Many of the wall and ceiling treatments described below can be seen in this picture.  The as-yet unfaced high frequency panels (yellow) and finished bass traps hang on the far wall.  A 16-inch deep broadband absorber can be seen at the left edge of the diffuser array.  Two of the polycilinders can be seen suspended from the ceiling, along with two of the as-yet un-covered broadband "cloud" absorbers, suspended from the ceiling by chains.

     If you've read the previous chapter about how the studio was laid out then you know that I decided to keep the walls parallel, that is, no splayed walls.  My reasons were partly for space considerations.  There are good reasons to make the walls non-parallel, mostly because of flutter echo, the repeated echoes heard as sound bounces repeatedly between two parallel surfaces.  Its like standing in a room with mirrors on opposite sides, only sound travels much more slowly than light and we can actually hear the individual reflections as a very rapid "flutter".  While splaying the walls would largely solve any flutter echo problems that solution comes at the price of making standing waves much more difficult to predict.  Since standing waves and low-frequency issues in general are much more difficult to address I decided to keep the walls parallel so that I could control the room response in the low frequency range (standing waves) at the expense of having to deal with flutter echo. 

    When most people think of wall treatments they tend to think of things like thick curtains, or, in a studio, various forms of acoustic foam treatments.  Both work quite well at taming the high to mid frequency range and (if thick enough) do remove most of the flutter echo, but do little to rein in low frequency energy.  Hence rooms treated with acoustic foam tend to sound "boomy"; all the highs have been attenuated but most of the lows are still there.

    Ideally one would like to be able to attenuate the entire frequency spectrum evenly.  In essence the goal is to reduce the room's reverb decay time, expressed as its RT60 time (the time it takes for a sound to attenuate by 60 decibels, or by a factor of 1 million), equally across the entire frequency spectrum, to some reasonable value.  What is reasonable depends to some degree on what one plans to record.  In my case that tends to be rock music, for which I tried to achieve an RT60 time of something between .25 and .33 seconds.  That gives the room a reasonably live feel without letting things get washed out or muddy sounding.

    Given that it's pretty easy to remove the high frequency energy with some sort of foam or insulation, the toughest challenge becomes getting rid of the lows and low-mids.  I decided on a multi-faceted approach to attack each range of frequencies with materials and devices best suited to each range. 

    The high frequencies are tamed using panels fashioned from Owens Corning 703 compressed fiberglass insulation.  This stuff comes in 2-foot by 4-foot panels, either one or two inches thick.  By placing the fiberglass in a wood frame, which holds the panel off of the wall an inch or two, one can achieve high and high-mid frequency attenuation that is at least as good as that offered by commercially available foam panels.  And, I might add, at a much lower cost.  The only drawback is that you have to build them (so its back to the table saw!) and you need to use something to prevent the fibers from entering the work environment.  I decided to use double layers of burlap as an outer cover, which I was able to find in several colors at Walmart. 

The panel frames were made from 1x4 pine board.  Inside the frame and against the wall I attached small pieces of 2x2, to which I then glued a 2"-thick piece of OC703.  Hence the fiberglass is held 1.5 inches off of the wall, and even with the front edge of the frame.  I then glued and stapled two layers of colored burlap over the frame, and trimmed it out with 3/4-inch screen bead around the edge.  The picture at left shows a  couple of the panels in Isolation Room 1 before the burlap was attached to the front of the frames.

    To tame the low-frequency beast I decided to build a bunch of panel traps, also called membrane absorbers or bass traps.  The idea is that to create an enclosure whose front surface (the membrane) vibrates at some known frequency.  Then fill the enclosure with something to impede air movement within the enclosure, attach to the wall and create an airtight seal.  In fact what the material inside the enclosure also does is it broadens the response curve for the front panel membrane (lower "Q").  The central frequency that membrane absorbers work at depends on the surface density of the material used as a membrane, and the depth of the enclosure.

    Each trap has an outer frame cut from 1x3, 1x4, 1x5, or 1x6 pine, depending on the depth desired.  OC703 rigid fiberglass is attached inside the frame in a way that holds the fiberglass off of the wall an inch or so and avoids touching the front panel.  The frames were cut so that the front membrane would be at a slight angle to the wall.  Since a majority of the wall surface are would be covered with either bass traps or high-frequency absorbers I figured that since I didn't splay the walls then splaying the individual bass traps at varying angles would help reduce flutter echo as well.   Additionally, since the depth of each trap varies across its width, splaying the front surface broadens the effective frequency as well  - at the cost of efficiency at any given frequency, of course - no free lunches here!

This model shows how the bass traps are constructed.  The one shown here is 24 inches wide and 48 inches tall, similar to the ones that ring the upper wall in the large recording room

    It also turns out that one gets a fair amount of low frequency absorption from the sheetrock walls.  The studio walls are all 1.25-inch thick sheetrock hung 24 inches on-center over steel stud, and backed with fiberglass insulation.  Additional absorption in the large recording room comes from the large diffuser array, which works as a low frequency absorber at frequencies below where it is effective as a diffuser (that is, below about 300 Hz). 

    On each side of the diffuser array I also built 16-inch deep bass traps fashioned entirely from multiple layers of 2"-thick OC703, with each layer separated from its neighbor by 3/4 inch.  These traps should provide broadband attenuation from very high frequencies down to well below 80Hz. 

    Finally, I also built a few simple polycylindrical diffusers made by simply mounting 48-inch by 96-inch Luan pannels pinched into cleats set 46.5 inches apart.  Forcing the panels between the cleats forces them to bow outward presenting a convex surface toward the room (scatters high frequency sound) and leaves it to vibrate at a fairly low frequencies (around 60-100 Hz).  These tend to be better at scattering the high frequencies, but do help somewhat absorbing lower frequency energy.

The ceilings in each of the isolation rooms are mostly covered with the polycylinders described above.  In the main recording room, however, we added four "clouds", each about 64 square feet (8 ft by 8 ft).  These clouds are simply some more of the high frequency absorbers suspended varying distances (from 6 to 14 inches) from the ceiling.  They actually provide absorption across a pretty broad range including some lows.    The picture, at left, shows the finished treatments in Isolation Room 1.