What Is AAC? Guide to Advanced Audio Coding
AAC (Advanced Audio Coding) is a lossy audio codec standardized as part of MPEG-2 and MPEG-4. Designed as the successor to MP3, AAC delivers significantly better audio quality at equivalent bitrates through more advanced psychoacoustic modeling and signal processing. It is the default audio codec for YouTube, Apple Music, Spotify (on mobile), and virtually every modern streaming platform.
How AAC Encoding Works
AAC improves upon MP3 in several fundamental ways. It uses a pure Modified Discrete Cosine Transform (MDCT) without the hybrid filterbank that limits MP3, allowing better frequency resolution. AAC supports block sizes up to 2048 samples (versus 1152 for MP3), giving it more precise control over the time-frequency trade-off.
The codec includes tools like Temporal Noise Shaping (TNS), which redistributes quantization noise in time to mask it behind transient signals, and Perceptual Noise Substitution (PNS), which replaces noise-like spectral content with synthetic noise at the decoder, saving bits. AAC also supports Spectral Band Replication (SBR) in its HE-AAC profile, extending high-frequency content at very low bitrates by deriving harmonics from the lower frequencies actually encoded. These techniques combined allow AAC to deliver transparent audio quality at roughly 128 kbps, where MP3 typically needs 192 kbps or higher.
Advantages and Limitations of AAC
- Delivers better audio quality than MP3 at the same bitrate across all quality levels
- Default codec for YouTube, Apple Music, iTunes Store, and most streaming services
- HE-AAC profile provides usable quality at extremely low bitrates (32-64 kbps) for speech and streaming
- Supports multichannel audio up to 48 channels (surround sound, spatial audio)
- Natively supported on all modern devices, browsers, and operating systems
- More computationally expensive to encode than MP3, requiring more processing power
- Some older portable devices and car stereos may not support AAC playback
- Multiple AAC profiles (LC, HE, HE-v2) can cause confusion about compatibility
AAC vs MP3: The Technical Comparison
AAC was specifically designed to address the limitations of MP3 while maintaining practical compatibility.
| Feature | AAC | MP3 |
|---|---|---|
| Quality at 128 kbps | Near-transparent | Noticeable artifacts |
| Quality at 256 kbps | Transparent | Transparent |
| Max sample rate | 96 kHz | 48 kHz |
| Max channels | 48 channels | 2 channels (stereo) |
| Low bitrate efficiency | Excellent (HE-AAC) | Poor below 96 kbps |
| Encoding complexity | Higher | Lower |
| Legacy device support | Good | Universal |
Best Use Cases for AAC
- Streaming services where bandwidth efficiency directly impacts user experience
- Mobile audio playback where storage space and data usage matter
- Video soundtracks in MP4 containers (the standard for web video)
- Voice calls and VoIP using HE-AAC for excellent quality at very low bitrates
- Podcast encoding where file size impacts download times and hosting costs
- Any application where better quality per bitrate than MP3 is desired
How to Convert AAC to MP3 or Other Formats
- 1
Identify your conversion goal
Convert to MP3 for broader device compatibility. Convert to WAV for uncompressed editing. Convert to FLAC only if starting from a lossless AAC source (rare).
- 2
Upload your AAC file
Use WeLoveConvert to upload your AAC or M4A file. Both raw .aac files and AAC audio in M4A containers are supported.
- 3
Configure output settings
For MP3 output, use 256-320 kbps to preserve maximum quality. Keep in mind that transcoding between lossy formats always involves some generation loss.
- 4
Download and use
Save your converted file. The output will be compatible with virtually any device or platform.
Frequently Asked Questions
Is AAC better than MP3?
Yes, AAC delivers better audio quality than MP3 at the same bitrate. This advantage is most pronounced at lower bitrates (96-160 kbps). At 256 kbps and above, both formats are effectively transparent for most listeners.
What is the difference between AAC and M4A?
AAC is the audio codec (compression algorithm), while M4A is a file container. M4A files typically contain AAC-encoded audio, but the terms are not interchangeable. A raw .aac file has no container, while .m4a wraps the AAC audio in an MPEG-4 container with metadata support.
What is HE-AAC?
HE-AAC (High Efficiency AAC) is a profile that adds Spectral Band Replication to extend frequency response at low bitrates. HE-AAC v2 adds Parametric Stereo for even better efficiency. These profiles are used for streaming at 32-96 kbps with acceptable quality.
Do all browsers support AAC?
Yes, all major modern browsers (Chrome, Firefox, Safari, Edge) support AAC playback. Firefox added AAC support in 2016. AAC is the most universally supported codec in HTML5 audio and video elements.
What bitrate should I use for AAC?
For music, 192-256 kbps AAC-LC delivers transparent quality. For podcasts and speech, 64-96 kbps is sufficient. For low-bandwidth streaming, HE-AAC at 48-64 kbps provides acceptable quality.
Is AAC lossy or lossless?
AAC is a lossy codec — it permanently removes audio data to achieve compression. For lossless audio in the Apple ecosystem, ALAC (Apple Lossless Audio Codec) is the corresponding lossless option, often stored in the same M4A container.
Why does YouTube use AAC?
YouTube uses AAC because it provides excellent quality at the bitrates used for web streaming (128-256 kbps), supports both stereo and multichannel audio, and is universally decoded by all browsers and mobile devices.
Can I convert MP3 to AAC to get better quality?
No. Converting MP3 to AAC transcodes already-compressed audio through another round of lossy compression, which degrades quality. To benefit from AAC quality, you must encode from a lossless source (WAV, FLAC, or ALAC).
AAC is the modern workhorse of digital audio, powering everything from YouTube and Apple Music to phone calls and podcasts. Its technical superiority over MP3 is well-established, delivering transparent audio quality at lower bitrates and supporting features like multichannel audio that MP3 cannot match. While MP3 retains an edge in universal legacy device support, AAC is the clear choice for any modern audio application. When you need to convert AAC files, WeLoveConvert processes them quickly and privately in your browser.