Architectural Helmholtz Resonators

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Tapiola Lutheran Church in Espoo, Finland, Opened in 1965

Helmholtz-type resonators are built into the walls of the church.  The slots between brick pairs and cavities behind them act as both absorbers and diffusors of sound.  The absorption reduces the reverberation time as desired for speech intelligibility during sermons.  Some reverberation is desirable, however, to enhance organ music.

A Helmholtz resonator is a volume of air which is enclosed in a container with at least one opening. It is also called a cavity resonator.  The air in the container’s neck acts as a mass. The air in the volume acts as a spring. The Helmholtz resonator thus behaves as a mechanical spring-mass system.

Bankstown-court-house-Decor-Trend-ceiling-by-Decor-SystemsCourtroom in Bankstown, NSW, Australia, with slotted panels for reverberation reduction.  https://decorsystems.com.au/

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Slotted block absorbers in a gymnasium.

BT240 5

The BT240 slotted modular bass trap panel provides excellent absorption down to 65 Hz.  https://soundacoustics.com.au/

– Tom Irvine

Nissan Versa Side Window Buffeting

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versa

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I recently drove a Nissan Versa rental car similar to the one shown above.

The Versa made a strong throbbing sound when I drove down a highway at 65 mph with the front side windows closed and the rear windows open.

Here is the sound recording that I made with my Samsung Android: Versa Sound

Please listen to the sound file using a headset with good low frequency response to hear the full effect.

The Fourier transform is shown above with its fundamental frequency at 21.2 Hz and integer multiples thereof.

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Dr. Stephen Granade explains:

That “whum whum WHUM WHUM” noise happens because the wind passing over the small window opening… forms tiny tornadoes as it moves past the front edge of that opening. When those tornadoes, or vortices, reach the opening’s back edge, they make a wave of pressure that pushes air into and out of the car.

Since sound is nothing more than waves of pressure, this makes noise… The vortices keep pressing on the air in your car just at the right time to make big pressure waves that we can feel and hear.

The technical term for this effect is the Helmholtz resonance, though car people call it “side window buffeting”…

…As you drive faster, the rate at which the whums occur speed up and the loudness goes up.

Interestingly enough, Granade goes on to theorize that “It’s more noticeable in modern cars because they’re more aerodynamic,” the thinking being that cracking a window is more disruptive to a smoothly-tuned airflow. If that’s true, it would mean cars with boxy shapes would suffer less from Hemholtz resonance.

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A more technical explanation is:

Buffeting (also known as wind throb) is an unpleasant, high-amplitude, low-frequency booming caused by flow-excited Helmholtz resonance of the interior cabin. Vortex shedding in the shear layer over the cavity opening (side window) couples with the cabin acoustics, leading to a self-sustained oscillation of shear layer and cabin pressure.

Reference

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– Tom Irvine