Hmm, that isn't bad sounding... I am pretty sure it is reaching it's peak.... it cuts off right after it peaks... but it is getting there. It is a little bit slower on the wind up but it isn't out of range for a normal STH-10... 99% of the STH-10's you see and hear have the 7 1/2 hp motor. The only real way to tell a 7 1/2 from a 10 is to look at the motor... the 10hp motor is a little bit bigger but it has a cap on it. Here are two pictures for comparision:
7.5HP
10HP
As a side note, Kyle made this video a really good way... he was a distance from the siren... siren's sound good when you are right on top of them... but when you are a 1/4 mile away.. you get the full ambience and sound. Good video Kyle!