Clarity measure

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The degree of clarity C is a measure derived from the sound impulse test to determine the clarity of speech (C 50 ) or the transparency of music (C 80 ).

It puts the sound energy arriving at the listener's seat in an auditorium up to a point in time t C in relation to the room energy arriving later, evaluates this ratio logarithmically and gives it in decibels :

The following mean:
C = degree of clarity
t C = integration limit (50 ms or 80 ms)
p = sound pressure which is delivered by an impulse at the listener's position in time t after the arrival of the direct sound (at t = 0)

For the assessment of rooms for music and speech use, two separate assessment measures have become established that take into account the different requirements. These differ only in the choice of their integration limit. This is t C = 50 ms for speech and t C = 80 ms for music .

Clarity measure C 50

The measure of definition (also Sprachklarheitsmaß ) has been developed as a criterion for rooms with language use. A value of 0 dB means that the energy from the direct sound and all reflections arriving within = 50 ms is exactly as great as that of all remaining reflections. The range in which speech intelligibility is good is between –3 dB and +2 dB.

Clarity measure C 80

The clarity measure (also music clarity measure or transparency measure ) with = 80 ms is used to evaluate rooms for musical use.


If the values ​​are above 0 dB, i.e. positive, then there is sufficient clarity or transparency .

For cinemas, discos or event locations with frequent playback recordings, a degree of clarity of C 50 = 3… 4 dB should be achieved. A degree of clarity of −3 to +4 dB is optimal in concert halls. By increasing the degree of clarity and distinctiveness, the acoustic impression of the room is generally reduced; conversely, the dimensions decrease as the reverberation time increases . With values ​​below 0 dB, the listener has the impression that he seems to be “enveloped” by the music.


  • Walter Reichardt et al .: Relationship between clarity measure C and other room acoustic criteria . Journal for electrical information and energy technology, 5, 1975, page 144.
  • Wolfgang Fasold et al .: Building and room acoustics . Verlag für Bauwesen, Berlin, 1987.