Let’s face the facts — all machines generate sound and vibration. Noise levels are affected by different properties within the compressor room, from the size of the room to the materials used for walls and ceilings, and the presence of other equipment. While engineering and quality of materials can damp the sound and vibration of individual pieces of equipment, this article will focus on how the design of the compressor room can attenuate the sound radiated into other parts of the plant.
Most sound proofing materials will have an Sound Transmission Class (STC) rating to quantify how well a material attenuates airborne sound. STC is a common measurement used to give designers an estimate of attenuation; however, the actual amount of attenuation can vary based on sound frequency. For instance, methods effective for mitigating sound at higher frequencies like high-speed drills may not be as effective on lower frequencies such as home theaters or idling trucks. The same principle can be applied to damping equipment vibration because sound is inherently caused by vibrations.
4 important components for mitigating sound transference include decoupling, damping, absorption and adding mass.
- Decoupling involves eliminating a direct physical pathway from one room into the other. In a standard room built with 2 x 4s and drywall, the sound from the compressors will hit the drywall on the compressor room side, which vibrates the drywall, followed by the studs and drywall on the outside of the wall. The outside wall will effectively act as a speaker and transmit the sound out to the rest of the plant. If you were to build two entirely separate walls not physically connected and with space between them, the inside and outside walls would be decoupled because the vibrations on the interior wall would not vibrate the exterior wall via physical contact. Soundproofing channels and clips are also available as a less effective option.
- Damping involves using a material such as an elastomeric glue or sheets of rubber to reduce the amount of vibration transmitted into the sound wall. Building on our previous example, we would damp the connection to the wood studs so that fewer sound vibrations hit the drywall and transmit into the stud wall. In turn, fewer vibrations from the wood studs would be sent back into the room from the other side of the wall. Damping differs from decoupling in that a physical connection between the two materials still exists, (in this case our drywall and wood studs) but we are reducing the vibrations transmitted from one material to the other.
- Absorption involves adding materials inside the wall to decelerate the sound waves inside the wall itself, which is common practice in adding insulation to wall cavities. When sound hits one side of the wall, it will vibrate and the space between the two walls will act as a drum. Adding insulation reduces the speed and amplitude of these sound waves.
Certain types of insulation have better absorption properties. For example, mineral wood (Roxul) insulation works better than fiberglass and spray foam insulation. However, spray foam is very effective in sealing open gaps, which would otherwise allow the sound to have a direct path through the wall. These gaps are commonly found where plumbing or electric is run from one room to another. It is advised to not compress the insulation as this will reduce both its thermal and sound insulation effectiveness.
- Adding mass is a critical element in soundproofing. The heavier something is, the less it will move, given that the same about of force is applied. This translates to less vibration of the wall itself. A 4.5 inch poured concrete wall will have a much higher mass then our drywall and wood-studded wall. It is possible to add mass to your existing wall without having to pour concrete or use concrete blocks. Extra layers of drywall, plywood or even cement board will all help in mitigating sound transference.
While it can be costly to implement all of these techniques and logarithmic nature sound volume can reach diminishing returns at a certain point, it is worth investing in cost effective techniques that fit your budget and sound attenuation needs.
Have you tried any noise damping tactics in your facility? Leave us a comment below, and let us know your best practices for mitigating sound.