APPENDIX 6: THE CAR AS HELMHOLTZ RESONATOR
Here are some results from measurements I made of HR in a friend’s car. First we got the HR going at around 80kph by winding down a rear window. I recorded the resonance, which we could feel as an unpleasant buffeting in our ears – the low frequency pressure variations driven by the passing turbulent airstream outside the window.
I then waited by the side of the road as my friend drove past a couple of times with the resonance going, and again recorded the sound of the car passing.
Analysing the inside signal gave a frequency of 15.5Hz. Analysing the signal from the side of the road showed a signal of the same frequency, even though it wasn’t audible to me at the time.
Helmholtz resonance from inside and outside a car
The Helmholtz signal shows as a spike at 15.5Hz in both lines. From outside the car the spike is about 15 to 20dB (up to 100 times) stronger than the background noise of the car engine and tyres.
I was surprised that the HR was detectable at a considerable distance from outside the car, and for a while I expected to find the same for a guitar soundhole. Of course, if you’ve read my report you’ll know I found the opposite.
[I surmise that for the car, the intensity of the HR driving impulse – the turbulent airflow blowing forcefully over the open window – is proportionally much greater than for a guitar soundhole. Whether this is true or not I don’t know.