This whistle is LOUD - so loud that I didn't find it very usable even in a decent-sized park in the city.
I think there could be a bit more variability in the tones, and it is nearly impossible to tell which side has the ball and which doesn't - not ideal when you want to act fast. The shape of one side should be different or there should be some other way to quickly tell them apart.
NOTE: The point of this whistle is NOT to stop barking - it is to signal your dog. If you want to stop barking, it should be a part of more comprehensive training. Look up books by Karen Pryor to get started.