The CXC700 features Sennheiser's NoiseGard / digital system Ear-Canal headphone, which allows travelers to choose from three noise-canceling modes optimized to reduce low, medium and high-frequency noise for frequent business travelers and regular commuters on trains. The noise cancellation tech obviously requires an additional source of power, which is why you?ll need an AAA sized battery for whenever you're travelling. If the battery dies though, no need to worry as the CXC 700 will just perform as a pair of decent earphones, sans all the noise cancellation. Silver/Gray