels in front of the existing foam in the 2 front corners?
The ones I can get are 2400mm x 1200mm ?????
That would be floor to ceiling around 4 foot wide across the corner.
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25th January 2009
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If you can show me measured scientifical evidence of a tuned bass trap, similar to RPG MODEX, measured in a lab, just using rockwool I will be happy because I have not found data on that subject so far.

That kind of absorption as far as I'm concerned acts as a broadband and not a tuned one. If may have a larger peak at a certain frequency but it is not as precise as tuned systems.

For instance one of BBC papers has a tuned system where rockwool is placed in a box and even so it is not as tuned as the MODEX. Yet the principle is similar, membrane systems

Of course we can start discussing what is resonant and broadband absorption.

Check more info at:

RPG Diffusor Systems
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25th January 2009
Old 25th January 2009
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Quote:
Originally Posted by anguswoodhead View Post
Could I put the fibreglass panels in front of the existing foam in the 2 front corners?
Yes, this is what you should do. 70 to 120 Hz as stated in the thread subject is easily within the effective range of porous (fiberglass-only) bass traps. You just need a lot of them.

--Ethan
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25th January 2009
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Hi, we have been using a tuned resonance absorber from MB Akustik for a week now in our mix room. We placed the biggest one wich works from 30 to 90 Hz, wich is the area where the first three standing waves are in that room. It is placed against the back wall a few meters behind the mix spot.

It works very well, bass response is much clearer through out the room and especially in the mix spot. I have some more "normal" traps coming too to finetune the room some more, straddle ceiling corners etc.

I think both methods might work, low freq traps or resonance absorbers, I made this decision because I wanted to play absolutly safe with certain frequencies.

My 2 eurocents. thumbsup
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27th January 2009
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Thanks for your info guys.
I am starting to get an understanding of what to do.
Course if money was no object I would just pay an expert to come and 'fix' my problems - lol.

I think I will start with the fibreglass in areas where the bass is built up then go to phase 2 - Real Traps, Auralex, MB, more Fonic etc. if I need more help.
There are a lot of different products on the market - seems to be a debate of high density foam vs membrane type absorbers

At the end of the day for me it is going to be a matter of cash flow efficiency.
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27th January 2009
Old 27th January 2009
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Quote:
I think I will start with the fibreglass in areas where the bass is built up then go to phase 2 - Real Traps, Auralex, MB, more Fonic etc. if I need more help.
There are a lot of different products on the market - seems to be a debate of high density foam vs membrane type absorbers.
Not sure if there is much of a debate. Foam can work for the early reflections in the room but for bass trapping, rigid fiber glass or mineral based products leave it in the dust.
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1st February 2009
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Hi Glenn,

It's not really a matter of fibreglass or foam but rather the properties of each. We have a foam formulation that works as well as and in some ways better than fibreglass as bass traps. I do have certified lab test data if required.
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2nd February 2009
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Quote:
Originally Posted by AndrewSteel View Post
Hi Glenn,

It's not really a matter of fibreglass or foam but rather the properties of each. We have a foam formulation that works as well as and in some ways better than fibreglass as bass traps. I do have certified lab test data if required.
It'd be great to see that just for posterity/curiosity sake.

Frank
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2nd February 2009
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I dont know much about membrane absorbers but these coefficient graphs dont seem very realistic,Such a wide Q while still maintaining very high efficiency...
I want to see some real measurments
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2nd February 2009
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Quote:
Originally Posted by anguswoodhead View Post
Hi guys.
I haven't searched around much, but....
I had an acoustics guy here today looking at my control room.
I have a big dip between 70 and 120 approx. (Seems this is quite common )
Everything is else is kinda sitting ok which is nice. (Not perfect but workable)

Short of re-building / re-designing the whole downstairs of our home (which is the control room, piano room which is open with the control room, bathroom and laundry he said there wasn't much I could do, without spending a lot of cash.
He suggested gluing 4mm ply on the back of my existing 6 wall mounted foam panels and mounting them 50mm off the walls.
The foam panels are 1.2m x 0.7m (75mm thick) - see photo.

