Efforts to control noise and vibration in a wide range of mechanisms and devices inevitably require the use of passive acoustical materials. To achieve the greatest performance per dollar requires not only the correct choice of materials, but also an understanding of how they work, and of how and where to install them. All passive noise control systems use at least one of the following material types: barriers, absorption materials, vibration isolators, and damping materials.
- Barriers: enclosures, weighted materials, walls (examples shown right)
- Absorption materials: acoustical foams, fibrous batts or blankets, acoustical tiles
- Vibration isolators: equipment mounts, plastic or rubber-based bushings and grommets, steel spring equipment supports
- Damping materials: plastic sheets, mastic solutions, adhesive films.
A sound barrier is usually a solid material which, by virtue of its mass, acts as an acoustical reflector, interrupting the path of a sound wave. Absorption materials (examples shown left) are almost always used in conjunction with a barrier of some type, since their porous construction permits noise to pass through relatively unaffected. The porous nature of absorption materials renders them susceptible to contamination, moisture retention, and deterioration due to physical abuse. To avoid these problems, facings may be attached to at least one side of the absorber.
Vibration, like sound, travels in all directions away from a source to surfaces where it can be radiated as noise. Use of vibration isolators (examples shown right) can stop the flow of vibration from one point to another and reduce noise. While isolators are available in a very broad variety of designs, all have one characteristic in common: they provide a means of connecting two structures so as to provide relative motion between them under dynamic loads. The amount of motion required depends on many variables, the chief one being the range of frequencies over which the isolator must be effective.
Structural damping (damping materials shown left) is to structural vibration what absorption is to airborne sound. That is, it provides a means for eliminating mechanical energy by converting it to heat. Coverage need not be total in order to be effective, and impact noise reductions of 10 dB(A) or more can be achieved with as little as 25% surface coverage. For simple free-layer installations, sometimes called extensional damping, four parameters determine the amount of damping and noise reduction:
- Base material and thickness (e.g., steel or aluminum);
- Damping material characteristics at temperature and frequency of the application;
- Ratio of damping material thickness to base material thickness; and
- Percentage of surface area covered.
