What is AAC?
AAC (Advanced Audio Coding) was developed in 1997 as part of the MPEG-2 and MPEG-4 standards. It was designed to achieve better sound quality than MP3 at similar bit rates, and has become one of the most widely used audio codecs, particularly in Apple's ecosystem and for streaming services.
Key Features
AAC implements more efficient coding strategies than MP3, including:
- Higher frequency resolution for better audio representation
- More advanced psychoacoustic modeling
- Better handling of frequencies above 16 kHz
- Improved stereo coding efficiency
- Native support for multichannel audio (up to 48 channels)
When to Use AAC
AAC is particularly well-suited for:
- iTunes and Apple Music ecosystem integration
- Streaming services where quality-to-size ratio is important
- Videos on platforms like YouTube and social media
- Broadcasting applications (used in digital radio standards)
- Situations where better quality than MP3 is needed at the same file size
Variants and Profiles
AAC comes in several profiles that optimize for different use cases:
- AAC-LC (Low Complexity): Most common variant, balances quality and computational requirements
- HE-AAC (High-Efficiency): Uses Spectral Band Replication for better quality at low bitrates
- HE-AAC v2: Adds Parametric Stereo for even better low-bitrate performance
- AAC-LD and AAC-ELD: Low-delay variants for communications applications
Bitrates and Quality
AAC generally produces better quality than MP3 at equivalent bitrates:
- 96-128 kbps: Good quality for general listening, comparable to higher bitrate MP3
- 160-192 kbps: Very good quality for most listeners
- 256 kbps: Excellent quality approaching transparency for most audio material
Limitations
Some considerations when using AAC:
- Lossy compression: Like MP3, some audio information is permanently discarded
- Compatibility: Not as universally supported as MP3, though still widely compatible
- Container format variations: Can be packaged in different container formats (ADTS, MP4, etc.)
- Patent licensing: Subject to patent royalties in some contexts
Technical Details
AAC files typically use the .aac extension in raw bitstream format (ADTS container) or may be packaged in .m4a or .mp4 containers (MPEG-4 container). The MIME type is "audio/aac" for raw AAC or "audio/mp4" when in MP4 containers. AAC supports sampling rates from 8 kHz to 96 kHz and can encode from mono up to 48 channels of audio.
AAC vs MP3
When compared to MP3, AAC offers several advantages:
- Better audio quality at the same file size
- More efficient at very low bitrates
- Better handling of high frequencies
- Native support for multichannel audio
- Less "smearing" of transients (sharp attacks in the audio)
Conclusion
AAC has established itself as a modern standard for digital audio compression, offering excellent quality with efficient file sizes. Its adoption by major platforms like Apple, YouTube, and many streaming services ensures its continued relevance. For most digital music and streaming applications where file size matters, AAC represents a better choice than MP3 while maintaining broad compatibility.