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Do you want to listen to music on headphones while on an airplane trip? If so, you might have to turn the headphone volume very high to hear the music over the regular whirring sound of the engines and air circulation system. Inside the headphones, your ear is hearing both the music and the jet noises superposed on top of each other. Noise-canceling headphones can reduce those external sounds so that you hear the music more clearly—and at a lower volume level! 
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On the outside of noise-canceling headphones is a microphone that samples the sounds of the air and engine. Those signals are sent to a specialized computer chip called a digital signal processor that separates out the low frequencies. This chip then inverts the low-frequency sound—changes the sign from positive to negative or vice versa—and sends it to the amplifier. The amplifier plays this antinoise back into you ear at just the right amplitude to cancel with the low-frequency part of the sounds from the jet, but without affecting the sounds from your music. Thus noise + antinoise = quiet!
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Even though the headphones cancel the jet noises, you can still hear the voice of the person next to you. Why? The antinoise signal does not cancel voices because these are mostly composed of higher frequencies and they do not repeat over periods of a few seconds. Repeated, low-frequency noises are canceled; nonrepeated, high-frequency sounds are not.
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Like all technologies, active noise cancellation has trade-offs. The headphones require additional power for both the chip and the internal amplifiers. There is also a degradation of audio quality, particularly for low-frequency (bass) sounds, and sometimes also an audible, high-frequency hiss.
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Noise-canceling headphones are useful on airplane trips. In which of the following situations would noise-canceling headphones be most useful in eliminating noise? - at a rock concert
- in a lecture hall
- in an apartment above a busy street
- at the Super Bowl
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The correct answer is c. Noise-canceling headphones are best at eliminating repetitive, constant background noise. Street traffic is most likely to be constant and repetitive whereas noise at a concert, lecture hall, or sports event is not. 
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