When using a master tape to create records, one of the primary goals is to preserve as much of the audio as possible. This page explains why music simply sounds better when mastered to 45 RPM than to 33-1/3 rpm, and the simple answer is that more of the original music is preserved at that speed. As the discussion will make clear, speed isn't the only factor that comes into play. The distance from the record's edge to its center, and the shape and configuration of the record's grooves play their part as well. Most of this material comes from companies that master records for a living and was used to explain to customers the differences between 7-inch and 12-inch records and between 45 RPM and 33-1/3 rpm speeds.
Our "Sound Gallery" contains examples of the superior audio you can get from 45s, comparing them to the LP versions.

Why 45 RPM?

The 45 RPM speed is the only one to be decided by a precise optimization procedure (by RCA Victor in 1948). Calculations showed that the optimum use of a disc record of constant rotational speed occurs when the innermost recorded diameter is half the outermost recorded diameter. That's why a 7-inch single has a label 3 1/2 inches in diameter. Given vinyl groove dimensions and certain assumptions about bandwidth and tolerable distortion, a speed of 45 RPM came out of the formula. The 33-1/3 rpm, 12-inch format developed by Columbia was a compromise that attempted to fit more music on a single disc, accepting the limitations that will be discussed in this section on sound quality.

Issues of Platter Size and Rotation Speed

In record mastering, the higher the recorded level and frequency, the greater the curvature of the grooves that get created when making master plates. Curvature isn’t usually a problem, near the outside edge of a 12” 33 1/3 record, but as the groove moves toward the center, its relative speed slows and curvature increases. The record is still turning at 33-1/3 rpm, but one revolution takes 1.8 seconds. That 1.8 seconds at a 12” diameter is covering a lot more territory than at the minimum 4.75” diameter. The result is actually a loss in high frequencies -- an increase in distortion as the groove moves to the center. The problems start when the curvature of the groove equals or exceeds the diameter of the playback stylus. To get around this, one approach is to keep the recorded volume to a reasonable level. In addition, using an elliptical or line-contact stylus that has a smaller tip radius will help. The best solution is to make the record short enough to keep the music away from the very end of the disk, but when you're pressing an LP, that's hard to do.

A better approach is to spin the disk at 45 instead of 33-1/3 rpm. This gives you a 35% increase in groove velocity at any point on the disk, which is a huge advantage. Yes, the groove still slows down as it moves inward, but the effects are greatly reduced. The only problem is that the amount of recorded time is now also reduced by 35% ... hence one of the advantages of the 12-inch record format spinning more slowly. So, 45s are smaller and hold less music, but they sound better!  In addition to playing faster, they technically store "more" of the music. Not more music as in more songs or longer play time, just a more accurate copy of the sound.

More Music Per Inch

This may not be an intuitive conclusion, so let me try an analogy I found on the web: Think of drawing a flipbook character. If you were given 50 pages to draw on and flip through, you could create an animation. But imagine if you drew the same animation on 100 pages. You'd have to flip the pages twice as fast for the character to move at the same speed, but the changes from page to page would also be more slight, making the animation considerably smoother. In this analogy, a 45 has many more "pages" of information to reproduce the music it encodes. Another way of thinking of this is that, like the compression used for files like mp3s, whose quality can be measured in kilobytes per second (kbps), more RPM is like more kbps. Again, you end up with the conclusion that 45s contain more information per second, and more information per second equals better quality.

Since 45s travel faster than 33s, more waveform definition can be squeezed into the format, which takes up more room. More bumps and grooves created in pressing a 45 means better audio quality. As a hypothetical example, suppose  you were able to uncoil the grooves in your record. Let's say one minute of audio takes up one foot at 33-1/3 RPM's. Now, at 45 RPM's the same audio will take up a foot and a half since its traveling faster. At 78 RPM, that one minute groove be 3 feet long.

Again, it is similar to MP3 sound quality issues in the digital world. MP3 audio files encoded with higher kbps options (192, 256 or even 320 kbps) will sound significantly better than those encoded at 128 or 160 kbps. The reason is that the MP3 decoder can benefit from more sound information per second stored in the higher kbps MP3 files. Vinyl records (and magnetic tapes as well) sound better if higher speeds are used. The higher the RPMs, the more vinyl that passes under the playback stylus per second. Therefore, we gain more accurate reproduction of the sound stored in grooves.

