John Sayers' Recording Studio Design Forum Archive

I am currently creating a panel absorber with an airgap and wood front panel to resonate and dampen to create deep bass absorption.

Air Gap question in general. Are air gaps implemented to deep bass traps to substitue those that cannot afford to fill it up with absorbers.
I am making this design.

In an MDF 18mm, 30cm deep box using 88mm rockwool (Bradford Soundscreen r2.5 batts) insulation. I am making two types, the order goes. 9mm mdf front panel version as my low bass trap and a 4mm plywood front panel version as my mid bass trap. 15mm air gap before the 88mm rockwool goes next. After the rock wool is a 20cm airgap.

Would these two front panels , the 9mm mdf and 4mm plywood I am using work as a panel trap with wide to semi wide absorption. Can I use pegboard to replace one of the different type panel for creating a better trap. What will using a peg board, the normal pegboard type from the hardware store do to this design. What frequencies do you think I will be getting with these traps, estimation please.

Would it be better to fill the 20cm airgap with polyster batting or other absorption materials, maybe foam or will that wreck the traps.

My aim are bass traps and would like it to be wide or atleast below the general troublesome bass frequencies. What can I do to this trap to make it better.

Also, would rolling up a an acoustic foam into a roll be equivalent to a bass tube trap. This rolled up acoustic foam, will that be any good at absorbing bass and placing it on the corners of my room. I mean rolling it up will make it super thick and thickness for foam means lower bass absorption, is that correct? Will leaving an airgap on mounting an acoustic foam onto a wall be the same to leaving an airgap with panels against a wall principle meaning, better bass absorption.

Thanks

Generally; the air gap between the front panel and the absorbent is mandatory for the unit to act as a panel absorber(un-perforated, that is) - behind that, doing a complete fill front to back tends to broaden the traps response, but also lessen the absorption by lowering the Q of the trap.

Using the information from MHOA by Everest, you could expect about an octave either side of center frequency for usable absorption (Coeff. of at least 0.5) but formulas rarely equal real performance as far as center frequency is concerned; a lot of this depends on the front panel mounting methods, and is very difficult to measure, much less predict. If there's room, it's better to build several absorbers each with slightly different effective internal depths in order to cover the range of frequencies desired. This can be done by using "false bottoms" inside the absorbers, setting the deepest one as your lowest frequency trap and adding 1/2" layers of MDF inside at the back for shallower traps without having different physical depths.

Two traps, each with 300mm depth, the 9mm MDF front will give approx. 40-45 hZ hZ center; the 4mm PLYWOOD front will give approx. 75-80 hZ.

Perforation raises the Fo, standard pegboard with 1/8" holes on 1" centers = a perforation percentage of approx. 1.25%, and would result in about 120-140 hZ peak when placed over a 300mm air gap. Keep in mind that for best helmholtz action the absorbent (cloth covered) should be up against the panel instead of leaving an air gap as in panel (membrane) absorbers.

Foam and air gaps; foam won't absorb as much per unit thickness as rigid fiberglass or rockwool, but the principles are the same - an air gap behind will lower the frequency where absorption decreases, whether foam or rockwool or sheeps wool, etc. Rolling up foam, unless you already have it, won't do as much as 4" thick rockwool diagonally across the corners.

Also keep in mind that panel/membrane traps do NOT work the same way as absorbent/broadband traps - absorbent traps need to be at the location of maximum VELOCITY of sound waves for best absorption, which is 1/4 wavelength away from obstructions - panel/membrane traps need to be at the point of maximum PRESSURE, which is MINIMUM velocity; this will be nearest any obstructions (like walls, ceilings, etc)

The basic way a membrane trap works is that the membrane is vibrated by sound pressure - this movement causes air to try and move within the trap, then the inner absorbent IMPEDES that movement; the complexity of the interstices (fibers/air spaces) of the absorbent causes part of the sound energy to be converted into heat through friction, removing that part of the energy from the sound level. The two mechanisms that reduce sound at resonance are the energy required to flex the front panel, and the absorption inside due to sound trying to penetrate the absorbent.

In order to place panel traps effectively, you need to know which modes of the room are generating the frequencies you're trying to trap - this can get pretty challenging, because sometimes tangential modes can look a lot like axial modes but won't be at peak pressures in the same locations.

If you can do a basic sketch of your space, including height, we can probably get a better idea of what you need and where to put it... Steve

I am a little confused, on the SAE site, it sais not to leave a gap between the vibrating panel and the insulation. They say the insulation is attached to the panel, but yet on most sites it sais to leave a small air gap. Which one is true.

With the pegboard version can you please clarify with me that the pegboard is attached to the cloth and the insulation. Is that correct. There are no air gaps between the insulation, the cloth and the pegboard.

If I was to stuff the 30cm airgap with insulation, it would broaden my frequency absorption range by 1 octave, is that correct?

