Ultrasonic stack components (horns and boosters) are designed to be tuned to a specific frequency. There is an allowable range at each frequency.
Here are the tuning ranges:
1) 15 kHz – 14,965Hz to 15,035 Hz
2) 20 kHz – 19,959 Hz to 20,050 Hz
3) 30 kHz – 29,950 Hz to 30,050 Hz
4) 40 kHz – 9,900 Hz to 40,100.
Horns and boosters are tuned to frequency not length.