While 8-tracks and cassettes are as relevent to the digital world as wax cylinders, the vinyl LP is still being steadily produced and collected despite, or perhaps thanks to, their imprecise warm analogue acoustics. Here's how LPs get their unique sound.
Thomas Edison is credited with inventing the phonograph, the first device to both record and play back sounds, in 1877. It recorded sound directly as indentations on a sheet of tin foil wrapped around a grooved cylinder, which could then be played back by rotating the cylinder, which would moved the needle across the indentations, vibrate the mechanical diaphragm accordingly and reproduce the sound. By the 1880's Volta Labs had devised a superior wax-coated cardboard cylinder whose stylus vibrated laterally, like a seismometer, to inscribe the wax with recorded sound rather than vertically, known as the "hill-and-dale" method, like Edison's. It wasn't until the early 20th century that the cylinder was finally replaced by the flat disc record we use today. But even then, sounds were recorded directly, mechanically onto the disc. "Acoustical" recordings like these relied on a large horn connected to a stylus. As the sound waves caused the horn to vibrate, so too did the stylus, which transcribed the vibrations to a hand-cranked wax disc. As you'd expect, these lo-fidelity recordings sound like a guy yelling into a horn — the maximum volume was severely limited to the acheivable physical effects of the horn and the cutting stylus. The extra effort required to generate additional volume quickly wore down both the record and the stylus.
The problem is that the amplitude of a bass note is so much taller than that of a higher-pitched note, the lower-frequency sound takes up more physical space in each grove, literally crowding out the mid and upper range sounds that would be lost during playback. This resulted in a distorted, bass-heavy playback. It wasn't until about 1925 that the audio input was run through a mic and amp to boost the dynamic range and volume without relying on the physical attributes of a horn and the era of "electrical" recordings began. These recordings intentionally rescaled (read: boosted) upper register sounds and suppressed the bass, "equalising" the sounds during playback. 78s — records that spun at 78 RPM — were seen as a great technological leap over the hand-cranked predecessor. Per Compton Pakenham of The New York Times in 1930,
The time has come for serious musical criticism to take account of performances of great music reproduced by means of the records. To claim that the records have succeeded in exact and complete reproduction of all details of symphonic or operatic performances ... would be extravagant ... [but] the article of today is so far in advance of the old machines as hardly to admit classification under the same name. Electrical recording and reproduction have combined to retain vitality and colour in recitals by proxy.
That quality only increased in the 1950's with the introduction of 33 RPM discs, which held about 25 minutes of music per side, and audio tape, which could be easily edited and its sound optimised in post production, then dubbed onto a master disc for reproduction. This master disc is known as a laquer or acetate disc. Acetate is actually a bit of a misnomer, as the discs are actually lacquer coated with cellulose nitrate. And unlike mass produced vinyl records which are molded, the master disc is physically cut, its grooves gouged out on by a record-cutting machine's needle whose mechanical motion is guided by an audio input. The grooves are routinely checked during the cutting process to ensure a proper depth of cut and to adjust volume and pitch on the playback. Once the audio has been transcribed and the playback quality verified, the cut laquer is sent to a production company.
The laquer is far too maleable to be used in the actual production process, so a metal cast must be made of it. Originally, the laquer was coated in an extremely fine layer of electrically-conductive graphite to which silver or nickel was then plated. Once the plate was thick enough, the wax was stripped away leaving a sturdier metal mold. Modern techniques involve dipping the disc into a solution of stannous chloride and then spraying on atomised silver. The conventional plating bath technique is also still widely used but is not as acoustically accurate.
After the initial silver plating has hardened, a layer of fine-grain copper is applied, followed by a layer of more robust, coarse-grain metal like steel. Fine-grain metal, wherein the individual grains are relatively small (packing 85 or more grains per square inch at 100 diameters magnification) and feature improved strength and toughness than coarse grained steel, which is more easily machinable. Once enough metal has been applied to support the silver plate, the laquer is smashed free of the mold, leaving the metal negative, or "matrix."
The matrix is still pretty maleable so it must be recast once more before it can begin pressing vinyl. A metal record known as "the mother," is cut from the matrix and then replated to create another negative known as the "stamper." And since the mother is cut from a metal such as copper, it can withstand multiple recastings to churn out multitudes of stampers, which are then used to press the actual production records.
Each stamper, which is attached to an autonomous 100-tonne hydraulic press, works essentially like a waffle iron and can churn out thousands of records before needing replacement. A sheet of preheated vinyl roughly half the area but three-times the thickness of a finished disc, known as the biscuit, is slipped between the jaws of the press, 150 degree C steam is applied to further soften the material before the jaws close, and squeeze the hot vinyl into its final shape while imprinting the audio. The disc is then cooled and hardened in a water bath and labelled — all of which is done automatically, the only human input is the loading of the two disc labels sandwiched around the biscuit.
Once it comes off the stamper, the record's ragged edges are trimmed into a neat circle on an automated trimmer table and the record is inspected, both visually and audibly. Many, many, pressings don't pass muster and are subsequently melted back down into biscuits for another go. But the LPs that are quality enough are then packaged and shipped out to your local independent music retailer.