Acoustic Glossary


Speech : Definitions, Terms, Units and Measurement

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 : STI : Speech Transmission Index and STIPA Public Address systems
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 : Privacy Index 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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