Measuring speech intelligibility with STIPA – how it works
When assessing the intelligibility of speech in spaces such as classrooms, offices, railway stations or public buildings, STIPA plays an increasingly important role. STIPA stands for Speech Transmission Index for Public Address systems and is a standardised method for measuring speech intelligibility objectively.
What exactly does STIPA measure?
STIPA is a measurement method based on the broader Speech Transmission Index (STI), but simplified and speeded up for practical situations. The measurement gives a value between 0 and 1, where:
-
< 0.45 means: poorly intelligible
-
0.45 – 0.6 means: adequate
-
> 0.6 means: good to excellent intelligibility
STIPA measures how well a spoken message is conveyed to a listener through a space or a public address system. In doing so, factors such as reverberation, background noise, frequency response and amplitude modulation are taken into account.
Why is measuring speech intelligibility important?
Good speech intelligibility matters in spaces where information has to be conveyed or where safety plays a role. Think of:
-
Classrooms and educational institutions
For the effective transfer of course material. -
Offices
To reduce the cognitive load caused by poor acoustics. -
Hospitals
Where fast and clear communication is vital. -
Stations, airports, shopping centres
For clear public address systems during emergencies.
In many cases a minimum STIPA value is mandatory under NEN standards and Dutch Building Decree requirements, certainly for public buildings and schools.
How should you measure speech intelligibility?
For a STIPA measurement you need two elements:
-
A simulated voice signal (STIPA test signal), often played through a public address system or loudspeaker such as the SV 90/303.
-
A class 1 sound level meter, such as the Svantek SV 971A or SV 977D, which measures the degradation of the signal by the space.
With this combination you measure in less than 30 seconds per position how well a spoken message comes across. Thanks to integration with an app or PC software, the results can be analysed and reported immediately.
👉 Also read our blog about measuring reverberation time for background on RT60 and room acoustics.
The best equipment for measuring speech intelligibility
For reliable STIPA measurements:
- Svantek SV 977D – Class 1 sound level meter with STIPA function, integrated acoustic analyses and app support.
- Svantek SV 971A – Compact, class 1, with RT60 and STIPA capabilities.
- SV 90/303 Noise Source – Lightweight omnidirectional loudspeaker, specially developed for room acoustics and STIPA applications.
SV 90/303 Noise source for speech intelligibility
Measuring speech intelligibility first of all requires a reliable and even sound source. The SV 90/303 from Svantek is specially designed for building and room acoustics measurements and meets international standards.
With its powerful 300W RMS speaker and a diameter of 330 mm, this sound source delivers a constant sound distribution in all directions.
What makes this sound source extra practical is the wireless operation via a remote control and the optional portable battery. This lets you carry out hundreds of measurements without depending on a power supply on site. The sound source is user-friendly and versatile in use, whether you want to test a classroom, office or public space.
SV 971A sound level meter for speech intelligibility
The SV 971A is the sound level meter for anyone who wants to measure speech intelligibility professionally and accurately in spaces such as offices, classrooms or public buildings. Thanks to its support for STIPA measurements, this compact class 1 instrument gives quick and reliable insight into how well speech is understood in a space.
The device complies with the IEC 61672 standard and features an intuitive menu, so that less experienced users can also get started with it quickly. In combination with the mobile app you carry out measurements, analyse data and generate reports, all with just a few taps.
For those who need more: the SV 977D
Just like the SV 971A, the SV 977D supports speech intelligibility measurements via STIPA, but it also offers extra functions for anyone analysing more complex acoustic situations. For instance, the 977D also measures floor vibrations (for impact sound), supports ultrasonic frequencies up to 40 kHz, and offers options such as audio recording, WAV export, and a faster logging interval. This makes it eminently suitable for projects in which speech intelligibility is only one part of a broader acoustic assessment.
Would you like to measure speech intelligibility yourself?
Contact our specialists for advice, demonstrations or to rent/buy the right measuring equipment.
📩 Get in touch
🔍 View the SV 971A
🔍 View the SV 977D
🔍 View the SV 90/303
📘 Also read: Measuring reverberation time
