Building a better silence

Originally published 24 June 1991

It is late night. Oth­er mem­bers of the house­hold are asleep. Street nois­es have sub­sided. The groan of planes mak­ing its way to Logan has come to an end. Even the refrig­er­a­tor has momen­tar­i­ly sus­pend­ed its almost cease­less purr. It is that time of the day when one hears the silence.

Silence is not a mere absence of sound. It is the black­board on which sounds are chalky scrib­bles. Thore­au had a good image. Sound, he said, is a bub­ble on the sur­face of silence which straight­away bursts. Below a froth of noi­some bub­bles silence flows like an infi­nite stream.

In post-mid­night hours silence becomes pal­pa­ble, flow­ing sound­less­ly out of the past into the future, a stream as broad and as deep as the uni­verse itself. It is a time for con­tem­pla­tion and repose. Thore­au said: “If the soul attends for a moment its own infin­i­ty, then and there is silence.”

It’s an old theme, com­mon to New Eng­land her­mits and medieval monks, to sec­u­lar philoso­phers and reli­gious teach­ers of all cul­tures: In silence we are in touch with some­thing infi­nite­ly greater than ourselves.

But silence has become rar­er than pure water or unpol­lut­ed air. The assas­sin is machin­ery. Even as I write, the disk dri­ve in my com­put­er spins into action, emit­ting an intru­sive, silence-offend­ing whine.

Hazards to our health

Machine noise is an ubiq­ui­tous pol­luter of our envi­ron­ment. It is a haz­ard to the phys­i­cal health of many of us, and a haz­ard to the men­tal health of all of us. Noise-abate­ment laws have made bare­ly a dent in the din. To baf­fle or reduce machine noise at the source costs more than most of us are will­ing to pay. Machines — noisy machines — are here to stay.

But a par­tial solu­tion may be at hand, and the solu­tion, iron­i­cal­ly, is more machines that make even more noise.

The name of this tech­nol­o­gy is active noise con­trol (ANC).

Here is how it works.

Sound is a vibra­tion, typ­i­cal­ly a rather com­plex mix of ampli­tudes and fre­quen­cies. If two sounds of equal ampli­tude and fre­quen­cy but oppo­site phase are com­bined, they can­cel out. The crests of one wave match the troughs of anoth­er. In effect, the eardrum of a lis­ten­er is pushed and pulled at the same time. The result is no sound at all.

Noth­ing new so far. Every high school sci­ence stu­dent has heard about this mutu­al can­cel­ing of out-of-phase waves. It’s called destruc­tive interference.

In an ANC sys­tem, a micro­phone is placed close to the source of an offend­ing sound. The sig­nal is fed to a spe­cial-pur­pose com­put­er chip that instant­ly analy­ses fre­quen­cies and ampli­tudes. The com­put­er then uses a speak­er to gen­er­ate a pre­cise­ly match­ing out-of-phase sound waves. The two waves can­cel. Two nois­es become silence.

An acoustic rep­re­sen­ta­tion of the envi­ron­ment can be built right into the ANC com­put­er chip, telling the com­put­er how sounds will make their way to the ears of the lis­ten­er. This too is fac­tored into the calculation.

Final­ly, a sec­ond micro­phone placed between the speak­er and the lis­ten­er picks up any residue sound result­ing from a mis­match, enabling the com­put­er to instant­ly refine its calculation.

It is the speed of the new chips that makes the tech­nol­o­gy prac­ti­cal. An ANC sys­tem must com­pute and pro­duce an out-of-phase repli­ca of the noise in mere thou­sandths of a sec­ond. For­tu­nate­ly, today’s com­put­ers-on-a-chip rou­tine­ly per­form mil­lions of cal­cu­la­tions per second.

Only a decade ago, com­put­ers cost­ing tens of thou­sands of dol­lars would have been required for the task. Built-to-pur­pose ANC com­put­er chips cur­rent­ly cost hun­dreds of dol­lars. The price of the chips will plum­met as the tech­nol­o­gy expands, poten­tial­ly to the point where a noise con­trol sys­tem will be tak­en for grant­ed with almost every machine.

The possibilities are endless

For exam­ple, it would be a sim­ple mat­ter for Apple Com­put­er to intro­duce ANC into my Mac­in­tosh, silenc­ing the both­er­some whine. There is already a speak­er in the com­put­er, with not much more to do than emit an occa­sion­al beep. A pow­er sup­ply is in place. Some­time with­in the next few years Apple could add an ANC chip and two tiny micro­phones at very mod­est cost. Cer­tain­ly at far less cost that what would be required to man­u­fac­ture a silent disk drive.

The appli­ca­tion pos­si­bil­i­ties for ANC are end­less — auto­mo­biles, air-con­di­tion­ers, fur­nace fans, refrig­er­a­tors — sig­nif­i­cant­ly silenc­ing our envi­ron­ment. ANC does not just mask noise, or dead­en its effect; it eras­es noise from the black­board of silence.

Even­tu­al­ly we can expect to see per­son­al ANC head­sets that can be pro­grammed to elim­i­nate sounds at the user’s dis­cre­tion: Traf­fic noise can be elim­i­nat­ed, for exam­ple, leav­ing bird songs, insect chirps and the sound of rustling leaves unaf­fect­ed. Then one will be able to expe­ri­ence the sounds of silence much as Thore­au did at Walden Pond in the days when the pond was far removed from the clam­or of car- and truck-clogged highways.