Section 3 Speech intelligibility
Clasification Society 2024 - Version 9.40
Clasifications Register Guidance Information - Ship Vibration and Noise Guidance Notes - Chapter 9 Audible alarms and public address systems - Section 3 Speech intelligibility

Section 3 Speech intelligibility

3.1 The transmission quality performance of a Public Address (PA) system with respect to speech intelligibility can be rated objectively by using the Rapid Speech Transmission Index (RASTI) method which is a simplified version of the Speech Transmission Index (STI)

3.2  Speech

Speech can be regarded as a flow of sound, with the spectrum varying continuously over time. Faithful speech transmission between any speaker-listener pair relies on preservation of these spectrum variations. For a PA system, speech transmission can be degraded by factors including the effects of the acoustic properties of the listeners’ enclosure such as reverberation and echoes, signal to noise level, equipment generated distortion and transmission bandwidth.

3.3  Modulation Transfer Function

Signal degradation in an enclosure can be quantified by determining the Modulation Transfer Function (MTF) or M(F). The test signal is a noise carrier with a speech shaped spectrum, intensity modulated with a modulation frequency F. The MTF is the ratio of the modulation index mo of the signal at the listener’s position to the modulation index mi of the test signal.

3.4  Speech Transmission Index

The Speech Transmission Index (STI) is based on a set of measurements of the MTF for 7 octave bands from 125 Hz to 8 kHz and 14 modulation frequencies at third-octave interval from 0.63 Hz up to 12.5 Hz, giving 98 data points. The result is normalised to give a value between 0 and 1. The STI has been correlated with intelligibility scores obtained by subjective speaker-listener measurements.

3.5  Rapid Speech Transmission Index

The RASTI method is restricted to two octave bands, centre frequencies 500 Hz and 2 kHz, and four or five modulation frequencies for the two octave bands, giving nine data points. This reduces the number of tests while covering a representative range of speech. The result is given as a value between 0 and 1.

3.6  RASTI Equipment and Measurement

A signal from the RASTI Transmitter unit is input into the PA system, either electrically, or by positioning the unit on a tripod so that the external loudspeaker is located at the speaker’s position at the P.A. microphone. The RASTI receiver unit is placed at the test location, and the measurement carried out over 8, 16 or 32 seconds, with the signal broadcast over the PA system.

3.7  RASTI Rating

The general relation between the quality of speech transmission with respect to intelligibility and the RASTI index is given in Table 8.8.5 RASTI Rating .

3.8  Sound Systems for Emergency Purposes

An STI of 0.5 or greater (measured using the RASTI technique) is required for sound systems for emergency purposes.

3.9  Diagnostics

If the RASTI value is unsatisfactory, then the other measured parameters can be examined in order to identify the cause of the signal degradation, in particular the graph of the modulation reduction factors against the modulation frequency:

  • A flat line is indicative of noise interference only.
  • A negatively sloped line, indicative of reverberation interference only.
  • A line with a sharp minimum can be indicative of a discrete echo or time delay.
Parent topic: Chapter 9 Audible alarms and public address systems
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