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Room Acoustics Speech Intelligibility STI

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Room Acoustics Speech Intelligibility STI

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    1. Room Acoustics Speech Intelligibility STI/RASTI Wilson Acoustics Limited (WAL) Internal Presentation

    2. Speech Intelligibility Inside a Room Depends on masking effects: Background Noise Reverberations of Speech

    3. Simple Testing of Speech Intelligibility Locate a Source & a Receiver Source produces a signal Receiver measures the quality of received signal E.g. Clap your hands & listen to the response of the room

    4. Systematic Method to Measure Speech Intelligibility Source emits a well-defined signal a function of direction & frequency Receiver has a well-defined sensitivity a function a direction & frequency

    5. DIRAC 4.0 (B&K) Software for Room Acoustics Standards: ISO 3382 (Room Acoustics) & IEC 60286-16 (Speech Intelligibility) Measuring Impulse Response -> Room Acoustic Parameters e.g. Reverberation Time, Sound Strength G, Energy Ratios, Signal to Noise Ratio (SNR), Speech Transmission Index (STI), etc.

    6. Modulation Transfer Function (MTF) Also named Modulation Reduction Factor Given that intensity of source is modulated by a low frequency sine modulation of 100% depth

    7. Modulation Transfer Function (MTF) Reduced by room reflections & background noise A function of Modulation Frequency F F: 0.63 12.5 Hz

    8. Modulation Reduction Matrix

    9. Speech Transmission Index (STI) One of the speech intelligibility parameters Weighted sum of Modulation Transmission Index (MTI) over each octave frequency band from 125 Hz to 8kHz RASTI (Rapid/Room Acoustic Speech Transmission Index) Subset of STI

    10. Different Methods to measure STI Modulated Noise Method / Impulse Response Method Closed Loop Mode / Open Loop Mode

    11. Modulated Noise Method Consisting of 7 (octave band freq.) X 14 (modulation freq.) = 98 noise signals Open Loop Relatively long time (about 30s on average) Error: may misinterpret background noise as signal modulations, hence overestimating STI/RASTI

    12. Impulse Response Method Giving a specific signal covering necessary octave bands through the source Measuring the Impulse Response, the response of a system to an impulsive signal Closed or Open Loop Used by DIRAC 4.0 Error in Open Loop: sample rate & playback speed mismatch

    13. Signal Types

    14. Impulse Response Response of a system to an ideal impulse with unit energy & infinite power An acoustic property of a system (room specific) Can be obtained by Deconvolution, when using other types of signal instead of impulse

    15. Convolution Convolution: A mathematical operation on the system response & signal to obtain the system response f(t) is convolved with g(t) : if f(t) is unit impulse, result is simply g(t), hence g(t) is named impulse response

    16. Deconvolution Concept: -> System response is a mixing of input signal & impulse response -> Deconvolution is a signal processing method to separate them

    17. DIRAC 4.0 (B&K) Recording the signal by microphone Processing on the signal to obtain impulse response All parameters are obtained from impulse response It just takes several seconds to obtain the STI/RASTI !!

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