Types of Acoustical Materials

Most commercially available acoustical materials are used to develop the following acoustical items : 

Prefabricated Units 

These include acoustical tile, (which is the principle type of material available for acoustical treatment), mechanically perforated units backed with absorbent material, and certain wall boards, tile boards and absorbent sheets. 

Prefabricated units are classified into three types. These groups include tile, absorbent material covered by mechanically perforated units, and certain building boards and sheets.  

The most outstanding feature of an acoustical tile is its 'built- in' absorptive value. The tile is a factory may product; the absorptivity is relatively uniform from tile to tile of the same kind. This makes it fool proof, a highly desirable characteristic. Another merit possessed by the acoustical tile is its relatively high absorptivity. In a factory made product it is possible to control such factors as porosity, (including the number and size of pores), flexibility, density and the punching or drilling of holes - factors which are paramount in determining the absorptivity of materials and factors which often are difficult to control in certain types of  acoustical platers. 

The principal disadvantages of an acoustical tile are its limitations regarding architectural treatment and its cost compared with that of other acoustical materials. It is quite impossible to conceal entirely the joints between adjacent tiles.

Acoustical tiles often are two or three rimes more absorptive than acoustical plasters and for this reason as much absorption may be attained with one square foot of tile as with two or three square feet of plaster. 

Acoustical Plaster and Sprayed-on Materials 

These material comprise plastic and porous materials applied with a trowel, and fibrous materials combined with binder agents which are applied (sprayed on)  with an air gun or blower. 

Acoustical plastic materials are satisfadory for treatment of offices, school rooms, corridors, and public buildings. The absorptivity of acoustical plaster is dependent on its thickness and composition and on the manner in which it is applied and dried. As the thickness is increased, the absorptivity increases particularly for low frequencies. However, for plasters of the type applied with a trowel, it is usually uneconomical to increase the thickness beyond".  If too much binder material is used, the plaster is not sufficiently porous. If an insufficient  amount of binder is used, the plaster does not set hard, and its tensile strength may be less than that required for adequate structural bond, and under such circumstances, it may dust or pop off the wall. Likewise, if the undercoats of plaster are too wet, the binder material forms an impenetrable film at the surface, whereas if the undercoats are too dry, the binder material is absorbed by the undercoats and the plaster. Since, the absorption coefficients of acoustical plasters are dependent on such factors as the suction behind the plaster, the pressure applied to the trowel, and the manner of floating, texturing or stippling, the workman should be instructed to exercise great care in applying and finishing these materials. 

In selecting an acoustical plastic material it is desirable to consider its adhesive and cohesive properties, its resistance to fire and abrasion, its ease of application, its texture and its maintenance (such as cleaning and decorator) as well as its coefficients of sound absorption. 

Acoustical Blankets 

Blankets are made up chiefly of mineral or wood wool, glass fibres, kapok batts, and hair felt. 

The materials used most commonly in the fabrication of acoustical blankets are mineral wool, hair felt, wood fiber and glass fiber. These materials are more absorptive m the low frequency range, principally because of their greater thickness, than most other types. Hence, blankets sometimes are useful for controlling the acoustical characteristics of studios and auditoriums that require 'balanced' absorption, including a considerable amount of absorption at low frequencies. 

The absorption coefficient of a blanket mounted against a wall depends on its density and thickness and on the frequency of the incident sound. Increasing the thickness of the blanket increases it absorptivity, principally for low frequencies, and slightly for high frequency. 

The fibres in certain types of blanket, especially some mineral-wool products, have a tendency to 'settle', often as a result of building vibration, this settling alters the acoustical characteristics of the blanket. For this reason, blankets fabricated of materials that tend to settle are frequently quilted at intervals of a few inches. In other cases, the materials are given additional structural strength by the addition of a binder material or by a wire-mesh screen or hardware cloth on one or both sides of the blanket. 

Perforated Facings 

A perforated facing such as plywood, metal, or fiber board constitutes a very practical covering for an acoustical blanket. Except for the small holes, the appearance of the plywood covering the patches of absorptive material does not differ from other portions of the wall. This type of facing has the advantage that it can be easily cleaned and decorated, and repeated painting does not reduce its absorptivity if the holes are not bridged with paint. 

Painting of Acoustical Materials 

Acoustical materials with small pores in the surface such as acoustical plaster and fiber boards will have their sound absorption coefficients reduced if covered by thick coats of paint. Such materials should therefore not have applications of viscous water or oil paints, varnish or calcimine. Where painting is required, thin coats of paints, lacquers, stains, or aniline dyes may be applied. The use of a spray gun is recommended rather than a brush. The surface pores should be cleaned by means of a solvent before painting. Some acoustical materials with large perforations or perforated facings may not be affected acoustically by painting if the holes are not covered.