Sound deadening

[HarryB]

I know, joke of a quest in an open car, but since I am doing a fair bit of highway commute couple of times per month, I have start wondering what I can do to reduce it a bit. Have done my fair bit of reading but there's a huge amount of people copying what others do (or Dynamating everything) thus unnecessarily increasing weight and cost. So what I will try to do is find what type of noise do I hear the most, and try to reduce that without adding too much weight (my top limit is less than 10kg).

Now what I feel is that a LOT of noise comes through the rear bulkhead/parcel shelf. If you think about it, it is an empty resonance chamber communicating with the cabin through two "tunnels" on both sides of the fuel tank, making it effectively a perfect bass chamber . I borrowed a decibel meter from work a while back, and my measurements confirmed that most of the noise indeed comes from back there.

Now the second part; what type/frequency of noise is that? I have done a fair bit of work analysing sound signals to identify cutting tools condition during material removal processes at work, so I wanted to borrow that (calibrated) equipment to play around a bit, but was not practical. Instead I resided to my smartphone and an app called "Sound Analyser" which might be less accurate but will do for my purposes. Did various experiments at various speeds, coasting with and without gear, and through FFT the frequency of 258Hz kept popping up, while most of the noise occurred at frequencies below 500Hz. The following image is from a test coasting at 100kph w/o gear.



Putting the car in gear did not affect much the readings so indeed most of the "annoying" noise does not come through the engine, rather from road excitation. Time to fix this...

There are a lot of ways to soundproof any structure; the most common one being mass loading. It is the classical example of having a large flat metal surface hit, producing that tinning sound, and then sticking a chunk of plasticine at the center; it now does not. Very effective, but should be used with care due to the additional mass. What has been found to work well at low frequencies is closed cell foam, which works by having multiple close voids that dissipate soundwave energy. A combination of the two sounds ideal for this occasion; mass loading the center of large panels while foam covering every other surface.

To be continued...