Articulation Class (AC) - measures how well a ceiling panel prevents sound from reflecting back down to adjacent work spaces in an open-plan environment.

Articulation Index (AI) - represents how well speech can be understood in a given space. AI is expressed as a decimal value between 0 (speech is unintelligible) and 1.00 (speech is easily heard and understood). AI can be measured using ASTM Test E-1130.

Ambient Noise - The sound pressure levels associated with a given environment. Ambient noise is usually a composite of sounds from near and far sources none of which are particularly dominant.

Audible Frequency Range - The range of sound frequencies normally heard by the human ear. The audible range spans from 20 Hz to 20,000 Hz, but for most engineering investigations only frequencies between about 40 Hz and 11,000 Hz are considered.

Background Noise - The sound pressure levels in a given environment from all sources excluding a specific sound source being investigated or measured.

Broad Band Noise - Spectrum consisting of a large number of frequency components, none of which is individually dominant.

CAC - Ceiling Attenuation Class - Indicates the ability of a ceiling panel to block sound transmission. A ceiling panel with a CAC of 40 will reduce transmitted sound by 40 dB. A ceiling panel with a CAC of 35 or higher offers significant sound attenuation properties.

Decibels - Ten times the logarithm (to the base 10) of the ratio of two mean square values of sound pressure, voltage, or current. The abbreviation for "decibels" is dB.

FIIC - Field Impact Insulation Class. A single-number rating of the insulation against impacts, provided by a floor (and associated structures) derived from field impact sound measurements in accordance with ASTM Test Method E989.

Frequency - The number of cyclical variations per unit time. Frequency is generally expressed in cycles per second (cps) also denoted Hertz (Hz).

Insertion Loss - The reduction in noise level at a given location due to placement of a noise control device in the sound path between the sound source and that location.

Noise - Unwanted sound which may be hazardous to health, interferes with communications or is disturbing.

Noise Isolation Class (NIC) - A single number noise reduction rating of a partition, room, or enclosure, where sound pressure level differentials are measured in one-third octave bands and compared with standard contours as per ASTM E 413.

Noise Criterion (NC) - An NC level is a standard that describes the relative loudness of a space, examining a range of frequencies (rather than simply recording the decibel level). This level illustrates the extent to which noise interferes with speech intelligibility. NC should be considered for any project where excessive noise would be irritating to the users, especially where speech intelligibility is important.

Noise Reduction (NR) - The difference in sound pressure level between any two points along the path of sound propagation. As an example, noise reduction is the term used to describe the difference in sound pressure levels between the inside and outside of an enclosure. Noise reduction is usually expressed as a function of full octave or one-third octave bands.

Noise Reduction Coefficient (NRC) - The arithmetic average, to the nearest multiple of 0.05, of the sound absorption coefficients in the 1/3 octave bands centered at 250 Hz, 500 Hz, 1000 Hz, and 2000 Hz. By convention, the maximum NRC used is 0.95, even though the published laboratory average may be greater.

Octave Bands - Frequency ranges in which the upper limit of each band is twice the lower limit. Octave bands are identified by their geometric mean frequency, or centre frequency.

One-Third Octave Bands - Frequency ranges which each octave is divided into one-third octaves with the upper frequency limit being 2 1/3 (1.26) times the lower frequency. Identified by the geometric mean frequency of each band.

PI - Privacy Index - is the inverse of articulation index. The PI represents how will the elements in and the properties of a space render outside conversations unintelligible.

Pink Noise - Sound with an uninterrupted frequency spectrum and a power which is steady within frequency band and proportional to center frequency. An example is constant power level per octave band.

Preferred Frequencies - Octave and one third octave band center frequencies are the geometric means of their respective lower and upper limits as defined by ISO and ANSI standards. The upper and lower nominal octave band limits are arrived at by multiplying the center frequency by 1.4142 and 0.7071 respectively. One-third octave band centre frequencies are given by 10n/10 where n is the one third octave band number. For instance, for 125 Hz the band number is 21 and 102.1 = 125.89. Lower and upper frequency limits are a function of 2-1/6 or 0.89 and 21/6 or (1.12) respectively.

Pure Tone - A sound emitted at a single frequency.

Reverberation - The persistence of sound in a particular space after the original sound is removed. When sound is produced in a space, a large number of echoes build up and then slowly decay as the sound is absorbed by the walls and air, creating reverberation, or reverb.

Sabin - a unit of acoustic absorption equivalent to the absorption by a square foot of a surface that absorbs all incident sound, (NRC=1.0).

Sound - Sound is a vibrational disturbance, exciting hearing mechanisms, transmitted in a predictable manner determined by the medium through which it propagates. To be audible the disturbance must fall within the frequency range of 20 Hz to 20,000 Hz.

Sound Power - The acoustic power of a sound source, expressed in Watts.

Sound Pressure - Fluctuations in air pressure caused by the presence of sound waves.

Sound Receiver - One or more observation points at which sound is evaluated or measured. The effect of sound on an individual receiver is usually evaluated by measurements near the ear or close to the body.

Sound Absorption Coefficient - The dimensionless ratio of sound energy absorbed by a given surface to that incident upon the surface.

Sound Source - Equipment or phenomena which generate sound. Source room: room containing sound source.

Sound Transmission Coefficient - The ratio of sound transmitted through a partition to that incident upon the partition.

Spectrum - Equipment or phenomena which generate sound. Source room: room containing sound source.

STC- (Sound Transmission Class) - A single number decibel rating of the transmission loss properties of a partition. Measured transmission loss data is plotted versus frequency and compared with standard contours according to rules outlined in ASTM E -90 and ASTM E- 413.

Structureborne Noise - Generation and propagation of time-dependent motions and forces in solid materials which result in unwanted radiated sound.

Threshold of Hearing - The lowest continuous sound pressure levels which will create an auditory sensation for the average human ear. Any sound below these levels will be inaudible and any sound above the threshold will vary in loudness dependent on intensity.

Transient Sounds - Sounds which are audible for a limited period of time e.g., sounds from airplane flyover.

Vibration Isolation - Reduction of force or displacement transmitted by a vibratory source. Often attained by use of a resilient mount.

Wavelength - The length in space of one complete cycle of a sound wave.

White Noise - A noise whose spectrum (level) density is substantially independent of frequency over a specified range and has equal power for any range of frequencies of constant band width.          


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