ANR does very little to affect the relatively higher frequencies of, for example, voices. You can also tell when a sound changes. So, if the rhythmic pulse of the propeller is altered by the addition or loss of power, your ears will tell you about it. Passive noise control is much more simple. It works by muffling noise, as when you put your hands over your ears.
Passive headsets use thick earpads, often of gel or foam, to create a seal around each ear.
Clamping action keeps the seal tight against your head, but sunglasses and even hair can get between the ear seals and the side of your face, significantly reducing the effectiveness of passive noise reduction. Since the primary reason for wearing a headset is to protect your hearing, you need to know how well any headset reduces the amount of noise reaching your ears. The Environmental Protection Agency requires headset manufacturers to test their products and determine by how much they reduce noise. Active noise reduction headsets may give two figures one for the noise reduction when the electronics are turned on and one for when they are not. After protecting your hearing, the greatest value of a headset is in allowing you to communicate more clearly and easily both inside the cockpit, through the use of an intercom, and outside the cockpit when talking to air traffic control, flight service, or other aircraft.