Types of Treatment

Types of acoustical treatments: absorption, diffusion, reflectors. How to place them in a room

Types of Treatment

Postby Scott R. Foster » Mon Feb 09, 2004 4:10 am

Types of Treatment.

There are two major forms of acoustic treatment, absorption and diffusion. Absorption can be broken down into two classes: broadband absorbers which work to absorb sound fairly evenly across the band; and narrow band absorbers that work on only a part of the sound spectrum.

Broadband absorbers generally are porous materials that absorb sound by converting the energy of the room’s air volume’s vibrations into heat by detouring the vibrations [the sound waves energy] it into a path separate from the air volume of the room and keeping it there.  In broadband absorbers this generally works by creating a room surface made of interwoven fibers which create a boundary with many interstices [little holes].  Generally, the absorption characteristic of a broadband absorptive material comes down to a matter of the density of the material and the topology of the interstices created by the size and orientation of the individual fibers. Density and hole size/configuration yield a property called gas flow resistance [will the air being pushed around by the sound waves be able to pass into the material, and if so, how much resistance will the air being so pushed encounter].

To get a maximum efficiency out of a broadband absorption material, these factors must be balanced - too much density and you've filled in all the little holes - lower the density to where the holes get really big and you got no gas flow resistance.

The simplest types of absorption treatment are merely panels of absorbent material mounted to the walls and ceiling. Common materials for introducing absorption in a room are upholstered furniture, people, suspended ceiling tiles, acoustic foam panels, and mineral wool panels. Porous material absorptive panels [like acoustic foam, semi-rigid fiberglass or rockwool] make excellent broadband absorbers and are the most economical means of reducing how "live" a room is.  

Obviously, the more square feet of absorptive panel added, the more sound is absorbed, but proper mounting can enhance this performance -  such panels, mounted with an air gap behind, will work with greater efficiency [for middle range and high frequencies - almost as though the panel were as thick as the thickness of the actual panel plus the air gap behind] and will work better at lower frequencies - though a solid panel of 6” thickness will work better – particularly low in the band – than a 2” panel with a 4” air gap.  The point here is that whatever square footage and thickness of panels you have on hand [or can afford], you can enhance the results you get from them by adding an air gap behind the panels when you mount them.

An effective and cheap material for this purpose commonly available in the United States is Owens Corning 703 fiberglass insulation boards – thus “703” is a material you will hear referred to ad infinitum when the topic of absorption panels is discussed. These panels work well, but are pretty ugly, so usually they are covered with cloth when installed.  Muslin, craft grade felt or even dyed burlap [hessian or jute] will all work and are very cheap – but any fabric you can breath through will work – chose fabrics as a decorative and practical use choice – as there is little acoustic difference provided they are breathable – though you should consider the wisdom of choosing a cloth which is treated with fire retardant for this purpose, or treating the fabric you select afterwards. Also, you might need to consider, durability, ease in cleaning, resistance to staining, etcetera, in a high traffic area.

http://www.owenscorning.com/comminsul/p ... &system=79

http://www.flameseal.com/

Or here is a DIY recipe for fire treatment:

"TEXTILE FIRE RETARDANT TREATMENTS - Many chemicals have been used as fire retardants. Some of these can be toxic, difficult to apply, or alter the quality of fabrics significantly. The 1977 edition of NFPA 701, included some sample uncomplicated formulas:

Formula 1: Borax - 6 parts, 6 lbs, Boric acid - 5 parts, 5 lbs, Water - 100 parts, 12 gallons. Steep fabric in cool solution until impregnated. Heavy applications by spray or brush are usually reasonably effective. Repeat if necessary. This is good for theater scenery fabric, and recommended for rayon and natural fabrics. Yields a 8 - 12 % weighting.

Formula 2: Borax - 7 parts, 7 lbs, Boric acid - 3 parts, 3 lbs, Water - 100 parts, 12 gallons. Water can be varied according to absorptive capacity of fabric. For rayon and sheer fabrics, these same amounts of borax and boric acid can be used with 17 gallons of water. Hand-wring for an 8 - 10% weighting on fabric. Flexibility and softness will be retained without dustiness, and also microorganism growth is prevented. "


