When I design a speaker or subwoofer enclosure with a passive radiator, I consider many factors to achieve the best possible sound quality. That’s why I created this Passive Radiator Calculator—to simplify the decision-making process by estimating key parameters and offering recommendations. Whether you’re an experienced audio engineer or just beginning, I’ll walk you through using the calculator effectively.
What is a Passive Radiator?
First, let me explain what a passive radiator is. A passive radiator is an unpowered diaphragm inside a speaker enclosure. Unlike the active driver (woofer), it doesn’t have a voice coil or magnet. Instead, it moves in response to changes in air pressure caused by the active woofer. By carefully selecting and tuning a passive radiator, you can achieve deeper bass extension and improved sound quality without using a long, space-consuming port.
Things to Know Before You Start
To use this calculator effectively, you need some basic information about your speaker components and goals. Here are the key terms you should know before you begin.
1- Active Woofer Parameters
- Driver Diameter: The size of the active speaker driver (woofer), typically in inches or centimeters.
- Driver Vas: The equivalent compliance volume of the driver in liters or cubic feet. This is often found in the driver’s specification sheet.
- Driver Fs: The resonant frequency of the driver in Hertz (Hz).
- Driver Qts: The total Q factor of the driver (unitless), which helps you understand how damped it is.
2- Enclosure Parameters
- Enclosure Volume: The internal volume of the speaker box, measured in liters or cubic feet.
- Enclosure Type (Sealed or Ported): Affects how the passive radiator behaves.
3- Passive Radiator Parameters (Optional)
All the things below are optional. If you know input them, it will help the calculator estimate your preferences. However, it will provide you all these values in the output whether you input them or not.
- Passive Radiator Diameter: If you have a particular passive radiator in mind, you should specify its diameter.
- Passive Radiator Mass: The mass of the passive radiator’s diaphragm in grams.
- Passive Radiator Compliance: How easily the passive radiator moves (in mm/N or similar units).
4- Target Tuning Frequency (Optional)
- Target Frequency (Hz): If you have a specific low-frequency tuning goal, enter it here.
Note: I have tried to explain all the important things above that will help you use the calculator. But below the calculator is a comprehensive guide that explains how to input values and how to read output. You should take a look at it for a better understanding of the results.
Passive Radiator Calculator
How to Input Values?
Here is a step-by-step guide to enter the required values.
1- Driver Parameters
Start by entering the values of your active woofer. If you have the manufacturer’s specification sheet, you’ll find the Driver Diameter, Vas, Fs, and Qts listed. For example, enter your:
- Driver Diameter (e.g., 8 inches)
- Vas (e.g., 30 liters)
- Fs (e.g., 35 Hz)
- Qts (e.g., 0.4)
2- Enclosure Parameters
Specify your enclosure volume and type. If you know your enclosure volume (e.g., 40 liters), input that value. Then select whether your enclosure is sealed or ported. Even though the passive radiator system is akin to a ported enclosure, this option can help the calculator provide more relevant suggestions.
3- Optional Passive Radiator Parameters
If you have a particular passive radiator in mind or certain design constraints:
- Enter its diameter (e.g., 10 inches)
- Enter its mass if known (e.g., 50 grams)
- Enter its compliance if available (e.g., 0.5 mm/N)
These inputs are optional. If you’re unsure, you can leave these fields blank, and the calculator will provide default or estimated values.
4- Optional Target Tuning Frequency
If you have a specific tuning frequency in mind (e.g., 30 Hz), enter that value. If not provided, the calculator will estimate a tuning frequency for you.
5- Calculate
Once all relevant fields are filled out, click the “Calculate” button. The calculator will process your inputs and display results below.
How to Read the Output?
After you have completed all the required inputs you will see a results section with several pieces of information. Here is the breakdown.
Recommended Passive Radiator Size
If you didn’t specify a passive radiator diameter, the calculator will suggest one based on your active driver size. For example, it might recommend a PR about 20% larger than your active driver’s diameter.
Recommended Passive Radiator Mass (grams)
This shows you the ideal mass needed to achieve your target tuning frequency. If you provided a target frequency and have the active driver’s parameters, it will give a calculated mass. Otherwise, it may show a default or approximate value.
Recommended Passive Radiator Compliance
The calculator provides a suggested compliance value. If you already entered a compliance value, it may simply re-display that. If not, it uses the relationship between the driver’s Vas, enclosure volume, and some assumptions to estimate suitable compliance.
Calculated Tuning Frequency (Hz)
If you left the target frequency blank, the calculator estimates a tuning frequency based on the driver and enclosure parameters. If you specified a target frequency, it confirms that value or shows the closest achievable number.
Recommended Enclosure Volume (L or ft³)
If you didn’t have an enclosure volume in mind, the calculator suggests an approximate enclosure volume to achieve a balanced system response with a passive radiator design.
Estimated Bass Extension (Hz)
The calculator gives you an approximate low-end extension. This is a rough estimate of how low your system’s frequency response might extend.
Estimated Efficiency (SPL)
An approximate SPL (Sound Pressure Level) rating is provided. This is a very rough guess based on the driver size and other factors. Actual measurements and fine-tuning are still necessary for precise values.
Passive Radiator Placement Recommendations
Finally, the calculator offers generic guidance on how to position the passive radiator relative to the active driver. For example, it might suggest placing the passive radiator on a different enclosure panel from the active driver and ensuring sufficient clearance.
Things You Should Be Aware of
Here are 3 things to keep in mind when using the results provided by my passive radiator calculator.
Refine Your Design
The results from this calculator are approximations. Use them as a starting point. Once you have estimates for PR size, mass, and tuning frequency, you can refine your design, possibly running simulations with speaker design software or consulting detailed engineering references.
Adjust Your Parameters
If the recommended tuning frequency seems off, try changing your enclosure volume or target tuning frequency. If the mass seems too large or small, consider choosing a different passive radiator or adjusting the suspension to meet that mass target.
Compare Results with Real-world Data
Always cross-check with manufacturer datasheets and, if possible, measure actual prototypes. The calculator’s results are best considered as guidelines, not absolute values.
Check my other helpful calculators:
Speaker Cable Gauge Calculator