Natural Orchestra

Natural orchestra

TDC

Kevin Cameron

FOR SOME REASON, WE HUMANS FIND the constant repetitive motions of nature soothing. People like the rush and return of waves at the seashore, the chuckling of a brook and the gliding of the clouds. I recently sat looking at trees stirred into motion by wind. Each leaf moves rapidly, constantly flipping over and back, while the tree as a whole sways slowly-an aerodynamic dancer.

Each part in motion has its own natural frequency; the tree as a whole moves slowly because its mass is great and its restoring force-the stiffness of the trunkis only moderate. Individual branches move more rapidly.

All this made me think of the “sound signature” of familiar objects. When someone is in the kitchen and I am in my office, I can readily identify the sound of a pie tin, a glass or a skillet being set on the counter. In the shop the same thing happens-a piece of wood vibrates in a specific and identifiable way. Pulling a finger across a cylinder’s cooling fins makes a distinctive papery singing. Sliding sheet stock out of a pile tells its particular sonic tale. An experienced mechanic knows at once from subtle aural cues that a universal joint is failing, or that wristpin clearances are out of spec.

How do we decode these signals? Like the tree, each of these items has its own particular vibratory modes and frequencies. In our ears are fibers of various lengths, each coupled to neurons that report the motions of the fibers. Like reed valves, each fiber has a natural vibratory frequency, higher for short fibers, lower for long, and they vibrate in step with sounds having the same frequencies. The pattern of nerve impulses reported to our brains from a sound we hear is rich in information-the more so as our experience and curiosity increase.

Pour water into a glass and the sound frequency seems to rise as it fills. This is the air-filled cavity above the water, acting as a resonator. When filling begins, the resonator is as long as the glass is tall, and the sounds it reinforces are low. On a larger scale, this causes rooms to have a “presence.” I phoned a new business recently and could sense a cavernous space. When the owner came on the line, he told me about his new 100-by-80-foot premises. I was “hearing” this space, as ambient sounds reverberated back and forth across it. The room reinforced particular sound frequencies and not others,

just as the water glass did. This is part of why motorcycle sound testing requires so-called “anechoic” chambers, designed specifically not to reverberate or reflect sound. They are the sonic equivalent of a stealth airplane, turned inside-out.

Guitarists know that it makes a difference where a string is plucked. Pluck in the middle and you get the simplest, lowest-frequency vibration. Pluck halfway between the middle and one end and you get the “fundamental” plus a double-speed vibration in which half the string goes north while the other half goes south. The closer to the bridge you pick, the more twang (high frequency) you add to the fundamental. In more complex ways, a pie tin, a 2x4 or any object (crankshafts can be a particularly troublesome example) can be excited into families of vibratory frequencies. And our remarkable brains can often recognize the source instantly.

When Robin Tuluie showed me a motorcycle on a two-post hydraulic shaker, and dialed it upward in shaking frequencies from low values, various parts of the machine were excited by particular frequencies. At a few cycles per second, the suspension responded. At 20-22 cycles, the front fork became a blur, whipping forward and back at its natural frequency-just like a reed petal or one of those tiny vibrating hairs in the inner ear. At other frequencies, various brackets stirred

into motion. Identifying and controlling such chassis motions are important problems in MotoGP tire traction and handling right now. Instead of petals or tuning forks, the concerns are the frequencies of steering-head twist, chassis bending or lateral flex of the swingarm. Like the tree’s trunk, branches and leaves, each has its own modes and natural frequencies.

Because they obey physical law, these vibratory motions can be modeled on computers. How can we redesign this body panel so it won’t drum in noisy resonance with vibration coming from radial tires, from a whirling crankshaft or from the subtle engage/disengage motions of gear teeth? Try a few tweaks and you may resolve this. Each object can have many possible modes of vibration-think of a drum head, which can move up and down as a whole, or can vibrate in various numbers of segments, or as circular ripples moving back and forth between the center and the rim.

Or perform the analysis from the opposite direction-record the actual sounds made by the vehicle, then take that noise apart into its component frequencies with the technique of Fourier Analysis. Then go hunting for those frequencies among the modes that are present in vibrating parts such as gas tanks, fairing panels, etc. Just as chassis tuners tune out a harmful fork vibration by making the tubes stiffer or more flexible, so small changes in the curvature of a panel can alter its natural frequencies enough to stop its motion and the annoying sound that results.

The sound of an engine can even have its sharp edges removed by changing the way exhaust is released from its cylinders. For highest performance, rapid valve opening is usually preferred, but the more suddenly exhaust is released, the greater its content of irritating high frequencies. Example: the two-stroke engine, whose piston-controlled exhaust ports expose flow area faster than any poppet valve can. The resulting sound has a sharp edge that is not easily quieted. But if you are building slow-turning lo-po engines for boulevard cruisers and have noise-compliance issues, slowing the opening rate of exhaust valves to let the smoke seep out could be just the ticket.

I’d still like to know why trees, stirred by wind into many complex motions, are so soothing to look upon. □