Air gap behind bass traps ~ how much is enough??

[College_Records]

I have heard that 4" off 703 with a 4" air gap behind it is an excellent bass trap. What would happen if I legthened the gap to 1', 3', 20' etc...

If my focal cms 50's only go down to 55hz, would I benifit from larger air gaps behind my panels?

Thanks so much - Andrew

[Jens Eklund]

http://www.gearslutz.com/board/5999249-post4.html

Not 703 and not 4" but you can extract the trends. I would not use a ratio larger than about 3-4 : 1. Also, If frequencies below about 100 Hz is to be absorbed effectively, I would definitely use pressure based absorbers.

[Glenn Kuras]

Putting a gap behind it will work and as Jens points out it can be even more, but when you are talking about 55hz I would straddle the panel (actually make it more like a 6" panel) or go with something tuned. Keep in mind though that even with something tuned it is going to take quite a bit of wall space.

[hendrikxix]

The purpose of the space between the wall (the reflective surface) and the fibrous absorber (703) is to move the absorber away from the wall where the velocity of the air particles is zero. Fuzzy material absorbs as a function of slowing down moving particles and converting that movement energy into heat energy. Newell (Recording Studio Design, Second Edition) uses the analogy of a kid on a swing to explain this concept. When the kid is at the highest point in the arc she stops moving before she goes back down, here motion is zero, but potential energy is the highest. If you want to slow the kid down with something absorbent (like a 2 inch wall of tapioca pudding or 4" of 703) you make the biggest impact when the kid is moving the fastest at the bottom of the arc not when the kid is moving the least at the top of the arc. So the most effective location to locate the material would be at the 1/4 wavelength of the frequency you are trying to absorb. This would be around 5.15 feet for 55 Hz which isn't practical, but you get the basic idea. You will still absorb some 55 Hz if you place the absorber closer to the wall.

Thicker is better with 703 to absorb lower frequencies, I definitely agree that 6" would be better.

The Newell book BTW is absolutely fantastic. Big props to Lou Clark from Sonic Space (Sonic-Space**?** Designing spaces that inspire the creative spirit) for recommending it to me.

[avare]

Originally Posted by hendrikxix

So the most effective location to locate the material would be at the 1/4 wavelength of the frequency you are trying to absorb. This would be around 5.15 feet for 55 Hz which isn't practical, but you get the basic idea. You will still absorb some 55 Hz if you place the absorber closer to the wall.

This is misleading at best by itself. The speed of sound in porous absorbent material is slower than in air. The 1/4 wavelength distance is shorter in the porous material. Sound waves at non-normal angles of incidence travel longer paths.

Andre

[avare]

Originally Posted by Jens Eklund

http://www.gearslutz.com/board/5999249-post4.html

Not 703 and not 4" but you can extract the trends. I would not use a ratio larger than about 3-4 : 1.

+1! or plus 3-4?

Andre

[junya-eskimo]

i just made some tuned traps..

The tuning worked bang on, 59 and 52 hrtz.

I have the fiber glass for the traps (12 inches) but i just made a new thread asking if i should leave air gaps in the tuned trap before i put it in and seal it.

Although i think i wont need as its tuned..But we will see./.

i will post my results soon on monday after i made them. But the traps are tuned to the frequency i wanted..

Might be the way to go.. I will let you know/.

John

[hendrikxix]

Originally Posted by avare

This is misleading at best by itself. The speed of sound in porous absorbent material is slower than in air. The 1/4 wavelength distance is shorter in the porous material. Sound waves at non-normal angles of incidence travel longer paths.

Andre

Well that's embarrassing! I must have misunderstood what I read, Andre. Or it's possible that what I read was wrong. I understand the idea that sound travels more slowly in the porous material because of the friction, but wouldn't the sound travel normally (~1130ft/s) in the air space both behind and in front of the fiber?

To make sure I understand what you're saying, let me do the math. Let's say that sound travels half as fast in fiber (like ~565ft/s.) So 565/55Hz = ~10.27. Divided by 4 for 1/4 wavelength is ~2.56 ft.

I wasn't able to find data for the speed of sound in fiberglass insulation only in solid fiberglass (2740m/s) which is not at all what we want.

So basically the slower speed of sound, the shorter the wavelength, and the shorter the distance needed from the wall to affect the frequencies that are desired.

I found in my own experience that fibrous materials in the middle of the room work much better for smoothing out frequency response and reducing room resonance than material on the walls. It's anecdotal evidence of course, but putting a half ton of fiberglass where the bass sounds the loudest in a space seems to work the best for controlling the bass in the room.

