Why do vinyl records always spin at 33 or 45RPM? Why not every any slower speeds that would allow for more music on each side?
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The slower a record turns, the worse the audio sounds. Due to this, in order to provide the best sound possible, the record needs to turn faster (higher RPM). However, when a record turns faster the amount of information it can hold on a record is reduced thus having a shorter playback time.
There’s a trade-off between the speed of the disc and the quality of the audio stored on it–if you spin it slower then less of the vinyl will pass under the needle every second, so to get the same quality you’d have to have more detailed vibrations in the groove.
Having said that, 33 and 45 are not the only speeds ever used. Many years ago, when manufacturing techniques weren’t as good as they are now, in order to get decent quality the records had to spin at 78rpm. There was also a brief-lived record format that spun at 16rpm, but the audio quality was so bad it could really only be used for spoken voice and the like, not music.
They had 16 2/3 rpm records, but the slower the speed the lower the sound quality.
The 78s had the issue of being capable of high quality sound, but short play time, as a 12″ disk gave you 4-5 min of play.
Other speeds exist. There were record played at 16⅔, 33⅓, 45, 78 and other RPM speeds. 33 and 45 were originally two different competing formats by different companies like BlueRay and HD-DVD or Beta and VHS, but the relatively simple implementation made record players that could do both possible and those became the norms for most of the time that record players were used.
One reason why the records couldn’t be slowed down too much more than what was used, is the fact that records were analogue mediums and at some point physics come into play.
Vinyl records use constant angular velocity meaning one rotation takes the same time no matter where in the record you look. (Audio CDs used constant linear velocity instead where the read head covers the same amount of ground per second no matter where.)
What this means that the tracks near the center of a record are much more compact and squished together.
With slower RPMs you may be fine near the rim but the squishing together near the center gets worse and worse.
To produce the same sound the needle still has to swing back and forth in the grove the same way no matter where you are.
Human hearing, especially in younger people, goes all the way up to 20 KHz.
There is a bit of math involved. But if you plug in stuff like the radius near the grove, the highest possible frequency and amplitude and the size of the needle. You will find that you simply can’t use such a system to play back some high frequency, high amplitude sounds.
Not without completely redesigning everything to use constant linear velocity and a number of other changes. That would make record players more complicated and lose compatibility.
The systems we had were a compromise between audio quality and record length and cheap technical implementation. There isn’t too much you could change while keeping the same system and get better results.
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