When trying to locate a source for 703, keep in mind that "Owens Corning 703" is just a brand name for a compressed fiberglass insulation board whose density is about 3 lbs per cubic foot [48 kg/m3].  Neither fiberglass in general nor OC 703 in specific are the last and final word when describing an absorption material which will be effective across the band [mineral wool products generally known as “rockwool” are made from basalt as opposed to spun glass – and are just as good if not better than fiberglass products].  Also, there are several acoustic equivalents to the OC 703 brand (made of either fiberglass or mineral wool) sold under different brand names – it is recommended that you shop for price and convenience of delivery not by brand name. Here is a link which lists the properties of a number of absorptive materials put together by StudioTips forum hero Bob Golds:

http://www.bobgolds.com/AbsorptionCoefficients.htm

If you are looking at fiberglass insulation board get something greater than 2 lbs. per cubic foot or 32 kg/m3 in density (OC 703 = about 48 kg/m3 or 3 lbs pcf.). If you are looking at a rockwool board something greater than 45 kg/m3 will work well (rockwool board which is 60 to 65kg per m3 is pretty much acoustically equivalent to 48kg /m3 fiberglass products like 703).  If you can find it, use un-faced board, but folks have used boards lined on one side with good result [with foil for example].  If your boards are faced, then for general applications you want to take the facing off – especially if you are stacking panels to make a thicker device – if only using one panel thickness, at least put the un-faced [fuzzy] side toward the room as the facing will reflect HF waves. Also, in a construction project consider hanging onto all the job site scraps of insulation panels and batts [there is always a ton this crap left over from the thermal insulation of a structure] as these materials can be used to build or augment the absorption devices built to treat the finished room(s).  

In addition to mineral fiber panels [fiberglass / rockwool], acoustic foam is an option.  Here is a rough comparison between glass fiber and acoustic foam, with the pros & cons of both written with the help of forum hero Ido:

Fiberglass and mineral wool are referred to as the wool family. The foam family consists of Polyurethane based foam, and the white colored foam known as Melamine / Sonex / Illbruck / Basotect.  Acoustic foams are of the open cell type - closed cell mattress type foam isn’t right for acoustics, because of it’s very limited absorption [closed interstices – 100% gas flow resistance].

Acoustic absorption:

Generally similar, which is overall excellent broadband absorption, with slight advantage to the wools. Melamine is slightly better than Polyurethane.  Absorption low in the band is mainly a function of material thickness for both wool & foam, and is also dependant on shape, so that a correct comparison of absorptive materials should be made with identical shape & thickness. It is incorrect to compare typical sculptured foam to full thickness wool. As a general rule, sculpturing foam (wedges, pyramids) makes it less effective in the LF range.  Though it is seldom seen in the home studio world, full, flat foam can be had. In any case, the sculptured limitations should be known, so that either relatively thick foams should be used, or air gaps incorporated, or full flat foams used.

Fire hazards: Wools meet all necessary regulations, and are especially suitable where these considerations are a must, such as public places.  Polyurethane foam is very limited in this case, it can burn and release dangerous gas.  Good quality foam is fire treated, which improves it to some extent, but it’s still not suitable for public places, or wherever fire hazard is a major issue.  Melamine foam is much better in that sense (can be considered for public places).

Application:

Wools release their fibers rather easily, and these fibers are irritating.  While these materials are safe to be around [they are inert - made from rock and sand] the fibers are sharp and scratchy
That means that wools need to be covered so as to capture the fibers in place.  Usually, they will also need some sort of framing as even “rigid” fiberglass aint all that stiff.  Foams can be set as is.  In other words, wools require work and preparation for installation, foam doesn’t.

Cost:

Wools are the cheapest.  Foam will always be more expensive, Melamine is more expensive than Polyurethane.

Longevity:

Properly covered wools can last a lifetime. Foam will probably last less than wool, and is much more sensitive to weather [even sun light from your window over time can cause foam to change color or even break down] and to physical bruising. Some types of Melamine are especially sensitive to physical bruising, and will bruise very easily.

Looks:

This could be a factor to some. You can do all sorts of stuff with wools, but foam will look like foam, which to some can be cool, to others, not. White Melamine can be painted.

Specific advantages:

Being cheaper, wool is more suited for building specific structures – such as corner mounted wedges for enhanced low frequency absorbers - though the time & energy put into working with wool should be taken into account when comparing costs.  You can theoretically do the above, but there is no sense in it, since foam has no constructional properties, and if you are anyways going to cover a surface, use wool.  The unique advantage of foam is that you don't have to prepare before hand [build a frame to hold it in place or cover it with cloth] - you just stick it to the wall.  Though, once you have prepared a wool panel – the difference disappears.

Overall considerations:

The major factor is whether you are DIY inclined or not. If you are, and you have the energy and time, go with the wools. It will cost you less, and you can get all out acoustics, with the fully professional feel and looks. It also depends on your situation: If you have found what looks like home sweet home (as in permanent), and you want to go acoustically all the way, do wool (you don't have to do the work, you could always get someone to do the work for you). However, if you aren't a DIY'er, you're not up to the work, you are in a temporary place (as in renting a place year or two), and you can get by with basic acoustics (no full scale LF treatment), foam is a strong option (also, foam can be removed and used over again).
Regarding costs, bear in mind that the basic treatment for the smallish room requires a limited amount of material, so is acoustic foam expensive? Yes, but not terribly so. You don't need to cover a whole room, rather, you shouldn't. Also, the extra work & time & fabrics etc. with the wools is not insignificant, it evens out the score somewhat. The big issue is not wool or foam, but knowing what to do with it. This is described both here and in various group messages. If you do know what to do, you can get good results with both.

