Passive Radiator Tuning Calculator
Enter values and click "Calculate" to find the required Passive Radiator Moving Mass.
function calculatePassiveRadiator() {
var enclosureVolume = parseFloat(document.getElementById('enclosureVolume').value);
var driverFs = parseFloat(document.getElementById('driverFs').value);
var driverVas = parseFloat(document.getElementById('driverVas').value);
var driverMms = parseFloat(document.getElementById('driverMms').value);
var desiredFb = parseFloat(document.getElementById('desiredFb').value);
var resultDiv = document.getElementById('result');
if (isNaN(enclosureVolume) || isNaN(driverFs) || isNaN(driverVas) || isNaN(driverMms) || isNaN(desiredFb) ||
enclosureVolume <= 0 || driverFs <= 0 || driverVas <= 0 || driverMms <= 0 || desiredFb <= 0) {
resultDiv.innerHTML = 'Please enter valid positive numbers for all fields.';
return;
}
// Formula: Mms_pr = ( (Fs_driver^2 * Vas_driver) / (Fb^2 * Vb) – 1 ) * Mms_driver
// This formula calculates the total effective moving mass of the passive radiator (Mms_pr)
// required to achieve the desired system tuning frequency (Fb).
var termNumerator = Math.pow(driverFs, 2) * driverVas;
var termDenominator = Math.pow(desiredFb, 2) * enclosureVolume;
if (termDenominator === 0) {
resultDiv.innerHTML = 'Calculation error: Denominator is zero. Check your input values.';
return;
}
var ratio = termNumerator / termDenominator;
if (ratio <= 1) {
resultDiv.innerHTML = 'The desired system tuning frequency (Fb) is too high or the enclosure volume (Vb) is too large for the given active driver. A passive radiator cannot achieve this tuning, or a negative/zero mass is required. Please choose a lower Fb or smaller Vb.';
return;
}
var requiredMmsPr = (ratio – 1) * driverMms;
resultDiv.innerHTML = 'Calculation Result:
' + 'Required Passive Radiator Moving Mass (Mms_pr): ' + requiredMmsPr.toFixed(2) + ' grams' + 'This is the total effective moving mass required for the passive radiator, including its inherent cone mass and any added weights.'; }Understanding Passive Radiators and Their Tuning
A passive radiator (PR) is essentially a speaker cone without a voice coil or magnet. It's used in conjunction with an active speaker driver in a sealed enclosure to enhance bass response. Unlike a ported (vented) enclosure that uses a column of air to tune the box, a passive radiator uses a physical mass to achieve the desired tuning frequency.
Why Use a Passive Radiator?
- Deeper Bass Extension: Passive radiators can help achieve lower bass frequencies than a sealed box of the same size, often extending below the active driver's natural resonant frequency (Fs).
- No Port Noise: Unlike ported designs, passive radiators eliminate port turbulence noise (chuffing) because there's no air moving through a narrow opening.
- Smaller Enclosures: They can sometimes allow for smaller enclosure volumes compared to ported designs for similar bass extension.
- Aesthetics: They can offer a cleaner look than a port, especially in compact designs.
Key Parameters for Passive Radiator Tuning
To effectively design a passive radiator system, you need to understand and measure several parameters:
- Enclosure Volume (Vb): This is the net internal volume of your speaker box, typically measured in Liters or cubic feet.
- Active Driver Resonant Frequency (Fs): The natural resonant frequency of your active speaker driver when measured in free air (not in an enclosure). Measured in Hertz (Hz).
- Active Driver Equivalent Volume (Vas): The volume of air that has the same compliance (stiffness) as the active driver's suspension. Measured in Liters or cubic feet.
- Active Driver Moving Mass (Mms): The total mass of the active driver's moving parts (cone, voice coil, dust cap, etc.). Measured in grams.
- Desired System Tuning Frequency (Fb): This is the target resonant frequency for your entire speaker system (active driver + passive radiator + enclosure). For bass extension, Fb is typically chosen to be lower than the active driver's Fs. Measured in Hertz (Hz).
How the Calculator Works
This calculator uses a common formula derived from Thiele-Small parameters to determine the required total moving mass (Mms_pr) for your passive radiator. The formula is:
Mms_pr = ( (Fs_driver^2 * Vas_driver) / (Fb^2 * Vb) - 1 ) * Mms_driver
Where:
Mms_pris the required total moving mass of the passive radiator (in grams).Fs_driveris the active driver's resonant frequency (in Hz).Vas_driveris the active driver's equivalent volume (in Liters).Fbis the desired system tuning frequency (in Hz).Vbis the enclosure volume (in Liters).Mms_driveris the active driver's moving mass (in grams).
The calculated Mms_pr represents the total mass the passive radiator needs to have, including its inherent cone mass and any additional weights you might add to it. It's crucial that the passive radiator's effective piston area (Sd_pr) is at least equal to, and preferably 1.5 to 2 times larger than, the active driver's effective piston area (Sd_driver) to prevent over-excursion of the passive radiator.
Important Considerations:
- Fb vs. Fs_driver: For typical bass extension, your desired Fb should be lower than your active driver's Fs. If Fb is too high, the formula may yield a negative or zero mass, indicating that a passive radiator is not suitable for that tuning with your given driver and enclosure.
- Passive Radiator Size: Ensure the passive radiator you choose has an effective piston area (Sd) at least as large as, or preferably larger than, your active driver's Sd. This helps prevent the PR from reaching its mechanical limits before the active driver.
- Adjusting Mass: Most passive radiators allow for the addition of weights to fine-tune their moving mass to achieve the precise Fb calculated.
- Real-World Testing: Always verify your design with real-world measurements (e.g., impedance sweeps) after construction, as theoretical calculations are approximations.