How do I turn off startup sound?

Every computer I ever had, the first thing I do is to turn off the startup sound because it is very annoying in a public place and sometimes in a quiet place. I have not been able to find out how to trun it off on my Air. I asked the guys at the stores and they just say you can't. That is really dumb. I turned off my speaker but that is not a practical way because I may not remember to turn it off when I shut down.

The question is how do I turn it off permenantly.

Love to know.

Thanks.

Glenn