Brüel & Kjær Sound & Vibration Measurement A/S

A New DIRAC Measuring Speech Intelligibility


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Speech intelligibility measurements in situations where there is a large distance between the receiver and the sound source, require the source and receiver to operate independently. While this leads more often than not to practical conflicts,
DIRAC 4.0 provides a unique solution.

Speech intelligibility relates to the sound intensity modulations in the talker’s voice, as described by Houtgast et al. [1]. This principle is used to determine speech intelligibility from the remaining modulation at a receiver position, using a modulated noise source at the talker position. The noise spectrum represents that of a human voice, and modulations at several frequencies represent the spoken words. Figure 1 depicts the principle for a single modulation frequency.

The Speech Transmission Index (STI) is a single number quantity derived from seven noise octave frequency bands ranging from 125 Hz to 8 kHz, each modulated by 14 frequencies ranging from 0.63 to 12.5 Hz. In other
words, the STI is calculated from 98 modulation reduction values.

To measure STI, you can play a CD with modulated noise at the talker position and measure and analyse the resulting signal at a listener position. With this modulated noise method the source is always independent from the receiver (open
loop mode, see insert), which is convenient for long distance measurements. On the other hand, due to the randomness of the source signal, measurements have to be relatively long to get reproducible results (some 30 s on average). Another disadvantage of the modulated noise method is the risk of the receiver misinterpreting background noise level fluctuations as signal modulations, hence overestimating speech intelligibility at low SNR values.

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