Noisy mechanical equipment can be more than just a nuisance. In many locations, including the New York metropolitan area where my acoustic consulting company is based, building codes place legally enforceable limits on the noise levels produced by air conditioning condensers, backup generators, pool pumps, exhaust fans, and similar units. In response, we’ve developed some basic soundproofing techniques that ensure quieter installations in both new construction and retrofits.
Soundproofing a pool pump is relatively easy: We mount it on vibration isolators to block structural noise and build a complete enclosure around it to block airborne noise. But soundproofing a generator or AC condenser is trickier. Since these units require airflow, you can’t enclose them completely, so you need to equip the enclosure with sophisticated silencers and baffles, an engineered solution we typically use only in commercial construction.
In residential construction, we can usually get good results by putting an acoustically insulated barrier between the noisy unit and the neighbors, as we did for the two projects shown in this article. One of those jobs involved bringing a standby generator into compliance with local noise ordinances; the other was precipitated by a dispute between neighbors over a noisy AC condenser. We use the same general techniques to muffle other types of outdoor equipment as well. Keep in mind, though, that whenever laws or codes are involved, you may need to consult with an acoustic engineer to get accurate measurements and effective results.
Two Types of Noise
Outdoor mechanical equipment produces both structure-borne and airborne noise. Because there are fundamental differences in how sound waves travel through air and through structures, we use different strategies and materials to manage each type of noise.
Airborne noise. A conventionally framed and plywood-sheathed enclosure or barrier will block a certain amount of airborne noise. But with a sound transmission class (STC) rating of only about 21, 1„2-inch plywood generally won’t eliminate all noise, and in fact could add to the problem by reflecting and amplifying certain frequencies. So we add acoustic materials to the barrier that will both reduce and absorb sound.
The acoustic blanket we use is made with a fiberglass absorber bonded to mass-loaded vinyl, which is a limp and heavy material that looks a little like sheet vinyl (see slideshow). The blanket has an STC rating of 32 and a noise reduction coefficient (NRC) of .85 (for an explanation of these and other terms, see the glossary on the facing page). Lining the barrier with this material can provide as much as 15 dB of additional noise reduction, a decrease in sound energy of more than 90 percent. That means that a condenser shielded by this barrier will sound less than half as loud as a freestanding unit.
Acoustic materials tend to be most effective at muffling higher frequencies (1,000 Hz and greater), but most outdoor condensers are noisiest in the lower frequencies (60 to 250 Hz). Though acoustic blankets can help make a barrier more effective, it’s the barrier’s height that makes the biggest difference in its overall acoustic performance. Just a couple of extra feet can make a surprising difference. We have a simple graph we use to estimate the noise-reduction levels we can expect from barriers of various heights (see “Calculating Noise Reduction”). With it, we can quickly determine whether we can help clients achieve their noise-reduction goals and still meet zoning height restrictions. Later, if we end up doing a detailed analysis and design, we use mathematical equations to fine-tune the barrier height.
Structural noise. Even though we focus on airborne noise, we also address low-frequency structural vibration in outdoor applications. Structure-borne noise happens when the unit is rigidly connected to a structural base like a concrete slab. When the equipment vibrates, the structure vibrates too, creating the sound. If the slab isn’t already isolated from the building foundation, the most effective way to eliminate this sound is to create a flexible disconnect between the unit and the base with either neoprene pads or spring-type vibration isolators.
Vibration isolators are rated by deflection, which correlates to the amount of vibration the isolator can absorb. Neoprene pads are less expensive and therefore more widely used than spring isolators, but they typically have only a .03-inch deflection, while some spring isolators offer as much as 8 inches of deflection.
Vibration isolators are also rated by weight capacity. Neoprene pads are load-rated, typically in pounds per square inch (psi). Since the efficiency of a spring isolator is optimized when the spring is compressed almost completely, spring isolators must be correctly sized for each mechanical unit and application.