The ultrasonic speaker runs at a slower frequency than the generator input. The input frequency is divided by 16 for high speed operation and 256 for low speed operation. At high speed setting an input of 3000 Hz will make the ultrasonic speaker run at 187.5 Hz. You will hardly hear it. To make the ultrasonic speaker run at 3000 Hz set the generator frequency to 3000 x 16 = 48000 Hz.
Source: Spooky Central Q&A