Speech Intelligibility Measurement Using STI and STIPA

What Is Speech Intelligibility?

Speech intelligibility describes how easily a spoken message can be understood by listeners within a given space.
Speech understanding does not depend solely on sound level.

Factors such as reverberation, background noise, the distance between the source and the listener, and the performance of the sound system can significantly affect message clarity.

In environments such as classrooms, auditoriums, railway stations, airports, or voice evacuation systems, insufficient speech intelligibility can hinder communication and reduce the effectiveness of transmitted messages.

How Is Speech Intelligibility Measured?

Speech intelligibility can be evaluated using objective methods designed to quantify how the acoustic conditions of a space affect the transmission of spoken messages.

The most widely used reference method is STI (Speech Transmission Index), which expresses the quality of speech transmission between a source and a receiver as a numerical value.

For practical applications and field measurements, STIPA (Speech Transmission Index for Public Address Systems) is commonly used. STIPA is a simplified version of STI defined in IEC 60268-16 and specifically designed for rapid assessment of public address and sound reinforcement systems.

What Is STIPA?

STIPA (Speech Transmission Index for Public Address Systems) is a standardized method defined in IEC 60268-16 for evaluating speech intelligibility under real operating conditions.

The method uses a test signal that simulates specific temporal and spectral characteristics of human speech. This signal is reproduced through the sound system or sound source under evaluation and measured at different listening positions throughout the space.

During the measurement, the analyzer evaluates how the acoustic conditions of the environment modify the transmitted signal. Factors such as reverberation, background noise, system distortion, and acoustic reflections directly affect the listener’s ability to understand spoken messages.

The measurement result is expressed as an STI or STIPA value representing the quality of speech transmission between the source and the listening positions.

STI and STIPA values are expressed on a scale from 0 to 1, where higher values indicate better speech intelligibility.

How Does a STIPA Measurement Work?

In a STIPA measurement, a standardized test signal is reproduced through the public address system or sound system of the venue.
The sound level meter analyzes the degree to which the amplitude modulations of the signal are preserved after propagating through the space and being affected by real acoustic conditions. Loss of modulation is directly related to factors that reduce speech clarity, including:

• background noise,

• reverberation,

• acoustic reflections,

• sound system distortion,

• distance between the source and the receiver.

Based on this analysis, an STI or STIPA value is calculated, allowing the speech intelligibility performance of the space to be evaluated and classified.

Relationship Between Speech Intelligibility, Reverberation, and Background Noise

Speech intelligibility is closely linked to the acoustic behavior of a room.

Excessive reverberation can cause syllables and words to overlap, reducing the clarity of spoken messages. Similarly, high background noise levels can mask important parts of the speech signal.

For this reason, speech intelligibility measurements are often complemented by other room acoustics parameters, such as reverberation time, background noise levels, and the acoustic response of the sound system.

Evaluating these parameters together provides a more complete understanding of the acoustic performance of a space.

Typical Applications of STIPA Measurements

STIPA measurements are commonly used in:

• public address systems,

• voice alarm and voice evacuation systems,

• railway stations and airports,

• auditoriums and conference rooms,

• classrooms and educational facilities,

• public and commercial buildings.

In these environments, the ability to clearly understand spoken messages may be critical for both everyday communication and emergency situations.

STIPA Measurement with Nor145 and Nor850

The Nor145 sound level meter enables portable, direct STIPA measurements in the field, making it possible to evaluate speech intelligibility at different listening positions under real operating conditions.

Nor850 Room Acoustics allows speech intelligibility measurements to be integrated into a broader workflow for acoustic analysis, documentation, and reporting, particularly in projects where multiple room acoustics and sound system parameters are assessed together.

Integrating intelligibility measurements within a single analysis platform simplifies the overall evaluation of a venue’s acoustic performance.

Applicable Standard

Speech intelligibility measurements using STI and STIPA are performed in accordance with IEC 60268-16.

This standard defines the procedures for the objective evaluation of speech intelligibility using the Speech Transmission Index (STI) and its simplified variant STIPA, providing a standardized basis for comparing results and verifying intelligibility requirements across different acoustic applications.

Benefits of STIPA Measurements

Key advantages of the STIPA method include:

• fast measurements under real operating conditions,

• objective and repeatable intelligibility assessment,

• practical application for public address and voice evacuation systems,

• easy comparison of different listening positions,

• integration with room acoustics and sound system studies.

Limitations and Practical Considerations

STIPA measurements should be performed under conditions that are representative of the actual use of the space. Background noise levels, sound system status, and room configuration can significantly influence the measured result.

In addition, STIPA provides a specific indicator of speech transmission performance but does not replace a complete acoustic assessment of the venue. For this reason, STIPA results are typically interpreted alongside other acoustic parameters, such as reverberation time, background noise levels, and the overall performance of the sound system.

Combining STIPA measurements with other acoustic parameters provides a more comprehensive and reliable evaluation of communication conditions within the analyzed space.