What is speech intelligibility, Speech Transmission Index (STI) and STIPA? Why is it important and how is it measured?

For Public Address & General Alarm (PAGA), Public Address (PA), Voice Address (VA) and alarm systems, it is critical that messages are clear and intelligible so that important instructions can be understood. This is particularly of concern for hazardous areas such as industrial sites, factories, and oil & gas platforms, but also areas with large crowds such as stadiums, train stations, airports and hospitals. The intelligibility of emergency announcements can be quantitively assessed by determining the Speech Transmission Index (STI), which is most easily measured using the Speech Transmission Index for Public Address (STIPA) testing method.

What is speech intelligibility?

In almost all situations where speech is transmitted between a talker and a listener, the speech is degraded somewhat by the transmission channel, and becomes less intelligible to the listener. This may be due to:

  • Acoustic factors e.g. reverberation
  • Environmental factors e.g. background noise, attenuation over distance or due to obstacles
  • Electro-acoustic factors e.g. electrical/PA system noise, frequency response of loudspeakers, speaker orientation

The two main variables that affect the intelligibility through a transmission channel are the Signal to Noise Ratio (SNR) and the reverberation, with high SNR and low reverberation required to achieve good intelligibility.

To achieve the best possible intelligibility, the SNR should be above 15 dB, with the sound pressure level of the message at the ear of between 60 and 80 dBA. Above 80 dBA, auditory masking may reduce the intelligibility of the message.

The Speech Transmission Index (STI)

The STI is a numerical value that quantifies the effect of the transmission channel on the intelligibility of messages from a person speaking to a listener. The STI is a good method of assessing PA systems as it does not take into account the talker or the listener, and only reflects how the transmission channel affects the speech intelligibility. It is calculated from the change in modulation of a test signal, and gives a value of between 0 and 1, with 1 being the most intelligible and 0 the least.

How to measure STI – Full STI or STIPA

There are two methods of measuring the STI: Full STI and STIPA.

Full STI

A Full STI measurement requires 98 separate test signals with 14 different modulation frequencies and requires 15 minutes per measurement. As hundreds of measurements are typically required for assessing large spaces, this method is very time intensive.


The STIPA method is a simplified version of Full STI and requires only one test signal, comprised of modulated pink noise with two modulation frequencies in each octave band. This allows a measurement to be taken in 15 seconds with similar performance and is hence a much more efficient method for measurement of the STI. For this reason, STIPA testing has all but replaced the Full STI method for measuring the STI in practice.

The test signal is transmitted over a transmission channel (e.g. a PA system in an airport or from a stage in a concert hall), either acoustically via a calibrated Talkbox or electronically via line input and a .mp3 file. The sound pressure level of the test signal is measured at the listener (receiver) locations, and the STI calculated from the change in modulation depth between the transmitted and received signals.

STIPA measurements – How Xi can help?

Xi Engineering Consultants have the acoustic expertise, experience, and equipment to conduct speech intelligibility measurements using the STIPA testing method in the harshest of environments, from lecture theatres to offshore platforms. We can also model the STI in your space and provide design advice for improving the intelligibility at all stages of the engineering lifecycle. Please contact us to discuss your project’s requirements and how Xi can help.