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Absorption

A Very Small Room
Some day I will have more space, but for now I'll work with what I've got. Sound Familiar? Probably. This discussion assumes a 9x10x8 room with 1 entry door, a pair of sliding closet doors, one large window, sheet-rock walls, carpet (now replaced with hardwood flooring), equipment and a bit of furniture. Not exactly a first choice, but a worthy challenge and likely very common. So, what surfaces are available for acoustic treatment?
  • The two longest flat walls, 10x8 are just bare sheet-rock with bare ceiling corners.
  • The closet end also has the entry door. The closet doors are tall so no ceiling corners are available.
  • The opposite wall has a large window with blind taking up most of the width.
  • The ceiling corner, above the window can receive treatment but not either side -- two narrow.
This room has the closet doors and the large window at opposite ends of the longer dimension. For this and other domestic reasons, that forces the speakers and listening/mixing position to be at opposite sides the other way. This is exactly the reverse of what is normally recommended -- the speakers should be at one end in the longer dimension.


The Problem(s)
So, what's the problem ? Reverberation, echo, flutter, sound coloration, too much bass, reflections, hot-spots and nulls, you name it. Oh well, these just makes the project more interesting.

The two plots below clearly show that the room response over frequency a far from uniform and the waterfall plots shows that at low frequencies the wall reflections will prevent clarity.

 
 
Frequency Response
(untreated room with gear)
Waterfall Reflections Plot
(untreated room with gear)


Right? So, what is the solution? Several things, but first, absorption !

Since there are many construction options to choose from, and they usually don't come for free, it is best to make the trade-offs early -- before you buy 3 bundles of Pink R11 insulation, just because it was marked down for the weekend. While there is no one right answer to improving room acoustics, just guessing is not one of them.

Absorber Options
Flat absorbers can cover much of the two longest walls, the wall-ceiling corners can have bass traps and flat absorber "clouds" can be ceiling mounted a) above the recording position and b) smaller, lower-frequency versions ceiling mounted in front of the closet door wall's ceiling corner (a modified 'corner bass trap').

Trap: a common term which refers to an acoustic absorber panel. Typically they will have a limit of about 2 kHz though that is only a rough reference and not an form of frequency cutoff.

Bass Trap: a common reference to an acoustic absorber but a terms which is over-used. Here, the term will refer to corner absorbers designed primarily for frequencies below 200 Hz.

Corner Trap: a bass trap specifically designed to fit into or cover a room corner and a design which acoustically would not be appropriate for wall mounting -- especially in a first-reflection position.

Resonators: or often called Helmholtz Resonators. They are (when designed correctly) chambers whose dimensions resonate like the pipes of a pipe organ except they have absorptive material inside to attenuate sound -- unfortunately, only at a single frequency. They have limited use and none here.

Wall Traps
In this small room, the 9-ft width has a first-order mode of (1115 ft/sec divided by 9 ft width) 125 Hz. Clearly, the open, flat, parallel walls need as much bass absorber treatment as is practical. So, what to install and where? First, consider absorber material.

Bass Traps
Since mostly wall-ceiling corners are available, these will likely have to be narrower than the wall traps just because of space constraints. However, they still need to work down to the lowest frequencies possible. So what's the right approach? As with wall traps, first consider absorber material.

Acoustic Absorption Materials
  • Foam - Forget it. The density is much too low to provide reasonable absorption below 500 Hz
  • Egg-crates - Acoustically transparent; they do nothing but look ugly
  • Rugs, mattresses, furniture - The effect is almost measurable so ineffective
  • Commercial bass traps - generally great, but not on a small budget
  • Fluffy R-19 wall fiberglass - Works, but you need 8-10 inches thick and it's messy
  • Rigid OC-703 fiberglass panels - Very absorbent, easy to work with, relatively inexpensive
  • RockWool - reasonable but heavy and hard to work with. there are better choices
Rigid fiberglass sheets are readily available for a moderate price and come in 24x48 inch dimensions of 1, 2, 3 and 4 inches thick. The -FRK versions have a think kraft paper or aluminum foil bonded to one surface. While this layer is added for moisture resistance, it has acoustic value but primarily in corner bass traps.

