Sound is the propagation of smallest pressure and density variations in an elastic medium (gas, liquid, solid-state body). For example, a noise is generated when the air in a specific spot is compressed more than in the surrounding area. Subsequently, the layer with changed pressure propagates remarkably fast in all directions at speed of sound of 343 m/s.
Acoustic frequencies between 16 kHz and 1 GHz are referred to as ultrasound; in industrial settings we call it “ultrasonics”. To clarify: people are able to hear frequencies between 16 Hz and 20 kHz; i.e. the lower frequencies of industrial ultrasonics are audible, especially if secondary frequencies are generated. And what is more, ultrasonics is palpable when touching the weld tool. For ultrasonic welding, the frequency range is between 20 kHz and 70 kHz.
Additional fields of application: Imaging ultrasound in the field of medical diagnostics ranges between 1 and 40 MHz. It is not audible or palpable. In the field of industrial material testing, ultrasonics is used at frequencies from 0.25 to 10 MHz.