[mschuep]

A few rules of thumb...

If you double the power you gain 3 dB, which would be the same as adding an additional horn. Every time you double the distance you lose 6dB. An increase of 10 dB is percieved as something twice as loud. Likewise, a loss of 10dB sounds about 1/2 as loud.

Some ratings may be listed as sound power level, instead of sound pressure level at a distance...You can't do an apples to apples comparison without first changing the sound power level to sound pressure level.

JCWhitney has a large selection of horns.