The -FRK Layer
The general consensus on the Web is:
  • Membrane Toward the Wall: Using the -FRK layer "toward the wall" will increase absorption "a little bit and it certainly won't hurt". There is little to no hard comparative performance data to support this and anecdotal evidence tries to compare results between OC-703, OC-705 with and without -FRK and with different thicknesses. There is no solid justification for the added expense for this case so there is only support for using this configuration if the material is already purchased. If several layers of rigid fiberglass are used, then ONLY allow ONE -FRK layer and ONLY place it furthest from the room side of the sandwich.
  • Membrane Between Layers: Using the -FRK layer "between other layers" will destroy performance. Don't do it. Technically, every layer boundary between different density materials is an acoustic impedance change and the more of these you create which are not designed for a specific purpose, the worse the results will be.
  • Membrane Toward the Room: Using the -FRK layer "toward the room" is useful, but ONLY for corner bass traps -- which are, by definition, never placed at "first reflection points". (First reflection points are places on walls where there is a "first-bounce" reflection wave which directly adds and/or cancels a direct sound wave; like from a speaker. Using a layer of -FRK foil at a reflection point will make the reflections worse; just the opposite of what a wall bass trap is supposed to accomplish. FRK also damages the absorption above 250 Hz so it is a very poor choice for wall traps.
So, why is an -FRK foil reflection layer of any use in a corner bass trap? Corner traps are designed to absorb, not reflect, right? Well, it's complicated .....

Limp Membrane
This term appears on the Web in acoustic discussions but is seldom described or defined. Limp is easy to image but don't think of wet noodles. In the case of traps it means a dense (not thick), loosely-held membrane covering a sealed air volume which is "filled" with absorber material. The limp membrane is physically separated from the absorber material.  There is low resistance to sounds energy so as to couple that energy to the airspace and then to the absorber. A membrane structure seems obvious when thinking of plastic sheets but in Web discussions the term is also applied to thin sheets of plywood. The point is fairly simple. At the frequency of interest (usually something below 200 Hz) the "limp membrane" is a large, relatively flat surface and the membrane and air cavity is a tuned, resonant chamber. Limp membrane designs seek to low the 'Q' (less sharply tuned) to provide absorption over a range of frequencies. This is usually acomplished by choice of membrane material -- limp and very dense.

Since most bass traps are not build with sheet limp membranes, this discussion will ignore those designs in favor of membranes of thin paper, foil or plastic -- and specifically the foil backing of OC-703/705 type -FRK rigid fiberglass panels. This does not mean limp membrane designs are not valuable, they are just much more complex and require much more engineering for each studio space -- too much for these pages.

Foil Membranes
There are many explanation on the Web for why the -FRK foil layer improves corner bass trap performance. There is some truth in many of them but mostly much misconception. However, there is some fairly good measurement data which supports performance improvement below 300 Hz. So, using -FRK exclusively for corner traps appears warranted. However, the improvement is not always dramatic. So, why might it work for a bass trap?

Rigid fiberglass panels like OC-703 and -705 are typically 24x48 inches. One wavelength at 48 inches is about 278 Hz but corner traps are targeted for 125 Hz and below. As noted above, 125 Hz is a wavelength of about 9 feet so the 24-in width of a panel is less than 1/4 wavelength. So, at these low frequencies, sound waves applying pressure to a panel are mostly plane waves which will diffract around the sides.

Without the -FRK foil, the pressure wave will lose energy within the panel by friction of the dense fibers. With the -FRK layer, the pressure wave is partially reflected by the high density foil. The foil being tightly bonded to the fiberglass is important because the acoustic pressure at the surface must be efficiently coupled to the panel. RealTraps apparently found a material and bonding method which surpasses the effectiveness of the -FRK foil + Kraft paper layer. This would not be surprising as the -FRK layer is applied as a moisture barrier, not for acoustic absorption. It just happened to help absorb low acoustic frequencies. So, as a result of the bonded -FRK membrane, several things will happen. Assuming the panel is "loosely suspended' (not securely mounted)
  • Sound pressure will lose energy from attempting to move the entire panel
  • Energy coupled, by the bonded membrane, will be additionally absorbed by the rigid fiberglass
  • Energy at higher frequencies will not couple as efficiently and so will be partially reflected back to the room
Air-Gap
Another factor affecting an absorber panel's low frequency performance is related to total surface area. Mounting a trap directly to the wall means two things; a) the trap surface (at the wall) can only act as a reflector and b) the trap surface area is limited to the front surface and maybe the edges. Attenuation is increased whenever there is an impedance mismatch between layers; fiberglass to air to wall and back to air to fiberglass, for example. If the trap is mounted away from the wall, the back side of the absorber will be exposed to sound reflecting off the wall near the trap. The trap will behave as if it were larger than it is.

The amount of spacing is frequency dependent as well as limited by practical room considerations. Again, the prevailing rule-of-thumb is setting the space equal to the thickness of the panel suggested by subjective observation that greater spacing had diminishing performance returns while becoming a more significant spatial problem -- especially in a small room.

Practical Guidelines
So, the above explanations provides some valuable guidelines when using OC-70X-FRK:
  • Only a SINGLE layer of -FRK foil -- foil side toward the room
  • Exposed surface area matters more than absorber depth at 3-inches or more
  • Do not restrict the mounting of the rigid fiberglass -- let it "float"
  • Expose as much of the panel's surface as possible -- including panel edges
  • Straddling a corner is efficient use of material and provides an air-gap
  • Leave an air-gap behind at least 3 inch thickness of fiberglass