Speech intelligibility

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In room acoustics, speech intelligibility describes the quality of the transmission of speech , either through electroacoustic systems or directly through one or more human speakers. In the free field , the transmission path between the sound source and the sound receiver is not disturbed by reflection , shadowing or absorption . The sound pressure level decreases with increasing distance, depending on the type of sound source. In closed rooms, however, it is almost independent of location outside a certain minimum radius. This is caused by the numerous reflections that occur on the boundary surfaces and equipment. The resulting diffuse sound field is superimposed on the original signal and thus influences speech intelligibility. Since speech intelligibility cannot be measured directly, various methods are used to assess speech intelligibility.

Measurement of speech intelligibility

Reverberation time according to DIN 18041

As the oldest criterion, the reverberation time is suitable for measuring speech intelligibility. In DIN 18041, the recommended reverberation times for various purposes are specified depending on the size of the room. If the reverberation time is longer than this recommendation, the room is too reverberant and speech intelligibility is poor. If the reverberation time is too short, on the other hand, the room is acoustically too dry and therefore the sound pressure level generated by the sound source is too low on arrival at the receiver.

In contrast to the reverberation time, the so-called energy criteria ( degree of clarity , degree of clarity and focus time) evaluate the strength of the sound reflections in the room. The degree of clarity and the degree of clarity relate the sound energy that arrives at the receiver within the first 50 ms to the total sound energy that arrives. This can be used to assess how strongly the original signal is overlaid by later reflections. The focus time, on the other hand, does not set an exact time limit, but calculates the time after which exactly half of the total sound energy has reached the receiver.

Other very common methods of measuring speech intelligibility are the speech transmission indices (STI and RASTI). Both measure the modulation transmission index in order to map the transmission of human speech as precisely as possible. With the STI, seven octave bands with 14 modulation frequencies each are measured and then averaged. The RASTI, on the other hand, only measures two octave bands and nine modulation frequencies each. It is becoming less important as the STI evaluates speech intelligibility more precisely.

Individual evidence

  1. Werner Schirmer: Technical noise protection: Fundamentals and practical measures to protect against noise and vibrations of machines (2006), p. 389, Springer Verlag
  2. ^ Michael Dickreiter: Room acoustics, sound sources, sound perception, sound transducers, sound reinforcement technology, recording technology, sound design Berlin (1987), p. 29, Walter de Gruyter.
  3. Johannes Barkowsky: Mathematical sources of musical acoustics (2007), p. 572, Noetzel Verlag (2006)


  • Ulf-J. Werner: Soundproofing and room acoustics - manual for theory and practice . Bauwerk Verlag GmbH, 2009.
  • Wolfgang Moll, Annika Moll: Soundproofing in residential construction: good criteria, possibilities, constructions . Ernst & Sohn GmbH & Co. KG, 2011.
  • W. Fasold, E. Veres: Noise protection + room acoustics in practice . HUSS-Medien GmbH, 2003.
  • Eckhard Momertz: Acoustics and noise protection: basics, planning, examples . Walter de Gruyter, 2008.

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