Passive Radiator Mass Calculator
Use this tool to estimate the required moving mass for your passive radiator system to hit a target box tuning frequency.
M = equivalent moving mass (kg), ρ = air density, c = speed of sound, S = total PR area (m²), f = tuning frequency (Hz), V = box volume (m³).
What Is a Passive Radiator Calculator?
A passive radiator calculator helps you estimate how much moving mass a passive radiator needs so your speaker enclosure tunes to your target low-frequency resonance. In practical terms, this tells you how much weight should be on the radiator cone (or diaphragm) so your bass extension lands where you want it.
If you have ever tuned a bass reflex box with a port, you can think of a passive radiator as an alternative “air mass” system. Instead of pushing and pulling a slug of air in a duct, the enclosure pressure drives a diaphragm with a suspension and a controllable moving mass.
Why Use a Passive Radiator Instead of a Port?
1) Compact enclosures
Very low tuning in small boxes often requires long ports. Those ports may not physically fit inside the cabinet or may need bends that add complexity.
2) Lower port noise
At high output, narrow or undersized ports can chuff. Passive radiators can reduce this issue when sized appropriately.
3) Flexible tuning
You can tune by adjusting mass. Adding washers or tuning discs is often easier than rebuilding a port.
How This Calculator Works
This calculator uses a simplified lumped-parameter model. You provide:
- Net internal box volume (after bracing, driver, amplifier module, and passive radiator displacement).
- Target box tuning frequency (Fb).
- Number of passive radiators.
- Effective cone area per passive radiator (Sd).
- Native moving mass per passive radiator (before extra weights).
It then calculates total equivalent required moving mass and converts that into per-radiator target mass and added mass.
Input Guidelines for Better Results
Use true net volume
Gross cabinet volume can be significantly larger than net volume. Always subtract displacement of drivers, passive radiators, braces, and electronics.
Use accurate Sd values
Effective radiator area should come from manufacturer data, not nominal diameter. A “10-inch” radiator does not have 10-inch piston diameter.
Confirm native mass from datasheet
Native moving mass varies by model and construction. If this number is wrong, your calculated added mass will be wrong too.
Quick Example
Suppose you want to tune a 35-liter cabinet to 32 Hz with one passive radiator:
- Box volume: 35 L
- Target Fb: 32 Hz
- PR count: 1
- Sd: 214 cm²
- Native mass: 80 g
The calculator returns a required total moving mass and tells you how much extra mass to add. If you switch to two radiators, the required mass per radiator changes because total effective area changes and the equivalent mass is shared.
Practical Tuning Workflow
- Design your enclosure and estimate net volume carefully.
- Choose radiator size and count with adequate excursion capability.
- Use this calculator for an initial mass target.
- Build and measure impedance (or near-field response).
- Adjust mass in small steps until measured Fb matches your target.
Common Mistakes to Avoid
- Ignoring excursion limits: Passive radiators can bottom out if under-sized.
- Using gross volume: This causes tuning errors that can be several hertz off.
- Skipping measurement: Final tuning should always be verified after assembly.
- Overweighting: Very heavy added mass can reduce efficiency and transient behavior.
FAQ
Can I use this for car audio and home audio?
Yes. The physics are the same. Just keep units accurate and use realistic enclosure volume.
What if the result says negative added mass?
That means your radiator is already too heavy for the target with the current area and box size. You may need a lighter radiator, a different Fb target, a larger box, or different radiator area.
Is this as accurate as full simulation software?
No. This is a useful first-pass tool. For final performance, use full loudspeaker modeling and measurement, including suspension nonlinearity, losses, and excursion limits.
Educational note: This calculator is a simplified estimator and does not replace full electro-mechanical-acoustical simulation.