It was almost comical. We bought this for travelling in the car with our dog, who has to bark at anything that moves. When she started barking at a string of motorcycles coming up beside us, I slipped it on. It was as if she didn't have it on at all, not even a change in noise volume. And yes, it had been fit for her before hand. I gave it a second star as maybe it's more effective for anything other than barking.