How a Helmholtz Resonator Improves Exhaust Note (And Gets Rid of Pesky Drone)

How a Helmholtz Resonator Improves Exhaust Note (And Gets Rid of Pesky Drone)

Last Updated March 20, 2024 | Meghan Drummond

Some high-end exhaust systems feature a Helmholtz resonator. They often replace the muffler and look like a sealed canister or tube welded onto your exhaust.

This tube may not look like much, but it’s able to modify your exhaust note and eliminate drone. This sounds almost too good to be true, but the principle it works off of is very simple. It’s actually something you experience in your daily life.

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What Is Helmholtz Resonance?

If you don’t care how your exhaust works (as long as it does), you can just skip this section. Trust us, the principles are sound. But, if you like to know exactly how your car works right down to the nuts and bolts, this is all pretty interesting.

Helmholtz resonance, also called “wind throb,” is what happens when there’s air resonance in a cavity. A good example of this is when you roll down a single window of your car. The inside of your car is full of compressed air, while the air blowing past outside is at a different pressure. The result is an unpleasant throbbing of sound that kind of hurts your ears.

Gif shows how air circulates around an open car window

But the same phenomenon is what makes an ocarina a pretty cool instrument. It’s also why there’s an air cavity on stringed instruments (like a violin or guitar).

Any container with a cavity for air collection and an open hole can act as a Helmholtz resonator. One simple example is a jug. If you blow across the top of a jug, it acts like a resonator. Large jugs create a low pitch, smaller jugs create a higher pitch. How fast you blow across the opening determines how loud the sound is.

How a Helmholtz Resonator Works in an Exhaust

As air moves past the resonator, air is pulled in and pushed out, changing the pressure and thus the sound. This changes the exhaust note. While the remaining exhaust note is richened and deepened, undesirable noises (like exhaust drone) get canceled out.

Gif shows how exhaust flows through helmholtz resonator

How Helmholtz Resonators are Tuned

To improve the tone of a Helmholtz resonator, you can change the diameter or length of the small section of pipe connecting it. Or, you can change the volume of the canister.

Exhaust System with helmholtz resonator

One of the reasons people prefer Helmholtz resonators to other exhaust modifications, like J-pipes, is that they’re easier to make compact.

Professional fabricators and modification shops will often design custom Helmholtz resonators. They can do this by analyzing the frequency where they experience an exhaust drone. Then they can make a Helmholtz resonator to directly address the drone at that frequency.

That’s why, perhaps more than with other exhaust modifications, you really want a Helmholtz that’s designed for your vehicle.

Helmholtz Sound

In this video, Bill Tumas installs an MBRP race cat-back with a Helmholtz resonator onto a Mustang GT. The exhaust was made specifically for this body, this engine, and the RPM range that most drivers will hit.

As you can hear in the video, the exhaust starts with a good deep idle. Then, it starts to get a high-pitched sound. But when the engine gets to 1,800 RPM at a 65 mph cruise, the exhaust goes back to its deep, signature tone. Above 2,000 RPM, the issues disappear.

The Helmholtz in this instance does exactly what it’s supposed to. It preserves the loud, crackling sound of the race-style system. But it also works to reduce drone and create a good powerful sound once you get into higher RPM ranges.

Other Ways of Eliminating Drone

When it comes to getting rid of drone, Helmholtz resonators are a great solution. But they’re usually reserved for premium exhaust systems. Given the effort that goes into tuning a Helmholtz, that makes sense.

But they’re not the only way to eliminate exhaust drone! Check out our article on mufflers and resonators for more ways to craft an exhaust note you’ll love.

Sources: Helmholtz Resonance, The University of New South Wales | Why Do Slightly Opened Windows Make That Terrible Sound?, Jalopnik

This article was researched, written, edited, and reviewed following the steps outlined in our editorial process. Learn more about CJ's editorial standards and guidelines.