The higher the PSI, the louder your horn will be; 200 PSI being the max. PSI Stands for Pound Per Square Inch).
The bigger the air tank, the longer your horn will sound; often times, also longer it will take to fill the air tank again to re-sound.
The lower the AMP draw, the better the compressor; max should be 30
Pay attention to the gauge of your air line. It DOES effect the volume and quality as well.
You want typically at least a 3 Gallon talk to run multiple horns.
A manual air valve, vs electric, will help you adjust the sound