I use the air horns sometimes daily in police vehicles, cars, boats and pickups. They are electric and loud. Unknown what the cost maybe. If it where me that is what I would install, than no need for a compressor. If you like your hearing mount it near the front bumper/grill area. If it is on the roof is as painful for you as the person you use it on. Trust me the horns and sirens where up there for several years during my career. After one use you found out why the old troops rarely used them.
2013 24RKSWE (27ft TT) Cougar 1/2 ton series SOLD 10-2021
2013 Ford F350 4x4 CC 6.7 engine, 8 ft bed, 3.55 rear end, lariat package
Retired from Oregon State Police in 2011 than worked another 9.5 years as a small town traffic cop:
As of 05-2020, I am all done with 39 years total police work. No more uniforms for me.