Glossary of Acoustical Terms
A Weighing-A special electronic filter, built into most sound level meters that approximates the hearing characteristics of the human ear.
Absorption Coefficient-See Sound Absorption Coefficient.
ASTM-The American Society for Testing and Materials. An organization that standardizes acoustical test methods. ASTM also has test methods for other materials and purposes.
dB-See Decibel.
dB(A)-A unit of sound loudness, in decibels, measured with a sound level meter that has an A Weighing filter. The most common unit of sound loudness. See also Decibel and A Weighting.
Decibel-The most common unit of sound level (loudness) measurement. Zero decibels is the threshold of hearing, the quietest sound that most people can hear under ideal conditions.
Diffusion-The scattering of sound. Usually in reference to the evenness of sound distribution in a room (no hot spot or dead spots). Also, a way to prevent echoes by scattering sound with surface irregularities rather than by absorbing it.
Echo-A discrete reflection of sound from a hard sound-reflecting surface resulting in an audible repetition of a sound. See also Reverberation.
Hertz-The unit of sound frequency: cycles per second.
Masking-The covering up or obscuring of one sound with another sound. This is commonly done in open-plan offices where a random (pink) noise is played quietly through ceiling-mounted loudspeakers to mask or partially interfere with conversation from other workers. Masking is used to prevent distraction from neighboring conversation and to give an impression of privacy in the midst of an open-plan environment.
NC-See Noise Criteria.
Noise Criteria-A standardized system of rating the ambient noise level (usually from HVAC) in a room. The NC value is roughly 10 points lower than the corresponding dB(A) level. A room with an NC-15 is very quiet such as a recording studio. An NC-35 is a typical quiet office level. See also Room Criteria. 95% of Acoustical Consultants use Noise Criteria.
Noise Reduction Coefficient-A single-number rating system (standardized by ASTM) used to classify the amount of sound absorbed by a material. The Noise Reduction Coefficient is the average of the individual sound absorption coefficients at 250, 500, 1000, and 2000 Hz rounded to the nearest 0.05. The NRC rating is most applicable to the absorption of speech sounds.
Noise-Unwanted, bothersome, or distracting sound. However, some noises can be useful: See Masking.
NRC-See Noise Reduction Coefficient.
Octave Band-A frequency band with an upper frequency limit equal to twice the lower limit. Octave band center frequencies used in architectural acoustics are at 125, 250, 500, 1000, 2000, and 4000 Hz.
Pink Noise-A type of random noise that sounds very much like the noise between stations of a radio or TV. Similar to white noise but with a different tone quality. See also Masking.
RC-See Room Criteria
Reverberation Time-The time it takes for a sound level to decrease by 60 dB after the sound is abruptly stopped. For most office and residential rooms the reverberation time is less than 0.5 second, in performance halls it is typically about 2 seconds, and in large cathedrals it can be 6 seconds or more.
Room Criteria-A standardized system of rating the ambient noise level (usually from HVAC) in a room. Both the Room Criteria and Noise Criteria systems have been in use during the past several years, but as of 1991, the Room Criteria system is the preferred method. For most purposes, the Room Criteria and Noise Criteria are equivalent. See also Noise Criteria. 95% of Acoustical Consultants use Noise Criteria.
RT,RT60-Abbreviations for Reverberation Time.
Sabin-A unit of sound absorption. One sabin is equal to one square foot of total (100%) sound absorption. The sabin was named for Wallace Clement Sabine (1868-1919), the “father” of architectural acoustics.
Sound Level-The loudness of sound, usually measured with a sound level meter. Sound levels are usually expressed in decibels (dB). See also Decibel.
Sound Absorption Coefficient-The percentage of incident sound that is absorbed by a material. The remaining percentage of incident sound is reflected by the material. The NRC value is a number between 0 and 1.00. Multiply the number by 100 to get the percentage of sound absorption. A rating of NRC-0.00 indicates no (0%) sound absorption; NRC-0.50 indicates 50% absorption; NRC-1.00 indicates perfect (100%) absorption.
Sound Transmission Class-A single-number rating systems (standardized by ASTM) used to rate the sound insulation performance of a material or assembly. The Sound Transmission Class is most commonly used to rate the sound insulation provided by common building partitions. Typical office walls (gypsum board on studs) are in the range of STC-40 to STC-50. The STC number is roughly the number of decibels that a sound would be reduced when passing through a partition or assembly.
STC-See Sound Transmission Class.
T60-Another abbreviation for Reverberation Time.
White Noise-A type of random noise that sounds very much like the noise between stations of a radio or TV. Similar to pink noise but with a different tone quality. See also Pink Noise.
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