Back to Building Envelope Theory Topics

Noise Reducing Barriers

Basics: What are Sound, Frequency, and Wavelength?

If you are not familiar with how sound works, the following brief refresher course may help.

Sound is a pressure wave traveling in air or water. A sound wave resembles the surface wave made when you throw a stone into a calm pool of water, except that the sound wave consists of tiny fluctuations in the air pressure rather than fluctuations in water height, a sound wave can travel in three dimensions rather than two, and the wave speed is much faster (340 meters per second in air).

Sound is usually generated by the vibration of an object or surface such as a speaker cone, a violin body, or human vocal cords. The vibrating surface “radiates” pressure waves into the adjoining air or water as sound.  (Sound can also be generated by turbulent airflow, by bubbles collapsing, or by many other phenomena.)

The frequency (number of wave crests per unit time that pass a fixed location) measures the tone or pitch of a sound. For example, a bass guitar plays lower frequencies than a violin. The wavelength, or distance between wave crests, is related to frequency: lower frequencies have longer wavelengths. In air, all frequencies of sound travel at the same speed. When bending waves travel through a flexible structure, however, low frequencies travel faster than high frequencies.

Brief Summary of Sound Control and FOAM-TECH's Role

The reduction of sound transmission from one space to another can be achieved in two ways – sound absorption and sound isolation. The absorption method relies on trapping all of the sound generated in one room in fissures on the room?s surfaces. This is generally not practical in normal building construction, as its effectiveness depends on treating all of the wall, floor, and ceiling surfaces with deep-textured finishes (an example for a typical building application would be carpeting all of the walls, floor, and ceiling) to achieve order-of-magnitude differences in the overall room absorption (see “Exposing acoustical myths” by Richard Strong). The latter method, isolating sound, is the solution more commonly used. To achieve this, it is important to first understand the ways in which sound is transmitted from one space to another.

Generally, sound is transmitted through the boundary walls, ceilings, and floors of the space in which it is generated in three ways:

Penetrations: sound waves are transmitted unimpeded from the first space through openings (doors, windows), or holes (electrical outlets, fan units, etc.) into the second space.

Airborne: sound waves are transmitted through the air from the surface in the first space to the surface of the adjoining space, then re-radiated in the adjoining room.

Structure borne: sound waves are transmitted from the radiating source in the first space or room and impact the sound barrier (wall or ceiling) and are then conducted through structural members to the surface of the second space, then are re-radiated in the adjoining room.

It stands to reason that stopping sound from traveling unimpeded through penetrations is the most important. If sound is able to move directly through openings, the intensity of the sound is not reduced and it will negate the overall sound reduction measures. Gaps around the penetrations from doors and windows, and from plumbing and electrical components can have a devastating influence on the overall acoustical performance of walls, ceilings, and floors. The most carefully designed partition will perform only as well as its weakest link-a gap around a door or electrical box. Standard practice is to stuff these gaps with insulation. Although this approach provides sound absorption, it is not a barrier to sound transmission through (penetrations) and around (flanking) the wall or ceiling. Penetrations can be sealed and flanking transmission can be controlled to some extent by use the of suitable construction details (see the Canadian Housing Information Center [CHIC] “Flanking Sound Transmission in Wood Framed Construction”). To put the importance of properly attending to penetrations in perspective, a miniscule gap of 1/64 inch around the perimeter of a 3' x 7' door will effectively reduce the Sound Transmission Coefficient (STC rating) of the door from STC-36 to a STC-29. A reliable recourse is to use a sound-rated door which is manufactured as an integral unit with its frame and gasket. Sealing around the unit where it is installed into the wall is also critical. To apply this concept to penetrations caused by electrical boxes and plumbing pipes – merely stuffing the holes with absorbent insulation instead of sealing the gap can limit a wall’s overall performance significantly. Polyurethane foam is especially advantageous in this application for isolating sound because it can be used to provide this essential seal which blocks direct sound transmission. Polyurethane’s unique sealing properties enable it to provide an effective barrier to the direct transmission of sound, at the same time it contributes to sound absorption.  This is an area where FOAM-TECH’s blower door, pressurized fog, and infrared testing capabilities can be used to locate and seal gaps and seams that compromise the performance of a sound isolation wall.

Airborne sound is more challenging to control than sound which travels through gaps, but easier to manage than structure borne sound. Foam and other fibrous insulation materials can be used effectively in reducing airborne sound. By filling the cavity inside an isolation wall or ceiling with any low-density material, the transmission of sound from one room?s surface through a wall or ceiling to the other room’s surface can be significantly reduced.  The advantage of polyurethane in this application is that it serves the dual purpose of sealing penetrations as well as preventing sound from traveling from surface to surface through the air.  Polyurethane foam also has the flexibility to be injected into, or spray-applied onto the surfaces in sound isolation bays of stud walls and between the joists of floors and ceilings.

A fact often overlooked when attempting to isolate sound is that structure-borne sound cannot be isolated in the same way as airborne sound. Insulating between studs with a particular insulation material to control airborne sound does nothing to control structure borne sound ? the sound transmitted through the framing. Structure borne sound cannot be controlled with foam or any other low- density infill materials. There are generally three approaches used to inhibit this type of sound transmission. One is nailing resilient channel directly to the studs/joists. Sound must then travel through this flexible channel, which weakens the intensity of the sound waves before they are transferred from the structural members to the drywall or other finishes. This practice is commonly used in the construction of ceilings in hotels, condominiums, and multi-family dwellings. A second method is the use of double-wall construction in which there is a minimum of structural bridging combined with an ample intervening air space. This is common in “party walls” between dwelling units. Finally, installing multiple layers of gypsum board or uninterrupted walls of solid masonry, where mass is the mechanism for reducing sound transmission, is an effective way to control structure borne sound.

Polyurethane Foam For Sound Control

It is important to realize that polyurethane foam can assist in reducing sound transmission, but must be used in conjunction with other measures to provide a totally effective sound barrier. Polyurethane foam, properly quality controlled, will stop sound leakage through mechanical penetrations and at open flanking paths around a sound separation wall, ceiling, or floor. Foam can also significantly reduce airborne transmission through the wall or ceiling cavity itself. Open-cell and closed-cell foams can both be used successfully in sealing penetrations; however, open-cell urethanes are more absorbent and are the best for reducing airborne sound.

To complete sound control measures, air sealing and airborne sound insulation must be supplemented by provisions to stop direct structure borne transmission through the framing. In most applications, this means the use of resilient channel or double wall construction details to break up the acoustic bridge created by structural framing.

Notes on High and Low Frequency Sound

It is much more difficult to keep low-frequency sound from going through a wall than high-frequency sound. Transmission loss (reduction) depends on the mass and stiffness of the surfaces and the thickness and the absorption characteristics of the air space between them.  Structural separation of the surfaces on the two sides of the boundary is important where mass is low. For this reason, heavy, stiff, reinforced concrete or masonry are often used as major sound separation barriers in larger buildings to avoid the cost of constructing double walls and ceilings. In lighter wood-framed structures, controlling low frequency sound requires heavier finishes, double wall, or resilient channel construction with special attention to flanking path details.

Notes on Mechanical System Details

To optimize acoustical performance, how the materials are put together is often as important as what materials are selected. For example; while the placement of electrical boxes can cause degradation of stud wall sound isolation, the greatest reduction of sound insulation occurs when there is a short unimpeded path between back-to-back boxes.  Larger cavity sizes which allow the installation of sound absorbing insulation between the boxes could reduce the effect of poorly placed electrical boxes, but planning locations spaced well apart will achieve the best results.

Back to Building Envelope Theory Topics