Convert M4B to AMR — Free Online Converter

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.