Convert AAC to MP3
AAC is the audio codec inside MP4, M4A, and many streaming platforms. A bare .aac file is a raw ADTS bitstream - just the audio, no container (see Codecs vs containers for the wrapper concept). Wave decodes the AAC frames in your browser and re-encodes to MP3 at 192 kbps VBR through libmp3lame. AAC vs MP3 trade-offs are covered in Audio codecs explained.
What actually changes
Raw AAC files carry the codec's ADTS (Audio Data Transport Stream) bitstream directly, without a wrapping container. That format was designed for streaming - each frame is self-delimiting with a sync word - so it appears in HLS captures, some legacy podcast enclosures, and streaming-server pipelines. The more common form of AAC in the wild is inside an MP4 container as .m4a or as the audio track of .mp4 video; for those, use the M4A to MP3 or MP4 to MP3 page instead.
Wave decodes AAC through ffmpeg-wasm's AAC decoder (which handles LC, HE-AAC v1, and HE-AAC v2 profiles) and re-encodes to MP3 at 192 kbps VBR through libmp3lame. AAC is generally more efficient than MP3 at lower bitrates - a 96 kbps AAC file can sound like a 128 kbps MP3 - but at Wave's 192 kbps target that efficiency gap closes to nothing. The MP3 preserves everything audible in the AAC source.
AAC was standardised in MPEG-2 (1997) and refined in MPEG-4 (1999) as the successor to MP3 within the ISO standards world. Apple made it the default codec of the iTunes Store from launch in 2003, YouTube adopted it for the audio track of most H.264 videos, and streaming services standardised on HE-AAC for low-bitrate mobile delivery. The result: essentially every video file people watch and every music file people stream through 2003-2015 has AAC audio inside somewhere.
The efficiency-vs-compatibility trade Wave targets makes MP3 the right output. AAC is superior at low bitrates but MP3 has been shipping on every consumer audio device since 1995. The MP3 output at 192 kbps VBR is transparent for typical AAC sources, plays on every phone / speaker / car head unit / voice assistant that plays sound, and doesn't need to convince anyone that it'll work. That reliability is the point.
Three steps.
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Step 1
Drop the .aac file onto the card.
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Step 2
Wave inspects the ADTS stream and confirms Decode-and-re-encode.
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Step 3
Convert. Save the MP3.
Answers, specifically.
Is my source an .aac raw file or .m4a container?
The extension names both. Raw .aac is a bare ADTS bitstream (this page); .m4a is AAC inside an MP4 container (see the M4A to MP3 page). Wave inspects the file and picks the right decoder either way, but the M4A page has more format-specific context.
HE-AAC (High Efficiency AAC) support?
Yes. HE-AAC v1 and v2 both decode through ffmpeg-wasm. These profiles appear in radio streams and low-bitrate podcast enclosures.
AAC efficiency vs MP3 - does the MP3 sound worse?
At 192 kbps VBR (Wave's target), no. AAC is more efficient at low bitrates (< 128 kbps); above that, both codecs are transparent on consumer playback.
HLS stream capture (.aac from an m3u8)?
Yes. Wave decodes ADTS streams cleanly regardless of source. If you captured audio-only .aac from a HLS live stream, Wave produces an MP3.
5.1 or 7.1 AAC?
AAC-LC supports up to 6 channels (5.1); higher-channel-count content is rare in raw ADTS form. Wave downmixes to stereo before the MP3 encode.
What sample rate does the MP3 land at?
The source rate or nearest supported MP3 rate (44.1 or 48 kHz for typical content). LAME resamples internally if needed.
Are metadata tags preserved?
Raw ADTS files don't typically carry metadata; the MP3 arrives untagged. If your source is .m4a (AAC in MP4), metadata does transfer - use the M4A to MP3 page.