I once got into a conversation regarding lasers as an assassin's weapon in a science fiction game world. I remember arguing then that lasers aren't silent weapons. The reason is that the ionization of air causes a loud crack (think an electrical arc or a lightning bolt).
I was reminded of this when an almost-argument took place on a message board I frequent. Here's the post:
Oh dear. It appears that I must dispel someone of the illusion that weapons-grade lasers are silent. Trust me, they're not. Once a laser hits the multi-kilojoule range they are in fact very loud. I used to work as a signmaker and one of the tools we used was an etching laser. If used on a human being it would have caused nasty burns, but would not be lethal in of itself. A friend of mine worked for the Dept. of Main Roads and used an industrial laser (which after it interpretted fire for 2 seconds at 20% power while traversing 9cm left as rotate 90 degrees left and fire at 100% power, they refuse to operate it under Windows) that even at the lower settings was quite loud.
You see, when a laser of that intensity burns a path through air (or anything else for that matter), it flash vaporises that path and ionizes it, creating a loud *CRACK* that can be heard for quite a distance. The lasers that most people are familiar with are millijoule range, from television remotes to laser target designators.
Here's a corroborating statement by someone else:
Yup. I once witnessed a CO2 laser punch a hole through a two-inch-thick block of seasoned oak. It was only a split second firing of the laser, resulting in a very neat hole drilled right through the oak, along with a very loud, very sharp BANG!. Not the same "bang" as the firing of a standard powder-propelled ballistic weapon, but very audible and very distinctive all the same.