Subsonic speed for horn

[Anonymous]

I have 4 independent loops on a modern zw with 2, 180 watt power houses. I am operating in conventional mode, and have added 2 Lionel sound activation buttons for the 2 loops that are not controlled by the big handles. The horns will not operate below 16 volts. The trains are on the verge of flight at this voltage. The track is clean, and the trains run fine at lower voltage. Any ideas on how to make the horns operate at a lesser voltage?