Mass required for a new wall to redefine the room ratio

[ggegan]

When building a new wall within an existing concrete block room in order to improve the room ratio, how much mass is required to create a boundary that will redefine the room modes in the 20Hz-200Hz range? The goal is to prevent bass frequencies from penetrating the new wall and reflecting off the original concrete block wall. Is a double stud wall with two layers of 5/8" drywall on each side capable of defining a new boundary for bass frequencies, or would a concrete block wall be required?

[boggy]

Hmmm... brick wall or concrete wall will do the job, but... why you
change a room ratio?
Why not put more absorption there instead of building a stiff wall?
If you change a room ratio, you will still have room modes (at different
frequencies), and you still need absorption to fix this, but
if room has volume under 100m3 (~3500ft3), you can't have much
space for absorption... I'm really not sure if changing room ratios
is a best first step to fix resonances in room. Absorption is.
If you have a liberty to build a new room from zero, room ratios is
good to follow. (IMO)

Cheers,

Bogic

[jhbrandt]

Gary,

Two layers of 5/8" type 'X' gypsum board with insulation behind and an air space of about 4" will have a MAM resonance of about 38Hz and therefore offers a good mass boundary down to about 40-60Hz. Below that it will act as an absorber. - this is a good thing.


Bogic,

Sorry, but I disagree with you. tutt
The idea is to shift the modes into a pattern more appropriate for music production. Modal energy should be evenly distributed so that all the notes on the piano keyboard are 'supported'. Damping is then added to the room in the form of trapping so the these modes do not 'ring'. You can not fix modes with absorption. But you can dampen them enough to broaden the 'Q' or resonance peak.
Often a change in dimensions of only a few inches or centimeter can turn an otherwise problematic room into a very workable and useful room for critical mixing, etc.
Adding absorption to a room that has multiple coincidences, IMHO, is like trying to make a silk purse out of a sow's ear. - Or (like an athlete on steroids) if that's all you've got.. do the best you can until you can buy or build a better room.

Cheers,
John

[boggy]

Originally Posted by jhbrandt

.........
Bogic,

Sorry, but I disagree with you. tutt
The idea is to shift the modes into a pattern more appropriate for music production. Modal energy should be evenly distributed so that all the notes on the piano keyboard are 'supported'. Damping is then added to the room in the form of trapping so the these modes do not 'ring'.

Ok, I agree, but if you decrease room volume, you will have smaller space for dampening room modes (and we always have that space too small in small rooms). I expect that ggegan don't have a big room.

Originally Posted by jhbrandt

You can not fix modes with absorption. But you can dampen them enough to broaden the 'Q' or resonance peak.

I probably use wrong term for same thing... in my mind, "fixing" room modes
means dampening it. Sorry.

Originally Posted by jhbrandt

Often a change in dimensions of only a few inches or centimeter can turn an otherwise problematic room into a very workable and useful room for critical mixing, etc.

Still, I don't agree... if we don't know dimensions of room, or at least info about room volume, it's hard to define that basic rule... (IMHO)

Regards

Bogic

[DanDan]

With a concrete wall behind the studded plasterboard one, where is the real boundary? One is lighter but presented directly to the wavefront, the other is massive but much of the energy will be lost getting through the first one.
This seems like it would break up and increase the modal density. Furthermore the 38Hz absorption would surely be welcome. I would be careful not to brace this stud structure well.
On the other hand. One could use this space and construction resources to create a deep fibre trap fronted with slats to create some helmholtz an some diffusive effect.
This latter seems to me to be a more direct route to the finish line.
DD

[ggegan]

Thanks John and Dan.

I'm trying to estimate costs in order to determine the feasability of moving my mixing system to a new location. Your input is very helpful.

Boggy, the room would be reasonably sized for my purposes, approximately 2650 cubic feet or 75 cubic meters, with a room ratio of 1:1.28:1.54. The room volume without building the wall is approx 3510 cubic feet or 99.4 cubic meters.

