The Howl

Everyone has heard it. A speaker steps to the microphone, the room is still, and then out of nothing a tone climbs the air — a rising wahhh that sharpens into a squeal until someone yanks a fader or steps away. We treat it as an interruption, a fault in the equipment, noise breaking into the proceedings. But the strange thing about that sound is where it comes from, which is nowhere. No one is making it. The room is not ringing with anything the room contains. The microphone is hearing the loudspeaker, and the loudspeaker is playing the microphone, and the tone is the sound of a system listening to itself with nothing in between. It is one of the few everyday sounds with no source outside the system making it. It is a machine mistaking its own output for the world.

The Danish physicist Søren Absalon Larsen described the principle early in the last century, and in much of the world the effect still carries his name; British broadcast engineers called it howlround, which is the better word, because it names the shape. Sound leaves the speaker, crosses the air, enters the microphone, runs through the amplifier, and returns to the speaker louder than it left — a closed loop with a gain on it. Whether that loop sings depends on two conditions, and both must hold at once. The returning signal has to come back at least as strong as it went out: loop gain of one or more. And it has to come back in phase — the round trip delayed by some exact multiple of the wave's own period, so the return arrives crest-on-crest with what's already there and adds instead of cancels. Meet both and the loop becomes an oscillator built out of a room. It is the identical criterion engineers use to design oscillators on purpose. The howl is an accidental one, assembled by chance out of a microphone, an amplifier, and the distance to the nearest wall.

That is why it picks a single pitch instead of a wash of noise. Of all the frequencies bouncing around, the loop seizes on the one where its own gain happens to be highest and the phase happens to line up — a frequency set by the resonances of the room, the peaks in the microphone and speaker response, and the precise air-gap between them. And once it seizes, it runs. Each pass around the loop is in phase, so each pass adds to the last; with loop gain above one, every pass is a fixed multiple of the one before — exponential growth, the whole climb over in a fraction of a second. What stops it is not restraint but collision: the amplifier reaches the ceiling of what it can produce and begins to clip, and clipping bleeds off gain, and the runaway stalls at the exact level where the loop gain has been dragged back down to one. The steady squeal you finally hear is not a system out of control. It is a system pinned to the precise edge of its own instability, held there by its own distortion — balanced, horribly, at the threshold it crossed.

The remarkable part is that the cure and the disease are the same circuit. On the second of August, 1927, a young Bell Labs engineer named Harold Black was riding the Lackawanna ferry across the Hudson to work, and — having nothing else to write on — sketched the idea across a blank page of his morning New York Times — a sheet, he liked to note, conveniently date-stamped by its own masthead. The idea was to take a portion of an amplifier's output, turn it upside down, and feed it back into the input so that it subtracted — so that any drift or distortion in the output returned as its own correction. Negative feedback. It is the howl's loop with the sign reversed: return the signal in phase and it reinforces until it screams; return it inverted and it cancels its own errors until it goes quiet and true. Black's colleagues were skeptical for exactly the reason you'd expect — everyone knew that feeding an output back to an input was how you got a squeal — and it took Harry Nyquist's mathematics to draw the precise line between the loop that corrects itself and the loop that destroys itself. They are millimeters apart. The entire coast-to-coast telephone network, hundreds of amplifiers in a row that somehow didn't accumulate into garbage, rode on keeping to the right side of that line.

And then, having spent decades learning to suppress the thing, people discovered it could sing. On the eighteenth of October, 1964, the Beatles were in the studio and Paul McCartney plucked a sustained A on his bass; the note set John Lennon's hollow-bodied guitar humming where it leaned against its amplifier, and the resulting low nnnwahhh — pure Larsen effect, the same fault that ruins a wedding toast — opens "I Feel Fine," kept because someone heard it and thought it was beautiful rather than broken. It was, by most accounts, the first feedback deliberately pressed onto a pop record, and Lennon defended it as his own to the end of his life. Hendrix made an entire vocabulary of it. And the instrument makers miniaturized it: the EBow you hold over a single string, the Sustainiac built into a guitar's body — both of them tiny, deliberate, switchable Larsen loops, a pickup driving a string driving the pickup, the runaway tamed down to an infinite singing sustain you can turn on with your thumb. The same decades that taught electronics to fear the loop taught music to play it.

So the loop itself was never the problem, which is the part worth keeping. You cannot cut the loop without also cutting the voice: the feedback that screams, the feedback that corrects, and the feedback that holds a note open forever are the same circuit, and the only thing dividing them is the sign — whether the returning signal is added or subtracted, taken as reinforcement or taken as error to be removed. The hardest thing for a system built as a loop is not to be loud or quiet but to know which sounds reached it from outside. The squeal is exactly what that failure sounds like: a system that can no longer separate its own returning output from a new signal, and so counts the one as the other, and climbs. Everything downstream of it — Black on the ferry, Nyquist's line, the narrow notch tuned to the room — is one project under different names. Not to stop the loop, but to learn its sign: to know your own returning voice from the world's.