Best Audio Format for Audiobooks: M4B, MP3, or FLAC?

Audiobooks Are Not Music

The first thing to understand about audiobook audio formats is that audiobooks have fundamentally different requirements than music. A three-minute pop song and a twelve-hour novel need different things from their container format.

Music needs high bitrate encoding to reproduce the full complexity of instruments, harmonics, and stereo imaging. Audiobooks need low bitrate efficiency because the content is spoken word, which occupies a narrow frequency range. Music files are played from start to finish in one sitting. Audiobooks are listened to over days or weeks, requiring bookmarking and chapter navigation. Music libraries have thousands of short files. Audiobook libraries have fewer files, but each file can be hours long.

These differences make the format choice genuinely important for audiobooks. The wrong format means no chapter markers, lost bookmarks, unnecessarily large files, or compatibility issues with your preferred listening app.

This guide compares every format used for audiobooks — M4B, MP3, AAC, FLAC, and Opus — with honest recommendations based on your devices, apps, and listening habits.

Format Comparison at a Glance

M4B: The Audiobook Standard

M4B is the de facto standard format for audiobooks. It is technically identical to M4A (AAC audio in an MP4 container) but uses the .m4b file extension to signal audiobook functionality to media players and operating systems.

Why M4B Exists

Apple created the M4B designation specifically for audiobooks. When iTunes (now Apple Books) encounters a file with the .m4b extension, it automatically:

• Places the file in the Audiobooks library (not Music)
• Enables bookmark/resume functionality
• Displays chapter markers in the player
• Shows audiobook-specific metadata (narrator, series, etc.)

This distinction matters. If you rename an M4A file to M4B, many players will treat it as an audiobook with bookmark support. If you keep it as M4A, the same file may be treated as a music track with no bookmark functionality.

M4B Technical Details

• Codec: AAC-LC (Low Complexity)
• Container: MP4 (MPEG-4 Part 14)
• Chapters: Native support via MP4 chapter atoms
• Metadata: Full MP4 metadata (title, author, narrator, cover art, series, genre)
• DRM: Supports Apple FairPlay DRM (used by Audible in older downloads)
• Recommended bitrate: 64 kbps mono for speech, 128 kbps stereo for dramatic readings

Creating M4B Files

# Convert MP3 audiobook to M4B
ffmpeg -i audiobook.mp3 -c:a aac -b:a 64k -ac 1 -f mp4 audiobook.m4b

# Convert WAV with chapters (using chapter metadata file)
ffmpeg -i audiobook.wav -i chapters.txt -map_metadata 1 -c:a aac -b:a 64k -ac 1 audiobook.m4b

For creating chapter markers, you need a metadata file:

;FFMETADATA1
title=The Great Novel
artist=Author Name

[CHAPTER]
TIMEBASE=1/1000
START=0
END=1800000
title=Chapter 1: The Beginning

[CHAPTER]
TIMEBASE=1/1000
START=1800000
END=3600000
title=Chapter 2: The Journey

[CHAPTER]
TIMEBASE=1/1000
START=3600000
END=5400000
title=Chapter 3: The Arrival

MP3: The Universal Fallback

MP3 is the most widely compatible audio format in existence. Every device, every app, every operating system plays MP3 files. For audiobooks, this universal compatibility is MP3's biggest advantage — and its lack of chapter support is its biggest weakness.

MP3 for Audiobooks: Pros and Cons

Advantages:

• Plays on absolutely every device
• Well-supported by every audiobook app
• Easy to create and manipulate
• Works with car stereos, old MP3 players, and budget devices

Disadvantages:

• No native chapter support (workaround: split into one file per chapter)
• Bookmarking depends entirely on the player app
• Less efficient than AAC at low bitrates — needs higher bitrate for equivalent quality
• ID3 tags have limited audiobook-specific fields

Recommended MP3 Settings for Audiobooks

For most audiobooks, 64 kbps mono MP3 is perfectly adequate. Human speech occupies a frequency range of roughly 300 Hz to 3,400 Hz for intelligibility, with overtones extending to about 8 kHz for naturalness. At 64 kbps, MP3 preserves this range well. Going higher than 128 kbps for a solo narrator is wasteful — the audio information simply is not complex enough to benefit from the extra bitrate. For a deeper understanding of how bitrate affects quality, see our audio bitrate and quality guide.