Technically speaking, the clarity of the record depends on groove velocity, how fast the vinyl passes under the stylus. The faster it does, the better the sound.

Difference in Grooves

Most people want their records as loud as possible, and that is directly related to the way the grooves get constructed when making a record. The recorded groove is a spiral starting from the edge, going to the center. It's very shallow, and is required to be deep enough for the needle to stay in it. The sound is recorded by vibrating the cutting needle which causes the groove to move from side to side in a mono recording, and somewhat up and down as well in a stereo recording. The more time that gets recorded on the record, the more revolutions the record turns in making the groove, resulting in a longer groove -- and hence a thinner and shallower groove.

Since louder recordings require more vibrations, the needle deflects sideways, and possibly into the adjacent groove causing a skip. To prevent the grooves from colliding, the device that makes the grooves adjusts the spacing between them to make sure they do not collide and cause the record to skip. Therefore, if the recording is long, the grooves are shallower. If the recording levels are louder, there must be more space between them.

Scientifically speaking, the longer wave-lengths, the smaller angles in the grooves and the less complicated geometry at 45 RPM help to cut very precise grooves with even the finest details. With a higher rotation speed, of course, the available playing time per side is reduced at 45 RPM. But if you want the  best possible sound quality, get your records cut at 45 RPM. This is true for 12-inch 45 records as well as the usual 7-inch format.

LP Format Compromises

When you master an LP, you have to put each groove as close to the other as possible. As a result, in loud passages you sometimes can hear bleed-through from the previous groove. (A similar thing can happen in the master tape when one layer of tape magnetizes the next layer on top of it in the reel.) Compromises in the sound must be made to press music in this format, and bass is what mastering engineers have to compress and attenuate the most.

To record as much information as possible on an LP, signals with a wide dynamic range or amplitude have to be reduced in level, otherwise they can damage adjacent channel grooves. In the case of 45s, the cutting engineer has more available surface area and a greater rotation speed to play with, since he only has one track to worry per side. The higher rotation speed of 45 RPM allows for a wider frequency response, and the larger available surface area allows for less compression of any signals with a wide amplitude. Bass is an example of a wide amplitude signal that sounds better on 45. Overtones and high treble are also better.

Exceptions aside, the same master tape can be used to cut an LP and a 45. However, the LP cut requires more compression than the 45 in order to prevent wall damage. Any large amplitude musical passages that can be pressed un-altered on a 45, would very likely have been to be altered (resulting in reduced dynamic range) in order to fit on an LP. And that's why a 45 sound better than a LP.

Loudness Comparison Between LPs and 45s

One mastering company has put together the following guidelines for how much the line level (in decibels) must be altered to accommodate music of different lengths on 45s and LPs. Here's a summary of their data.

12" at 33-1/3 RPM, 20 minutes:0db
12" at 33-1/3 RPM, 24 minutes:-3db
12" at 33-1/3 RPM, 28 minutes:-6db
7" at 45 RPM, 4 minutes:+3db
7" at 45 RPM, 5 minutes:0db
7" at 45 RPM, 6 minutes:-3db

This shows that while for a normal LP with 20 minutes of music, the master tape line level must be set at 0 db, whereas for a normal 45 with 4 minutes of music, the engineer can crank up the volume by 3db. This explains why LPs sound so much softer than 45s as a general rule.

12-inch 45s Are Now A Thing

As awareness of the sound-quality difference between pressing at 45 RPM and pressing at 33-1/3 rpm has grown, so have the vinyl products designed to take advantage of this "new" knowledge. The main evidence for this new awareness is the new availability of classic LPs in a high-fidelity 12-inch 45 RPM format that has accompanied recent years' renewed interest in vinyl. One site that sells such 12-inch records is Music Matters, but I also recently purchased two Bob Dylan albums from the 1960s in this format, made by Mobile Fidelity Sound Labs. Mobile Fidelity Sound Labs now has 26 albums and album sets that are pressed on 12-inch vinyl and mastered to spin at 45 RPM. Clearly, the downside of "convenience" is not an issue when you're willing to pay more and get two 12-inch records instead of one in order to enjoy the music mastered for greater fidelity.

* This information comes from various sources on the web, including Acoustec Mastering, QRates, and Aardvark Record Mastering.