Archive | October, 2010

Riding the Groove

My all-time favorite explanatory passage from the literature of hi-fi appears in Laura Dearborn’s sadly out-of-print 1987 guide to audio, Good Sound. In it, she contrives to describe what’s happening when a turntable cartridge’s stylus rides an LP groove, and she pulls it off in a way that makes it sound like a marvel akin to a Star Wars jump into Hyperspace. Now and then, when I’m playing a great-sounding record via my trusty Ortofon Kontrapunkt A cartridge I remember Deaborn’s thrilling explication.

So let’s cue up Good Sound:

Visualize the fineness of a record groove, and then consider that it combines two distinct channels of information, each with completely different modulations. Some of the signal modulations in the groove are on the same order of size as a wavelength of light, which means the stylus has to “read” a signal as small as a millionth of an inch…

For the half a mile or so of record groove per LP side, the stylus must precisely trace abrupt changes in the direction of the undulating groove, sometimes traveling at speeds several times the acceleration of gravity, without ever losing contact with either wall or blurring together the modulations.

Groove friction heats the stylus up to 350 degrees Fahrenheit and the groove vinyl momentarily liquefies each time the stylus passes over it. (This is why one should let a record rest for at least 30 minutes before replaying it, and preferably for 24 hours.)

Even though the cartridge tracking weight is commonly set at only about 1.5 grams, the entire weight is supported on the minute edges of the stylus. As a result, the downforce applied to the groove on a per-square-inch basis is several TONS.

Combine these extreme conditions of weight, heat, speed, and need for exquisite maneuverability, then add in the scale of environmental vibrations that interfere with the stylus as it retrieves the music from the groove, and it’s extraordinary that ANY music (as opposed to noise) is heard through an audio system.

For the grand finale of her bravura account of LP playback, Dearborn gets all Brobdingnagian, blowing up the stylus-and-groove action to outsized gynormousness. Technology as minute as an LP record groove is typically measured in microns. One micron equals 0.0039 inch. Dearborn walks us through what would happen if you could convert the micron scale upward to inches, borrowing a thought-experiment originally devised by the Boston Audio Society’s magazine, The Speaker.

Using the inch scale, a stylus is 30 feet long, affixed to a cantilever 50 feet thick and 275 feet long, which extends from a cartridge body 2,000 feet long, sitting 80 feet above the record. The tonearm, 450 feet in diameter, crosses 1,500 feet above the record from its pivot point four miles away… The stylus downforce temporarily deforms the vinyl by as much as an inch (20 times the size of a violin harmonic), leaving a stylus footprint on the groove wall measuring 10 inches long and 4 inches wide. A typical midrange signal demands that the stylus move 16 inches from peak to peak of the wave form. A deep bass note 10 dB louder requires the stylus to move 10 feet 6 inches whereas for a high-frequency harmonic at a very low sound level , the stylus must move only 0.68 inch. Even the simplest piece of music is likely to contain, at any one time, enormous numbers of frequencies at different levels.

The next time you hear someone try to dismiss vinyl as a primitive, antiquarian, and thoroughly Luddite approach to sound reproduction, just remember that analog vinyl sound reproduction is, and always will be, miraculous. Perfect Sound Forever, suckas!

Electron microscope photograph by Chris Supranowitz / Institute of Optics, University of Rochester

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October 27, 2010 at 6:56 pm

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