compressed air horns

Question
Mr. Blanchet,

I'm afraid that this is a ridiculous question for someone with your credentials, but I'm also pretty sure that you'll be able to help me, and I'm not sure who else would be able to.

The question is about those compressed air emergency horns that boaters use.  I have been tasked by the conservation club I belong to to come up with a way to shut down the shooting ranges in an emergency.  The fastest way to do that at all five ranges would be a signal, such as three blasts from a horn.

The distance is not terribly far - all the ranges are within a half-mile distance.  If you are able, I would like a recommendation for an air horn (my head is spinning at the number of choices) that fit these criteria:

1)It should able to be heard from 1/2 mile away (my estimation is between 115-120 dB)

2)It will be outdoors, but on a covered porch inside a metal box. If possible, it should last indefinitely (this is an emergency tool, which will be used very little).

3) Is should work in temperatures from 30 - 110 F.

4) It should fit into a box 3 3/8 X 10 x 6 1/2 deep. If you have a horn in mind, but it won't fit these dimensions, let me know and we will get a bigger box.

Again, this is a ridiculous question for a man with your qualifications, but I have no knowledge of these horns, other than that boaters use them.  None of the websites I've gone to indicate dimensions, and horns range in price from $7 to $50.  

Any help you could give me would be greatly appreciated.

Answer
Hi CH...Thank you for choosing the undersigned to respond to your question.

I recommend you explore falconsignalhorns.com and edwards.ca, then consider the construction/mines models. Their specifications appear to be within your preferred scope, and freezing temperatures ought not to disable or minimize the pressure.

Regards,
Ron Blanchet