|There are two types of traditional full size headphones: open and closed. Closed headphones are basically a sealed cup around your ears that tries to block out external noise. Open headphones don't have a sealed enclosure, and the sound from the rear of the driver can radiate out into open space. Generally, open headphones sound better because the designers don't have to deal with the resonances of a sealed enclosure.Over the last 5 years or so, we've seen manufacturers produce much better sounding sealed headphones, and you can now get very good sound while shutting out some outside noise.[caption id="" align="alignright" width="298"] Noise reduction data for and open (AKG K701), closed (Sennheiser HD 280 Pro), noise cancelling (Bose QC15), and in-ear (Etymotic ER4P) headphones.[/caption]Unfortunately, while sealed headphones do a pretty good job of getting rid of higher frequency outside noise, they are virtually transparent to low frequencies. (<~200Hz). Noise cancelling headphones are specifically designed to get rid of this lower frequency noise.The graph on the right shows how much reduction in outside noise is achieved by four different headphone types. The open headphones (AKG K701 in this case) provides no reduction in noise at all below 1 kHz. The sealed headphones (Sennheiser HD280 Pro) start to provide some isolation at around 200Hz, and provide about 25dB of isolation above 1kHz. The noise cancelling headphones (Bose QuietComfort 15) are 15dB down almost immediately at the low end, and average about 20dB down over 100Hz. The best isolation of all headphone types are the deep fitting in-ear headphones (Etymotic ER4P) which completely block the ear canal. In this case almost all noise reaching your ears is coming through bone conduction and other secondary acoustic pathways.|
|How Noise Cancelling Headphones Work [caption id="attachment_718" align="alignleft" width="300"] Noise cancelling headphones product an equal and opposite acoustic signal to cancel outside noise.[/caption] Inside each earpiece of noise cancelling headphones is a small microphone (or two) that listens to the noise outside the headphones. The signal from this mic is sent to a signal processing circuit to produce essentially an inverted signal, which is then mixed with the incoming music, amplified, and sent to the headphone driver. Once the sound is emitted from the driver the noise cancelling part of the sound mixes with the outside noise entering the headphones and, because they are equal and opposite, both dissapear in a silent "poof" of destructive interference. The only remaining audio is the music. This technology has come a long way in the last two decades since it was initially developed for headphones by Dr. Bose. In early versions the "signal processing" function was a simple analog circuit that merely filtered and inverted the input from the microphone. Currently, the best noise cancelling headphones use sophisticated digital signal processing techniques to achieve noise reduction and work amazingly well.|
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