It also changes the directivity pattern of the mic. Since rear sounds now have to travel to the side of the mic, it changes the pick-up of rear sounds. And as the front sound is reflected on the plate, it hits the rear of the mic out-of-phase, which creates some attenuation. Depending on the type of mic that it is used in conjunction with, it gives different results. It takes some experimentation to really master the use of these contraptions.