Fixing the Drywall Box

A low budget solution to a common acoustical problem

Theory and Practice – By Dale Shirk

With today’s financial pressures, many churches and schools are trying to build their facilities as inexpensively as possible. Almost invariably, this leads to the dreaded “drywall box”, a hideous acoustic monstrosity that is suitable only for warehouse use. The best way to completely fix it is to tear it down and start over. Now they want you to install a sound system in it and make recommendations for acoustic treatment, all on a very limited budget, of course. Let’s see what can be done.

We need to look a little closer at why this room behaves the way it does. Typically every wall and ceiling surface is made of gypsum wall board, commonly known as drywall. The floor is covered with thin commercial carpet. The drywall absorbs the lowest frequencies fairly well by panel action. The carpet and perhaps furnishings such as padded chairs, absorb the upper mid and high frequencies fairly well. This leaves the lower mid frequencies with very little absorption, and results in a reverberation characteristic with very long times in this range. This is sometimes referred to as drywall hump.

The second major problem in the typical drywall box is flutter echos between parallel reflecting surfaces. These can make the reverberation seem like a problem at high frequencies as well.

Sometimes the uninformed try to fix the room by carpeting the walls. This of course only absorbs high frequencies. While it can make a small improvement, mainly by knocking down the flutter echoes, it leaves the room with a very dark and boomy sound, since there is still too little lower midrange absorption, and now there’s too much high frequency absorption.

Adding full range absorption does tame the room. Be sure it’s thick enough to be effective in the 250 to 500 Hz range where absorption is most needed. Adding enough absorption to control this range will result in a room that’s pretty dead. This may be just fine if the room is a gym or multipurpose room. However for music, and especially congregational singing, it will likely be perceived as too dead.

What is needed is a panel that absorbs the lower midrange well, and diffuses the upper midrange and high frequencies without absorbing them. This can be achieved by using a panel with a fiberglass or mineral wool core, but with a perforated cover. I use ordinary lumberyard 1/4″ pegboard, with 1/4″ holes on a 1″ square pattern.

It is a loosely tuned Helmholtz absorber, where the peak frequency is determined by the hole size, panel thickness and cavity depth. A deeper cavity tunes to a lower frequency and a shallower cavity tunes to a higher frequency. I curve the pegboard which varies the depth, making a wider tuning peak. The fiberglass fill lowers the Q of the resonator and widens the effective frequency. Using 1/8″ pegboard would raise the tuned frequency since there is less air mass inside the hole, and would require greater cavity depth to be effective in the frequency range we want. Various acoustics texts cover this in all it’s gory mathematical detail.

The curved face diffuses high frequencies, breaking up flutter echoes and increases the reflection density, without losing all the high frequency energy. The step edge of the panel helps diffuse midrange frequencies as well.

I usually recommend 2-3 inches depth at the edge and maybe 6 inches in the middle. I install them with the curves alternating up/down and side to side. They can be covered with cloth if desired, using spray adhesive.

How much square footage of this panel you need will be determined by the size of the room, the desired reverberation level and the amount of existing sound absorption. Typically somewhere in the vicinity of 30% of the surface area of the room needs to be covered with some sort of sound absorption.

Since these panels break up flutter echoes between parallel surfaces, they should be located on those parallel surfaces at ear level. They should also be located on the surfaces hit hardest by the sound system, such as the rear wall or balcony front. Additional panels should be spread around the room.

Since this panel is built of wood, it will add to the fire load of the room.

This simple panel is a solution well matched to the problems of the drywall box, and can be installed for far less cost than most commercial products, particularly when volunteer labor is available. ds

It is easier to be wise for others than for ourselves.
Francois De La Rochefoucauld