Weights on exhaust pipes

I've seen some later model cars with what look like weights hanging off the exhaust pipes. Any ideas as to what they are for? Vibration? 

Yes, it's a damper to cancel out resonance

Nvh 

There must be science behind where in a system these would be placed??? I'm surprised with all the discussions of resonance and drone on this and other sites that this isn't proposed as a solution. 

The exhaust gas driven and the exhaust mechanical resonances happen in completely different frequency ranges. They may amplify each other but they do not generally occupy the same frequency ranges. The exhaust gas drone happens at something like 80 hertz (if I recall correctly). The mechanical vibrations are at significantly higher frequencies, but if these become resonant they can damage the mechanical components and cause exhaust leaks. Adding mass to the mechanical stops/moves the resonant ranges on the pipes away from the critical range.

In reply to stafford1500 : But the end result, resonance or drone, is emitted by the vibrating pipe/muffler metal, correct? 
Would thicker wall tubing/muffler body help? Either with containing the exhaust driven resonance or withstanding the mechanical resonance? 
Have I got that question right?

It's funny I learned about these exhaust dampers for the first time a few weeks ago. It can out of a discussion about how FiSTs have mass dampers on the front struts. Dampers for the dampers. They're a pound each. I removed mine and could not discern a difference.

Note that the dampers are usually attached with a rubber mounting.  They're more akin to harmonic dampers than a simple mass addition.  Heck, they spend the money for dual mass flywheels to get a harmonic damper at both ends of the crank, instead of just using a much cheaper heavy flywheel.

What gets me are the damper masses on STRUTS.  And, sometimes, there are two different ones per strut.

The thinner the metal used the lighter the component, the lighter the vehicle is as a whole and the more likely efficiency will increase. Also thinner metal is often less expensive to buy, form and produce.