rapid speech transmission index (acoustics) (RASTI)

There are several measurement methods for evaluating speech intelligibility in rooms with sound. In addition to the Rapid Speech TransmissionIndex (RASTI), there is the Speech Transmission Index( STI), the Speech Intelligibility Index( SII) or ALcons.

RASTI has been standardized as IEC standard 60268-16 and evaluates speech intelligibility using modulated noise. It is a simpler method to the relatively elaborate Speech Transmission Index (STI).

RASTI uses a modulated noise signal that is radiated through loudspeakers and picked up and analyzed by measurement microphones. The signals received by the microphones are compared with the loudspeaker signals and frequency-weighted. In contrast to the STI index, the frequency-wise measurement is performed only in two octaves with the center frequencies of 500 Hz and 2 kHz. For modulation, RASTI uses a signal similar to a speech signal and makes the statement about speech intelligibility based on the changed modulation index.