HOW IT WORKS

WHY THE HEAR­ING PRO­TEC­TION ANR DE­LIV­ERS IS SO IM­POR­TANT

Flying - - DEPARTMENTS - By Rob Mark

Ac­tive noise-re­duc­tion (ANR) head­sets and why they’re im­por­tant

In the early days of air­borne com­mu­ni­ca­tions, ear­phones were cre­ated to make de­ci­pher­ing ra­dio con­ver­sa­tions for pi­lots easier above the roar of the en­gines and wind. Some of the first ear­phones crafted from hard Bake­lite used no pad­ding and were ex­tremely un­com­fort­able to wear for long pe­ri­ods of time. Ad­vances in acous­ti­cal re­search im­proved the wear­ing com­fort of head­sets by adding flex­i­ble cups that to­tally cov­ered the ears, although they only muted some of fly­ing’s more an­noy­ing noises.

While ex­perts claimed our sense of hear­ing was nearly as crit­i­cal to fly­ing as a keen sense of sight, it wasn’t un­til about 30 years ago that engi­neers re­al­ized the im­por­tance

of blend­ing com­fort with ad­vances in hear­ing pro­tec­tion. Re­search, in fact, shows un­pro­tected ex­po­sure to noise lev­els greater than 90 db, equiv­a­lent to what a hu­man would en­counter stand­ing near a run­ning lawn mower for long pe­ri­ods of time, can cause per­ma­nent hear­ing loss, ini­tially in the lower fre­quen­cies out­side the con­ver­sa­tional range. Early ear-cup head­phones blocked high-fre­quency noise rather ef­fec­tively but did a poor job on low-fre­quency sounds, such as dron­ing pro­pel­lers. Engi­neers learned that sim­ply press­ing the ear cups more tightly to the per­son’s head did lit­tle to solve the prob­lem.

En­ter ac­tive noise-re­duc­tion tech­nol­ogy in the mid-1980s, which is avail­able to­day in sev­eral high-end avi­a­tion head­sets from com­pa­nies such as Bose, Light­speed, David Clark and oth­ers. Bose cre­ated the first suc­cess­ful ANR avi­a­tion head­set in 1989 and de­buted its pop­u­lar A20 model in 2010.

Rather than sim­ply at­tempt­ing to block an­noy­ing cock­pit noise, the heart of the ANR sys­tem cre­ates an al­ter­nate elec­tronic sig­nal that’s fed back into the head­set’s elec­tron­ics to ac­tu­ally can­cel out the un­wanted sounds. ANR head­phones ac­com­plish this us­ing acous­ti­cal build­ing blocks such as feed­back and the feed-for­ward loop sys­tem, dig­i­tal or ana­log pro­cess­ing and a full fea­ture driver/speaker.

ANR sys­tems op­er­ate by us­ing one or more mi­cro­phones placed near the pi­lot’s ears ei­ther in­side or out­side the head­phones to ac­cu­rately hear what the pi­lot does. In a sound feed­back sys­tem, for ex­am­ple, that out­put is com­pared to the sound the pi­lot wants to hear, usu­ally ra­dio com­mu­ni­ca­tions or pos­si­bly mu­sic. The can­cel­la­tion sig­nal cre­ated though am­pli­fi­ca­tion and fil­ter­ing then builds the sig­nal fed to the driver. With just the right amount of fil­ter­ing, the re­sult is noise can­cel­la­tion that ef­fec­tively re­duces sound pres­sure on the ear. Au­dio engi­neers say noise can­cel­ing “de­struc­tively in­ter­feres” with un­wanted noise. Pi­lots sim­ply call it near-per­fect quiet un­til a wanted sound is de­tected and crisply de­liv­ered to the ears.

The best ANR units fo­cus on more than sim­ply the elec­tron­ics of quiet­ness, and em­ploy head­bands de­signed with soft con­toured foam pads to min­i­mize pres­sures on the head since ANR head­sets are a one-size-fits-all cre­ation. The best ANR head­sets also of­fer con­trol of the feed­back level to cus­tom­ize a near-per­fect blend of con­trol over what the pi­lot even­tu­ally hears.

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