Madeline Darnell, a visitor from Athens (GA, USA), asks :-
When the windows in the car are down and I'm driving at a higher speed (say 60 kmph or more) a pulsating sound is created that really hurts my ears. When I put the windows up or when I slow way down, the sound goes away. Why is that?
Colin Fox, a physicist at the University of Otago who earlier specialised in the acoustics of buildings, responded.
This effect is the same as when you blow over the top of a bottle and make a tonal sound. The air inside the car taking the place of the air in the bottle, and the air rushing past the partly open window providing the excitation. Only now your head is inside the bottle!
Just as with blowing across the bottle, the speed needs to be sufficient to get the effect, which is why you only experience the effect at higher driving speeds. Similarly, the effect goes away when you close the window. The air in the car is acting as a Helmholtz resonator, named after the scientist who used the effect to identify individual tones in music.
The main physical process is the oscillation caused by a mass and spring, like the bobbing up and down of a jumper at the end of a bungy. The air provides the mass, and also the spring though compression of the air. It is the oscillating compression of the air that affects your ears.
As with most physical systems, the larger the system the lower the frequency of oscillation. The volume of the interior of a car is large enough to make the resonance below an audible frequency, creating the painful pulsation that you mentioned.
I have noticed that this effect is more prevalent in modern cars that are well sealed, and with aerodynamic design that causes the outside airflow to hug the car. That gives some clues that the effect can be stopped by opening other windows or by deflecting the outside airflow in front of the window to break up the laminar flow. I have a van with flaps that raise at the front of the moon-roof opening, for just that purpose.
I should also note that Helmholtz resonance is used to improve the quality of sound around you. For example, the pan pipes are made up of tuned Helmholtz resonators, and the effect is also the basis of panel absorbers that acoustic engineers use to efficiently damp the annoying tonal noise radiated from machinery.
Send questions to: Ask-A-Scientist, PO Box 31-035, Christchurch 8444 Or email: email@example.com