If in doubt, and if the specific foam you are considering absorbs less LF than the wool you are considering, you can always use thicker foam (not to mention usage of air gaps). There are also corner foams to be had for LF treatment. In any event, you won't get broadband absorption which works all the way down to low in the band by sticking 2"/ 5 cm of something to the walls, whatever it's made out. Two good applications for the general wall surfaces would be ~ 10 cm /4 wool or ~ 15 / 6” cm sculptured foam (which would probably have similar overall volume to the wool anyhow).


Whatever type of absorptive material or device you use, the placement of an absorber will effect its efficiency – for example, LF waves get folded up when they reach the room’s corners, and doubly so when they reach tri-corners [like where the floor or ceiling and two walls all come together].  A wide / thick panel of 703 mounted across the diagonal of a room corner from floor to ceiling will capture LF waves much better than the same panel mounted flush to the wall because: a) it has a large air gap behind it; and b) it covers a room corner and two tri-corners [one at the floor and one at the ceiling].  If the gap behind the front panels is filled with additional 703 [or even scraps of mineral fiber left from insulating the walls and ceiling] the device’s efficiency in the low part of the sound spectrum will be further enhanced to some degree.  A thickness of 4” [100 mm] is a good compromise of acoustic effectiveness versus efficient use of materials and space.

Such panels are very cost efficient - an excellent balance of "bang for the buck".

viewtopic.php?t=534

Alternatively, you could cut triangles of mineral fiber and stack them to build a solid wedge in the corner from floor to ceiling… while this would require quite a bit of material, the results make for an effective absorber that works well - but for a 34” faced wedge, it requires 300% the material required to cover the same corner with a 24” wide panel yet the wedge will yield much less than a 300% increase in absorption performance - see this post for a comparison graph:

viewtopic.php?t=536

There are number of types of narrow band absorbers which are “tuned” to work on specific problem frequencies.  Generally these types of devices are too small in overall effect to be useful in treating a room of any size and too big to fit into a small room – but nonetheless they are made and can work very effectively if you can find room to place them – some designers go so far as to build attic spaces and/or false walls to create sufficient additional space to install such devices “outside” the room.  

Also such devices can reintroduce the frequencies they are absorbing back into the room as they hum in sympathy – thus extending the reverb time of such frequencies even while reducing their loudness. The need to use such devices often arises from a failure to balance the absorptive properties of the materials making up the major surfaces of the room [usually through the use of things like carpeted floors that absorb highs but leave all the lows to be a “problem” that must be treated with a targeted low frequency absorber – a better solution is to remove the carpet and treat the whole band].  By carefully choosing the materials for the surfaces of a room [painted concrete floor with a couple of area rugs vs. wall to wall carpet?] and then installing sufficient broadband absorbers, the need for narrow band absorbers can often be avoided altogether.

Nonetheless narrow band absorbers are sometimes what is needed to solve a problem resonance, but if you find yourself thinking this is the case with one of your rooms, you should go back to square one and see if you can’t figure out a shorter, cheaper, smarter way around the problem than build tuned boxes.

One type of narrow band absorber is the diaphragmatic absorber.  The diaphragm absorbs the frequencies which match its resonances. They work by vibrating at these frequencies and turning the sound energy into heat. The drywall in wall construction acts as a resonant panel and absorbs a considerable amount of sound at 125 Hz. Simple panel absorbers can be built by mounting a sheet of plywood in a frame.  Just build a frame, and fill it mineral wool, but leave a small gap between the panel and the insulation so that the panel is free to resonate. Panel resonation can be enhanced by reducing the point of connection between the panel and the frame by means of narrow spacer material such as a loop of wire or welding rod run along the edge of the frame so that the panel is perched on a thin edge. Approximate Plywood Panel absorption peaks on a 1x4 frame 3.5 " deep are:

1/8" = 150 Hz
1/4" = 110 Hz
3/8" =   87 Hz

Membrane absorbers are diaphragmatic absorbers with thin membranes, and are used to absorb lo-mids and higher frequencies. The Kraft paper backing on common residential grade insulation has an absorption peak of 250 Hz and will absorb very efficiently at this frequency. Another type of narrow band absorber is the Helmholtz resonator, which is simply a cavity with a tuned port (sometimes referred to as a Helmholtz bottle – btw a thick walled glass vessel like a coke bottle is a very effective Helmholtz resonator).  Sound that matches the frequency of the cavity’s resonances are trapped by the cavity where their energy is spent (turned to heat) vibrating the air in the cavity like a spring. Tube traps are Helmholtz cavities, with broadened Q’s (a wider range of frequency are effected) and can have a membrane wrap that reflects highs from a circular (mildly diffusive) surface.  

