What is a Passive Radiator?
Even if you are not familiar with passive radiators, you have most likely heard a speaker that includes one. Passive radiators (originally called drone cones) are used in many speaker systems to allow woofers to more efficiently reproduce low frequencies. A passive radiator is a device that looks like a woofer but acts similar to a port. Like a woofer, a passive radiator has a cone with suspension and has a resonant frequency that is determined by compliance and mass. Also, a passive radiator is a resonant device that is driven by the air pressure changes inside an enclosure, like a port. With today’s constant push towards smaller systems and higher displacement, passive radiators are commonly used in many speaker designs. The biggest advantage of a passive radiator is that they give you the capability to tune enclosures lower than possible using a reasonable-sized port (more on this below). Also, we have to admit it… a system that includes active woofers and multiple passive radiators looks pretty cool.
Parts of a Passive Radiator
The two main parts of a passive radiator are the diaphragm (cone or mass) and suspension. In some cases, this can be as simple as a flat steel plate attached to a surround, but most passive radiators available on the market are more similar to a woofer without the active parts (such as the magnet and voice coil). Simple versions will almost always have a problem maintaining linear movement and start rocking near their excursion limits due to weight distribution issues. Higher quality passive radiators use a conventional-looking cone that is suspended from the front by a surround and the back by a spider, all mounted in a rigid frame that holds everything together. The dual suspension ensures that the excursion remains mostly linear in order to make sure that unwanted distortion remains as low as possible and that output is predictable. In addition, most passive radiators will have either a threaded post or screw hole allowing you to easily add mass and adjust the tuning frequency to fit your needs.
Note: Adding mass to a passive radiator will lower the resonant frequency (Fpr) and increase the mechanical damping value (Qmpr).
When to Use a Passive Radiator
In most cases, any driver that is suitable for use in a vented enclosure will also work well in a passive radiator design and produce similar results from the same sized enclosure. As mentioned earlier, a compact tower speaker with 3 "woofers" on the front has a really good look (1 active woofer with 2 passive radiators), so in some cases a passive radiator is just an aesthetic choice. However, with today’s high displacement drivers, using a passive radiator will actually allow you to build a smaller enclosure compared to a vented design since you will not need to account for the space taken up by the port, while still delivering similar response.
High excursion drivers are capable of moving a large amount of air, and as that air is forced through a port tube (or vent) it will become turbulent and begin chuffing audibly. In order to avoid this chuffing, you can increase the diameter of the port (more surface area), unfortunately, as the diameter increases, you need to increase the length of the tube to maintain the same tuning frequency. This increase in diameter and length can result in ports that can be extremely large, significantly increasing the overall size of the enclosure. If you use a passive radiator then you can get nearly the same results without needing to account for the port displacement. Here is a good example:
The popular RSS390HE-22 15" Reference Series High Excursion Subwoofer from Dayton Audio is capable of delivering incredible low-frequency output from a fairly compact 4.5 cubic foot vented enclosure…but let’s take a look at the actual enclosure once the vent is added to it.
With a 27 mm Xmax the RSS390HE-22 is easily capable of displacing over 4.3 liters of air with each stroke. In order to allow that extreme amount of air to travel through a port, without chuffing, you will need a port that is 9" in diameter x 70" long (or 2 ports at 6.5' D x 73')! That would require a minimum of 2.5 cubic feet for just the port. With that factored in, the overall enclosure will be more than 7 cubic feet.
You could achieve a nearly identical response using the Dayton Audio’s RSS460-PR 18" Reference Series Aluminum Cone Passive Radiator in a 4.5 cubic feet enclosure. Both systems will have an F3 of 22 Hz, with identical overall output capability. In this situation, even though the "enclosure volume" is the same, the passive radiator allows at least a 35% reduction in overall size.
