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Are You Sure You Want To Paint Those Bricks?

Many acoustical nightmares exist, and many of them were accidentally created when well-meaning people tried to "clean things up" or make a few minor changes to a room. One little known mistake centers around interior concrete block walls and brick walls. Many churches built in the past and even present are constructed with brick and concrete block. This is a very strong, sturdy method of construction, and can often be very beautiful.

Over time, however, brick can become dirty, and gray unpainted concrete block walls just don't look very nice. The solution for brick walls is to have them cleaned, but this is very expensive--more often than not someone will suggest just painting the bricks. Concrete block walls are most often covered with drywall or just painted.

So what's the big deal? Painting just changes the color of the object, right? Sorry, that's not true. Objects such as brick and concrete block have porous surfaces. As hard as they may be, their surfaces are still pitted with tiny and sometimes large holes. These holes actually help absorb sound!

Applying paint is almost like putting on a thin coat of rubber over an object (and a few people have told us most quality latex paints have some sort of rubber material in them). Painting a porous material like a brick closes up the pores, sealing them. If you've ever painted brick or concrete blocks you've found how much paint it takes--usually at least 3 coats of paint are needed to cover unpainted brick. This is because the brick is porous and absorbs the paint readily. At the same time, you're "clogging" it's pores! Once these pores are clogged, they can no longer aid in absorbing sound.

How much of a difference does it really make between unpainted and painted brick? More than you think!

When dealing with acoustics and the acoustical properties of objects and surfaces, objects are measured to see how much sound they absorb and how much sound they reflect. Based on these measurements, they are assigned an absorption coefficient. This coefficient is simply a number that relates to how much sound the object absorbs (or reflects). A rating of 0 (zero) means all of the sound is reflected. A rating of 1 (one) is total absorption--as compared to a one-foot square window to the outside (an open window in a wall allows the sound to escape, thus an open window can not reflect sound at all).

Absorption Coefficients of Various Materials
(one square meter)
125Hz 250Hz 500Hz 1KHz 2KHz 4KHz
concrete block, unpainted .36 .44 .31 .29 .39 .25
concrete block, painted .10 .05 .06 .07 .09 .08
window glass .35 .25 .18 .12 .07 .04
carpet on floor, 1/4" pile .05 .10 .15 .30 .50 .55
carpet on floor, 3/4" pile .15 .17 .12 .32 .52 .57


Since objects absorb and reflect different frequencies of sound in different amounts, several absorption coefficients (at different frequencies) are given for the same object. Look at the chart at the bottom of the previous page. On the left is a listing of materials, and to their right is the absorption coefficients at various frequencies. Take a look at the first line which lists the absorption coefficients of unpainted concrete block (note that all numbers refer to absorption of a one-square meter sample of the object). The average absorption is .34--this is considered a sound-absorbing surface*. Now take a look at the absorption coefficients of painted concrete block (second line). The average absorption here is .075 (and is considered a sound-reflecting surface*)--this is quite a difference than the unpainted block! Now that you can see this difference, it's easy to understand why painting concrete block makes it more reflective. The same argument holds true for brick, too--although the absorption coefficients are different.

Also included in the chart are absorption coefficients for different thicknesses of carpet. I've included these simply to make a point (on another subject, no less!). Most people think carpet is very sound absorbent. This isn't really true--not at all frequencies, that is. At lower frequencies (below 500Hz) the presence of carpet isn't a great effect. At the higher frequencies, especially those over 2,000Hz, the carpet becomes a pretty good absorber. The numbers in the chart refer to just carpet laid on a flat surface (no underpad). Full carpet in a large room tends to make the room bass heavy (assuming there's no acoustical treatment). This is because the carpet absorbs much of the higher frequencies quickly, while the low frequencies are left to decay at a much slower rate in the room. Rooms like this are said to sound muddy and speech is often unclear or garbled sounding.

The coefficients for standard window pane glass are listed too--these numbers are for typical single-pane glass. You'll notice it appears the glass absorbs more low frequencies than higher, how can this be? The best explanation is that based on the type of glass, the thickness, and how it's mounted, the glass will move at low frequencies--thus some of the energy is lost through heat, and some is lost to the other side of the glass (like to the outside or another room).

This short article isn't just to tell you not to paint your brickwork, nor is it to tell you how much sound your carpet absorbs. The real intent of this article is to bring more awareness to the fact that sound and acoustics is a science--and it’s not just something to play with. Because it deals with the physical world, physical laws apply and we must be sure we know what those laws are and how to treat them to get the results we desire. Not taking the time to investigate matters like this can result in an unhappy congregation and future expenditures that could have been avoided.

A few dollars in paint can do thousands of dollars damage to a room. If you plan to do any construction in your sanctuary, it would be wise to ask an expert what the end result could be if you make the changes you propose. It may not matter at all--then again, it may cost you more than you can afford.

*Architectural Acoustics, M. David Egan