New Hampshire Chapter of the Construction Specifications Institute

Soundproofing Techniques
By  Thomas House, AIA, CSI
Principal, THA Architects

​​Called "sound-transmission" in the trade, noise between units is getting to be a big issue, particularly in renovated mill buildings with wood floor construction. Even solid concrete buildings can transmit noise from floor to floor.

As more and more people move into existing mill buildings the problem has gotten a lot of attention. Noise-reduction strategies are typically discussed and debated at the very beginning of the design process. Should the floor be covered, or should a new ceiling be dropped below the existing ceiling? Is it really necessary to build double walls between units?

In an existing mill building noise is often generated from the neighbors and outside, and to a certain extent this is just part of being in a mill building. But when you are trying to sleep and the neighbor’s every action is transmitted through the wall or ceiling in vivid detail you are justly aggrieved. You can tell your neighbor not to run the power sander at 2 am, but you really can’t tell him not to flush the toilet, watch a DVD, or get a mid-night snack.

Noise is differentiated between "air-borne" and "impact":

A radio playing creates air-borne noise. High heels on a hard floor create impact noise. You want to deaden, stop, kill, frustrate, etc. these vibrations. Systems that are good at stopping airborne noise don’t necessarily stop impact noise. A thick concrete slab will dampen the airborne sound of a radio or a skill saw, but it will accurately transmit the impact sounds of high heels, scraping metal chair feet and the like.

IIC (Impact Insulation Class) ratings refer to the ability of a system to reduce impact noise transmission.

STC (Sound Transmission Class) ratings refer to the ability of a system to reduce airborne noise. To give STC’s some context, a 55 STC rating indicates that loud speech is not audible between rooms, while a 25 STC rating indicates that normal speech can be clearly understood.

Air-borne noise can be stopped in the following ways:

  • Adding mass between spaces – a 12" concrete floor absorbs more sound energy than a 4" concrete floor. Extra layers of drywall reduce transmitted noise through both walls and ceilings.
  • Putting gaps in the structure – a typical sound insulating wall between units may be composed of two sets of studs, one set for each unit. This eliminates structure-borne noise transmission between the units. A flexible support system (sound isolating channels) can cut down on structure-borne noise passing from a structural beam to a drywall ceiling.
  • Closing air-holes between spaces – sound waves carry through key-holes, around pipes, between a wood floor and a brick wall, through outlets placed back to back, and over-and-under walls which are not thoroughly caulked at the top and bottom.
  • Putting dampening elements in the voids between spaces – fiberglass or cellulose insulation absorbs air-borne noise and also dampens vibrations in the wall surface, much as stuffing a sock in a guitar would make it quieter. The air between two sheets of drywall can’t transmit noise efficiently if it is filled with fluff.
  • Calming air-borne noise before it enters the structure – a radio playing in an empty apartment will transmit a lot more noise than one playing at the same volume in a unit filled with furniture, window coverings, paintings, clothing and other possessions. New buildings are "noisier" than long-occupied buildings for the same reason (although that is hard to tell a new occupant). Carpets do more than just reduce impact noise – they also absorb sound within the space. Music rehearsal spaces make use of many sound deadening technologies, including various foam products. (Always check the flame-spread rating of any foam insulation products.)
Impact noise is best stopped with dampening elements close to its source:
  • Stopping at its source - old-fashioned apartment leases used to call for tenants to cover a certain percentage of their wood flooring with carpets, for instance. Noisy machinery can be mounted on neoprene pads to prevent its vibrations from entering the floor.
  • Separating surfaces - if new hard-surface floors are going to be installed, a layer of "gypcrete", homasote, or recycled rubber matt between the sub-floor and the new floor can reduce impact noise transmission by preventing it from re-transmitting into the structure below.
  • Reducing "retransmission" once impact noise enters the structure - If an exposed hard-surface floor is retained (many old mill floors are beautiful maple, with the patina of age), impact noise can be abated lower down in the system if the ceiling is hung below the beams and topped with insulation. This is a good idea especially in the areas of bathrooms and kitchens, even if the ceilings elsewhere in the unit are exposed structure.
Some typical systems currently in use in mill buildings include:

