The Spectrum

Humans are most familiar with audible sound, which is sound between 20 and 20,000 Hz. However, tremendously useful things can be done in other, higher frequencies. It has been estimated that the maximum possible frequency of sound in any medium is close to 1.25 x 1013 Hz. Obviously, that's way out of the ballpark for this discussion. Here are the frequency ranges that other animals use:

Species Low Frequency (Hz) High Frequency (Hz)
Humans 20 20,000
Cats 100 32,000
Dogs 40 46,000
Elephants 16 12,000
Bats 1,000 150,000
Grasshopper 100 50,000
Rodents 1,000 100,000
Whales and Dolphins 70 200,000

Here's a quick sampling of some applications that use ultrasound, and their associated frequencies:

Sonar - 50 KHz
Cleaning - 50 - 100 kHz
Ultrasonic Imaging - 300 kHz to 2 MHz
Flow Meters - 5 MHz
Material Thickness Measurements - 6MHz

For reasons that I will better explain further on, we will mainly use the frequency region between 50 kHz to about 500 kHz. If the frequency was below 50 kHz, dogs, cats, and other household pets might get infuriated by the noise. At the upper end, around 500 kHz, a very strongly emitted signal will be completely absorbed by air in just a few feet.