November 18, 2009 3 min read
"A painter paints his pictures on canvas. But musicians paint their pictures on silence. We provide the music, and you provide the silence." - Leopold Stokowski "Soon silence will have passed into legend. Man has turned his back on silence. Day after day he invents machines and devices that increase noise and distract humanity from the essence of life, contemplation, meditation...tooting, howling, screeching, booming, crashing, whistling, grinding, and trilling bolster his ego. His anxiety subsides. His inhuman void spreads monstrously like a gray vegetation." - Jean Arp The headphones themselves seem a bit too humble to boast, so we tend to overlook their almost magical ability to deliver on a paradox: to give you both sound and silence. Headphones are both a sanctuary where we draw close to the sacred music of our soul, and refuge away from the chaotic din of modern life. Traditional sealed headphones have long been delivering on this promise, and have gotten better and better at delivering greatsound over the last few decades, but they haven't really gotten much better at delivering greatsilence. Noise cancelling headphones attempt to remedy that, and largely succeed ... largely, but not completely. Hold on, I'm getting ahead of myself here, let's start at the beginning ... Providing Silence with Headphones
|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. (|
|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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