NEW MEANING TO "ACOUSTIC PLASTER"
There has been a resurgence of the use of plaster in modern construction where sheet materials were the typical product of choice. How is the sound in a room affected by the use of plaster as a finish material?
"Plaster" is such a general term that it is not enough to talk about the acoustic properties of plaster; we must really consider the many applications, techniques, and use of plaster in construction projects. Plaster is a tremendously versatile material. Historically, the most common use of plaster had been as an interior finish material applied directly to masonry or applied to a thickness of 3/4 to 1-inch over wood or metal lath supported away from the exterior or structural wall the way that gypsum wall board is used now. The primary continuous use of plaster throughout its long history probably has been mostly in ornamental applications. Among its many variations of products is one known as acoustical plaster, which is a plaster product intended to
absorb sound.
In our work, plaster is always an acoustic material intended to absorb, reflect, diffuse, or shape the sound that will arrive at the front or rear of the plaster surface. Plaster applied at greater thicknesses or applied directly to masonry construction has very different acoustic performance from plaster on lath, and painted plaster will perform differently from unpainted plaster. Plaster can exhibit both porous and resonant absorptive properties, and can be effectively used for its reflective properties as well.
ABSORPTION IN PLASTER
In acoustic plaster, the plaster is aerated so that the finish surface is perforated with many tiny holes, exposing a labyrinth of pathways through the material that cause the vibrations in airsound to experience greater flow resistance than would be present at the surface of a smooth or polished material. This increased flow resistance is the mechanism that causes porous absorption. Similar techniques have been employed with cast tiles such as Akoustolith and its predecessor, Rumford Tile, which were used to create neo-Gothic churches that are intentionally much less reverberant than the buildings on which they were modeled. Examples of these churches include Rockefeller Chapel at the University of Chicago, Riverside Church and Church of the Heavenly Rest in New York City, and the Chapel at Duke University. Porous absorption tends to increase with increasing frequency and is therefore most effective at high frequencies.
Plaster as a predecessor to gypsum board as the standard interior finish material often was installed on lath furred out from the structural wall surfaces. This approach leads to low frequency resonant absorption. The relatively lightweight plaster surface vibrates in response to the sound energy, and the energy used in the vibration is lost to heat. Resonant absorption occurs at the resonant frequency (determined by the density, thickness, and bracing of the plaster) and diminishes above that point. Frequencies below this resonant frequency are typically transmitted through the plaster, and are therefore lost to the room as well. Resonant absorption tends to be a low-frequency phenomenon.
Varied acoustic properties are found in stages of plaster application as well. The back side of plaster applied to lath is sometimes exposed above a visual ceiling in a theater or concert hall where the tech gallery occurs above the architectural ceiling while still being part of the acoustic volume. This surface can be very rough because of the shape taken on where the plaster "pushes" through the lath. This surface is moderately absorptive. Early coats of plaster, either a "scratch coat" or a "brown coat" are more absorptive than the finish coat or "white coat." The greater porosity of these surfaces that allows successive layers to adhere well, also makes the surfaces absorptive, should they ever be left exposed to the room.
MAXIMIZING THE NATURAL
REFLECTIVE PROPERTIES OF PLASTER
In order to use plaster as a reflective surface, the finish surface must be smooth and the construction massive to minimize both porous and resonant absorption. Porous absorption can be reduced by painting the surface, resonant absorption can be reduced by applying plaster directly to masonry or by building up thick assemblies of plaster on heavy lath systems tied firmly to the structure. The thicker, stiffer, and heavier the surface, the better it will reflect low frequency sound.
Even white-coat plaster has some porosity that will increase as the material dries. Painting seals these pores in order to maintain high-frequency reflection. In the case where a brown coat plaster surface might be exposed to the acoustic volume of a room, it will be more absorptive by virtue of the greater porosity of the surface. The back side of a lath-applied plaster surface can be especially rough, and should be back-plastered if it will be exposed to the acoustic volume, if reflection is desired from that surface.
By applying the plaster directly to masonry, the mass of the masonry system becomes an effective part of the reflector system. A word of caution however; if sound isolation is a concern additional measures need to be provided to keep the rigid connection
of structural wall to interior finish material.
If plaster on masonry is not a viable approach because the interior wall is shaped differently from the structural or exterior wall behind, or if isolation or thermal concerns require an airspace in the wall system, thick applications of plaster (4 inches or more) can greatly reduce the low frequency absorption present in a thinner, lighter plaster surface.
ADVANTAGES OF PLASTER AS AN ACOUSTIC PRODUCT
Plaster can be gracefully applied in achieving many goals often set for good acoustics. Diffusive shaping can be accomplished either on a small scale, as in ornamental plaster, or on a large scale by providing a bowed or angled surface. Compound curves and other complicated shapes that are difficult to construct using sheet products are easy in plaster. Plaster is also more dense than gypsum and therefore provides greater mass in a similar thickness.
It is only in recent history with the progression from plaster towards the use of sheet materials that plaster construction has become an acoustic product in the absorptive sense. Plaster-like materials were first used directly applied to stone and masonry and provided the opportunity for decorative shaping on a small scale. It is in these applications that the natural properties of plaster come through as effectively acoustically reflective. The versatility of plaster in the construction world today may just give us one less reason to say, "They don't build 'em like they used to!"
