Advanced Acoustic Metrics – the Science of Speech: Decoding STI, C50, and C80
In architectural acoustics—such as concert halls, houses of worship, and conference centers—subjective impressions like “it sounds good” are not sufficient documentation. Objective, standardized metrics are required to describe how a space performs. In Smaart v9 Suite, these metrics are evaluated using Impulse Response Mode, which derives room-acoustic parameters from measured impulse responses.
Three commonly used and complementary metrics are STI, C50, and C80.
1. Speech Transmission Index (STI)
STI is a standardized measure of speech intelligibility ranging from 0 (unintelligible) to 1 (excellent intelligibility). For many public-address and speech-reinforcement applications, values of approximately 0.50–0.60 or higher are commonly targeted, depending on use case and local standards.
STI evaluates how well amplitude modulation in speech is preserved as sound travels through a system and a room. It is influenced by:
- Reverberation
- Early and late reflections
- Background noise
- System frequency response and distortion
If STI is limited, increasing level alone typically does not improve intelligibility. Improvements usually require acoustic treatment, noise reduction, or changes in loudspeaker coverage and directivity.
2. Clarity Index C50 — Speech Clarity
C50 is the ratio of sound energy arriving within the first 50 milliseconds after the direct sound to the energy arriving later, expressed in decibels. It is primarily used to evaluate speech clarity.
Early reflections arriving within this time window are perceptually integrated with the direct sound and tend to reinforce intelligibility. Higher C50 values generally correspond to clearer, more articulate speech, while lower values indicate excessive late energy that can mask consonants.
3. Clarity Index C80 — Music Clarity
C80 uses an 80 millisecond time window and is commonly applied to music performance spaces. This longer window allows more early reverberant energy to contribute positively to the listening experience.
Appropriate C80 values depend on repertoire and room function, but the metric is widely used to assess the balance between definition and reverberant support. Measuring C80 at multiple listener positions helps evaluate how consistently that balance is maintained throughout a venue, from stalls to balcony.
Practical application
When completing a system installation or acoustic commissioning, including STI, C50, and C80 measurements in the final documentation provides objective evidence of performance. Together, these metrics demonstrate that the outcome was not only functional, but intentionally engineered to meet intelligibility and clarity goals appropriate to the space’s use.
They do not replace listening—but they ensure that listening results are repeatable, defensible, and measurable.
Smaart is excellent on its own, but most users find they get much more out of it after some structured training. That’s where our seminars come in. At TZ Audio we run practical seminars, both online and in-venue. We offer seminar-only or full “all you need packages” including software & hardware. It’s simply the fastest way to become comfortable and confident with the measuring a sound system.
If you’re in Norway, Sweden, Denmark or Iceland – or elsewhere – we offer is online seminars and traveling to Norway is a valid option too of course. We’re here if you have any questions about the software or upcoming seminars.
Thanks for reading!