||Acoustical Performance Terms
Absorption – When sound waves contact a room surface such as a ceiling, wall or floor, a portion of the sound energy is reflected back
into the room, and the rest is considered to be absorbed primarily by conversion into heat due to friction within a porous
material, or by vibration of a solid material.
Absorber – Any material that absorbs at least 50% of the sound energy that strikes it. The NRC is a measure of a sound absorber performance.
Acoustical Material – Any material that affects the sound level or direction of travel of a sound wave. Some acoustical materials are specifically
characterized as sound absorbers, or sound reflectors, or as barriers to sound passage.
Ambient Noise – The lowest background noise level in a space, which is a composite of sound from sources both near and far, but none of which
are identifiable as being from a specific source, such as a nearby ringing phone.
Amplifier – An electronic component that is used in sound system applications to increase the power of a low level Source Signal (microphone,
CD player, etc.) into a high power Output Signal to drive the loudspeakers.
Amplitude – The level of a sound wave, which is perceived by a listener as the “loudness” of that sound.
Analog – In sound system applications, an analog electrical signal represents the sound in its exact continuous form (non-digitized).
Likewise, an analog device is an electronic component that processes analog signals in their continuous form.
Articulation Class (AC) – A measure for rating the speech privacy performance of a ceiling in an open plan environment where sound is reflected off
the ceiling between two adjacent spaces divided by partial-height furniture panels. A ceiling system with AC < 150 is low
performance, whereas one with AC > 200 is high performance.
Articulation Index (AI) – A measure for rating speech intelligibility, where an AI < 0.05 is representative of very poor speech intelligibility, and
an AI > 0.80 represents good speech intelligibility. The Speech Privacy Index (PI) is derived from the AI.
Attenuation – A reduction in sound level (loudness) as a result of any of the following 1) with increasing distance away from a source
(expanding waves); 2) as sound is partially absorbed upon reflection off a surface; or 3) as sound passes through a barrier
material such as drywall or glass.
Audible Frequency Range – The range of frequencies which can be detected by an individual with normal hearing sensitivity. The range of audibility
spans from 20Hz to 20 kHz for someone with perfect hearing, and is often reduced significantly at high frequency as a result
of “hearing loss”.
Background Noise – The noise level in a space, which is a composite of sound from HVAC, equipment, activity noises, etc., from both near and
far, but excluding specific sources of interest such as a person talking in an adjacent space (which would be considered the
Barrier – Any material or system that blocks or reduces the transmitted sound level as it passes through the material. A poor barrier
material will provide a transmission loss TL < 10 dB, whereas a good material will have a TL > 20 dB for partial height barriers
and TL > 50 dB for full height barriers, at the frequencies of interest. Both STC and CAC are measures of acoustical barrier
Broad Band Noise – A noise that is characterized by a broad range of frequency components including low-mid-high frequencies, wherein none is
individually dominant such that the noise sounds indistinct, unlike an electric transformer which has a very distinct humming
Ceiling Attenuation Class (CAC) – A measure for rating the performance of a ceiling system as a barrier to airborne sound transmission through a common plenum
between adjacent closed spaces such as offices. A ceiling system with a CAC < 25 is consider low performance, whereas one
with CAC > 35 is high performance.
Decibel, dB, dBA – A measure for rating the level of a sound, which uses a logarithmic scale. The sound level in dB is often represented as
a dBA, where the “A” indicates a specific frequency weighting used to represent how we perceive loudness as a function of
frequency since our hearing is non-linear with frequency. A sound level of dBA < 30 is a very low level, whereas dBA > 90
is a high level.
Diffraction – The frequency-dependent ability of sound waves to “bend” over or around a partial height barrier such as a furniture panel,
which results in an effective “acoustical shadow” only at high frequencies.
Digital Processor (DSP) – Digital signal processor, an electronic component that is used in sound system applications to digitally generate signals
(such as masking sound), as well as filter, equalize, and mix program signals (such as masking, paging or music).
Directivity Index – A measure for rating the directionality of a particular sound source in a specific direction relative to the level that would
be radiated by a perfectly symmetrical sound source of the same power.
Direct Sound – Sound that reaches the listener location by direct line-of-sight, without being reflected off any surface or transmitted
through any material, and thus is unaffected by the room conditions.
Furniture Panels – The partial height sound screens used to enclose an open office cubicle. Three factors have acoustical importance: the NRC,
STC and height of the panels, where panels < 48” are poor, and panels > 72” are very good.
Frequency – The repetition rate of a sound wave measured in cycles per second, which is usually expressed in Hertz (Hz). The audible
frequency range for normal hearing individual spans from 20Hz to 20 kHz. Frequency is that characteristic of a sound which
is perceived by a listener as the “pitch”.
Masking Sound – Electronically generated background sound of a specified level and frequency content, that is introduced into occupied environments
to provide masking of intrusive noises and to enhance speech privacy.
Noise Reduction Coefficient (NRC) – A measure for rating the overall sound absorption performance of a material when used in an enclosed architectural space
such as an office, where sound is being reflected at many angles of incidence. Specifically, it is the 4 frequency averaged
absorption coefficients @ 250, 500, 1000 and 2000 Hz, rounded to the nearest 0.05. A material with NRC < 0.50 is a poor absorber,
and NRC > .80 is a very good absorber.
Noise Criteria, NC curves – A family of noise rating curves specifying the level and frequency content of background noise which will have a varying
degree of acceptability for occupied architectural spaces such as offices. For example, a background noise of between NC 25
and NC 35 is appropriate for a private office environment.
NC 40 curve – This specific NC curve has been used for years as the basis of design for electronic masking sound systems. The NC 40 curve
is typically “rolled down” below 100 Hz, and above 5 kHz.