  • Exposed concrete over original wood decks. This system is being perfected as it gives a good-looking, hard-wearing surface and allows the ceiling below to be exposed. The system includes a generous layer of sound-absorbent matting firmly adhered to the existing wood floor. The new concrete floor should be continuous under partitions, as wood walls can pick up sound and send it into the structure. The joint between the concrete and the exterior walls needs to be caulked ahead of the pour, as the concrete can shrink and pull away from the walls leaving an air space for sound to enter. Since the slab depth needs to be kept thin (about two inches), there is no room for conventional reinforcing, so fiber reinforcing is used instead. The concrete mix, sealant, and curing methods are also critical to avoid cracking and curling. Control lines are sawn into the slab after pouring. They should be directly above the beams so that the slab does not randomly crack as the building structure flexes. This becomes an expensive system when done right. When done wrong it can become a disintegrating mess, so it has to be carefully designed and installed.
  • New wood floors over existing wood floors. When existing floors are beyond salvaging, or when the ceiling below is too beautiful to conceal, new wood floors are sometimes the solution. A sound absorbent material, such as rubber matting, gypsum concrete "gypcrete", or a layer of homasote – will reduce airborne and impact noise transmission into the space below. All of the underlying layers must be continuous under partitions and caulked at the edges. Sound transmitting short circuits are easy to thwart at the design and construction stage, and very hard to fix later on.
  • A note on gypcrete: Typically, wood floors, resilient floors or carpeting are fastened to the gypcrete with an adhesive product. The durability of this system is only as good as the gypcrete below. It is not recommended for areas receiving rugged treatment such as the heavy moving loads in a wood shop or sculpture studio, since this can cause the relatively weak gypcrete to break up and disintegrate. Various sealers and primers are available to prepare existing flooring to adhere to the gypcrete. Caulking around openings and edges does more than just stop noise; it also keeps the very liquid gypcrete from spilling into the space below. Gypcrete will seek its own level, so a heavily sloped floor can use up large quantities of the material at the low points.
  • Dropped ceilings below the structure. This is the only line of sound defense if existing wood floors are retained. If the ceiling is doing all the sound deadening, it should be well-engineered. Carefully install sound insulation. Seal/caulk all holes and beware of openings around recessed light fixtures and gaps at ceiling edges. If structural elements are left exposed they will transmit some sound, particularly impact noise. Since large beams may be too beautiful to hide, sometimes noise must rank below visual impact. Use "resilient" channels to hang the drywall – don’t let noise move directly from structure to the drywall. An additional sheet of sound deadening material made of wood fiber is sometimes installed directly above the drywall, at an additional cost of about $1 per SF. More than one layer of drywall improves performance, but always put the resilient channels above both layers of drywall.
  • Drywall partitions between units. Walls between units (or along corridors) can have one or two rows of studs, and several layers of drywall on each side, as well as insulating infill, sound clips and other devices. A simple display of various stud "demising" walls and their STC ratings can be seen at
The STC rating of a simple metal stud wall with 5/8" drywall on each side is 43-44, which means that loud speech is audible but hard to understand. Increasing the distance between studs from 16" to 24" improves the STC rating to 36 – a rare instance of things getting better when you build cheaper. Adding resilient channels on one side and adding batt insulation in the cavity increases the STC to 46, which is a tolerable level for units, but definitely not "sound-proof".

Note that wall and ceiling assemblies between units also need to be fire rated per applicable codes.

While wall and floor construction does the bulk of the soundproofing, don’t forget these other important things:

  • Make sure that electrical receptacles are not located back to back.
  • Caulk the upper and lower edges of all walls between units, leaving a small gap at the base and the top of drywall to receive the caulk.
  • Try to keep sleeping areas away from common hallways and stairways. Use of industrial carpeting on hallway and stairway floors can deaden footfalls and eliminate echoes.
  • Do not allow washers or driers to be located over sleeping areas.
  • Avoid exposed plastic drain lines at ceilings – they very accurately transmit plumbing sounds.
  • Encourage noisy activities to locate on ground floors so no one is below them.
  • Encourage occupants to put area rugs in their living areas.
  • Encourage the use of industrial rubber mats or pads under machinery located on upper floors.
  • All suspended HVAC equipment should be hung with vibration isolators.
  • A length of sound-insulated duct should separate the furnace from the (often exposed) ductwork. Installing this later on to calm down an obnoxiously noisy heating system is much more expensive than putting it in initially.
  • Overhead garage door openers can be very noisy and are especially irksome to people trying to sleep or concentrate near them. Pay attention to the noise level of the door opener, and if a noisy system is picked for cost or functional reasons, add layers of sound deadening and use vibration isolators. Make sure sleeping areas are as far away from overhead doors as possible.
  • Carefully caulking around window frames on both the interior and exterior can reduce the amount of sound infiltration from the exterior.
  • When replacing wood sash with new aluminum windows, noise infiltration can be reduced by filling the old sash weight cavities with blown-in cellulose insulation. This also reduces air infiltration through these substantial hollow areas.

Many of the techniques for sound reduction require close construction supervision. They also may require education of the sub-contractors. Caulking around air leaks is routinely forgotten or done poorly. Sound insulation needs to be continuous and seamless to be effective. As construction progresses it is necessary to keep up inspections and to make sure that previously installed items are not pushed out of place, or otherwise damaged.

The result of attention to sound attenuation is a more peaceful building, and satisfied occupants. Remember, when describing your building; NEVER say a floor or wall is "sound-proof". Affordable systems cannot totally eliminate the louder ranges of living and working noise, but they can make them infrequent and tolerable.