Studiotips Corner Absorber

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

Studiotips Corner Absorber

Postby Scott R. Foster » Thu Jun 03, 2004 1:31 pm

The Studiotips Corner Absorber is the AK-47 of acoustic treatment devices - the ultimate in cheap but effective. See comparison here:

Comparison of 4 Absorption Devices

The StudioTips Corner Absorber ["SCA"]is accomplished by simply mounting medium density mineral fiber insulation panels - of sufficient thickness - across the diagonal of a corner. 

Panels.jpg
The core idea of the SCA without upholstery
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The corner can be a vertical wall/wall corner, or a horizontal ceiling/wall corner, or a floor/wall corner as shown above.

When built and installed properly this device exhibits a large increase in performance in the low frequency band between 63 and 125 Hz where typically porous absorption panels begin to drop off in effectiveness. This rise in low frequency performance [frequently referred to as the 100 Hz peak] is apparent in the graphs shown in the above linked 4 devices thread and is a perfect match for the treatment needs of typical small rooms.

Installing a number of SCA's in a room will address the key non-trivial problem of low modal density in the lower band and cure the "muddiness" of low frequency resonance typical in such spaces.

Panel thickness for the SCA can vary but a minimum of 4"s [100 mm] is recommended because panels made thinner than 4" begin to lose the ability to perform well on low frequencies.  

Generally Owens Corning 703 is a great choice for the core material - but materials of equivalent [or better] absorptive properties may be the "smart" buy for you in your market. Owens Corning 703 is a semi-rigid fiberglass insulation board which has been used with success, but other mineral fiber products [either fiberglass or rockwool] may be cheaper and work just as well.  It is recommended that price and availability, and material handling properties [durability of the panel, weight, etcetera] guide you in making a material choice  - provided you use some sort of open pore [no paper or foil covering] medium density mineral fiber insulation board and make you panels thick enough, you will get satisfactory results.  

If you are looking at a semi-rigid fiberglass board product, look for something in the 3 lbs. per cubic foot range like 703.  For rockwool products made by firms such as Roxul look for something with a slightly higher density such as 6 lbs. pcf RHT80.  Use the Bob Golds' collection of absorption coefficients to shop wisely in your local market for mineral fiber to make your own Studiotips Corner Absorber - try to lay hands on the available material before purchasing - consider the mechanical properties [flakiness, floppiness], weight, and other properties along with price per board:

bobgolds.com Absorption Coefficients

The raw materials are abrasive and should be covered for both this reason and aesthetics - usually SCA's are upholstered to meet this need. The panels can be covered by cloth pinned or glued to the back of the insulation board - or a light weight timber frame can be built to carry the panels and fabric stapled to the frame. Decorative grade burlap is a cheap choice which works fine - but any cloth you can breath through with no more than modest effort will work well. Cover your mouth with a layer of Tee shirt fabric and blow through it and you are experiencing the breathability of a perfectly suitable fabric - but basically any fabric which would not make a decent rain coat or windbreaker jacket will work.

Given the panel is an architectural element you should consider the wisdom of choosing a cloth which is treated with fire retardant for this purpose, or treating the fabric you select afterward. Also consider, durability, washability and stain resistance of your choice of fabric for upholstery.

Here is an example of a commercial fire treatment.

http://www.flameseal.com/

Or here is a DIY recipe fir 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. "


Denser weaved cloth will cause an impedance jump that will increase the 100 Hz peak in absorption seen in the measurements shown at the "4 Devices" thread linked above, and may slightly reduce HF absorption.  Also, using a rim of spray adhesive around the edge - to fix a membrane of plastic film [4 to 6 mil plastic sheeting for example] - to the face of the panels will cause the device to be more reflective of HF waves and give rise to a peak of absorption at the resonance of the membrane [if you do this - test the panel in situ as exact results may vary a great deal according to how and where you mount the panel and other factors].  For most users and applications such membranes are not required or beneficial - a "vanilla" SCA works perfectly for most uses without this complication.

