As a cat person for some 40 years now, I can attest that the dog whistle (along with the reward of course) works well.
My clowder (yesteryear’s name for a group of cats) consists of indoor males. The DO however love to venture outdoors for exercise and a sense of freedom (mostly in a large fenced-in yard.. but one or two do venture beyond).
You can just imagine the experiences of “herding” the crew back indoors.. usually one at a time… and with tons of patience. You KNOW that cats respond to negativity with stark fear or panic, and not respect or obedience… the opposite of the desired effect. So patience is an absolute MUST to train your cat AND keep your sanity!
Enter a “trigger” (like with Pavlov’s dogs) and the reward. Tap on a food bowl to call the group for a meal (ringing the “dinner-bell” so-to-speak), and they’d all come-a-running.
Enter the dog whistle. Now instead of walking about the grounds with a bowl & spoon in hand, I toot the dog whistle… much more discreet and easy. Of course the transfer of “dinner bowl sound” trigger to the dog whistle had to be done but that’s another story.. Again in dealing with your domestic felines, the emphasis is on PATIENCE and not anger, gets positive results.