GLOSSARY
ABSORPTION COEFFICIENT The effectiveness of a sound‐absorbing material can be expressed by its absorption coefficient, (alpha). The coefficient describes the fraction of the incident sound energy that a material absorbs. It can vary from 0 (for no sound energy absorption) to 1.0 (perfect absorption with all incident sound energy absorbed).
AIRBORNE SOUND TRANSMISSION All sound transmission brings into play waves which eventually reach our ears through the air.
DAMPING RATIO Damping is the dissipation of energy with time or distance. Critical damping is the minimum viscous damping that will allow a displaced system to return to its initial position without oscillation. The fraction of critical damping (damping ratio) for a system is the ratio of actual damping coefficient (c) to the critical damping coefficient (cc).
FREQUENCY It is the rate of repetition of a periodic event. The frequency of a sound wave is determined by the number of times per second a given molecule of air vibrates about its neutral position. The greater the number of complete cycles, the higher the frequency. The unit of frequency is Hertz (Hz).
NOISE Noise can be considered as unwanted or undesirable sound, a definition which conveys how highly subjective is the conception of noise.
NOISE CRITERIA (NC) CURVES It can be used to evaluate existing situation by measuring sound levels at the loudest locations in rooms. They also can be used to specify the background noise levels needed to help achieve satisfactory sound isolation. Each NC curve has defined sound pressure levels at eight octave‐band center frequencies. The NC rating for a noise situation usually means the lowest NC curves that is not exceeded by any octave‐band sound pressure level measured inside a room.
NOISE REDUCTION COEFFICIENT (NRC) The noise reduction coefficient, NRC is the arithmetic average of sound absorption coefficients of a material at 250, 500, 1000 and 2000Hz. It is intended as a single‐number rating of sound‐absorbing efficiency at mid‐frequencies. It should not be used to evaluate material for sound absorption at rooms where music or speech perception is important. Where low‐frequency absorption may not be an important factor (e.g. small office), the NCR can be an appropriate rating to compare materials.
REVERBERATION In a confined space we will get reflection, and far from the source the reflections will dominate. The reflection dominate region is called reverberant field. The more absorption in a room, the less the buildup of sound energy in reverberant field.
REVERBERATION TIME The reverberation time, T60 is defined as the time required for the sound pressure level in a room to decrease by 60dB from an initial level, i.e. the level before the sound source is stopped.
SOUND ABSORPTION When sound hit the surface of an element, sound energy is absorbed when it is converted to another form of energy, e.g. heat. The results from the actions of friction and the resistance of various materials to movement and deformation.
SOUND MASKING The addition of background or masking sound to an open‐office environment is a critical component to obtaining overall privacy in the workplace. When properly done, masking sound raises the noise level in an unobtrusive way that increases privacy without being noticed by the occupants. Masking sound is produced by using an electronic pink noise generator and a filter that result in a spectrum that falls off at about 5 ‐ 6dB per octave at the receiver. The overall level is set between 43 and 49dB(A) in general. Loudspeakers can be located above the ceiling tiles, out of sight, and pointed up to increase their effective distance and widen their coverage pattern. They may also be built into the tiles or under the raised flooring system.
SOUND TRANSMISSION CLASS (STC) It is a single‐number rating system to characterize the properties of a construction element in sound transmission between adjacent spaces. The Sound Transmission Class is such a system and is calculated in accordance with ASTM E413 or ISO/R 717. It begins with a plot of measured third‐octave transmission loss data versus frequency. The STC curve is compared to the measured data by sliding it vertically until following criteria are met : (1) no single transmission loss may fall below the curve by more than 8dB, and (2) the sum of all deficiencies may not exceed 32dB. When the curve is positioned at its highest point consistent with previous criteria, the STC rating is then determined.
SPEECH INTERFERENCE LEVEL Signal‐to‐noise ratio is the key to speech intelligibility, and we obtain more precise estimates of the potential interference by studying the background noise in the speech frequency bands. The Speech Interference Level (SIL) is a measure of a background noise’s potential to mask speech. It is calculated by arithmetically averaging separate background noise level in the four octave bands, namely 500, 1000, 2000 and 4000Hz. The SIL can then be compared to the expected speech sound pressure level to obtain a relevant speech to noise ratio.
SPEECH INTERLLIGIBILITY Speed intelligibility is a direct measure of the fraction of words or sentences understood by a listener. The most direct method of measuring intelligibility is to use sentences containing individual words or nonsense syllables, which are read to listeners who are asked to identify them. These can be presented at various levels in the presence of background noise or reverberation. Both live and recorded voices are used, however recorded voices are more consistent and controllable.
SPEECH PRIVACY It is a measure of the signal‐to‐noise ratio between the intruding speech level (signal) and the steady background sound (noise). The intruding speech level largely determined by the vocal effort of the speaker and the noise reduction between enclosed rooms. When the intelligibility of speech is low, it follows that the speech privacy is high. In an office environment, where speech privacy is the goal, we can strive to achieve the required signal‐to‐noise ratio in several ways.

(1) Control the sound source, (2) increase the path attenuation, and (3) raise the masking sound level.
STRUCTURE‐BORNE SOUND TRANSMISSION On the way from source to receptor, sound waves can be propagated through one or several solid objects, with or without attenuation.
TRANSMISSION LOSS (TL) It is a measure of how much sound energy is reduced in transmission through materials. Sound waves impinging on materials produce back‐and‐forth motion. The magnitude of this motion or vibration depends on the mass of the material. The more massive a material, the higher its TL. However, due to coincidence effects, the TL at some frequencies will be far less than would be predicted by only considering the mass of a material.
VIBRATION Vibration is an oscillation wherein the quantity is a parameter that defines the motion of a mechanical system. Oscillation is the variation, usually with time, of the magnitude of a quantity with respect to a specified reference when the magnitude is alternately greater and smaller than the reference.