# Optimal MP3 encoding for a solo narrator audiobook
ffmpeg -i recording.wav -c:a libmp3lame -b:a 64k -ac 1 \
  -metadata title="Book Title" \
  -metadata artist="Author Name" \
  -metadata album_artist="Narrator Name" \
  -metadata genre="Audiobook" \
  audiobook.mp3

AAC (M4A): Better Quality, Same Container

AAC (Advanced Audio Coding) is the successor to MP3 and provides measurably better audio quality at the same bitrate. At 64 kbps, an AAC file sounds noticeably cleaner than a 64 kbps MP3 — less artifacting, better high-frequency reproduction, and more natural sibilants.

For audiobooks, AAC in an M4A container supports chapters (just like M4B) and has excellent metadata support. The only reason to use M4B over M4A is the automatic audiobook categorization behavior in Apple's ecosystem. If you are not using Apple Books/iTunes, M4A with chapters works identically.

# High-quality AAC audiobook (M4A container)
ffmpeg -i recording.wav -c:a aac -b:a 64k -ac 1 -f mp4 audiobook.m4a

Our AAC converter can handle the conversion from any source format.

FLAC: Overkill for Most Audiobooks

FLAC is a lossless format — it preserves every bit of the original recording. For music, this matters. For audiobooks, it almost never does.

A 10-hour audiobook in FLAC takes up approximately 1.8 GB. The same audiobook in 64 kbps AAC takes up 290 MB. Can you hear the difference between the two on a voice recording played through earbuds during your commute? No. The frequency range and dynamic complexity of spoken word simply do not benefit from lossless encoding in any practical listening scenario.

The only legitimate use cases for FLAC audiobooks:

• Archival masters: If you are a publisher or narrator preserving the original recording for future use
• Dramatic productions with music: Full-cast productions with orchestral scores might benefit from lossless quality on high-end systems
• Personal preference: If storage is not a concern and you want the psychological comfort of lossless audio

If you do use FLAC, convert to M4B or MP3 for actual listening. Our FLAC converter handles this efficiently.

Opus: The Efficiency Champion

Opus is the most efficient lossy audio codec available today. It produces excellent speech quality at bitrates as low as 32 kbps — half the bitrate of equivalent-quality MP3 encoding. This makes Opus extremely attractive for audiobooks, where file size compounds over hours of listening.

A 10-hour audiobook at 32 kbps Opus: approximately 140 MB. The same audiobook at 64 kbps MP3: approximately 290 MB.

The challenge with Opus is compatibility. While most modern devices support it (Android natively, iOS through apps, all major browsers), not all audiobook apps recognize Opus files. If you use a dedicated audiobook player that supports Opus (like Voice on Android or any app that uses the system audio decoder), it is an excellent choice. For maximum compatibility, stick with M4B or MP3.

Platform-Specific Recommendations

Audible

Audible uses its own proprietary formats (AA and AAX) with DRM. You do not choose the format — Audible delivers it. If you are purchasing from Audible, format choice is not relevant.

If you are producing audiobooks for distribution through ACX (Audible's self-publishing platform), they require:

• WAV or FLAC uploads (lossless)
• 192 kbps or higher CBR MP3 (if uploading MP3)
• 44.1 kHz sample rate
• Mono or stereo
• Peak levels at -3 dB
• RMS levels between -23 dB and -18 dB
• Noise floor at -60 dB or below

Apple Books

Apple Books natively supports M4B with full chapter and bookmark functionality. It also plays M4A and MP3 audiobooks. M4B is the recommended format for Apple Books — it provides the best experience with automatic bookmark saving, chapter navigation, and library categorization.

Google Play Books

Google Play Books accepts MP3 and M4B audiobook uploads. MP3 files should be one per chapter with sequential naming. M4B files can be single files with chapter markers.

Libby / OverDrive (Library Apps)

Library audiobook apps stream in their own formats — you do not choose. Downloaded titles are typically in an encrypted MP3 or M4B format that only works within the app.

Self-Hosted (Audiobookshelf, Plex)

If you run your own audiobook server, M4B with chapters is the best format. Audiobookshelf (the leading open-source audiobook server) has excellent M4B support with chapter scrubbing, per-user bookmarks, and metadata management. It also supports folder-of-MP3s format, where each file is a chapter.

