Audio Joiner (Merge MP3)

About the Audio Joiner (Merge MP3)

Join several audio files into one, in the order you choose: drag the files by their handles (or use the arrow buttons or keys) to arrange them, then press Join.

When every file is MP3 with the same sample rate and channels (or every file AAC with the same settings), they are joined without re-encoding: the audio frames are copied one after another, so it takes moments and the quality is untouched. Otherwise — or when you add silence between files, crossfades, a volume change or loudness matching — the files are decoded and saved as MP3, M4A, WAV or FLAC, with files of other sample rates resampled cleanly.

Match the files’ loudness measures each file with the ITU-R BS.1770 loudness method and turns quiet and loud files towards a common level (each other’s, or a target such as −16 LUFS), never pushing peaks above −1 dBFS. Tags and cover art for the result start as the first file’s and can be edited. Nothing is uploaded.

When files can be joined without re-encoding

MP3 is a sequence of frames and every file starts afresh, so MP3 files with the same sample rate and channel count can simply be put one after another — their bitrates may even differ — and the result plays in every player; the page writes a fresh Xing header so lengths and seeking are right. AAC files can be joined the same way when their encoder configuration matches (the same AudioSpecificConfig: profile, sample rate, channels). The page checks this and lists any file that differs and why.

WAV, FLAC and other formats are decoded and encoded again, which for WAV and FLAC is lossless; transitions, volume changes and loudness matching always need re-encoding because they change the sound.

Gaps, crossfades and loudness

A silence puts that many seconds of digital silence between files. A crossfade overlaps the end of each file with the start of the next and blends them with equal-power curves (cosine out, sine in), so the level does not dip in the middle; every file must be at least twice as long as the crossfade.

Loudness matching measures each file’s integrated loudness as ITU-R BS.1770 defines it: K-weighting, 400 ms blocks with 75 % overlap, an absolute gate at −70 LUFS and a relative gate 10 LU below the level of the blocks that pass the absolute gate. Each file is then set to the target — the files’ median loudness (which moves every file as little as possible) or a fixed level: −16 LUFS is common for podcasts and spoken word, −14 LUFS for music streaming, and −23 LUFS is the EBU R 128 broadcast level. Boosts stop where a file’s sample peak would pass −1 dBFS (such files are marked), and your own volume setting is added on top.

Limitations

• Joining without re-encoding is possible only for MP3 files with matching sample rates and channels, or AAC files with matching encoder settings, and only without transitions or volume changes.
• With AAC joined without re-encoding, the first moments after each join can differ slightly from the originals, because each AAC file starts with its own encoder priming.
• Each MP3 starts with a few milliseconds of silence that its encoder adds (about 25 ms with LAME) and is padded at the end; players skip these only at the start and end of a whole file. A join keeps them, so there is a gap of a few dozen milliseconds at each join: unnoticeable between separate songs or spoken parts, but audible in music that should flow without a break.
• Peaks are measured as sample peaks, not true peaks (BS.1770 Annex 2); leave some headroom when boosting files with very sharp transients.
• Up to 200 files at a time; very long results take time to encode and are kept in memory until you download them.
• Formats browsers cannot decode (WMA, AMR, ALAC outside Safari) cannot be added.

Privacy

Everything happens in your browser. What you enter or open here is not uploaded or stored by MySmartCoPilot. The audio engine (and, when you re-encode, the MP3 or AAC encoder) downloads from MySmartCoPilot the first time you use it. Your files are never uploaded.