What Makes an MP3 Different From an AAC?

People often call any music file an "MP3," but that's not accurate. MP3 is a specific type of audio file and not every digital audio file is an MP3. If you use an iPhone or other Apple device, there's a good chance that most of your music isn't MP3s at all.

So what kind of files are your digital songs? This article explains the details of the MP3 filetype, the more-advanced AAC format used by Apple, and some of the other common audio file types that do and don't work with iPhones and iPods.

What MP3s Are and How MP3s Work

MP3 is short for MPEG-2 Audio Layer-3. It is a digital media standard designed by the Moving Picture Experts Group (MPEG), an industry group that creates technical standards.

Songs saved in the MP3 format take up less space than the same songs saved using a CD-quality audio format like WAV (more on that later).​​ They do this by compressing the data in the song. Compressing songs into MP3s involves removing parts of the file that won't impact the listening experience, usually the very high and very low ends of the audio. Because some data has been removed, and an MP3 doesn't sound identical to its CD-quality version, MP3 is referred to as a ​​​"lossy" compression format​. The loss of some parts of the audio has caused some audiophiles to criticize MP3s as damaging the listening experience, though many people can't hear the difference.

Because MP3s are compressed, more MP3s files can be stored in the same amount of space than files using lossless compression formats. Generally speaking, an MP3 takes up about 10% of the space of a CD-quality audio file. So, if the CD-quality version of a song is 10 MB, the MP3 version will be around 1 MB (this can change based on your audio encoding settings).​

Understanding Bit Rates and MP3s

The audio quality of an MP3 (and all digital music files) is measured by its bit rate. Higher bit rates mean the file has more data that the MP3 sounds better. The most common bit rates are 128 kps, 192 kbps, and 256 kbps.

There are two kinds of bit rates used with MP3s: Constant Bit Rate (CBR) and Variable Bit Rate (VBR). Many modern MP3s use VBR, which makes files smaller by encoding some parts of songs at a lower bit rate and others at higher bit rates. For example, a section of a song with only one instrument is simpler and can be encoded with a lower bit rate. Parts of a song with more complex instrumentation need to be less compressed to capture the full range of sound. By varying the bit rate, the overall sound quality of an MP3 can stay high while the the file size is reduced even more.

How MP3s Work With Apple Music and iTunes

MP3 may be the most popular digital audio format online, but neither Apple Music nor the iTunes Store offer music in that format (more on that in the next section). Despite that, MP3s are compatible with Apple Music, iTunes and with all iOS devices, like the iPhone and iPad. ​​You can get MP3s from:

• Digital download stores.
• Ripping songs from CD, depending on your music-conversion settings.
• Many music-file-sharing services.​

All About AAC Audio Files

AAC stands for Advanced Audio Coding. It's is a digital audio file type that 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, but that's wrong. AAC was developed by a group of companies including AT&T Bell Labs, Dolby, Nokia, and Sony. While Apple has adopted AAC for its music, AAC files can actually be played on many non-Apple devices, including phones running Google's Android OS, game consoles, and others.

How AAC Works

AAC is a lossy file format, just like MP3. To compress CD-quality audio into files that take up less storage space, data that will not impact the listening experience—again, generally at the high and low end—is removed. As a result, AAC files do not sound identical to CD-quality files, but generally, sound good enough that most people don't notice the difference.

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

How AAC Works with Apple Music and iTunes

Apple has adopted AAC as its preferred file format for audio. All songs streamed or downloaded from Apple Music, or sold at the iTunes Store, are in the AAC format. ​All AAC files offered by Apple are encoded at 256 kbps.

Other Kinds of Audio Files That Work with iPhone, iPad, and Mac

While MP3 and AAC are the most popular kinds of audio files used with iPhone, iPad, Mac, and other Apple products, they're not the only ones that work. Here's a look at some other widely used Apple-compatible audio formats.

WAV Audio Files

WAV is short for Waveform Audio Format. This is a high-quality audio file often used on as CDs. WAV files are uncompressed, and therefore take up more disk space than MP3s or AACs.

Because WAV files are uncompressed (known as a "lossless" format), 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. By comparison, an MP3 needs about 1 MB for every 1 minute.

WAV files are compatible with Apple devices but are not commonly used, except by audiophiles.

WMA Audio Files

WMA stands for Windows Media Audio. This is the file type promoted by Microsoft, the company that invented it. It is the default 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. It is not compatible with the iPhone and iPad.

AIFF Audio Files

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. Since it was invented by Apple, it's compatible with Apple devices.

Apple Lossless Audio Files

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

FLAC Audio Files

FLAC (Free Lossless Audio Codec​) is an open-source audio format that's Popular with audiophiles. It can reduce the size of a file by 50-60% without reducing audio quality too much. FLAC is not compatible with iTunes or iOS devices out of the box, but it can work with additional software installed on your device. ​

Audio Files That Are Compatible with the iPhone, Mac, and Other Apple Products