Passive Radiators Explained: The Acoustic Trick Behind Big Bass in Small Speakers

Look at almost any modern portable Bluetooth speaker or compact soundbar. You’ll see the main speakers, the ones that produce the vocals and guitars. But often, you’ll also spot another speaker-like cone, maybe on the end or the back, that just seems to vibrate silently. It has no magnet, no wires leading to it. It’s an imposter. A drone.

And yet, this unpowered, “dummy” cone is responsible for one of the most impressive feats in modern audio: producing deep, satisfying bass from a box that physics says has no right to do so. This is a passive radiator, and it’s less magic and more a supremely clever exploitation of physics.

To understand its genius, we first have to understand the fundamental problem of bass.

The Physics Problem: Why Small Speakers Struggle with Low Notes

Creating sound is about moving air. To create high-frequency sounds (treble), a speaker cone just needs to vibrate back and forth very quickly. But to create low-frequency sounds (bass), it needs to move a large volume of air. This means either a very large speaker cone, or a smaller cone that can move a very long distance (this is called “excursion”).

When you put a speaker (an “active driver”) in a small, sealed box, you create another problem. As the cone moves backward into the box, it compresses the air inside; as it moves forward, it creates a vacuum. This trapped air acts like a stiff spring, resisting the cone’s movement and limiting its ability to make those long, slow movements needed for deep bass.

For decades, speaker designers had two main options:
1. Use a huge box: This gives the speaker plenty of air to “breathe,” but defeats the purpose of being portable.
2. Use a ported box: This involves cutting a hole, or “port,” in the box.

This second option is where things get interesting. The port isn’t just a random hole; it’s a carefully tuned tube that turns the entire speaker box into a resonator, much like how you create a low note by blowing across the top of a bottle. This bass reflex design uses the energy from the back of the speaker cone—which would otherwise be wasted—to push air through the port, reinforcing the bass. It’s a great trick, but it comes with downsides: port turbulence (wind noise), the potential for dust and water to get inside, and the fact that a port needs to be quite long to tune for very low frequencies, making it difficult to implement in a small enclosure.

So, what if you could get the benefits of a port, but without the hole?

Enter the Passive Radiator: An “Acoustic Lever”

A passive radiator is the elegant answer. Imagine you take that ported box, and instead of a hole, you cover it with a new, unpowered speaker cone—our “drone.” This drone, the passive radiator, consists of just the cone and its flexible surround, with a specific weight added to it.

Here’s how the dance works:
1. The main, powered speaker (the active driver) moves backward into the box, compressing the air inside.
2. This pressurized air, our “air spring,” pushes outward on the only other thing that can move: the passive radiator.
3. The active driver then moves forward, creating a vacuum that pulls the passive radiator inward.

The passive radiator is now moving in sync with the trapped air, moved by the energy from the back of the active driver. By carefully “tuning” the passive radiator (by adjusting its weight and compliance), engineers can set it to resonate strongly at a very specific low frequency. At this frequency, it takes over from the main speaker, moving a large amount of air and producing the deep bass that the small active driver couldn’t manage on its own.

It’s a closed, sealed system that acts like a ported box, giving you that powerful low-end boost without the drawbacks of an open hole.

Case in Point: The “Racetrack” Advantage

Now let’s look at a modern implementation, like the JBL RallyBar XL. Its spec sheet doesn’t just list passive radiators; it specifies four 7.2” x 3” racetrack passive radiators. Why that shape?

It comes down to surface area. To move a lot of air for bass, you need a lot of surface area. In a long, thin enclosure like a soundbar, you might not have the height for a large, 8-inch circular radiator. But an elongated “racetrack” or oval shape allows you to fit a much larger surface area into that same slim profile. A single 7.2” x 3” racetrack radiator has a surface area roughly equivalent to a 5-inch circular cone. By using four of them, JBL is able to create a massive amount of low-frequency radiating surface in a very constrained space.

This design is a key reason why a relatively slim soundbar can achieve a claimed frequency response down to 45Hz, a level of bass extension you’d typically expect from a much larger system or one with a dedicated subwoofer.

Passive Radiator vs. Port: The Verdict

So which is better? It depends on the application.

• Ports are simple, cheap, and very effective in large speakers where there’s plenty of room for a properly designed tube.
• Passive Radiators shine in compact devices. They allow for deep bass tuning in small enclosures where a port would be too long or impractical. They also offer a sealed design, which is a huge advantage for outdoor and portable products, preventing ingress of dust, dirt, and water. And they completely eliminate the “chuffing” or wind noise that can sometimes be heard from a port working hard.

For a rugged, all-weather soundbar designed to be blasted with mud and water, the choice is obvious. The sealed system enabled by passive radiators is not just an acoustic choice; it’s a durability choice.

The next time you pick up a small speaker and are shocked by the depth and quality of its bass, look for the silent partner—that vibrating, unpowered cone. It’s not a gimmick; it’s a passive radiator, the unsung hero of compact audio, turning trapped air into thunder.