How Do Small Speakers Make Big Bass? The Magic of Passive Radiators

The “Magic” Trick: How Do Small Speakers Get Such Big Bass?

It’s a feeling we’ve all had. You’re at a friend’s house, and they’re playing music from a small, stylish portable speaker. The bass drops, and it’s so deep and powerful you can feel it in your chest.

You look at the small box and think, “How is that possible?”

For decades, the first rule of physics for audio was simple: if you want deep bass, you need to move a lot of air. And to move a lot of air, you need a big speaker cone (a woofer) in a big box.

So, how do modern speakers like the Bang & Olufsen Beolit 20, which is the size of a lunchbox, produce bass that claims to go down to 37 Hz (which is really deep)?

They use a clever engineering trick called a Passive Bass Radiator.

The Trampoline Analogy

Imagine two people on a trampoline.
1. The “Active Driver”: This is your main speaker. He’s the one actually jumping. He’s powered by electricity (the amplifier).
2. The “Passive Radiator”: This is his friend, who is just standing still on the other side of the trampoline.

When the “Active Driver” (your speaker) jumps down (moves inward), he pushes air down and stretches the trampoline. This instantly launches his “Passive” friend up into the air.

The passive radiator is the friend who gets bounced. It’s an unpowered, “dummy” speaker cone. It has no magnet, no wires. It just sits there, ready to be moved.

How It Actually Works in a Speaker

In a small, sealed speaker box, your “Active Driver” (the main speaker) moves back and forth to create sound waves. * When it moves outward, it creates sound you hear. * When it moves inward (back into the box), it creates a buildup of air pressure inside the box.

In a cheap speaker, this internal air pressure is just wasted energy. It fights against the driver and creates distortion.

But in a high-end design, engineers install a Passive Radiator. That internal air pressure (from the active driver moving in) pushes the passive radiator out.

It recycles the wasted energy.

This recycled energy creates more sound waves, specifically, very low-frequency bass waves. It’s like getting a second bass driver for free, without needing any more electricity or space for a magnet.

Why This Matters

This is the secret to almost all premium small speakers. When you look at the specs for the Beolit 20, it lists “four active drivers and two passive bass radiators.” Those two “passive” guys are the reason it can produce chest-thumping bass from such a small enclosure.

Now, a passive radiator isn’t a magic bullet. It needs to be paired with a very powerful and well-controlled “active driver” and a perfectly tuned, rigid box. You can’t just slap one on a cheap speaker and expect miracles.

But when you see “Passive Radiator” listed on a spec sheet, you now know you’re looking at a clever piece of acoustic engineering. It’s the “magic” that finally beat the old rule of physics, allowing a lunchbox-sized speaker to sound as big as a sound system.