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Date: March 27, 2002 at 17:46:41
From: CDN, [cpe00045ad31b4a.cpe.net.cable.rogers.com]
Subject: Just to clear this up a little


As an aerospace engineer, I have some training in structural analysis, particularly in dealing with steels and aluminum. When an object begins to rattle, it is doing so because the sound waves are exciting the object at is natural frequency. Think of using a swing. The swing will not move much if you flail your legs rapidly or very slowly. Only if you match the swing's natural frequency will it begin to oscillate. The same principle applies here. If an aluminum panel and a steel panel have different natural frequencies, they will vibrate when exposed to different sound frequencies. I simulated vibrating a 1m by 1m aluminum plate as well as a 1m by 1m steel plate. I guessed the thickness so the actual numbers may not be applicable, but serve the purpose to compare the two metals. The aluminum plate vibrated with a fundamental natural frequency of 672Hz and the steel, 685Hz. Not a very big difference. This is because an object's natural frequency is proportional to sqrt(K/M) where K is derived from the object's stiffness and M, its mass. This shows that a material with a lower stiffness will vibrate at a lower natural frequency, and an object of lower mass will vibrate at a higher natural frequency. Since Aluminum is both less stiff and less massive than Steel, the two properties effectively cancel to result in very similar vibrational behavior.

Conclusion: Aluminum cars are not more susceptible to rattling than steel cars.

Hope that helps.


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