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.