叫noise cancelling的分两种。
1 noise isolating的，in-ear耳塞。例如Sennheiser CX-300。原理就是把耳朵堵上。
。。
2 "active" noise cancelling。例如Sennheiser PXC 450。原理是探测到background
noise然后耳机发射另一种噪音来offset。需要用电池。
自己科普： http://en.wikipedia.org/wiki/Active_noise_control
A noise-cancellation speaker “emits” a sound wave with the same amplitude
but with inverted phase (also known as antiphase) to the original sound.
