Just for giggles, I just ran it at 6VDC, 8VDC, and 15VDC using the same setup.
At 6VDC, it was 93dB, drew 7.2A, and had a significantly lower tone.
At 8VDC, the current draw and volume were the same as the 13.5VDC values, though the tone varied slightly.
At 15VDC it started to distort (much higher tone), went up to 15A, and hit 103dB.
I would say the linear range on the duty cycle is 8VDC-13VDC and it is safe to operate anywhere in that range as long as you wanted to.
Above that and you would probably want to widen the points gap to prevent clipping. That might or might not bring the average current down. I suspect it would.
