Active noise cancellation (ANC), also known as noise cancellation (NC) or active noise reduction (ANR), is a method for reducing unwanted sound by the addition of a second sound specifically designed to cancel the first. ANC technology is used in headphones and earbuds to reduce unwanted background noise. The system is based on microphones that "listen" to the sounds outside and inside of the earphone, an ANC chipset inverting the soundwaves, and a speaker inside the earphone cancelling the outside sound by the neutralizing soundwaves. The process of active noise cancellation involves recording the background noise, inverting the noise signal to create "anti-noise," and then adding it to the original sound. The noise-cancellation speaker emits a sound wave with the same amplitude but with inverted phase relative to the original sound, and the waves combine to form a new wave, in a process called interference, and effectively cancel each other out – an effect which is called destructive interference. Modern active noise control is generally achieved through the use of analog circuits or digital signal processing. Adaptive algorithms are designed to analyze the waveform of the background noise, then based on the specific algorithm generate a signal that will either phase shift or invert the polarity of the original signal. Active noise cancelling is best suited for low frequencies, while for higher frequencies, the spacing requirements for free space and zone of silence techniques are not practical.