It seems like regardless of the speed of sound in any one material we are always trading off space to use for people and gear for space to dedicate to absorption or other treatment.

[liquidrum]

Any thoughts on effectiveness / efficiency on the "Super Chunk" with an air gap vs straddleing the corner with 703/705?

Should have clarified...100Hz and down...

[jwl]

Originally Posted by liquidrum

Any thoughts on effectiveness / efficiency on the "Super Chunk" with an air gap vs straddleing the corner with 703/705?

Should have clarified...100Hz and down...

It will be better, given an equal amount of surface area exposed in the room. The flip side is that it requires more 703/705. If you are working with limited quantities of 703/705 you are likely better off using panels straddling the corner (assuming they are thick enough) to get more coverage area in the room.

[localhost127]

Originally Posted by hendrikxix

I found in my own experience that fibrous materials in the middle of the room work much better for smoothing out frequency response and reducing room resonance than material on the walls. It's anecdotal evidence of course, but putting a half ton of fiberglass where the bass sounds the loudest in a space seems to work the best for controlling the bass in the room.

your ears are sensitive to pressure. porous insulation is a velocity-based absorber (not a pressure-based absorber). you shouldn't be blindly placing porous absorption at areas in 3D space where the particular frequency is "loud" as "measured" by your ears..

not to mention, your fibrous materials (traps) in "the middle of your room" need to be large with respect to wavelength - otherwise the waves are simply diffracting around the absorbers...

[College_Records]

Originally Posted by localhost127

your ears are sensitive to pressure. porous insulation is a velocity-based absorber (not a pressure-based absorber). you shouldn't be blindly placing porous absorption at areas in 3D space where the particular frequency is "loud" as "measured" by your ears..

not to mention, your fibrous materials (traps) in "the middle of your room" need to be large with respect to wavelength - otherwise the waves are simply diffracting around the absorbers...

Is there an optimal ratio or a program i can use to find the optimal measure of 703 to airgap then? I do not like the idea of difraction...

I'm working in a medium size room and buliding a small mixing booth Would I benifit from filling the airgap with fluffy R15?

[College_Records]

Originally Posted by junya-eskimo

i just made some tuned traps..

The tuning worked bang on, 59 and 52 hrtz.

I have the fiber glass for the traps (12 inches) but i just made a new thread asking if i should leave air gaps in the tuned trap before i put it in and seal it.

Although i think i wont need as its tuned..But we will see./.

i will post my results soon on monday after i made them. But the traps are tuned to the frequency i wanted..

Might be the way to go.. I will let you know/.

John

Looking forward to seeing the results! - Andrew

[hendrikxix]

Originally Posted by localhost127

your ears are sensitive to pressure. porous insulation is a velocity-based absorber (not a pressure-based absorber). you shouldn't be blindly placing porous absorption at areas in 3D space where the particular frequency is "loud" as "measured" by your ears..

not to mention, your fibrous materials (traps) in "the middle of your room" need to be large with respect to wavelength - otherwise the waves are simply diffracting around the absorbers...

I don't blindly put the fiberglass in the middle of the room as a wide bandwidth trap, it's just where it starts when I am working on treatment. I pile the materials in the center of the room leaving access to the walls. I assemble the frames and soffits and install the 703 into the framing. I was just making an observation based on my experience, that's why I was calling it anecdotal. I do trust my ears!

When I test scientifically I use ARTA, an Earthworks omni and a Burr-Brown preamp. That said, the best way I have found to choose placement is by playing filtered pink noise through the monitors and listening with my own two ears. If my science is inconsistent with my ears then I look deeper, but most of the time my ears are right on.

You can also do a fair amount of guerrilla testing by filling up a couple of contractor bags with pink insulation and duct-taping them to the walls or corners to see where the best locations are. It certainly isn't going to help much in the bottom couple of octaves, but it gives a pretty good idea of what will happen when you use some carefully constructed absorbers.

[junya-eskimo]

Originally Posted by College_Records

Looking forward to seeing the results! - Andrew

DUDE..

Just put it together..No air gap inside the trap. But about 4 cm off the wall because its ona chair and the back wont let me push it further.

-18 DB on target frequency... and in ratio for ones around it.

I just dont believe it..But AMAZING result.

Dude.. Helmholtz are (I know I shouldnt say it) THE SHIT !!!!
John

Good bye Room Mode