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function calculateBandpass() {
var fs = parseFloat(document.getElementById('fs').value);
var qts = parseFloat(document.getElementById('qts').value);
var vas = parseFloat(document.getElementById('vas').value);
var qtc = parseFloat(document.getElementById('qtc').value);
var errorMsg = document.getElementById('error-msg');
var resultsDiv = document.getElementById('results');
errorMsg.innerHTML = "";
if (!fs || !qts || !vas || !qtc) {
errorMsg.innerHTML = "Please enter all values to calculate.";
resultsDiv.style.display = "none";
return;
}
if (qts >= qtc) {
errorMsg.innerHTML = "Qts must be lower than the Target Qtc (standard 0.707). Try a higher Qtc or check driver specs.";
resultsDiv.style.display = "none";
return;
}
// Calculation Logic
// 1. Calculate Rear Chamber Volume (Vr)
var vr = vas / (Math.pow(qtc / qts, 2) – 1);
// 2. Center Frequency (fc)
var fc = (qtc * fs) / qts;
// 3. Front Chamber Volume (Vf) – Standard alignment S=0.7
// Vf = Vr * (qtc^2) is a common flat-response formula
var vf = vr * Math.pow(qtc, 2);
// 4. Bandwidth calculation
// f_low and f_high based on Q-factor of the box
var q_bp = qtc; // Simple approximation for the bandpass Q
var bw = fc / q_bp;
// Simplified cutoff calculations for the web tool
var f_low = fc * (Math.sqrt(1 + 1 / (2 * Math.pow(q_bp, 2))) – 1 / (2 * q_bp));
var f_high = f_low + bw;
// Gain calculation in dB
var gain = 20 * (Math.log10( (2 * qtc) / qts ));
if (gain < 0) gain = 0;
// Display results
document.getElementById('v_rear').innerText = vr.toFixed(2);
document.getElementById('v_front').innerText = vf.toFixed(2);
document.getElementById('fb').innerText = fc.toFixed(1);
document.getElementById('gain').innerText = gain.toFixed(2);
document.getElementById('f_low').innerText = f_low.toFixed(1);
document.getElementById('f_high').innerText = f_high.toFixed(1);
resultsDiv.style.display = "block";
}
4th Order Bandpass Enclosure Calculator
Enter your driver's Thiele/Small parameters to calculate the optimal sealed and ported chamber volumes.
0.707 is standard for flat response.
Design Specifications
Rear Chamber (Sealed)
0.00 L
Front Chamber (Ported)
0.00 L
Tuning Frequency (Fb)
0 Hz
Efficiency Gain
0.00 dB
Lower Cutoff (F3 Low)
0 Hz
Upper Cutoff (F3 High)
0 Hz
What is a 4th Order Bandpass Enclosure?
A 4th order bandpass enclosure is a specialized speaker box design where the subwoofer is mounted inside a dual-chamber box. One chamber is sealed (acting as the rear air spring), and the other is vented or ported. The sound only exits through the port, which means the enclosure itself acts as a natural acoustic filter.
Benefits of This Design
- Natural Low-Pass Filter: The enclosure naturally rolls off high frequencies, reducing the need for complex electronic crossovers.
- High Efficiency: Within its specific frequency bandwidth, a 4th order bandpass can be significantly louder than a standard sealed or ported box.
- Cone Protection: The sealed rear chamber controls the driver's excursion at very low frequencies, preventing damage.
Critical Parameters Explained
To use this calculator effectively, you need the Thiele/Small parameters from your subwoofer's datasheet:
- Fs (Resonant Frequency): The frequency at which the driver's moving parts naturally vibrate.
- Qts (Total Quality Factor): A measurement of the driver's damping; drivers with a Qts between 0.3 and 0.45 are generally best for 4th order bandpass designs.
- Vas: The volume of air that has the same stiffness as the driver's suspension.
Example Calculation
If you have a 12-inch subwoofer with an Fs of 30Hz, Qts of 0.40, and Vas of 50L, a standard 4th order alignment (Qtc = 0.707) would yield:
- Rear Chamber: ~23.5 Liters
- Front Chamber: ~23.5 Liters (for 0dB gain)
- Tuning: ~53 Hz
This setup provides a balanced response between deep extension and punchy output.