Audio Precision Adds STI Measurement Capability to APx500 Software

BEAVERTON, OR -- Audio Precision introduces Speech Transmission Index (STI) plug-in for APx500 audio test software. Created for engineers developing public safety (police, fire, emergency) communications and public address systems, as well as flight recorder devices, the APx STI plug-in provides measurement capabilities conforming to international standard IEC 60268-16 (2011).

Quantifying the Intelligibility of Speech

Developed as a method for quantifying the intelligibility of speech with respect to a transmission medium—for example a room, an intercom system, a radio link—STI is an objective measure that provides a single rating between 0 and 1 that predicts the likelihood of syllables, words and sentences being comprehended within the intended medium. The rating is broken down into qualification bands ranging from “bad” (0 to 0.36), to “excellent” (0.75 to 1.0).

Of the four direct measurement methods detailed by IEC 60268-16 (2011), the APx STI plug-in employs the most commonly used technique, STIPA (Speech Transmission Index for Public Address). STIPA uses 14 modulation frequencies across seven octave bands, with two frequencies combined in each band, and all band-frequency combinations contained in a single signal. Using this signal, a single measurement is made, with total measurement time taking ~10-20 seconds. The STIPA method shows excellent correlation with the “Full STI,” the most comprehensive measurement method covered by the standard. In contrast to STIPA’s measurement time of about 15 seconds, the Full STI takes over 15 minutes to complete.

Integrated Audio, Electro-Acoustic and STI Measurement

Using the APx STI measurement enables developers to verify the STI performance of their designs with AP’s industry-leading instrumentation, and use that same test equipment for the rest of their audio and electro-acoustic test requirements. Once the plug-in is installed, the STIPA measurement can be easily incorporated into any measurement sequence. Additionally, the plug-in includes a Speech Level measurement that conforms to Annex J of IEC 60268-16, for proper adjustment of the STIPA signal level.

“With the release of our APx STI plug-in, system developers now have a lab-quality instrument for validating their designs’ STI performance,” said Dave Schmoldt, Audio Precision CEO, “an instrument that delivers unmatched versatility with electronic audio and electro-acoustic testing, up to 16 input channels, a variety of digital interfaces, and a highly intuitive user-interface.”

The APx STI plug-in is a software option that retails for US$500. It is compatible with all analyzers in the APx500 Series. For more information, visit

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