Speech clarity

Speech clarity concerns the quality of speech transfer to the listeners. In a reverberant room with disturbing background noise, it can be difficult to pick up speech.

Unclear

Clear

The sound that reaches the listener first is called direct sound. This is followed by early reflections. The early reflections that reach the listener within 50 ms are integrated with the direct sound and thus have a positive effect on speech clarity. The reflections that come later may be perceived as disturbing.

The Clarity measurement (C50) compares the sound energy in early sound reflexes with those that arrive later. It is expressed in dB. A high value is positive for speech clarity.

Speech Transmisson Index

Another measure of speech transfer is the Speech Transmisson Index (STI). When speech transmission is perfect, STI = 1. In a normal-sized classroom, STI should be greater than 0.75. In the case of poor speech transmission, variations in speech are perceived less well. Factors that impair speech transmission, thus contributing to a lower STI index are, for instance, background noise, long reverberation and echoes.

Illustration of bad speech transmission — Bad speech transmission

Perceived attribute	Objective descriptor	Designation	Unit	Explanation	Standard
Speech clarity	Speech clarity	C50	dB	Measures the ration between early and late reflections.	ISO 3382-1
Speech clarity	Speech Transmission Index	STI	Index (0-1)	Measures quality of speech transfer from speaker to listener.	IEC 60268-16