The introduction of LPs and PVC records came alongside the development of microgrooves. These are much more compact grooves that contain as much information as previously while allowing for a much lower play speed (33rpm) compared with 78. The average LP has nearly 500 metres of grooves per side!
The next bit gets pretty complex…
In 1954, a standard of equalisation was introduced in the USA (the RIAA) which ensured that all music was mastered to the same specifications.
In simple terms, the louder the bass frequencies are the wider the record grooves must be. By boosting the high frequencies when recording, and then deamplifying them during playback the opposite can be done with the lower frequencies - cutting them quieter and boosting them through amplification in the same proportions. This is depicted in the image above.
This way, the grooves can be kept at a certain (micro) width and smaller grooves equal longer runtimes. Thanks to the RIAA time per side could be increased to 22 minutes.
The anatomy of a record groove is best thought of in terms of sound waves. The groove is essentially a 3D representation of the waves – and so the stylus needle brushes along them and reproduces the track. Check out the video above where a scientist has used an electron microscope to get up close and slow-motion views of record grooves.
As record technology improved, more complex sound was able to be recorded and represented on records. Stereophonic recording came about in 1931, which we still use today to provide a greater balance and sense of space to a recording. It essentially records on two planes rather than one (Monophonic needles only move horizontally).
Stereophonic needles move vertically as well as horizontally. These vibrations are picked up by two coils (left and right) which then convert those vibrations into the stereo sound we hear.
Occasionally you might notice you hear an echo before or after a loud sound – particularly at the start of a record. This is when some of the cutting stylus accidentally transferred some of the information from another groove through the ‘groove wall’ – this is one of few drawbacks of having such thin grooves.
In the 70s there were attempts to introduce Quadraphonic records, but they didn’t hit it off. As you may have guessed, this method used four channels for recording. They weren’t successful due to technical issues with the discs, they were more expensive, and anyone who had a collection would need four speakers instead of two! They look just like a normal record unless you get up really close to the grooves.
It was still an important development, however, and has been essential for surround sound in places such as cinemas.