April 2012

The Silencer
Article By Grey Rollins

Difficulty Level

  From the beginning of time, unwanted noise has been an annoying burden to those who wanted to listen to the music, and only to the music. It takes little to imagine the annoyance of Ug, the caveman, listening raptly to the ululations of Ul, the belle of the tribe, as she cries out for love. Rudely interrupted by the mocking snickers of the teenagers in the back of the cave, she breaks down in tears. Ug, an early forerunner of today's passionate aficionado of live performance, applied the Neanderthal version of a mute button to the noisy brats — a stout oak club upside the head — then grunted for his inamorata to resume her love song. The cave-teenagers were concussed, but lived, although one had a permanent crease on the left side of his cranium.

The mead-quaffing Vikings enjoyed music no less and enjoyed interruptions no more than their distant ancestors. Having a club-footed slave trip, leading to the crash and tumult of a roast boar falling to the floor, in the process upending the barrel of mead from whence the mugs were filled, caused enough racket to be heard clear across the valley. Setting aside the tragic loss of meat and libations, the disturbance interfered with the lusty singing of Ulthera the Busty. Being somewhat more liberated than her forebear, she attended to the problem herself, seizing the first thing that came to hand — Ugly the Hammerhand's sword — with unfortunate results for the poor slave. In deference to those possessed of delicate constitutions, it would probably be best if we hurry onwards. Being smote by Ulthera the Busty is not necessarily as pleasant to contemplate as being smitten with her. Suffice it to say that the club-footed slave did not survive, but replacement slaves are easy to come by when you're a Viking; weekly raids on Thursdays, longship leaves promptly at dawn, rain or shine.

Time passed and new instruments were invented. New music was invented to take advantage of the new instruments. New listeners developed new musical tastes to take advantage of the new music. But still, noise was a problem. The glockenspiel made its way into Tchaikovsky's Nutcracker not long after its invention. Sadly, its light, tinkling tones were no match for the boorish man who insisted on bragging of his immense wealth to his mistress during the performance. Solution: Choose weapons, seconds, a location, and a time. Duels, however, sometimes had the unintended consequence of death for the offended, leaving the miscreant to offend again. Unpleasant, messy, and inefficient.

Finally, with the invention of recorded music, mankind progressed to the point where music could be enjoyed far from the cave, the mead hall, or the concert hall. In the privacy of his own home, the modern listener could not only leave noisy audience problems behind, but, courtesy of a 200W Marshall stack reproduced at concert levels, the music itself finally took an active role in noise suppression—or at least in masking it. The problem here being that the noise it was masking was the approach of the shotgun-wielding next door neighbor, muttering imprecations about that damned long-haired hippie. The police, during their investigation of the shooting, asked if anyone had heard the shot. To a man, the other neighbors swore they thought it was a rimshot, not a gunshot. The crime was never solved.

The curious thing is that over time, we see the listener progressing by gradual steps from being the aggrieved party to being the offender. The invention of recorded music allowed the listener to either distance him-or her-self from noise or to drown it out, but it did not prevent the music from wafting through the air and into the open windows of the neighbors.

This being the 21st Century, we now have the technology to solve the problem. With uncharacteristic modesty and a charming smile, I hereby present The Silencer. No longer does the listener need to endure the racket, intentional or otherwise, made by others and others need not be offended by the listener's choice of music. In effect, a virtual soundproof wall is erected between the listening room and the outside world to the benefit of all.

How is this magic accomplished? In a word, feedback. No, not the sort that made Jimi Hendrix famous. That was positive feedback. By the clever use of negative feedback, we can eliminate unwanted Stratocasters, glockenspiels, and the persistent cough of the blue-haired old lady in the fifth row during Cage's 4'33". Sadly, lacking a time machine, we cannot go back and prevent Ug the caveman from hearing the heckling of the teenagers leaning against the stalagmite in the back of the cave. Their destiny has long since been fulfilled.

The circuit is simplicity itself:

In application, a microphone is used for the input and the output is applied to an amplifier of any arbitrary output power capability. The amplifier is, in turn, hooked to one or more speakers.

Conceptually, the microphone "hears" a sound, The Silencer inverts the waveform, the amplifier amplifies the signal, and the speaker emits an inverted soundwave that, being out of phase with the original sound, cancels it. The result? Silence. Pure, unadulterated, blissful silence — a real world version of the fabled Cone of Silence much adored by science fiction writers.

For those who want to take the capabilities of The Silencer to the theoretical limit, I suggest using panel speakers (e.g. electrostats, planar magnetics) in place of the walls of the room. The virtual soundproof walls become the literal walls of the listening room! For the truly daring (and wealthy), replace the ceiling and floor as well. In fact, the ultimate listening room would be a spherical speaker of, say, eight meters diameter, with the stereo and listening chair suspended in the interior.

Discrete circuit versions are, of course, possible, and for the purist, desired, and those skilled in the art will realize that other implementations are within the scope of the present invention.

DISCLAIMER
Lest anyone miss the fact that it is April 1st, I'd like to note that this circuit, while arguably functional, has limitations. One being the unfortunate fact that high frequencies, such as Hendrix's screeching rendition of the bombs bursting in midair during The Star Spangled Banner, will not be suppressed, or at least not in a predictable manner. The problem is that high frequencies have such short wavelengths that the portion of the sound wave present at the microphone will not match that escaping through the window on its way to annoy the next door neighbor. In short, the sound won't cancel. Worse yet, the feedback will change from negative to positive at select frequencies, leading to wild, uncontrolled oscillations. Hendrix might (or might not) approve, but your neighbor certainly won't. And it's completely inappropriate for Tchaikovsky. In both cases, the musicians are dead, so you are free to experiment, but note also that the speaker used in the sound canceling rig will quickly lose its tweeter. Very expensive. Very annoying.

Another potential pitfall is the implicit assumption that the amplifier (and any other associated electronics you might choose to throw into the chain, such as preamps, etc.) is non-inverting. If, perchance, the amplifier in question should be inverting, it will change the negative feedback to positive and once again summon the ghost of Hendrix from his grave. Should you be foolish and/or daring enough to try the experiment, while he's available please ask Jimi whether the lyric is "kiss this guy" or "kiss the sky." Inquiring minds want to know.

Any deviation from flat response on the part of the microphone or nulling speaker would, of course, lead to imperfect cancellation at the frequencies in question. For the time being, we'll take it as a given that the amplifier and The Silencer have frequency responses sufficiently near perfect in the usual audio ranges that problems will most likely originate in the transducers.

In fact, this sort of circuit will only work for low frequencies, where the portion of the long wavelength perceived at the microphone's position comes reasonably close to being the same as that near the canceling speaker, near the window, or near the listening position, depending on how the system is deployed. In any event, the circuit will work best when the microphone and speaker are in close proximity and the listener (or neighbor, again depending on application) is, in turn, seated nearby.

If you're trying to stop midrange or high frequencies, your best option is to absorb them with curtains, pillows, or specialized room treatments; in other words, the mundane stuff that you're already using.