The physics of the turntable
Textbooks rarely discuss the physics of the phonograph anymore, since the CD and other digital formats have largely replaced the older format. To make up for that loss, here’s a very brief explanation of how sound comes out of a vinyl plastic disk. The reader is invited to look elsewhere for additional details.
First, the basics. Sound is a vibration in a medium such as air. These vibrations can make objects, like your eardrum, vibrate in sympathy. Thomas Edison in 1878 perfected a machine that could take the vibrations from someone talking VERY LOUDLY into a horn-shaped receiver, translate them to a vibrating needle, and finally onto a wax or tinfoil covered cylinder. The wiggles of the needle imitated the vibrations of the air. Playback used the s
ame equipment. The vibrating needle would excite the airhorn, and sound could be heard coming from the horn. The process — captured by the classic corporate logo of the RCA Victor company (right) — was entirely mechanical.
[You can recreate the process today with a sewing needle taped to a homemade paper cone. Choose an album that won't break your heart if it gets scratched, place it on a turntable and start the platter going. Hold the wide end of the paper cone at one point and let the needle drag along the record. If you listen very carefully , you'll be able to hear the tracks.]
The cylinder format was eventually replaced by the familiar phonograph disk, once developers learned how to compensate for the changing linear velocity of the disk’s grooves under the needle. While the entire record has the same angular velocity (rpm’s), a point on the outer edge has to cover a greater distance in the same time as a point on an inner track. Cutting the disk requires the manufacturer to compress the wiggles of the cutting needle into an ever-tighter linear space, so that the pitch of the sound doesn’t change.
Beginning in the 1920s, phonograph makers abandoned direct mechanical playback and adopted the electromagnetic system that survives today. Since the early 1800s scientists understood that moving electric charges (currents) create magnetic fields, and moving (or changing) magnetic fields set up currents in nearby electric circuits.
On a record, we have a needle vibrating side-to-side within a wiggly groove. Attach a small magnet to the other end of the needle and suspend a small coil of wire (or two coils for stereo sound) near it. The vibrations of the magnet create tiny currents in the coil(s), which can then be amplified for much louder sound. You can also mount a small coil on the butt end of the needle, and have it wiggle within a small magnet’s field. The effect is the same.
Aside from the obvious advantage of making playback louder, the electromagnetic system also has a better frequency response. Electrons are a lot easier to push around than the molecules of a wooden or metal airhorn, so higher frequencies are more audible.
Regardless, phono records suffer from their dependence on the mechanics of that wiggling groove. Low frequencies mean bigger wiggles, and bigger tracks, while high frequencies require the needle to move back and forth very quickly. In the 1950s the Recording Industry Association of America (RIAA) — the same group that now relentlessly hunts down audio pirates — agreed on a standard frequency response curve for all records. Records are deliberately cut with the difficult frequencies muted, and the playback amplifier then unmutes them, so that the final product is faithful to the original audio source.
The RIAA curve requires modern digitizers like this author to buy a special phono preamplifier to correct the muted frequencies of the album before the signal reaches a computer’s sound card or a modern amplifier without phono input jacks. Otherwise, the resulting output sound will be too quiet and too thin.
It all sounds kludgy, but for more than a century, the whole phonograph recording and playback system worked remarkably well, based securely on basic principles of mechanics and e&m.
