What does this knob on the horn do?

I've noticed that the horns in my car have little knobs on them that look like they're adjustable. These knobs thread in and out of the horn structure. They're visible in the photo of the horns below. Does anyone know what these knobs are for? Is it possible that they adjust the tone or volume of the horn? Have you ever adjusted one, and did it do anything when you did? Thanks!