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Drone on Mr. Helmholtz | Fabspeed Motorsport


(A short 4 minute read)

By: David Van Allen, COO

Fabspeed Motorsport

July 2019

If you ever rode in a car with a window partially opened and heard that horrible rumbling head-vibrating awful sound, you’ve experienced what is commonly called “drone”.

Drone is a phenomenon that occurs when audible sound resonates or bounces around within a closed area. The resonating frequencies bind together and create an annoying sound and an odd physical sensation inside the ear. Drone, caused by resonance, is often perceived as an unnatural sound and it can be quite overwhelming.

To illustrate a very visual example of the power of sound resonance, watch this video of a wine glass shattering simply by aiming the sound of a specific frequency, known to resonate with glass, at it. Notice that the specific sound – all by itself – is able to warp the shape of the glass and eventually cause its destruction, due to fatigue.

It's important to understand that it is not just the loudness or intensity of the sound that caused the glass to break, the frequency of the sound wave is what started the oscillations. If a different frequency was used, either higher or lower, the glass would not resonate and it would not shatter.

Every type of material: be it glass, metal, plastic or eardrum tissue, possesses a set of characteristics that if excited by a certain frequency will cause some type of resonance to occur, and that resonating effect is likely unpleasant.

In the closed cabin of a car, we create a sound room. If we alter that sound room by opening a window; putting the top down; or even adding people or luggage to the area, we can change the resonating properties. Now you understand why a partially opened window may drone, yet a fully opened one may not.

You may have read or heard something like, “That exhaust system really droned.” Technically, the exhaust system didn’t “drone” at all, rather certain frequencies that emitted from that exhaust system caused that cabin area in that particular type of car to drone. Open the windows, close a single window, put the top down, or add people inside and you’ll cause the drone to lower in volume, disappear, or maybe even increase. Cabin drone in a car always vanishes as you change RPM and is usually only present around one speed or in a narrow RPM range. Nevertheless, drone is bad and nobody likes it.

At Fabspeed Motorsport USA we fully understand acoustics and sports car sound. Through proper design and calculated engineering, we can remove the resonating frequencies from exhaust systems that could cause cabin drone in certain models of cars. We’re able to do this thanks to the genius mind of a late 19th-century era German physicist, Hermann von Helmholtz.

Helmholtz imagined a way to make a simple resonator device that could determine pure tones, but his invention could also be used to minimize unwanted tones such as the lower frequency tones that could cause cabin drone in automobiles.

Using Helmholtz’s theory, Fabspeed engineers map the frequency range of our exhaust systems using spectrum analysis, which is a fancy term for measuring each sound range emitted from the tailpipes. During testing, if we hear droning in the cabin of the test vehicle, we use magic stuff (I can’t tell you everything!) to determine what excitation frequencies are causing the drone to occur.

Using the information we get from our testing and then applying the math that Helmholtz defined, our engineers determine the exact design characteristics needed to create a custom ‘anti-resonator’ specifically for that unique model of car.

An anti-resonator takes away the frequencies that cause the cabin to drone without impacting the performance of the system in any way — only if they are designed properly. You may see an anti-resonator look like a small hooked tube or a bulb shape appendage just hanging out like it has no purpose. Sometimes they’re referred to as “Helmholtz Resonators” which is cool because it celebrates the man whose theory we are applying, however, the reference is actually inaccurate. It would be better to say “Helmholtz Anti-Resonator”.

The good news is that drone is relatively rare and the vast majority of the supercar models do not need anti-resonators — but when they’re needed, they’re really needed.

By adding a properly designed Helmholtz theory anti-resonator to our performance exhaust system for cars that will likely drone, Fabspeed totally eliminates the problem — scientifically, accurately and completely, so all you're left with is that sweet Fabspeed sound.