Soundproofing near you

There are two kinds of sound to stop and they need different treatments, which is why there are two ratings. Airborne sound is speech, television and music travelling through the air and then through the assembly. Structure-borne sound is vibration transmitted through a material and re-radiated at another point, which is what footsteps upstairs are.

Seattle's adopted building code sets the airborne requirement at a sound transmission class of not less than 50 for separating assemblies, or not less than 45 if field tested, where tested in accordance with ASTM E90, and allows the rating to be established instead by engineering analysis comparing the assembly with others whose ratings were determined by that test procedure. Field testing, where permitted, must be done in accordance with ASTM E336 or ASTM E492 under the supervision of an acoustical professional experienced in acoustical testing and engineering, who forwards certified results to the building official.

Entrance doors are treated separately and far more leniently, which tells you something about how much a door can realistically achieve: dwelling unit entrance doors from interior corridors and interconnecting doors between separate units must have perimeter seals and a sound transmission class rating of not less than 28. If you are trying to quieten a room with an ordinary interior door in it, that is the ceiling you are working under.

Transmission loss through an assembly is a property of the assembly's mass, its stiffness, its damping and whether the two faces are structurally connected. That is why the code rates assemblies rather than products, and why the useful measures are structural.

A separating assembly only performs as well as its weakest bypass, and the code text reads like a list of the bypasses installers actually create. Joints in the perimeter of a separating wall or floor-ceiling assembly must be acoustically sealed with a permanent resilient material approved for the purpose, and the assembly must extend completely to and be sealed to another separating assembly or to an exterior wall, roof or floor assembly. A wall that stops at a dropped ceiling and leaves the plenum open above it is, acoustically, not a wall.

Electrical boxes are the classic residential failure. Seattle's adopted code requires that outlet boxes not be placed back-to-back and be offset by not less than 12 inches from outlets in the opposite wall surface, and that the back and sides of boxes be sealed with resilient sealant and backed by at least 2-inch-thick fiber insulation material or an approved equivalent. Two receptacles sharing a stud bay on opposite faces is a hole through the assembly with a cover plate over it.

The code also recognizes that a tested design can be undone on site. It permits field- or laboratory-tested wall or floor-ceiling designs rated 50 or more to be used without additional field testing when, in the building official's opinion, the tested design has not been compromised by flanking paths, and allows the official to require tests where evidence of compromised separations is noted. Duct runs get their own line: metal ventilating and conditioned air ducts passing between dwelling units must be fabricated and installed to maintain the required sound transmission ratings.

Two different problems get sold under the same word. Absorption treats how a room sounds from the inside, soaking up reflected energy so speech is clearer and the space feels less live. Isolation treats how much sound passes from one space to another, which is what the sound transmission class measures across a complete assembly.

Foam panels and fabric-wrapped absorbers are absorption products. Stuck to the face of a finished wall they add almost no mass, they do not decouple the two faces of the wall from each other, and they do not seal any of the penetrations the code lists. They are the correct tool for a recording booth, a conference room or a hard-surfaced open-plan kitchen that echoes, and the wrong tool for a neighbor's television.

The practical test to apply to any soundproofing proposal is whether it changes the assembly. Does it add mass to the face, separate the faces, damp the panel, fill the cavity, or seal a documented flanking path? If the answer to all five is no, the proposal will change how the room sounds and not what you can hear through the wall.

Soundproofing a separating wall does not license you to compromise it in other respects. Seattle's adopted code carries the point as a note to the sound transmission section: design and materials for sound transmission control shall not impair the fire-resistive integrity of separating walls or floor-ceiling assemblies required to be of fire-resistive construction. Resilient channels, added layers and penetrations all interact with the tested fire assembly, which is a reason to use listed assemblies rather than improvised ones on any wall between dwellings.

Where the noise is coming from outside rather than from a neighbor, there is a federal yardstick worth knowing about. HUD's noise regulations at 24 CFR part 51 state the policy of protecting citizens against excessive noise in their communities and places of residence, and set the day-night average sound level thresholds that HUD uses to judge a site: not exceeding 65 decibels is acceptable, above 65 but not exceeding 75 decibels is normally unacceptable, and above 75 decibels is unacceptable.

The attenuation figures attached to those bands are a useful sense of scale for anyone living near a highway, a rail line or an airport. HUD requires a minimum of 5 decibels of additional sound attenuation, beyond that provided by buildings as commonly constructed in the area, for sites above 65 but not exceeding 70 decibels, and a minimum of 10 decibels of additional attenuation for sites above 70 but not exceeding 75 decibels. Those are federal site standards rather than a homeowner requirement, but they tell you that reducing exterior noise meaningfully is measured in a handful of decibels, hard won.

The scope drives the number far more than the materials do. What moves a soundproofing quote is the area of assembly being treated, whether the wall or ceiling is opened up or treated from one face, how many penetrations and boxes need sealing and relocating, whether the ceiling assembly needs decoupling for impact noise, whether ductwork or plumbing has to be isolated at its supports, whether doors and windows are included, and whether the finished assembly has to hit a stated rating and be field tested to prove it.

Ask for the proposed assembly to be named and for the rating it is expected to achieve, and ask which tested design it is based on. Ask what will happen at the perimeter joints and at the top of the wall. And if a proposal consists mainly of products attached to the face of a finished wall, ask which of mass, decoupling, damping, cavity fill or sealing it is providing.

LokalMatch handles the search step. You describe the noise problem once, contractors working in your area see it, and those interested contact you directly. Homeowners pay nothing and pros pay for the requests they receive. LokalMatch does not do acoustical work, does not set prices and does not rank or recommend any company, so checking credentials and the written scope remains with you.