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
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)
|concrete block, unpainted
|concrete block, painted
|carpet on floor, 1/4" pile
|carpet on floor, 3/4" pile
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 its
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