Whistles

My two "horrors" seem to respond really well to dog whistles. Does anyone have any advice on the best kind to get?

I was thinking of getting one for Lou Lou too, she get to focus on things sometime, so I was thinking that would catch her attention better. Did you get one yet? I am going to get one this weekend, don't have a clue what kind or anything yet though!

Ended up with a straight forward dog whistle where the noise level can be changed.

Have been using it to get them to come to me when I whistle and sit in front of me before I give them a treat - doing quite well so far once you get their attention!