Any other suggestions?
How does his suggestion sound?
Any help would be appreciated please.
I am going away for a weeks holiday but hopefully can find an internet connection on the island to check in for possible answers.
You also may be able to improve this deficiency by moving your monitors closer together. One of the big issues with near/mid field monitoring systems is that many people put up multiple LCDs and pull their monitors too far apart and lose too much of the low end coupling between drivers. Mark your speaker positions and then just try this before dismissing it. Your photo seems to support this, they are very far apart. I had a similar issue and corrected about 2/3 of my 80-100hz deficiency by doing this.
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2nd February 2009
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Quote:
Originally Posted by Beyersound View Post
You also may be able to improve this deficiency by moving your monitors closer together. One of the big issues with near/mid field monitoring systems is that many people put up multiple LCDs and pull their monitors too far apart and lose too much of the low end coupling between drivers. Mark your speaker positions and then just try this before dismissing it. Your photo seems to support this, they are very far apart. I had a similar issue and corrected about 2/3 of my 80-100hz deficiency by doing this.
Thanks - I will try that - but....
We ran test tones etc. with just a single monitor running and the problem was still there. (and with both running too)
These are Mastering grade speakers and are pretty damn flat - should be no big dips or peaks.
JLM Active Monitors
I have tried some of the things suggested but still no joy.
I am getting a new stand made out of metal for my desk so once that is built I will try the correct position (33% or 38% seating position) to monitor from.
Maybe the wooden table that my desk is currently sitting on (which is closed in on 3 sides) might be part of the problem.
The new stand will be a frame only and all open underneath - hopefully trouble free.
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5th February 2009
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Hi Weasel,

all listed here - Bass Traps

essentially if something was behind a screen so you didn't know what it was and it has the same properties as glasswool in terms of flow resistivity, it would be the same in every way. Porosity and tortuosity do make some difference but it is minor. We work to optimise all of these parameters in the foam manufacturing process. For the same amount of material we can get more LF absorption than glasswool.

Andrew
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5th February 2009
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Quote:
Originally Posted by anguswoodhead View Post
Thanks - I will try that - but....
We ran test tones etc. with just a single monitor running and the problem was still there. (and with both running too)
These are Mastering grade speakers and are pretty damn flat - should be no big dips or peaks.
JLM Active Monitors
I have tried some of the things suggested but still no joy.
I am getting a new stand made out of metal for my desk so once that is built I will try the correct position (33% or 38% seating position) to monitor from.
Maybe the wooden table that my desk is currently sitting on (which is closed in on 3 sides) might be part of the problem.
The new stand will be a frame only and all open underneath - hopefully trouble free.
That dip is also a by-product of standard drywall type construction. When you measure a single monitor you will see a bigger dip. Coupling obviously happens between two monitors and will help to fill in that dip. Try not spreading them more than 3-4 feet apart and correct your triangle for it. That will also put more in the nearfield. That is the system answer to the problem, the room is a different story. That is why you have people like Ethan and Glenn, that is their area of expertise.
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5th February 2009
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Quote:
Originally Posted by Beyersound View Post
That dip is also a by-product of standard drywall type construction. When you measure a single monitor you will see a bigger dip. Coupling obviously happens between two monitors and will help to fill in that dip. Try not spreading them more than 3-4 feet apart and correct your triangle for it. That will also put more in the nearfield. That is the system answer to the problem, the room is a different story. That is why you have people like Ethan and Glenn, that is their area of expertise.
Thanks - the walls are only a single layer of standard gyprock and have no glass in them at all.
Once my room is re-arranged I will try the closer distance - worth a try I guess.
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7th February 2009
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OK - got the stand built for my desk and axed the old table.
Put 5 sheets or 50mm semi-rigid glass around the front of the room.
Problem is virtually gone.
Also shifted my monitoring position to somewhere between 33% and 38% of the rooms length.
Experimented with having the monitors closer - didn't seem to make any difference and they were a bit blocked by my 2 x 22" screens when they were closer together.
I feel that the room looks a whole lot uglier than it did before but the sound is a lot flatter so worth it I guess.
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7th February 2009
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Broadband LF Control

