Convert MPEG-4 Audiobook (.m4b) to Adaptive Multi-Rate Audio (.amr) online for free. Fast, secure audio conversion with no watermarks or registration.
Upload your .m4b file by dragging it into the upload area or clicking to browse.
Choose your output settings. The default settings work great for most files.
Click Convert and download your .amr file when it's ready.
Converting M4B audiobooks to AMR produces extremely compact spoken-word files optimized for telephony networks. AMR (Adaptive Multi-Rate) encodes at 4.75-12.2 kbps — even lower than the M4B source (typically 32-64 kbps AAC). Since M4B content is almost always spoken word, AMR's speech model handles it reasonably well, producing intelligible narration at a fraction of the original file size. Chapter markers and bookmarking are lost.
Phone-based audiobook or educational delivery systems using IVR (Interactive Voice Response) may require AMR format. Distribution of spoken-word content via MMS has file size restrictions that AMR's extreme compression satisfies. Some IoT devices used for audio playback in remote locations have AMR as their only supported codec.
FFmpeg decodes the M4B's AAC to PCM, resamples to 8 kHz mono (AMR-NB requirement), and encodes via libopencore-amrnb. The conversion discards frequency content above 3.4 kHz and reduces sample resolution dramatically. For a 10-hour audiobook at 64 kbps M4B (~28 MB), the AMR at 12.2 kbps output is approximately 5.5 MB — an 80% size reduction.
AMR at 12.2 kbps produces telephone-quality speech. For well-recorded audiobook narration with clear diction, the result is perfectly intelligible. Background music, sound effects, and nuanced vocal performances lose substantial detail. The quality is comparable to listening to the audiobook over a phone call.
| Device | M4B | AMR |
|---|---|---|
| Windows PC | Partial | Partial |
| macOS | Partial | Partial |
| iPhone/iPad | Partial | Partial |
| Android | Partial | Partial |
| Linux | Partial | Partial |
| Web Browser | No |
Resolution: N/A
Bitrate: 320 kbps
OGG Vorbis preferred
Resolution: N/A
Bitrate: 256 kbps
AAC format required
Resolution: N/A
Bitrate: 128 kbps
Lossless FLAC/WAV for best quality
Resolution: N/A
Bitrate: 128 kbps
MP3 mono for spoken word
M4B to AMR serves telephony-based and bandwidth-constrained distribution of spoken-word content. Quality is adequate for understanding narration but far below the M4B original.
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| No |
| Funktion | M4B | AMR |
|---|---|---|
| Fulde navn | MPEG-4 Audiobook | Adaptive Multi-Rate Audio |
| Filendelse | .m4b | .amr |
| Bedst til | Chapter support | Speech optimized |