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April 2012 The Silencer 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 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.
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