So, passive radiators can reduce the overall size of large high output subwoofers, but this same concept applies to extremely small systems like portable Bluetooth speakers and sound bars. The compact drivers in small systems naturally aren’t capable of efficiently producing low-frequency output, but a driver’s excursion is well controlled near the tuning frequency of a system. This makes it possible to tune small systems lower than "optimum" and significantly boost output near the tuning frequency, essentially "forcing" the drivers to extend low-end capability. The same rules apply, meaning the drivers need to move as much air as possible to achieve significant low-frequency output. Also, these systems need to be tuned very low in order to control excursion enough to allow the power required to "force" the small drivers to play low frequencies. This can be achieved with a vented enclosure, however, the same physics still applies to small enclosures, meaning the high displacement and low tuning still results in excessively large ports. Thanks to passive radiators we have systems that are extremely small, yet can still produce satisfying low-frequency output. Something like our AudioBar sound bar kit would need to be almost twice the size if it weren’t for the use of passive radiators.
Like ported designs, passive radiator systems will unload below the tuning frequency and lose the ability to control a driver’s excursion. This means that these ultra-compact systems will require some sort of protection below the tuning frequency. This is achieved with a high-pass (or subsonic) filter, usually using an extremely steep slope. Because of the extreme amount of boost needed, followed almost immediately by a sharp cutoff, almost all these ultra-compact systems use some sort of DSP (digital signal processing) to manipulate the signal before amplification. Thanks to readily available compact amplifier boards with integrated DSP and a variety of passive radiator sizes we have available it is possible for anyone to build their own custom DIY Bluetooth speaker or sound bar that is capable of competing with most of the finished systems available on the market.
How to Choose a Passive Radiator
There is a general rule to thumb for choosing a passive radiator:
Start with a passive radiator that has twice the displacement capability of the driver you will be using.
This means that you want to make sure that your passive radiator (or radiators) can move twice as much air as your driver. So, if both your driver and passive radiator have the same surface area ("Sd" in the specifications, or the same diameter) then you want a passive radiator with twice the excursion ("Xmax" for the driver, "Xmech" for the passive). In many cases, there will not be a passive radiator that has twice the excursion capability of the driver, in that situation, you will need to consider a larger passive radiator or multiples. In that case, you can look at the volume displaced ("Vd" in the specifications, or Xmax x Sd), the passive radiator should have twice the displacement of the driver. Always verify that you are comparing the same specs, some manufacturers will use peak-peak numbers for the Xmax and Vd specification, and others will use one-way… to make comparisons on our website easier we try to always list one-way specifications (to the best of our knowledge).
Now, the above rule is just a guideline that will usually give good results. Passive radiators don’t always need to have twice the displacement of the driver. Using enclosure modeling software, you can design systems on your computer to create an incredibly accurate model of a passive radiator design. This will allow you to predict the actual excursion of the driver and passive radiator to determine if a certain unit is suitable for what you are trying to achieve. Software like BassBox Lite, BassBox 6 Pro, or SoundEasy will allow you to optimize your passive radiator designs and make sure that everything will remain within its limits.
Pros and Cons of Passive Radiator Designs
Pros:
- Improved low-frequency extension and efficiency over sealed enclosures
- Allows significantly smaller enclosures for extremely low-tuned designs
- Doesn’t create unwanted resonances (such as pipe resonance) for a more predictable overall response
- Due to the physical mass, a passive radiator system controls driver excursion below the tuning frequency better than a ported design
- Makes it easy to tune the smallest enclosures to low frequencies
- Maintains a sealed system, so they are ideal for designs that might be exposed to water and dust
- Sometimes the only option to tune an enclosure to desired frequency while keeping enclosure size manageable
- Can be used to create a visually impressive system with the look of multiple drivers
Cons:
- Price – a port tube is significantly less expensive than a passive radiator
- Generally, a steeper roll-off slope below the system’s F3
- Passive radiators have moving parts that could wear out over time
- Due to the area required for passive radiators, you will need to plan your design to ensure they will fit