I use earmuffs (I don't think it's necessary but I tend to be extra careful..) so that I don't hear the noise that the device emits, and I don't notice any occlusion effect. It is simply a spurt of water against the teeth/gum. I don't think it would be an issue if your earplugs seal the ear completely.
By the way, Michael, I was wondering: Do you use the "plaque seeker" tip or the regular tip?