dog whistle
A whistle that produces a high-frequency sound audible to dogs but often inaudible to humans, used for training or calling dogs.
犬笛を吹くと、遠くにいた犬が走って戻ってきた。
When I blew the dog whistle, the dog that was far away came running back.
Compound of 犬 (inu, 'dog') and 笛 (fue, 'whistle').