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Speech Intelligibility

The term intelligibility here, can also be defined as “speech clarity”, which refers to the proportion of a speaker’s output that can clearly be understood by a listener.

In the perspective of acoustics, it is imperative that the speech intelligibility is sufficiently high in buildings (public areas) such as airports, train stations, hospitals and others. This is because with high speech intelligibility, emergency announcements can be heard clearly and easier to be understood. To identify the intelligibility of one area, it is best to carry out Speech Transmission Index (STI) measurements.

Speech Transmission Index (STI)

The STI for Public Address (PA) systems, sometimes also known as STIPA, is a quick and high-accuracy method to measure the STI of an area. It takes account of the quality of speakers, signal interference, room acoustics, and many more aspects. STI gives a numerical value to present the speech transmission quality, which normally can rate anywhere between 0 and 1. STI of 1 means perfect intelligibility (which only happens in ideal cases, meaning virtually impossible). So the lower the STI value, the lower the speech intelligibility and quality.

Speech intelligibility measurement can be done by playing a known test signal (e.g., STIPA signal) through the PA system of the test area. It is always recommended to use a calibrated signal source (e.g., BTB65 Talkbox) to ensure the result accuracy. Once the signal starts playing, a STI measurement meter (e.g., SM50) can be used to analyse the signal from various locations. The device will display a numerical reading of STI within 18-25 seconds, depending on the type of STIPA test that is run.

Who started STI?

In 1971, Tammo Houtgast and Herman Steeneken invented the Speech Transmission Index in the Netherlands, and it has been evolving ever since. The STI is most useful for the room acoustics of large environments, for example churches, auditoriums or big conference rooms. It functions as well in smaller environments. STI can also be used to indicate the speech intelligibility over telecommunication channels, such as radio transmissions or telephone lines.

STI Measurements with Bedrock

Let’s be honest, measuring STI has not always been practical even though it has been around for quite a while. Taking a full STI measurement usually takes about 15 minutes, but not anymore!

Bedrock has proudly announced that they have discovered a method of taking Full STI readings by using the direct method, which only needs about 65 seconds. Although there are other accurate methods to measure STI quickly (i.e., STIPA), a Full STI measurement is still the absolute most comprehensive measurement there is. With 14 modulations per octave band, it is sampled much more than STIPA (2 modulations only) in the modulation domain. These readings that take over a minute can be done on any Bedrock SM50 or SM90 devices.