How MP3 and AAC Are Different, and Other iPhone File Types

Discover the audio file types that do & don't work on iPhone and iPod

Definition of MP3 and AAC
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In our age of digital music, MP3s are so common that the people often call any music file an "MP3." But that's not necessarily accurate. MP3 refers to a specific type of audio file. If you use an iPhone or iPod, there's a good chance that most of your music isn't in the MP3 format at all.

What kind of file is it, then? Read on to learn about the MP3, the more advanced and Apple-preferred AAC, and some other common audio file types that do and don't work with iPhones and iPods.

Understanding the MP3

MP3 is short for MPEG-2 Audio Layer-3, a digital media standard designed by the Moving Picture Experts Group (MPEG).

People use MP3s for digital music because songs in that format take up less space than the same songs using CD-quality audio. As a result, more MP3s can be stored in the same amount of space than CD-quality files. Though settings can change this, generally speaking, an MP3 takes up about 10% of the space of a CD-quality audio file (e.g, if the CD-quality song is 10 MB, the MP3 version is 1 MB).

How MP3s Work
MP3s save storage space by compressing the data that makes up the file. This is done by removing some of the audio from the original. Because some data is lost during the conversion, MP3 is called a lossy compression format. Compressing songs into MP3s involves removing parts of the file that won't impact the listening experience (often very high and very low notes).

Because some data has been removed, an MP3 doesn't sound identical to its CD-quality version. This has caused some audiophiles to criticize MP3s as damaging the listening experience.

Bit Rates
The audio quality of an MP3 is measured by its bit rate, rendered as kbps. The higher the bit rate, the better the MP3 sounds.

The most common bit rates are 128 kps, 192 kbps, and 256 kbps.

There are two types of MP3: Constant Bit Rate (CBR) and Variable Bit Rate (VBR). Many modern MP3s use VBR, which makes files smaller by encoding some parts of a song at a low bit rate (for instance, those with only one instrument can be simpler and more compressed), while others are encoded using higher bit rates.

iTunes & MP3s
While MP3 may be the most popular digital audio format online, the iTunes Store does not use it. You can get MP3s from:

iPod/iPhone/iPad compatible: Yes

Understanding the AAC

AAC, which stands for Advanced Audio Coding, is a type of digital audio file and has been promoted as the successor to the MP3. AAC generally offers higher-quality sound than an MP3 while using the same amount of disk space, or less.

Many people think AAC is a proprietary Apple format. It's not. It was developed by a group of companies including AT&T Bell Labs, Dolby, Sony, and Nokia. AAC files can be played on a range of non-Apple devices, including the Microsoft Zune, Sony PlayStation 3 and PSP, the Nintendo Wii, and mobile phones running Google's Android OS, among others.

How AAC Works
Like MP3, AAC is a lossy format. In order to compress CD-quality audio into smaller files, data that will not impact the listening experience of the file is removed. As a result of the compression, AAC files do not sound identical to CD-quality files.

Like MP3s, the quality of an AAC file is measured based on its bit rate and common AAC bitrates include 128 kbps, 192 kbps, and 256 kbps.

The reasons that AAC produces better sounding audio than MP3s are complex. To learn more about the details of this, read the Wikipedia article on AAC.

iTunes & AAC
AAC is the format of all songs sold at the iTunes Store.

With the introduction of iTunes Plus in May 2007, all AAC files sold at iTunes are 256 kbps.

AAC files purchased from iTunes can be authorized to play on a maximum of 5 computers at once. Computers can be authorized and deauthorized regularly, depending on your needs.

iPod/iPhone/iPad compatible: Yes

Other Audio Filetypes: WAV

Stands for Waveform Audio Format. This is a high-quality audio file generally used for applications that require high-quality sound, such as CDs. WAV files are uncompressed, and therefore take up more disk space than MP3s or AACs, which are compressed.

Because WAV files are uncompressed, they contain more data and produce better, more subtle, and more detailed sounds. A WAV file generally needs 10 MB for every 1 minute of audio. An MP3 needs about 1 MB for every 1 minute.

iPod compatible: Yes

Other Audio Filetypes: WMA

Stands for Windows Media Audio. This is the file type promoted most by Microsoft, the company that invented it. It is the native format used in Windows Media Player, both on Macs and PCs. It competes with the MP3 and AAC formats and offers similar compression and file sizes as those formats.

iPod compatible: No

Other Audio Filetypes: AIFF

Stands for Audio Interchange File Format. Another uncompressed audio format, AIFF was invented by Apple in the late 1980s. Like WAV, it uses about 10 MB of storage per minute of music. Because it does not compress audio, AIFF is a higher-quality format preferred by audiophiles and musicians.

iPod/iPhone/iPad compatible: Yes

Other Audio Filetypes: Apple Lossless

Another Apple invention, Apple Lossless Audio Codec (ALAC) is a successor to AIFF. This version, released in 2004, was originally a proprietary format. Apple made it open source in 2011. Apple Lossless balances reducing file size with maintaining sound quality. Its files generally are about 50% smaller than uncompressed files, but with less loss in audio quality than with MP3 or AAC.

iPod/iPhone/iPad compatible: Yes