Sounds like a very typical situation. I spent a couple of decades working this out and seem to have it well wired. The PRESSUREZONE™s are contributing to the cancelations and augmentations of the frequency range you are noticing and also the ones that are less pronounced. If you were to tune out a specific frequency, you would surely notice another problem. Also adding people, equipment and or furniture will shift/change things in a small space because of the relativity to the cubic dimension. This is why BROADBAND LF control is the answer. In small spaces, the crossover from ray acoustics to wave acoustics is typically around 400Hz-500Hz. This is where the BROADBAND absorber need to start working. 80+% at a minimum.

In a rectangular space, the zones are easily identified. Primary areas will be all corners including where the wall meets the ceiling. Putting LF absorption where the pressure is not loading will take the punch and efficiency out of the room so understanding the zones is critical. If you take an RTA and measure around the room while running pink noise, you find a rise in LF energy on most all of the boundaries but the majority of the SPL will be in the corners including where the ceiling meets the wall. The floor to ceiling occurrence should also be treated to further focus and tighten the response. A simple drop ceiling of 6lb fiberglass tile with a good foot or two of space above loosely filled with insulation will have a huge benefit as it can address the acute angle and the floor to ceiling occurrence in one swipe. Placing some diffusion tiles up there behind the listening position will further define the response. I like the RPG BADD panel for this because they will also contribute to the LF absorption while adding a bit of diffusion. The Skylines are more diffuse and mixing a few of those in works great too.

I will be publishing a study on the effects of this kind of treatment in the near future and hopefully it will help clear up some of these problems for folks. thumbsup
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8th February 2009
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Quote:
Originally Posted by Pelonis View Post
Sounds like a very typical situation. I spent a couple of decades working this out and seem to have it well wired. The PRESSUREZONE™s are contributing to the cancelations and augmentations of the frequency range you are noticing and also the ones that are less pronounced. If you were to tune out a specific frequency, you would surely notice another problem. Also adding people, equipment and or furniture will shift/change things in a small space because of the relativity to the cubic dimension. This is why BROADBAND LF control is the answer. In small spaces, the crossover from ray acoustics to wave acoustics is typically around 400Hz-500Hz. This is where the BROADBAND absorber need to start working. 80+% at a minimum.

In a rectangular space, the zones are easily identified. Primary areas will be all corners including where the wall meets the ceiling. Putting LF absorption where the pressure is not loading will take the punch and efficiency out of the room so understanding the zones is critical. If you take an RTA and measure around the room while running pink noise, you find a rise in LF energy on most all of the boundaries but the majority of the SPL will be in the corners including where the ceiling meets the wall. The floor to ceiling occurrence should also be treated to further focus and tighten the response. A simple drop ceiling of 6lb fiberglass tile with a good foot or two of space above loosely filled with insulation will have a huge benefit as it can address the acute angle and the floor to ceiling occurrence in one swipe. Placing some diffusion tiles up there behind the listening position will further define the response. I like the RPG BADD panel for this because they will also contribute to the LF absorption while adding a bit of diffusion. The Skylines are more diffuse and mixing a few of those in works great too.

I will be publishing a study on the effects of this kind of treatment in the near future and hopefully it will help clear up some of these problems for folks. thumbsup
Good to have you here Chris! I've heard great things from several different people that have worked in studios you've designed.

I'm interested to keep reading more of your posts.
#28
8th February 2009
Old 8th February 2009
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adding people, equipment and or furniture will shift/change things in a small space because of the relativity to the cubic dimension. This is why BROADBAND LF control is the answer.
That's a great point. Even if you measure and get everything tweaked "just so" with tuned absorbers, as soon as two people walk into the room all that careful tuning goes out the window.

--Ethan
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