Every indoor and outdoor space affects how speech and other noises carry and affect the surrounding environment. In order to measure this, scientists created the Speech Transmission Index in 1971, which was a quicker way to determine the speech intelligibility of a space than the current methods at the time.

The sound masking experts at Building Systems Solutions explain the Speech Transmission Index and how you can improve the STI of your office building or other noisy space.

What Does Speech Transmission Index Measure?

In short, speech transmission index (STI) is a measure of speech transmission quality. The STI measures characteristics of a transmission channel, such as a room, telephone line, etc., and denotes the ability of the channel to carry across the characteristics of a speech signal.

There are many factors that influence a transmission channel, which has an effect on speech intelligibility.

These include:

  • The speech level
  • Frequency response of the channel
  • Non-linear distortions
  • Background noise level
  • Quality of the sound reproduction equipment
  • Echos
  • Reverberation time
  • Psychoacoustic effects (masking effects)

How Is Speech Transmission Index Represented?

The STI is a numeric measure of the communication channel that varies from 0 = bad to 1 = excellent. It can predict the likelihood of syllables, words, and sentences being comprehended.

According to this scale, an STI of at least .5 is considered desirable for many applications.

How to Measure STI

There are two methods for measuring STI. One is called Full STI, and the other is STIPA.

Full STI

This measurement requires 98 separate test signals with 14 different modulation frequencies and requires 15 minutes per measurement.

This method is very time-intensive, particularly for large spaces.


The Speech Transmission Index for Public Address (STIPA) is a simplified version of Full STI and requires only one test signal.

The signal is comprised of modulated pink noise with two modulation frequencies in each octave band. A measurement can be taken in 15 seconds with a similar performance to the Full STI.

The test signal is sent over a transmission channel, such as a PA system, and the sound pressure level of the test signal is measured at the listener location. The STI is calculated from the change in modulation depth between the transmitted and received channels.

How to Improve Speech Intelligibility

There are a few ways to improve the speech intelligibility of a channel or space.

Room Acoustics

A room or location’s acoustic characteristics have an important influence on speech intelligibility. The primary factor to consider is whether the direct sound to the listener is dominant over any sound reflections that might occur.

To correct this, sound-absorbing objects can be installed, such as curtains, carpets, upholstered furniture, or special acoustic panels.

Background Noise

A lot of ambient noise can deteriorate speech intelligibility. This happens when a room is insufficiently shielded from nearby noise sources.

To correct this, better windows can be installed, and noise barriers or similar measures can help reduce external noise.

Voice Alarm Systems

For a voice alarm system, any distortion, defective components, or incorrectly wired speakers can cause poor speech intelligibility.

A bad layout of the system can also cause bad STI measurements. Too few speakers can cause “holes” in certain areas where the message cannot be heard. To correct this, check all components of a voice alarm system for functionality.

It is also a good idea to add more speakers to ensure the entire space is covered.

Protect Speech Privacy With Sound Masking & BSS

Sound masking is essential for many industries, as it provides speech privacy and increases comfort for those in the space, such as hospital patients. Sound masking provides ambient background sound to a space that can reduce or mask noise distractions and protect conversations. While it does not cancel out other sounds, it is a sound that is engineered to match the frequencies of human speech.

While having good STI is important in many spaces, sound masking can help reduce the noise produced in an environment where speech and other noises carry throughout the space, causing distractions. Sound masking also has a high ROI.

At Building Systems Solutions, we provide sound masking services, including space analysis and installation of Soft db sound masking products. We can also provide emergency warning and paging systems to ensure building occupant safety during emergency situations.

For more information, give us a call today at 763-502-1515 or send us a message online.