Another major form of treatment is diffusion. Diffusion varies the angle of the sound reflections off a surface so as to scatter the direction that the sound will take after the bounce. This effect is achieved by either turning a flat surface into a curved one, or a faceted/slotted one.  Curved diffusers work best when the curve they describe is not a pure circle section / arc.  Faceted / slotted diffusers come in several varieties generally based on the concept of a reflection phase gratings. Diffusers can reduce the need for absorbers because they act to spread reflections around the room making the modes more evenly spaced and thus there is less need to dampen them. Also, almost all diffusers act to some degree as absorbers – for example, grating diffusers are absorptive even if they are so stiff they do not vibrate [in fact industrial smokestack absorbers use gratings tuned to the noise they seek to diminish], and some types of treatment are purposefully designed to be significantly both absorptive and diffusive by adding elements of porous absorber material to bent/faceted surfaces or gratings.  

An easy to make diffuser is a curved panel often called a “polycylindrical”, or “Poly”. These can be simple hemi-cylinders [cut a large tube length-wise or bend a panel around semi-circular ribs, and fix it to the wall], or better yet a form a Euler-Bernoulli buckled plate [bend a panel by squeezing its edges].  Polys are basically a sheet of plywood bent into an arc – and the name comes from the idea of arraying many curved panels or cylinder sections to form a diffusive wall (thus polycylindrical or many cylinders). The best examples are not cylinder sections, instead they are buckled plates that describe sort of an ellipsoidal curve as this sort of curve is more irregular than a simple circle section and better serves to scatter reflections. Making buckled plates sounds complicated, but they are actually easier to make - just bend a panel into a simple frame that binds two of its edges. When treated thus, a sheet of plywood [or similar material] will not form a circular curve, but rather will buckle in curve with a radius that varies across the panel - in essence a concatenation of an infinite number of discreet cylinder sections of varying radius.  Such a curve is defined by the Euler-Bernoulli beam equation.

http://en.wikipedia.org/wiki/Buckling

http://en.wikipedia.org/wiki/Euler-Bern ... m_equation

In a Poly, the plywood acts as a diffuser by reflecting the incident sound waves across a wide variety of reflection paths that varies significantly dependent on where on the panel the wave impinge.  Also, if mounted in a frame so that it is left free to vibrate with insulation strung through the back of the frame, it is also a mild diaphragmatic absorber.  Making an optimized poly is simply a matter of building a frame that squeezes a plate into an Euler-Bernoulli buckled curve. Usually the frame is dimensioned so that the plywood is held only by the two vertical edges so that it is left free to vibrate, but the panel could be made stiffer by adding braces or even filling the cavity with stuffing or expanding foam.  A pure cylinder would present a fixed radius curved surface to the sound waves that hit it - Euler-Bernoulli buckled plates present many curves in one object and thus are more diffusive than pure cylinder sections.

The size of the poly determines the lower limit of the frequencies which it will diffuse.  A simple design for an optimized poly is to take a full 4’x8’ sheet of 1/4” plywood bent along its long axis and stuffed into a rectangular frame built of 1x4’s. Heavier plywood could be used but will be very hard to bend (you might need to make kerfing cuts to get the bastard in place). The interior dimensions of the 1x4 frame should be less than the width of the panel, for a 4’x8’ panel try 47” wide (and 8’-2” tall - 2” longer than the panel - if you want to leave the ends free to wiggle).  Put a pair of horizontal cross braces at the back of the panel and then squeeze the panel into the frame so that it bows out [you will probably need an assistant – and don’t get your finger stuck]. To install just hang the panel from the ceiling or mount to the wall with the bowed sided facing out.

If you wish to both absorber and diffuse, you can add a thin strip of porous absorbent to either the interior or the the face of a poly - or both.

Generally diffusion is best reserved for larger rooms.

Here are some polys built by forum members:

viewtopic.php?t=1851

viewtopic.php?t=2830

viewtopic.php?p=45275

viewtopic.php?p=45486#45486
Attachments
poly-2.jpg
Poly
poly-2.jpg (10.68 KiB) Viewed 48927 times
Panels.jpg
Panels of OC 703
Panels.jpg (7.44 KiB) Viewed 43815 times
Chunks.jpg
Wedge of 703
Chunks.jpg (7.09 KiB) Viewed 43798 times
diff2.jpg
An Acoustic Grating
diff2.jpg (17.25 KiB) Viewed 43805 times
Scott R. Foster
 
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Location: Jacksonville, FL USA

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