Understanding the Speech Intelligibility Index (SII)
The Speech Intelligibility Index (SII) is a standardized method (ANSI S3.5-1997) used in audiology to predict the proportion of speech cues available to a listener. It replaces the older Articulation Index (AI) and is a critical tool for hearing aid fitting and diagnostic assessment.
Key Importance Factors
Not all sound frequencies contribute equally to speech understanding. The SII calculation applies weightings based on the importance of specific frequency bands:
Low Frequencies (500 Hz): Provides vowel information and voice pitch.
Mid-High Frequencies (1000 – 2000 Hz): Most critical for consonant recognition and clarity.
High Frequencies (4000 Hz): Essential for distinguishing fricatives like "s," "f," and "th."
How to Interpret Your Score
The SII score ranges from 0.0 (no speech cues available) to 1.0 (all speech cues available).
SII Score
Potential Understanding
Above 0.75
Excellent – comparable to normal hearing in quiet.
0.40 – 0.70
Fair to Good – may struggle in noisy environments.
Consider an individual with a high-frequency hearing loss (Prebycusis). If their thresholds at 2000Hz and 4000Hz are 50dB and 60dB respectively, while 500Hz is 10dB, their SII score for average speech (65dB) will drop significantly because the high-frequency "importance" weights cannot be accessed. This explains why people often say, "I can hear people talking, but I can't understand what they are saying."
function calculateSII() {
// Get Input values
var h500 = parseFloat(document.getElementById('hz500').value);
var h1000 = parseFloat(document.getElementById('hz1000').value);
var h2000 = parseFloat(document.getElementById('hz2000').value);
var h4000 = parseFloat(document.getElementById('hz4000').value);
var speechLevel = parseFloat(document.getElementById('speechLevel').value);
// Validate inputs
if (isNaN(h500) || isNaN(h1000) || isNaN(h2000) || isNaN(h4000)) {
alert("Please enter valid hearing threshold numbers for all frequencies.");
return;
}
// ANSI Simplified Weights for 4 bands (derived from Octave importance)
// 500Hz: 0.15, 1000Hz: 0.25, 2000Hz: 0.35, 4000Hz: 0.25 (Total = 1.0)
var weights = [0.15, 0.25, 0.35, 0.25];
var thresholds = [h500, h1000, h2000, h4000];
// The "Count the Dot" concept or simplified SII math:
// Speech peaks reach approx 15dB above the nominal SPL level.
// Speech range is 30dB total (from -15 to +15 relative to RMS).
// We assume speech level SPL to HL conversion is accounted for in this clinical approximation.
var totalSII = 0;
for (var i = 0; i < thresholds.length; i++) {
// Effective audibility is the proportion of the 30dB speech window above the threshold
// Speech Peak at frequency band approx = speechLevel
// Speech Floor at frequency band approx = speechLevel – 30
var peak = speechLevel;
var floor = speechLevel – 30;
var audibilityFactor = 0;
if (thresholds[i] = peak) {
audibilityFactor = 0.0; // None of the 30dB range is audible
} else {
// Partial audibility: (Peak – Threshold) / 30
audibilityFactor = (peak – thresholds[i]) / 30;
}
totalSII += (weights[i] * audibilityFactor);
}
// Final result formatting
if (totalSII 1) totalSII = 1;
var siiDisplay = document.getElementById('siiValue');
var percentDisplay = document.getElementById('siiPercent');
var interpDisplay = document.getElementById('siiInterpretation');
var resultsArea = document.getElementById('resultsArea');
siiDisplay.innerHTML = totalSII.toFixed(2);
percentDisplay.innerHTML = (totalSII * 100).toFixed(0) + "% Speech Audibility";
var interpretation = "";
if (totalSII >= 0.75) {
interpretation = "Excellent speech audibility. Most cues are available for understanding.";
} else if (totalSII >= 0.45) {
interpretation = "Moderate speech audibility. Understanding likely clear in quiet, but very difficult in noise.";
} else if (totalSII >= 0.25) {
interpretation = "Poor speech audibility. Significant cues are missing; hearing technology is likely required.";
} else {
interpretation = "Minimal speech audibility. Speech will sound extremely muffled or unintelligible.";
}
interpDisplay.innerHTML = interpretation;
resultsArea.style.display = "block";
resultsArea.scrollIntoView({ behavior: 'smooth', block: 'nearest' });
}