Noise-cancelling headphone
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Noise-cancelling headphones reduce unwanted ambient sounds (i.e., acoustic noise) by means of active noise control. Essentially, this involves using a microphone, placed near the ear, and electronic circuitry which generates an "antinoise" sound wave with the opposite polarity of the sound wave arriving at the microphone. This results in destructive interference, which cancels out the noise within the enclosed volume of the headphone.
Keeping noise low at the ear makes it possible to enjoy music without raising the volume unnecessarily. It can also help a passenger sleep in a noisy vehicle such as an airliner.
Retail noise cancelling headphones typically only cancel the lower-frequency portions of the noise; they depend upon traditional noise suppression techniques (such as their earcups) to prevent higher-frequency noise from reaching the interior of the headphone. This approach has three benefits:
- The demands on the electronic circuitry are vastly simplified.
- Because of the shorter wavelength of the high-frequency sound, active cancellation is much less effective at higher frequencies. To truly cancel high frequency components (coming at the ear from all directions), the sensor and emitter for the cancelling waveform would have to be adjacent to the user's eardrum, an impractical situation.
- For the same reason (shorter wavelength), effective passive isolation becomes easier with rising frequency.
[edit] Disadvantages of noise-cancelling headphones
Noise-cancelling headphones have several disadvantages (that vary from vendor to vendor):
- They consume power, usually supplied by a battery that must occasionally be replaced or recharged.
- They may not cancel all sound effectively. Some noise-cancelling headphones can be overloaded by low-frequency pressure waves, distorting the desired signal. They work well for sounds that are continuous, such as the hum of a refrigerator, but are rather ineffective against speech or other rapidly changing audio signals.
- They may introduce additional noise, usually in the form of high-frequency hiss.
- They are generally more bulky than traditional headphones.
- They may pick up and convert stray electromagnetic fields, such as a mobile phone signal, into unwanted noise.
[edit] Some models of noise-cancelling headphones
- Around-ear style (and approximate noise canceling effect, when available):
- Aiwa HP-CN6
- AKG K 28 NC
- Altec Lansing AHP712
- Jensen JHNC51
- Bose QuietComfort® 2, Aviation X
- Coby CV-191
- Creative HN505, HN700
- David Clark (professional communications headphones, not audiophile)
- Jabra C820s
- JVC HA-NC100
- Koss
- Logitech Noise Canceling Headphones
- Maxell HP/NC-II
- Panasonic RPHC300, RP-HC150-S, RP-HC70E-A
- Philips HN110, HN050, HN060, SHN 5500, SHN 9500
- Pilot Communications PA-1779, PA-1771, PA-1781
- Plane Quiet
- RadioShack 33-1218
- Sakar International DVD-90 (-12 dB @ 300 Hz)
- Solitude (-18 dB @ 300 Hz)
- Sony NC-20, NC-50
- Sennheiser PXC-150, PXC-250, (-15 dB for f < 1000 Hz), PXC-300
- Targus AWM02US
- Turtle Beach ANR-10, ANR-20
- Skullcandy SC-NC (-18 dB @ 300 Hz)
- Helium Digital HDNC-303
- Over-ear style:
- In-ear style
(These usually apply the passive noise isolation technique which is similar to that of ear-plugs. This is supposed to be able to reduce ambient noises for almost all sound frequencies, to a degree even better than that of active noise-cancellation headphones [1] ):
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- Etymotic Research ER-4 MicroPro, ER-6isolator, 6isolator
- Panasonic RP-HC50E-A, RP-HC75-S
- Philips SBC HN060
- Philips SBC HN065
- Philips SHN 2500
- Sony NC-10, MDR-NC11 (-10 dB @ 100-400 Hz)
- Shure
- Ultimate Ears
- Westone
Additionally, a number of airlines in their business and first class cabins supply noise-canceling headphones (as mentioned above, this works very well with the "continuous hum" situation). In these cases, the headphones are about the same size as normal headphones, although they are identifiable by the double-pronged connecting plug. The actual electronics to take the sound from the microphone behind the headphone, invert it, and add it back into the audio signal is done in the plane handrest.
These systems often have a special channel allocated with no underlying audio signal, thereby only using the noise cancellation. This can make sleeping easier, as the noise is significantly quieter than with no headphones on at all.
