Helmholtz Resonance might be driving you nuts

Silicon Valley Highway 101 Traffic Hell Original Photo by Richard Masoner Flickr (CC BY-SA 2.0).

Have you ever been in a car travelling along the highway with one of the windows rolled down and you’ve noticed a low frequency throbbing that makes your eardrums feel like they’re throbbing along with it? This bass-like phenomenon is not life-threatening (albeit annoying) however you have probably also noticed that by slightly opening another of your car windows the throbbing stops and everyone in the car is happy again.

Well as it turns out there are a few different terms for this effect. Mechanics will often refer to the sound as ‘car window buffeting’ while acoustic enthusiasts might call it Helmholtz resonance.

A Helmholtz resonator is a container of gas (usually air) that has a single opening to allow entry or exit of the gas. If the air in and around the neck starts vibrating it can create small vortex pockets which interact with the opening and cause the air inside to compress and decompress rapidly in a spring-like fashion.

Helmholtz Resonator Original Photo by brian0918 Wikipedia (CC BY-SA 2.5).

You can test this yourself with any open glass or plastic bottle. I’m sure most of you have tried this for yourselves already but by blowing across the top of the bottle you can create a Helmholtz resonator which results in an audible frequency. The same thing is happening as you’re hooning down the Hume Highway with one window down.

A moving car will also create vortices moving past the open window when at high speeds. The frequency of these vortices is determined by the speed of the car, the size of the car and the car shape.

Why doesn’t the glass bottle create a “wubwubwubwub” sound but instead creates a steady hum?

In truth the bottle does create a “wubwubwubwub” sound but the frequency is too high for us to notice. If you record yourself playing the bottle and slow it down you’ll notice the oscillation.

So why does it seem so much louder in a car than coming out of a bottle?

Well that’s because the amplification is dependent on the frequency of the vortices and how closely they match the frequency produced by the Helmholtz resonator. You should notice that when blowing across a bottle, the loudness changes depending on how hard you’re blowing. The louder the noise produced, the closer the frequencies are. The same thing happens in a car.

The frequency caused by Helmholtz resonance depends on the volume of gas within your container. The bigger the volume of air, the lower the frequency produced. You can also test this with a full, half-full and empty bottle which will all produce different frequencies given the changes in volume (How many beers did I have to drink to come up with that idea?).

Bottles on Blue Original Photo by Chris_J Flickr (CC BY-NC-ND 2.0).

Because of the far greater volume of air in a car compared to a standard glass bottle the frequency produced by the Helmholtz resonance is far lower and we get the “wubwubwubwub” effect. There’s a handy equation for determining the frequency produced by taking into account the Volume of gas, size of the opening, length of the opening and a few other variables if any of you guys are interested.

And as for why a Helmholtz resonator stops working as soon as you allow another outlet for the gas, when the volume of gas initially begins to compress it can easily escape out this new outlet preventing any oscillation. Don’t try to poke a hole in the bottom of a beer bottle though as it may get messy.