There is an additional reason to add the wall beyond improving the room ratio - to create a VO/ADR booth. I'm trying to kill two birds with one stone. I could create a room ratio of 1:1.28:1.9 by building a wall that would only reduce the volume to about 3100 cubic feet or 88 cubic meters, or provide for a massive bass trap/diffuser like Dan describes that is approx 2' or 61 centimeters deep, but then there would be no room for a VO/ADR booth.

[ggegan]

Here's a sketch of the way I was thinking of dividing up the room. The outside dimensions of the room are roughly 11' x 14' x 24'. With the new walls in place the inner dimensions of the mix room would be about 14' wide x 17' long before any treatment. The VO booth would be about 6' x 9', all rough dimensions. Don't pay too much attention to the room treatment in the pic, they are just placeholders until I can test the final build and figure out what is really needed where. I figure I may not need a cloud if I do a drop ceiling with a couple feet of fluffy pink in the space above. Also, may go for all built in treatment rather than surface mounted panels if the facility owner is willing to invest the money.

[boggy]

Originally Posted by ggegan

Here's a sketch of the way I was thinking of dividing up the room. The outside dimensions of the room are roughly 11' x 14' x 24'. With the new walls in place the inner dimensions of the mix room would be about 14' wide x 17' long before any treatment. The VO booth would be about 6' x 9', all rough dimensions.

It's ok... I still believe that is not good for small rooms to spend more volume to reflect back (more) sound energy. Use it for absorption. Room modes in small rooms are more concentrated in low and mid audible frequency band... where you can't do virtually anything in such small space.

Originally Posted by ggegan

Don't pay too much attention to the room treatment in the pic, they are just placeholders until I can test the final build and figure out what is really needed where.

I see... loudspeaker placement looks ... hmmm.... I'm not sure.... but ... you will decide placement by measurements. For reference looks at:
"Multichannel surround sound systems and operations", AES Technical Council, Document AESTD1001.1.01-10, New York.

Originally Posted by ggegan

I figure I may not need a cloud if I do a drop ceiling with a couple feet of fluffy pink in the space above.

Good idea.
Usually that ceiling treatment is enough to calm down a big part of RT60 in untreated room

Originally Posted by ggegan

Also, may go for all built in treatment rather than surface mounted panels if the facility owner is willing to invest the money.

Built in treatment is better.
We use screws in some studios... most of construction/design can be moved elsewhere (with modifications, of course). All construction stand on floor, btw, and don't tuch walls/ceiling.

Good luck!

Bogic

[DanDan]

If you move your computers and other fan cooled noisy gear into a machine room/closet, the CR will be silent.
As it is treated and big it would probably be a much better sonic environment for voice. More social too. Unless you have a real purpose for the booth, e.g. privacy or whatever, I am inclined to prefer the idea of the bigger room.
However swings and roundabouts. Stud walls can be used to great effect at LF.
Try to find Jeff Hedback's design which was featured in Mix. I think the build is here somewhere. He did exactly what you are hoping for.

ModeWiz is a fabulous tool for virtually trying all variations of room dimensions, while applying up to 4 criteria, e.g. Bolt etc.

DD

[ggegan]

Originally Posted by DanDan

If you move your computers and other fan cooled noisy gear into a machine room/closet, the CR will be silent.
As it is treated and big it would probably be a much better sonic environment for voice. More social too. Unless you have a real purpose for the booth, e.g. privacy or whatever, I am inclined to prefer the idea of the bigger room.
However swings and roundabouts. Stud walls can be used to great effect at LF.
Try to find Jeff Hedback's design which was featured in Mix. I think the build is here somewhere. He did exactly what you are hoping for.

ModeWiz is a fabulous tool for virtually trying all variations of room dimensions, while applying up to 4 criteria, e.g. Bolt etc.

DD

I don't need a VO booth for myself, it was requested by the facility. There is another facility that has a slightly smaller room available. There would be no requirement for a VO booth there. In either location, the computers, expansion chassis and other equipment with fans would definitely have to be in a separate machine room, it's way too noisy for the mix room.