When mounting the panel be careful to fix it into the corner so the both long backside edges touch the room boundaries [walls, ceiling or floor depending on how you orient the panel].  Gapping the panel away from the walls as little as a couple of inches will make the 100 Hz peak demonstrated in the "4 Devices" comparison collapse. But, you do not need to "seal" the panel to the walls, and very small gaps can be ignored. Here is a graph demonstrating the collapse of the 100 Hz peak caused by improper mounting - note the reduction in low frequency performance caused by a 2" gap between the backside edges of a 4" panel versus resting these edges against the wall.

Corner_RT424-vs-Gap-600.jpg
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See below for a PDF format - Bass Trap Build Tutorial by Joel DuBay which describes core concepts for choosing materials and has lots of pics of a simple and sharp looking implementation of the concept of a broadband absorption panel suitable for corner mounting.

DIY_Bass_Trap.pdf
DIY Broadband Absorber - DuBay
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Links to suppliers of panel covering can be found here:

DIY Acoustic Treatment Parts Suppliers

With these products one can simply stuff a panel of mineral fiber into an upholstered bag and hang it diagonally in the corner of one's room and the result is a superbly cheap and easy SCA style low frequency capable / broadband absorber.

Low frequency performance can be enhanced by filling the cavity behind the panels with additional absorbent material... which approaches the idea of a StudioTips SuperChunk... but is short of filling the entire corner as in the SuperChunk. This saves labor and material. Just about any such addition will be helpful. For example a "vee" of folded wall or attic insulation in the apex of the corner behind the panel would be an inexpensive addition which would give a modest increase in performance.

Along these lines - an alternative to the basic SCA design is an idea developed in this forum and frequently known as the "DP Panel" named for a member Dave Portocarrero.  This clever design uses mitre cut panels, cloth, and spray adhesive to build a frame-less triangular cross-section corner device which lines the two walls and then provides a diagonal panel across the face.  This face is upholstered, then quarter round moulding used to provide a neat edge to the face of the device.
DP_Panel.gif
A DP panel
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Here is a picture of an installed DP Panel:

DaveP.jpg
A finished DP panel
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An alternative to the DP trap and the full SuperChunk is to align a narrower nested panel behind the front face panel... in a dual layer nested format like this which gives excellent low frequency performance. Below is a depiction of this concept as implemented by my firm Ready Acoustics in its NEST dual panel array product.

cm-b.jpg
A Nested Corner Panel
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The SCA when implemented in a thickness of 4" [100 mm] yields a broadband absorber with very good low frequency capabilities and highly efficient use of both labor and material. Increasing the thickness to 6" [150 mm] and adding filler behind the panel such as as shown above in the NEST dual panel array provides a means to approximately double performance at 63 Hz with only a small additional consumption of time and materials [specifically a 4" thick - 1x4' nested panel]. A similar concept many have used is to augment existing foam corner wedges which are of a dimension too small work well below 100 Hz [for example foam wedges which have a face dimension of 17" or less - measured across the diagonal of the corner] is to cover such units with a facing panel of 4 to 6 inches in thickness creating a concatenation similar to the fully filled wedge of the SuperChunk. Such combinations have proved to be very effective.

Basically, as long as you use a core material with low enough gas flow resistance [don't use a material with too great a density], the thicker the panel, and the more full the void behind the panel, the better the unit will perform at very low frequencies. Here is a graph of absorption tests done on three units manufactured by my firm Ready Acoustics - a 4" panel, a 6" panel, and a nested array of a 6" face panel and a 4" nested panel. This graph will give you an idea how various SCA designs you might build will funtion at very low frequencies.

NEST-v-RT426-v-RT424-800.jpg
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Scott R. Foster
 
Posts: 3854
Joined: Mon Feb 02, 2004 12:41 pm
Location: Jacksonville, FL USA

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