To reduce sound pressure levels in an enclosed space in which a sound source is present, one approach is to cover all exposed hard surfaces with a soft, non-reflecting sound absorbing material such as a compressible open cell foam. A common misunderstanding is that such sound absorbing materials also are good acoustical barrier materials. But, acoustical barrier materials have the opposite property from acoustical absorbing materials, i.e., barriers are highly reflective to sound, and do not absorb it.
Similarly, although some materials are used as acoustical barrier materials and also as acoustical dampening materials, the function of a barrier material differs significantly from the function of a dampening material. In order for a material to provide efficient viscous dampening to a composite panel, it must be adhered or coupled to the panel. The same material provides better performance as a barrier when it it isolated or decoupled from the panel.
Thus, a noisy piece of office equipment within a room could be enclosed within a barrier. But, it would be ineffective to leave the office equipment exposed within the room and line the room with acoustical barrier, as the noise will be reflected back to the inhabitants of the room. A better approach would be to line the room with acoustical absorber material, e.g., acoustical ceiling tiles, carpeted floors and absorbing materials mounted on the walls. By contrast, a meeting room adjacent a noisy factory could be lined with acoustical barrier material to prevent the factory noise from entering the room.
The differences in performance can be explained by considering the operation of each type of material. The essential physical characteristic of an acoustical absorber is controlled porosity. The process of absorption depends on sound entering the material where it is converted to heat by friction on the porous surface and cells of the material. Since sound waves must flow through the absorbing material, the effectiveness of the absorbing material as an acoustical barrier is very limited.
Thus, the prior art teaches that acoustical barrier materials should be non-porous, massive and limp in order to be effective. Acoustical barriers are ineffective when they are placed over an area which is not a significant noise source or path. In order to provide a noticable improvement (3 dB reduction in sound level), the treated area must be the source or path of half the acoustical energy of the targeted noise.
If design limitations require holes to be cut into an acoustical barrier material, the effectiveness of the acoustical design is reduced significantly. However, such holes are usually necessary for structural supports, electrical wiring, control cabling, and the like that support a piece of equipment representing a noise source.
Furthermore, acoustical barrier materials can be ineffective in controlling structural borne noise, which readily propagates through any portion of a structure due to the typical high density of structural materials.
To increase the transmission loss of an acoustical barrier material, the prior art teaches to increase the mass per unit area of the barrier, and to use a limp material, i.e., a material which is not so rigid that it will shake or vibrate in a sound field, thus transmitting vibration and regenerating sound on the other side of the barrier.
For a composite barrier system, the prior art teaches multiple massive layers, layers of highly absorbing material (e.g., a limp material such as glass-based thermal insulation) between layers of barrier materials, and air gaps between layers of barrier material.
The techniques are often combined. Each technique, however, has disadvantages at low frequencies (0.1-1.0 kHz). To achieve large acoustical loss at 0.1 kHz by adding mass alone, the barrier weight per unit area would have to be more than about 4800 N/m.sup.2. Thus, a dense material such as lead is suggested, and limp lead sheeting is often used to prevent resonances. However, limp thermal insulation and air gaps, while lower in weight than dense materials such as lead, provide only excellent high frequency (5.0-10.0 kHz) transmission loss, but are marginally effective at low frequencies.
U.S. Pat. No. 4,079,162, issued Mar. 14, 1978, discloses a composite material comprising hollow glass microspheres interspersed into a curable resin base. The microspheres support vacua within themselves. The cured resin is flexible, relatively soft and has a relatively low indentation hardness.
French Patent Application No. 8908982, published Jan. 11, 1991 as publication No. 2649356, discloses a composite honeycomb material comprising roughly bonded hollow microspheres and a solid binder forming menisci in contact zones located between the microspheres. The menisci insure mutual bonding among the microspheres while leaving the rest of the interstitial volume between the microspheres as void.