Public address (PA) and Voice alarm (VA) systems in building complexes (airports, railway stations, shopping centres and concert halls) are designed to inform members of the public about evacuation directions in case of emergency. However, if such announcements are misunderstood due to the poor quality of the system, this can lead to tragic consequences. Therefore, it is very important to design, install and correctly verify the speech intelligibility of such sound amplification systems. In addition, public announcement systems may have legal or contractual requirements for the level of speech intelligibility, for example in a medical, industrial or commercial environment.

The Speech Transmission Index (STI) is applied in the deterioration of the speech intelligibility induced by the transmission channel. The STI method involves the use of a test signal applied to the transmission channel, and an analysis of the received test signal.

The speech transmission quality of the channel is determined and expressed in values from 0 to 1 in the form of STI. Using the obtained STI value, it is possible to determine the potential speech intelligibility.

The applications of STI include evaluation of sound reinforcement and emergency systems, communication channels, speech intelligibility, communication in rooms and auditoria, etc.

STIPA (speech transmission index for public address systems) is a STI derivative approach that was developed for fast measurements of electro-acoustic and acoustic environments effects that affect the speech intelligibility in the room acoustics and/or public address systems.

STIPA calculations are realized by sound level meter in accordance with the IEC 60268-16:2011 standard. Special smartphone/tablet application “BA Assistant” simplifies the measurement process and generates a report which can be sent to the other device via e-mail, wireless connections or communication applications.