The pegboard is I think made of melamine board, white on one side, brown on the other and the holes are 5mm and 25mm apart from one another. Are those frequecies you calculate correct, and what kind of result estimates would I get with stuffing the airgap with fluffy insulation.
Also with the resonating panel with fluffy insulation in the airgap.

Also, does the vibrating panel have to be sealed. I was thinking of just screwing it on and be able to change different density and types of wood wherever I position the traps when I need to.

With the cloth for the pegboard, can I put it on the outside of the pegboard rather than the inside where the insulation is. Will that make a difference to its function besides cosmetic reasons.

On a standard room, 4 meters by 6 meters and 9 foot ceilings, with plasterboard walls and ceiling and brick outer. What are the general troublesome frequencies for that type of room.

Are wide absorbing traps best used on the sides of the mix position or will bass traps be better in general on the sides. The traps on top of the mix position, what usually are best on that position. Bass traps, Midtraps, Diffusers, etc. All I am asking are just general answers without being to overaccurate where it requires measurements and whats contained in the room.

Would hanging traps on the ceiling using chains, unadvisable. Will it cause resonance. Should I place acoustic foam on the adjoining of wall to ceiling.
I was thinking of rolling up acoustic foam into tubes and have it run along the top of the walls where the wall and ceiling meets around the room to prevent bad acoustic build up. Is that a good idea. I cannot afford to buy those rectagular corner bass traps to run along those areas.

I just need a general idea answers, without being too specific with what my room consitists of.

Can you post a link to the SAE page you're referring to, so I can see where the confusion is happening?

Also, you seem to be asking me to recommend a cure for an unknown disease in an un-disclosed species; it's difficult enough when we KNOW a few things about a space - it's not possible when you don't want to give information.

NO, your pegboard won't work the same as what I quoted; mine was for 1/8" holes on 1" centers, with 1/8" thick pegboard. your 5mm holes are .2" diameter on 1" centers, and you didn't specify the thickness of the pegboard; this is part of the helmholtz equation, it affects the acoustic impedance of the holes which changes the frequency.

If you want specific answers, you can't be vague about your end of things... Steve

Sorry, i thought all pegboards were one type. O.K. to be more accurate, for I await your expertise response before I proceed with these traps.

The pegboards are:
- 5mm thick melmine, white side on front and brown at back. Reminds me of those backings that I use to get on those tv, video entertainment units. It becomes cardboard like flaky if you wear out the edges.
- the holes are about 4.8mm and spaced 25mm apart.
The depth of airgap after the insulation precisely would be 20cm on one and 18.5cms on the other.

Knightly or some other acoustic expert, using their ingenious acoustic calculating mind, can you please give me an estimate on what frequencies this trap will cover and what would I need to do to it to absorb below 200hz.

Are there anything else missing from my questions that prevent you from answering them.

Sorry, but for Helmholtz calculations we need the TOTAL depth from the inside of the front panel to the inside of the rear panel. So your total air gap would be 30 cm in each case.

Plugging your values into Everest's formula, I get approx. 50 hZ center frequency; this is mainly due to the larger holes; even the 1/8" holes in pegboard commonly found in the US, with the same spacing as yours, is too much perforation for good helmholtz action.

Again, can you post a link to the SAE page that has you confused? Thanks... Steve

i might just make panel absorbers with the 4mm plywood with an air gap between the front panel and the rockwool. Is it right to use mesh to keep that rockwool in place between the front panel and the rockwool. Does the front panel have to be sealed.

Il also make another panel which is just the same, but the gap between the front panel and the rockwool will be replaced with polyester baffling and i will remove the front panel and just place burlap to cover it. What frequencies do you think I will get knigthtly with this. What would I do to make any of these panels hit the 80-140 hz area.

In regards to the SAE article. I will have to look for it again. It was a result from one of my millions of searches i made on google, in regards to aocustic related seraches.

I am thinking of making a pyramid foam trap. Its where I will glue an acoustic foam into a hollow pyramid and then hang them up in the corners intersection. Will this work? I will make a tube version as well. Anyone have success with this.

Yes. If you're making resonant panel type traps they have to be sealed.

Right; panel (membrane) traps need to be sealed - and yes, the insulation inside can't touch the front panel, or it damps the movement and the trap doesn't work (or at least not noticeably.) Steve

how do most people prevent the insulation from touching the front panel to leave that 10mm airgap between the front resonating panel and the insulation? Do they just glue it or use a wire mesh to hold it in place.

If you place metal mesh to hold it in place, doesnt it affect the way the front panel works or resonates, even though the metal mesh isnt touching the front panel? Will it resonate the metal mesh or add unwanted noise or artifacts?

Not a problem; you can use something like is used for the bottoms of rabbit cages; it's called "hardware cloth", it's typically 1/2" x 1" mesh galvanized screening.

The reason resonance isn't a problem is because the insulation being held in place also acts to damp any vibrations... Steve