Our hybrid audible/silent devices play both high frequency (silent) sounds with the option to use audible sonic sounds.
Using both high frequency an audible sounds together are works powerfully to confuse and frighten pest birds and some animals away.
Audible sounds can range from bird distress and predator calls to alarm calls depending on the unit.
The great thing is the bird distress calls and predator calls just sound like regular bird chirping sounds to us humans. You also have full control over the volume.