Construction Methods and Materials for Noise Control

by Nick Gromicko and Ethan Ward

In any heavily populated area, there is enough activity going on at once during the day to generate all kinds of sounds across the audible spectrum of human hearing. Planes take off and land, traffic moves along roadways, construction crews repair roads, dogs bark, music blares, sirens sound, lawns are mowed, etc. Even within a building, mechanical noise from heating or air conditioning can be audible, phones ring, and voices, radios and TVs are heard through walls, and so on. We rely on construction practices and materials to provide a sufficient barrier from the loud goings-on that surround us every day. Inspectors may be interested to learn how building materials and techniques influence the transmission of sound.

How Sound Moves

Sound energy travels from a source through air, water and solid objects. When sound waves hit the eardrum and cause vibration, we perceive them through our sense of hearing. The path a sound takes before it reaches our ears can be either airborne or structure-borne. Airborne sounds are radiated from a source directly into the air, such as a loud jackhammer. Structure-borne sound is sound that travels through solid materials before we hear it. An example would be heavy footsteps audible from the next room. This is why structure-borne sound is often referred to as "impact noise." Sound waves radiate around the source and decrease in loudness as they travel farther away. The energy of a sound wave is reduced by half when the distance from the source doubles.

Sound has three properties: frequency, wavelength and amplitude. Frequency is the number of cycles per second the sound wave itself produces, and it determines the pitch of the sound we hear. Frequency is measured in hertz. The range of human hearing goes from 16 to 20,000 Hz, with 16 being the lowest detectable pitch, and 20,000 the highest. Wavelength is the distance between the start and the end of a sound wave cycle. Amplitude is the perceived loudness of the sound.

Systems of amplitude measurement have been developed in order to quantify sound objectively. When amplitude is measured, it is actually the pressure of a sound wave striking a surface that is being measured. The unit of measure is the decibel (dB). A larger vibration at the source will cause a larger sound pressure level and, thus, a greater perceived volume. The human voice is usually in the range of 55 to 60 dB, a loud truck or motorcycle is 80 to 100 dB, and a jet taking off or a gun firing is measured at 120 dB or more.

Reducing dB Levels in Buildings

There are several categories of sound control for interiors: sound absorption, airborne sound transmission, and impact-sound transmission.

Sound Absorption

Sound absorption is the capability of a surface, or building material, to absorb sound instead of reflecting it. Sound waves will continue to bounce around a room for a time after they are created if the majority of surfaces in a room is reflective. Surfaces that absorb sound better will not allow for reflections to bounce around as much, and will deaden the sound wave more quickly. Many common building materials, such as gypsum board, wood, concrete, brick and tile, are fairly reflective and do not absorb much sound. Softer materials, such as carpet, foam padding, and fiberglass insulation, are far better at absorbing sound.

The use of absorptive materials can be helpful in controlling sound. Fiberglass insulation is very absorptive and can be used where sound control is a concern. Thick carpet with padding is also very absorptive, and acoustical ceiling tiles are designed to absorb rather than reflect sound. Even in cases where these options are not viable, absorptive materials can be added to finished rooms in other ways: furniture with thick cushioning is extremely absorptive, as are thick and heavy curtains and drapes. Items such as these can be added or arranged in ways that will allow for greater sound absorption. Acoustical baffles with absorptive materials can be purchased for use in areas where sound is a major concern, and most are designed to be unobtrusive and visually nondescript so as to allow for installation without drastically altering the aesthetics of a room.