What Is the Room Speech Transmission Index (STI & RASTI) Calculator?
The Speech Transmission Index (STI) is the international standard (IEC 60268-16) metric for quantifying speech intelligibility in classrooms, auditoriums, and airport terminals.
Acoustic reverberation causes temporal smearing of speech modulations, while background ambient noise masks acoustic signals.
This calculator evaluates the Houtgast and Steeneken Modulation Transfer Function (MTF) from reverberation time (RT60) and signal-to-noise ratio (SNR), computing the overall STI score and intelligibility rating.
How Does the Room Speech Transmission Index (STI & RASTI) Calculator Work?
The calculation evaluates user-provided measurements using recognized domain equations, converts between measurement units, and adjusts for real-world efficiency factors.
Room Speech Transmission Index (STI & RASTI) Calculator Formula & Variables
The core mathematical equation utilized by this calculator is expressed as:
IEC 60268-16 modulation transfer formulation compounding room reverberant decay with signal-to-noise masking.
How to Use the Room Speech Transmission Index (STI & RASTI) Calculator
- Enter your primary measurements in the input fields above.
- Select your preferred units (e.g. metric or imperial) if applicable.
- Review or adjust operational assumptions such as field efficiency.
- Click Calculate to instantly generate the full results breakdown and visual chart.
- Use the Reset button at any time to clear the form and test a new scenario.
Step-by-Step Example Calculation
University Lecture Hall Acoustics (RT60 = 1.2s, SNR = +12 dB)
Input Values:
Understanding Your Result
Your calculated result represents the realistic operational capacity or baseline output under the specified conditions. Comparing theoretical and effective outputs reveals the direct impact of turns, overlap, and practical downtime.
Factors That Affect the Result
Field terrain, operator experience, equipment maintenance, overlap margin, and weather conditions can significantly influence real-world output.
When Should You Use This Calculator?
Use this calculator whenever you need quick, verified estimates for job planning, budgeting, equipment sizing, or project timelines.
Assumptions & Limitations
- Houtgast-Steeneken MTF: MTF(F) = [1 / √(1 + (2π·F·RT₆₀ / 13.8)²)] · [1 / (1 + 10^(-SNR/10))].
- Apparent SNR: (S/N)_app = 10 · log10[MTF / (1 - MTF)], clamped between -15 dB and +15 dB.
- STI = [clamped (S/N)_app + 15] / 30. Values ≥ 0.60 represent good intelligibility (IEC 60268-16).
Frequently Asked Questions
Calculation Accuracy & Reference Note
This calculator implements verified, deterministic mathematical equations based on published standards. Results should be treated as professional engineering estimates; always verify critical operations with local equipment manuals and site inspections.