Acoustic Glossary


Speech : Definitions, Terms, Units, Measurements..

Articulation Class : AC indicates the speech privacy performance of a ceiling in an open plan environment, such as a space divided by half height walls. A rating below 150 indicates poor speech privacy performance; a rating above 180 indicates good performance.

Articulation Index : AI a measure of Speech Intelligibility influenced by acoustical environment rated from 0 to 1. The higher the number the higher the intelligibility of words and sentences understood from 0-100%. An Articulation Index of less than 0.3, generally suggests unintelligible speech and one over 0.7 indicates excellent intelligibility.

Articulation Intelligibility percentage of speech units correctly received out of those transmitted.

CIS : Combined Intelligibility Scale

Clarity Speech Clarity describes the quality of speech heard by a listener. In a Reverberant room and/or disturbing Background Noise, the Speech Intelligibility may be very poor
See also the Haas Effect

Preferred Speech Interference Level : PSIL under speech interference Level below.

RASTI : Rapid Speech Transmission Index is an objective way of measuring speech intelligibility. It is measured by placing a loudspeaker, which transmits sound from the location of the person speaking, and a microphone where the listeners are situated.

See also Instruments : RASTI and STIPA

Room Criteria : RC a single-number for rating room noise. Based on the Preferred Speech Interference Level : PSIL values. Suffixes are added, R for Rumble, RV for Vibration and Rattle, H for Hiss and N for Neutral.

This system is considered by some to more effective than the Noise Criteria (NC) system.

The B&K 2250 Sound Analyser, measures the RC values.

Speech Clarity describes the quality of speech heard by a listener. In a Reverberant room with disturbing Background Noise, the Speech intelligibility may be very poor - see also the Haas Effect

Speech Intelligibility is directly dependent on the level of background noise, Reverberation Time and the shape of the room. Different methods are used to evaluate speech intelligibility, the most common are the STI : Speech Transmission Index and CIS : Common Intelligibility Scale - see below.

Threshold of Speech Intelligibility : Sound Pressure Level of speech measured in a stated frequency band, using Fast (F) Exponential Time Weighting, at which 50% of relatively easy words can be clearly recognized.

Speech Interference Level : SIL degree to which Background Noise interferes with speech. The arithmetic average of Sound Pressure Levels at the 500 Hz, 1 kHz, 2 kHz and 4 kHz in Octave Band levels.

Speech Interference Level : SIL3 based on the highest three octaves is the arithmetic average of the 1 kHz, 2 kHz and 4 kHz octave bands.

Preferred Speech Interference Level : PSIL is the arithmetic average of the 500 Hz, 1 kHz and 2 kHz octave bands.

Note : although SIL, SIL3 and the PSIL Preferred Speech Interference Level are defined for octave band levels they are also calculated from the 1/3-octave bands within each octave before doing the average.

Speech Power
Average Speech Power for a stated time interval, the arithmetic mean instantaneous speech power during that interval.

Instantaneous Speech Power rate at which sound energy is being radiated by a speech source at any given instant.

Peak Speech Power maximum value of the instantaneous speech power within the time interval considered.

Speech Privacy the acoustic dissatisfaction in offices is frequently related to speech privacy - overhearing unwanted conversations, or feeling that one is overheard. See Speech Transmission Index below or our separate download file on Speech Privacy

Speech Privacy Index : PI is a measurement of speech privacy. PI can be derived from the AI : Articulation Index, measurement using the following formula:

PI = 100(1-AI)

PI = 0 indicates a complete lack of privacy
PI = 100 means complete privacy

Speech Privacy Potential : SPP combines the partition sound insulation performance, installed Dw : Weighted Difference Level with the measured Background Noise level in the receiving room (dBA). The higher the resulting SPP, the higher the level of privacy between the rooms.

Speech Reinforcement needs no definition but the Haas Effect phenomena may be of interest

STI : Speech Transmission Index is a method of measuring Speech Intelligibility in noisy and/or Reverberant locations. A number between 0 and 1 quantifies the speech intelligibility and is often implemented in two versions RASTI and STIPA.

RASTI : Rapid Speech Transmission Index method uses a transmitter to broadcast a special modulated noise test signal from a loudspeaker at the talker's location. A microphone at the listener's position gives a direct reading of the RASTI value.

The full version is derived from a family of Modulation Transfer Function : MTF curves

STIPA : Speech Transmission Index for Public Address systems : an alternative to RASTI with PA and other electro-acoustic devices in mind, was developed more recently and ratified in the BS EN 602682-16 2011 revision - see the NTi STIPA

CIS : Common Intelligibility Scale also in common use where CIS = 1 + log (STI), again resulting in a simple number between 0 and 1


RASTI Instruments and STIPA Instruments


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