They do use Sonics actually and its very straight forward.
Its very easy to take a sample of the sound an engine makes at peak revs and a sample of the engine noise when its on tick over in the pit lane. Say for arguments sake that the tick over pitch of the engine is a low C and at max revs the pitch hits a Hi F, the time it takes to ascend through the pitches relates directly to the weight of the fuel being carried as the labor on the drive trains differs under different fuel weights. Simples!
Itd be easier to track the acceleration with the timing/scoring system than with sound. Theres just better ways to do things than with something that has as many variables as sound.[/quote]
I would do it by using sonic and fouriertransform it to get the exact revs during acceleration. Then they need 2 exact times on that straight to make a good calculation.
If they knew the gear ratios they would not need any times at all. Just sound. Maybe they can calculate gears ratios by another mean and then they use sound to calculate weight.
The revs on TV during acceleration is done by sonic and it is extremely precise.
If they do it after quali they can use that info too to calibrate even more since they know they run minimal weight.