On Thu, 9 Feb 2012 04:49:04 -0800 (PST), NT wrote:

If the alarm's loud enough it'll be too painful for the thieves to
even want to go near it. Any half sensible burglar will pick a softer
target. High power piezo shriekers are available, but they require ...
I dont remember, but something like 2A at 24v IIRC, and I may well not
do.

Beat me to it.

"sound bomb alarm"

At random from 1st page:

http://www.tlc-direct.co.uk/Products/TSSB2.html

111dB @ 1m 12v 190mA. A small 12v SLA will keep that going for quite
a while...

It would need protecting by something sturdy but with holes to let
the sound out. Or perhaps fit several around the interior of the shed
as well as a proper exterior bell box/sounder.

--
Cheers
Dave.