Simple ear$-$protection devices use materials thatreflect or absorb sound before it reaches the ears. Anewer technology, sometimes called noise cancella $-$tion, uses a microphone to produce an electrical signalthat mimics the noise. The signal is modified electron$-$ically, then fed to the speakers in a pair of headphones.The speakers emit sound waves that cancel the noise. On what principle is this technology based? What kindof modification is made to the electrical signal?