Acoustic Glossary


Speech : Definitions, Terms, Units, Measurements etc., ..


the ability to express thoughts and feelings by articulate sounds. We have listed the common acoustical terms alphabetically on this page and links to other relevant terms

Articulation Definition

IEC 801-29-45, percentage of speech units correctly received out of those transmitted. Also known as intelligibility

Note 1 : the word articulation is used when the units of speech material are meaningless syllables or fragments; the word intelligibility is used when the units of speech material are complete, meaningful words, phrases or sentences

Note 2 : it is important to specify the type of speech material: phonemes, logatomes, syllables, words, sentences, etc. The kind of speech material used is identified by an appropriate adjective syllable articulation, vowel (or consonant) articulation, monosyllabic word intelligibility, discrete word intelligibility, sentence intelligibility.

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.

Average Speech Power Definition

IEC 801-29-43, for a stated time interval, the arithmetic mean instantaneous speech power during that interval

Common Intelligibility Scale (CIS)

, a method for ranking articulation based on a mathematical relation with STI (CIS = 1 + log (STI)).


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

Instantaneous Speech Power Definition

IEC 801-29-41, rate at which sound energy is being radiated by a speech source at any given instant.

Intelligibillity see Articulation and Speech Intelligibility

Peak Speech Power Definition

IEC 801-29-42, maximum value of the instantaneous speech power within the time interval considered

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.

See also • speech interference level.

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 ..more details

See also • speech transmission index

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 Romm Criteria 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 speech transmission index and the common intelligibility scale

See also • threshold of speech intelligibility and the articulation defintion

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.

See also • preferred speech interference level

Speech Power see Average Speech PowerInstantaneous Speech PowerPeak Speech Power

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 PDF 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 level difference 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.

STIPA - Speech Transmission Index for Public Address systems

an alternative to RASTI with public address and other electro-acoustic devices in mind, was developed more recently and ratified in the BS EN 60268-16 2011 revision .. see the NTi STIPA system

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

Threshold of Speech Intelligibility Definition

IEC 801-29-46, sound pressure level of speech measured in a stated frequency band, using fast exponential time weighting, at which 50% of relatively easy words can be clearly recognized

See also • Speech Intelligibility : under Articulation Definition : IEC 801-29-45

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