Noise Reduction – The difference in sound level between the source (talker) and receiver (listener) locations in a room or between rooms. The
NR is dependent on the distance between each, the absorptive materials in the space, and the intervening acoustical barriers
such as furniture panels or wall partitions.
Octave Band – This is a continuous band of frequencies which span a range such that the highest frequency in the band is twice the lowest
frequency. The audio frequency range is separated into octave bands as a matter of convenience, and each band is individually
specified by its center frequency, e.g. 63 Hz, 125 Hz, 250 Hz, 500 Hz 1 kHz, 2 kHz, 4 kHz, 8 kHz, and 16 kHz.
Open Plan Office – An office design which is characterized by the application of partial-height furniture panels to enclose a work area into
a cubicle, and the layout of many such cubicles within a large open room.
One-Third Octave Band – This is a further refinement of the octave band by dividing it into 3 equal one-third octave bands. For example, the 1000
Hz OB is divided into the 800 Hz, 1000 Hz, and 1250 Hz 1/3 OBs to cover the same frequency range of the original 1000 Hz OB.
Pink Noise – Electronically generated sound that has equal energy in each 1/1 octave band, and 1/3 octave band. It is often used as the
source signal for the design/tuning of a conventional masking sound system using loudspeakers installed within a ceiling plenum,
when the ceiling tile has low high-frequency TL or the plenum is shallow. Pink noise is always perceived to sound “hissy”
compared to masking sound.
Plan View – An architectural drawing showing the layout of a space e.g. open office, as would be viewed looking vertically down into
Plenum – The ceiling plenum is the volume defined by the area above the back of the ceiling tile, and below the bottom of the structural
slab above. Within this plenum is usually found a combination of HVAC ducts, electrical and electronic conduits, water pipes,
traditional masking sound speakers, etc.
Privacy Index (PI) – A measure for rating the speech privacy performance of an architectural space (or lack of speech intelligibility) where the
PI is calculated from the Articulation Index (AI) according to the following: PI = (1 – AI) * 100%. A privacy level of PI
> 95% represents confidential speech privacy, a PI between 95 – 80% represent normal or non-intrusive privacy, and PI < 80%
is poor privacy.
Reflector – Any material that reflects more than 50% of the sound energy that strikes it. The NRC is a measure of a sound absorber performance,
so (1-NRC) will give an indication of the reflector performance.
Reverberation – The buildup of sound within an architectural space, such as a room, as a result of repeated sound reflections at the surfaces
of the room. The level of the reverberant sound within a room is dependent on both the volume of the room and the amount of
sound absorption installed within the room, such that small hard surfaced rooms are “louder” than large well-treated rooms.
Reverberation time, RT60– A measure for rating the quality of the sound environment within an architectural space, and it appropriateness for various
uses. Specifically, the reverberation time is the time it takes for reflected sound within a space to decrease by 60 dB after
the sound was made, for instance, the time it takes before you cannot hear a loud clap. An RT60 < 1 sec. is beneficial for good speech intelligibility, whereas RT60 > 2.5 sec is appropriate for symphony music.
Room Criterion, RC curves – A family of noise rating curves specifying the level and frequency content of background noise from HVAC equipment which
will have a varying degree of acceptability for occupied architectural spaces such as offices. For example, a background noise
of between RC 25 and RC 35 is appropriate for a private office environment. The RC curves above 31 Hz are basically – 5 dB/OB
Section View – An architectural drawing showing the vertical layout of a space e.g. open office, as would be viewed looking horizontally
across the space.
Sound Attenuation – See Attenuation
Sound Source – Either an object such as a loudspeaker that produces sound, or an individual speaking.
Sound Masking – The process of using electronic masking sound to cover unwanted or intrusive sound such as speech or equipment noise, typically
used to enhance speech privacy and productivity.
Sound Paths – A trace of the path taken by specific sound waves as they move outward from the source of the sound to the receiver location.
This path could be the direct sound path, a transmitted sound path, or a reverberant sound path which may include multiple
Speech Weighting Factors – These are the Articulation Index weighting factors presented in ANSI S3.5 (1969). These factors
represent the frequency dependency of speech intelligibility being based on the vowels at low frequency carrying less intelligibility
than the consonants at high frequency.
Speech Privacy Level – See Privacy Index. Additionally, several levels of speech privacy are defined in the ASTM standards
including: 1) Confidential privacy - speech sounds can be heard but not understood, 2) Normal or non-intrusive privacy – speech
can be occasionally heard and understood but is generally non-intrusive, 3) Poor privacy – most nearby conversations can be
heard and understood.
Sound Spectrum – A graphical representation of sound, showing the level of the sound as a function of frequency over the audio frequency range.
Sound Transmission Class (STC) – A measure for rating the performance of a wall system as a barrier to airborne sound transmission
between adjacent closed spaces, such as offices. A wall system with an STC < 35 is consider low performance, whereas one with
an STC > 55 is high performance.
Transformer Tap – Refers to the power setting available on a step-down transformer (e.g. 1 watt, 2 watt, 4 watt, etc.) used in high voltage
distributed sound systems (e.g. 70v, 50v, and 25v amplifier systems). The transformer is usually located on the loudspeaker.
Sound Transmission Loss (TL) – A measure of the performance of a wall system as a barrier to airborne sound transmission between
adjacent closed spaces, as measured in 1/3 octave bands. The TL in 1/3 OB’s is used to calculate the STC, which is a single
number rating for the entire frequency range.
Watt – The power handling capability of amplifiers (output power) and loudspeakers (input power) is given in watts. Power in watts
is related to the combination of the driving voltage and the current handling capabilities of the devices.