Encoding Speech Optimally

Mono vs. Stereo

Most audiobooks should be mono. A single narrator recorded with a single microphone produces a mono signal. Encoding it as stereo doubles the file size while adding no spatial information — both channels contain identical audio.

Use stereo only when the content has genuine stereo elements: full-cast productions with panned voices, ambient recordings, or audio with musical accompaniment that uses the stereo field.

# Force mono encoding
ffmpeg -i input.wav -ac 1 -c:a aac -b:a 64k output.m4b

# Keep stereo for dramatic productions
ffmpeg -i input.wav -c:a aac -b:a 128k output.m4b

Sample Rate

Standard audiobook sample rates:

• 44.1 kHz: Standard for most audiobook production
• 22.05 kHz: Acceptable for voice-only content, produces smaller files
• 48 kHz: Common if the source was recorded alongside video

At 64 kbps, reducing the sample rate from 44.1 kHz to 22.05 kHz allows the encoder to allocate more bits per sample, which can actually improve perceived quality for speech content. However, this is a marginal improvement — 44.1 kHz at 64 kbps is already excellent for speech.

Noise Reduction

Background noise that is barely noticeable in a lossless format can become audible after lossy compression, especially at low bitrates. Apply noise reduction before encoding:

# Apply noise gate and gentle compression for clean speech
ffmpeg -i input.wav -af "highpass=f=80,lowpass=f=8000,acompressor=threshold=-25dB:ratio=3:attack=10:release=200" -c:a aac -b:a 64k -ac 1 output.m4b

The high-pass filter at 80 Hz removes low-frequency rumble, the low-pass at 8 kHz removes unnecessary high-frequency content (allowing the encoder to focus its bits on the important speech frequencies), and the compressor evens out volume variations.

Converting Between Audiobook Formats

MP3 Chapters to Single M4B

# Step 1: Create file list
for f in Chapter*.mp3; do echo "file '$f'"; done > filelist.txt

# Step 2: Concatenate
ffmpeg -f concat -safe 0 -i filelist.txt -c:a aac -b:a 64k -ac 1 audiobook.m4b

M4B to MP3 (for compatibility)

# Single M4B to MP3
ffmpeg -i audiobook.m4b -c:a libmp3lame -b:a 64k -ac 1 audiobook.mp3

# Split M4B into per-chapter MP3s (requires chapter metadata)
ffmpeg -i audiobook.m4b -f segment -segment_times "1800,3600,5400" -c:a libmp3lame -b:a 64k -ac 1 chapter_%03d.mp3

Audible AAX to M4B

Audible files require decryption before conversion. Tools like OpenAudible or inAudible can handle this conversion for audiobooks you have purchased. Once decrypted, the audio can be converted using our audio converter or MP3 converter.

Frequently Asked Questions

What format does Audible use?

Audible uses proprietary formats: AA (older, lower quality) and AAX (newer, AAC-based with DRM). AAX files are essentially encrypted M4B files. You cannot play them in standard media players without the Audible app or authorized software.

Is M4B the same as M4A?

Technically, yes. Both are AAC audio in an MP4 container. The only difference is the file extension. The .m4b extension tells media players to treat the file as an audiobook (enabling bookmarks, chapters, and library categorization). The .m4a extension signals that it is a music file.

What bitrate should I use for audiobooks?

64 kbps AAC mono is the sweet spot for most audiobooks. It produces clean, natural-sounding speech at approximately 29 MB per hour of audio. If you are using MP3 instead of AAC, use 96 kbps for comparable quality. For dramatic productions with music, use 128 kbps stereo.

Can I add chapters to an existing MP3 audiobook?

MP3 does not natively support chapters. Your best option is to convert to M4B with chapter markers. Alternatively, some audiobook players (like Smart AudioBook Player on Android) can create virtual chapter markers based on file splits or time intervals.

How do I make my audiobook smaller?

Three approaches, in order of impact:

1. Use mono instead of stereo (saves 50% if the source is stereo)
2. Use AAC or Opus instead of MP3 (better quality at lower bitrates)
3. Reduce bitrate (64 kbps is sufficient for speech; going lower is noticeable)

For related reading, check our OGG vs MP3 comparison for alternative format options, our best audio format for podcasts guide for similar speech-focused encoding advice, and our lossless vs. lossy compression explainer for understanding what lossy encoding removes.