Industrial insulation is generally thought of in terms of thermal performance, but an important, secondary benefit of insulation is its contribution to noise control. In many cases properly engineered industrial insulation systems can be used to reduce the noise level of noisy pipes, equipment and systems.
What we describe as sound is simply a vibration (mechanical pressure wave) that travels through a medium (e.g., gas, liquid or solid). Sound waves move longitudinally away from the source, just like water ripples away from a disturbance on its surface. For example, dropping a pebble into a pool of water will cause ripples to move outward, away from the disturbance. Sound waves travel in this very same manner.
Sound is described in terms of several different characteristics such as level (what we would interpret as loudness) and frequency. Level is measured in decibels and describes how much sound there is. Frequency is simply cycles per second, measured in “Hertz” (Hz). Frequency ranges for industrial environments can be described as a hiss (high-frequency, 1000-8000Hz), a roar (mid-frequency, 125-500Hz) or a rumble (low-frequency, 31-63Hz). In cases where ambient noise exceeds 85 decibels (dB), hearing protection is required to reduce the potential for long-term hearing loss.
In an industrial environment, we are concerned with two types of sound control: sound absorption and sound transmission. Sound absorption can be used to reduce reverberation in the industrial environment where hard, flat surfaces reflect sound. The results of sound absorption testing (ASTM C423) provide a Noise Reduction Coefficient or NRC value. Generally speaking, a product with an NRC value of 0.75 would absorb 75% of the sound that strikes its surface while reflecting the remaining 25%. The higher the NRC value, the more sound is absorbed. Some of the best sound absorbers are fibrous insulations such as mineral wool. This is because the dense fibrous structure dissipates sound by absorbing sound waves and then converting them into a small amounts of friction energy.
When too much sound is passing through a system, like pipe wall or equipment housings, sound transmission treatments are effective solutions for reducing noise. Adding weight and thickness to the system lessens the amount of sound traveling through the system, thus reducing the ambient noise level outside of the pipe or equipment. The more weight a system has, the more energy it takes to vibrate the system. Generally speaking, the thicker the insulation system is, the easier it is to dampen the mid-frequency (roar) and high-frequency (hiss) noise that radiates from the system. The best insulation materials for reducing sound transmission are heavier, thicker insulations like calcium silicate or perlite.
Finally, installation requirements for thermal applications apply to acoustical applications as well. Since sound can be transmitted through small openings and cracks, care should be taken to install the insulation so that it completely covers the pipe or equipment to prevent the radiation of sound. Although it’s difficult to eliminate all noise in an industrial environment, small changes can provide measurable reductions in background noise levels.