Speech Intelligibility

The Importance of Speech Intelligibility

It is absolutely vital that the speech intelligibility is high in buildings such as airports, hospitals, train stations, and other public areas. This ensures that emergency announcements will be clear and understandable. The best way to achieve this is by measuring the Speech Transmission Index (STI). The Speech Transmission Index for Public Address Systems, or STIPA, is a fast and accurate way to measure the STI of a given area, taking into account the quality of the speakers, signal interference, room acoustics, and much more.

STIPA Speech Intelligibility

Using STIPA to Test Speech Intelligibility

Speech intelligibility can be measured by playing a known test signal, such as the STIPA signal, through the PA of the area you wish to test. To ensure the most accurate results, it is recommended that you use a calibrated signal source, such as the BTB65 Talkbox. Once the signal is played, a measurement meter such as the SM50 analyzes the signal from various locations. Depending on the type of STIPA test you’re running, within 18-25 seconds this device will display a numerical reading of the STI.

The Benefits of Using Full STI

Although fast and robust, STIPA has its limitations. Only 2 modulation frequencies are measured per octave band with STIPA. The complete Speech Transmission Index measurement scheme calls for 14 modulations per octave band. This means that STIPA is under-sampled in the modulation domain. For many systems, the measured STI is not (or hardly) affected. However, some cases call for Full STI measurements, like areas with excessive echoing or systems that feature complex distortion factors.

Full STI measurements were available in commercial measuring tools up to the late 1990s, but have subsequently been absent from the market for two decades. The fact that a full STI measurement used to take around 15 minutes made Full STI measurements entirely impractical. Also, the introduction of STIPA around 2002 made a reasonable alternative available. The Full STI module now available on the SM50 and SM90 fully implements every aspect of the Speech Transmission Index model and can complete a test in about a minute, making it the most universally applicable speech intelligibility metric to date.

To learn more about speech intelligibility, click here to read a report from Bedrock’s own Sander van Wijngaarde and Jan Verhave.