only way to get really loud n low like a train or a real semi is gonna take a compressor, air tank and big horns
It takes a lot of air volume to make that much noise which means a compressed air tank, no way a little electric motor hooked to a plastic trumpet can do it.
I have real air horns on my JK but it was a PITA to find room for everything under the hood.
You're probably gonna be better off doing a real on board air system and running the horns off of it. Plus then you can air your tires up and down.
horns themselves are in the back right corner, the big round black thing next to them is a 1.5 gallon pancake air tank, the compressor is peeking around the side of the AiRAID shield. It keeps the tank at 125PSI
With the little pancake tank I can get about a 10 second blast before I'm out of air and the compressor has to refill the tank.
This is an old pic since then I've gone to an AEV snorkel so the factory airbox is back in and I had to make a compressor mount out of steel that stretches across from the air box to the drivers side fender and hangs the compressor between the radiator and the motor.
These sound more like a semi - train horns are HUGE and there's just not a good place to mount them on a JK where they won't get destroyed.
I left my stock horn hooked up to the steering wheel and only use the big guns for special idiots.
So yeah, REALLY loud, Low air horns is neither easy nor cheap.
The little ones with the electric motor compressor run to a couple of little red plastic trumpets REALLY sound like the roadrunner!
The other option is to get a pair of freeway blaster horns, high and low frequency and use them together. That's what Cadillac used to do and it's pretty loud and low, plus it's cheap and takes up very little space since they are "conventional" style horns.
I've seen them on amazon for about $17 each so you'd probably have $40 in that setup.
Or goto a junkyard and rob the horns off of a early 90's Fleetwood for even cheaper
That one is easy to test drive, just find someone in a parking lot with a early 90's Caddy and ask them to honk for ya - might get you a funny look, but you'll know what it sounds like