Speech - acoustic glossary

Speech Terms, Definitions, Units, Measurements ...

Speech 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

Articulate, the ability to speak fluently and coherently'

Articulation Definition IEC 801-29-45, percentage of speech units correctly received out of those transmitted.
● 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 is also known as 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.

Clarity describes the quality of speech clarity and/or music clarity heard by a listener. In a reverberant room and/or in the presence of disturbing background noise, the speech intelligibility may be very poor

See also • haas effect • music clarity • speech clarity

Common Intelligibility Scale (CIS), a method for ranking articulation based on a mathematical relation with STI : speech transmission index, where (CIS = 1 + log (STI)).

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 • speech intelligibility

Music Clarity is the objective measurement of clarity expressed in dB, based on the fact that late reflections, reverberation and background noise etc., are unfavourable to understanding music. Early reflections may enhance the clarity.

C₈₀ is a guide to music clarity and is a measure of the ratio of the sound energy arriving during the first 80 milliseconds and the sound energy arriving after 80 ms.
C₈₀ = 10·Log ((Energy(0-80ms)/(Energy(80ms-end)) dB

See also • haas effect • speech clarity

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 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 • 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 room criteria values.

Speech Clarity is the objective measurement of clarity expressed in dB, based on the fact that late reflections, reverberation and background noise etc., are unfavourable to understanding speech. Early reflections may enhance the clarity.

C₅₀ guide to speech clarity and is a measure of the ratio of the sound energy arriving during the first 50 milliseconds and the sound energy arriving after 50 ms.
C₅₀ = 10·Log ((Energy(0-50ms)/(Energy(50-end)) dB

See also • haas effect • music clarity

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 :
CIS • common intelligibility scale
RASTI • rapid speech transmission index
STI • speech transmission index
STIPA • speech transmission index for public address systems
STITEL • speech transmission index for telecommunication systems

See also • threshold of speech intelligibility and the articulation

Intelligibility Definition IEC 801-29-45, percentage of speech units correctly received out of those transmitted.
● 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.

Intelligibility is also known as articulation

Speech Interference Level (SIL), the 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.

Speech Power • average speech power • instantaneous speech power • peak 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 also speech transmission index 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.

Speech Transmission Index (STI) is a method of measuring speech intelligibility in noisy and/or reverberant locations. The noise source used for testing sound cover the octave bands from 125 Hz up to 8 kHz. A number between 0 and 1 quantifies the speech intelligibility and is often implemented in two versions RASTI and STIPA.

Speech Transmission Index for Public Address systems (STIPA) 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

Acoustic Glossary