Some horns have a small screw on the back of them that can be tweeked to "tune " the horn , but if yours goes ok when wired to the battery ckeck that it is earthed properly and also check the voltage to the horn , if there is not enough volts you are better off finding the problem and sorting it than just chucking a relay at it , which may well get the horn going but you are only hiding the problem which may well come back when the relay starts to not work .Having said that I have a relay on mine and there is a BF set of air horns ready to be bolted on ( if I face them directly forwards they will add to the braking in an emergency)....... I once ( when young and impressionable ) had an electric compressor ( 120 psi ) , a pressure tank ( old Freon cylinder ) , electric air valve and 4 sets ( 12 horns ) of horns on my car , scared the shit outa heaps of people but then the cops made me dismantle them on the side of the road...... good times
