2. Applications

2.1 Active noise-canceling headphones

Conventional noise-canceling headphones, which are effective at medium and high frequencies, do little to protect the wearer from low-frequency noise, as the thickness of the shells becomes too small for the wavelength. To improve their effectiveness at low frequencies, it was thought early on to use active control, since a single muted point in the helmet shell is enough to reduce noise in the ear canal. The first attempts to develop active noise-cancelling headphones date back to the 1950s, with work by Olson, Simshauser and Hawley