Key Acoustical Terms and Concepts

Understanding key acoustical concepts is critical in order to gain an understanding of the way in which sound is transmitted through walls, as well as human sensitivity to changes in noise levels.

Airborne Sound. Airborne sound consists of energy generated by a source, transmitted through a medium, and detected by a receiver. All three of these conditions must be in place or airborne sound cannot exist. The level of airborne sound is determined by the frequency of vibrations. This measure of frequency is given in hertz (Hz). The unit hertz can be thought of as the number of sound waves that pass a fixed point within a given amount of time. For example, frequencies between 20 Hz and 20,000 Hz are detectable by children. As people age, their ability to hear high-frequency sounds is significantly diminished. Humans are most sensitive to sounds in the range of 100 Hz to 5,000 Hz. Speech and other traditional sounds within a building range from 125 Hz to 4,000 Hz, which is the frequency range considered when calculating STC.

Decibels. The units decibels (dB) are used in acoustics to calculate the intensity or loudness of sound and to provide a relative measurement of sound level. Higher dB levels relate to loud sounds while lower dB levels relate to quiet sounds. A change of 3 dB would be barely noticeable to most human's ears, while a change of 5 dB would generally be noticeable to the average person. An increase of 10 dB would sound twice as loud, and a decrease of 10 dB would sound half as loud. It's important to note that the decibel scale is not linear. For example, compare the sound of a conversation (55 dB) with that of a lawnmower (105 dB). The lawnmower is not twice as loud as the conversation, but several orders of magnitude louder-a critical factor to observe when considering construction materials to prevent external noise from being heard inside a building.

Measuring Noise Reduction

Both government and industry standards were established to achieve acceptable noise levels in residential and commercial construction. U.S. standards are based primarily on a system that measures how well walls, floors, and doors prevent noise intrusion from adjacent rooms. That system is known as Sound Transmission Class, or STC. STC is a single-number rating of the effectiveness of a material or construction assembly to retard the transmission of airborne sound. It provides an indication of the volume at which transmitted sound is perceived by the listener. The higher the STC value, the more effective the assembly or material in reducing sound transmission.

For interior walls, STC values are derived by conducting a test according to a procedure outlined in ASTM E 90, "Standard Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions." ASTM E 90 is designed to measure how well building materials or specific wall assemblies reduce the transmission of airborne sound at 16 frequencies between 125 Hz and 4,000 Hz, which is consistent with the frequency range of speech and constitutes a wide range of airborne sounds at different pitches.

The ASTM E 90 test procedure is essentially conducted in the following way: In a controlled laboratory environment, a specified wall partition is built between two rooms-the source room, where airborne sound is emitted, and the receiving room, where the sound is received and measured in decibels. As airborne sound moves from source to receiving room, it naturally tends to decrease in decibels. The greater the level of decrease, the higher the STC rating.

ASTM E 413 calculates ratings for both laboratory and field measurements of sound attenuation. The test data collected would be analyzed using ASTM E 413, "Classification for Rating Sound Insulation." The result is a single-number acoustical rating. The higher the STC rating, the greater a partition's ability to reduce transmission of airborne sound from one side of a partition to another. Higher STC ratings will reduce typical noise from an adjoining room, including normal or loud conversation, a baby crying, dogs barking or sounds made by kitchen appliances.

Acceptable STC Ratings for Wall Partitions. In determining an acceptable STC rating, the industry generally turns to a prominent study by the National Research Council of Canada, "Deriving Acceptable Values for Party Wall Sound Insulation," by J.S. Bradley. Looking at the attitudes of residents in 600 multifamily units with 300 party walls between them, the study found that residents with lower STC-rated walls were more likely to be awakened by noises; have trouble falling asleep due to noises; and think their neighbors were less considerate. They were also more likely to want to relocate. The study concluded that an STC rating of 55 was a realistic goal for acceptable sound insulation and that an STC rating of 60 would virtually eliminate negative effects of noises from neighboring units.

The results of the Canadian study are reflected in the federal noise-reduction standards used by the United States Department of Housing and Urban Development (HUD). HUD uses a three-tier system for rating construction quality in terms of sound attenuation. An STC-55 or above is the criterion for the highest rated Grade 1 dwelling. Ultra-high-end dwellings may incorporate wall partitions of up to STC-65. HUD's Grade 2 building indicates an STC-52 rating; Grade 3 signifies a rating below STC-52. In the national building code, the minimum allowable rating for multifamily construction is STC-50.

Music-related sounds may require some of the highest STC ratings, which is important to note as loud music is the chief noise-related complaint reported by residents of multifamily dwellings. In practical terms, an STC-55 rating will enable the resident of a multifamily dwelling to remain undisturbed by a neighbor playing a loud sound system. With an STC-55 rating, loud music will be reduced to the level of a normal conversation by the time it passes through the shared wall. It is important to note that the STC ratings relate only to airborne sound; they do not apply to "structure" sounds such as footsteps or structural vibrations from street noise or airplanes flying overhead.