Every term you’ll meet when listening to, making or publishing audiobooks, in plain words, with a link to the tool or guide that goes further. Figures are from ACX, Audible, Spotify and Apple’s own pages,1,2,3,6,7,8,9 the format definitions from the MP4 and ID3 standards,4,5 and loudness from EBU R 128.10
A
- AAC
- Advanced Audio Coding, the compressed audio format inside most M4B and M4A files. It sounds better than MP3 at the same file size. M4B vs MP3 →
- AAX
- Audible’s protected audiobook format, described by third-party sources as an encrypted M4B. It plays only in Audible’s apps and approved devices. Audible AAX files →
- AAXC
- A newer protected Audible format, downloaded by Audible’s Android and iOS apps. Like AAX, other players can’t open it. Audible AAX files →
- Abridgedalso Unabridged
- A shortened version of a book, read from a cut text. Its opposite, unabridged, reads the whole book.
- ACX
- Audiobook Creation Exchange: Audible’s platform where authors put audiobooks on Audible, Amazon and Apple Books and hire narrators. ACX requirements →
- Audible credit
- A Premium-plan token that buys one audiobook from Audible’s Premium selection, whatever its price. Credits expire 12 months after they are issued. Audible credits explained →
- Audiobooks+
- Spotify’s add-on that gives Premium subscribers 15 more hours of audiobook listening a month ($11.99 in the US). Spotify audiobooks →
B
- Bitrate
- How much data a second of audio uses, in kilobits per second (kbps). It sets file size: 64 kbps is 28.8 MB an hour. Audiobook compressor →
C
- CBRalso VBR, constant bitrate
- Constant bit rate: every second of an MP3 uses the same number of bits. ACX requires CBR MP3s; variable bit rate (VBR) files fail. ACX checker →
- CHAP framealso CTOC, MP3 chapters
- The ID3 frame that marks one chapter in an MP3: its start and end time and, inside it, a title. A CTOC frame lists the chapters in order. Fewer players read them than read M4B chapters. Audiobook chapters →
- Chapter marker
- A named point in an audiobook file where a chapter starts, so players can list and skip chapters. M4B stores them in a chapter track, MP3 in ID3 CHAP frames. Audiobook chapters →
- Closing credits
- The short spoken ending of an audiobook. ACX asks for something final, such as “You have been listening to…”, the title, author and narrator, then “The End”. Credits generator →
D
- dBFS
- Decibels relative to full scale: digital audio levels, where 0 is the loudest possible and everything else is negative. ACX’s level rules are in dBFS. ACX levels explained →
- De-esser
- A processor that turns down sibilance, the hiss of “s” and “sh” sounds, only while it happens, usually with a dynamic EQ at the voice’s own sibilance frequency. Narration clean-up →
- Digital voice narrationalso AI narration, TTS
- Narration produced by text-to-speech or AI voices rather than a person. Stores label it differently: Virtual Voice, Synthesized Voice, Narrated by Apple Books, Narrated by Digital Voice. AI audiobooks →
- DRMalso DRM-free
- Digital rights management: encryption that stops a file playing outside approved apps. Audible’s downloads (AAX and AAXC) are protected; files from Libro.fm and LibriVox are DRM-free. This site’s tools work only on DRM-free files. Audible AAX files →
- Dual narrationalso Duet narration
- Two narrators who trade off by chapter or point of view. In duet narration both perform their characters’ dialogue throughout. Audiobook narrators →
E
- EBU R 128also R128
- The European Broadcasting Union’s loudness standard: programmes normalised to −23 LUFS, peaks no higher than −1 dB true peak. LUFS is the EBU’s name for the unit; the ITU calls it LKFS. Audiobook volume booster →
- Exclusive distribution
- On ACX, selling an audiobook only through Audible, Amazon and Apple Books, for a higher royalty (50% instead of 30% under the new model). Publishing routes →
F
- FLAC
- Free Lossless Audio Codec: compressed audio with no loss of quality. Good for masters, large for listening copies.
H
- Headroom
- The space between a recording’s loudest peaks and 0 dBFS. Recording with headroom (peaks around −6 dB) avoids distortion; levels are raised in mastering.
I
- ID3
- The tag format inside MP3 files for title, author, cover and, through CHAP and CTOC frames, chapters. Metadata editor →
L
- LibriVox
- A volunteer project that records public-domain books and gives the recordings away free as MP3 and M4B. Free audiobooks →
- Limiter
- A processor that stops audio going above a set ceiling. Mastering for ACX uses one at about −3.5 dB so peaks stay under −3 dB. ACX mastering →
- Loudness normalisationalso Normalization
- Turning each track up or down at playback so everything plays at the same loudness. Spotify does it for music, at −14 LUFS by default. It can’t help a file you play in an app that doesn’t, so a quiet book stays quiet. Audiobook volume booster →
- LUFSalso LKFS, integrated loudness
- Loudness units relative to full scale: loudness measured the way ears hear it (EBU R 128, ITU-R BS.1770), from 0 down. Apple asks spoken podcasts for −16 LUFS; ACX states its rule in RMS, not LUFS. Audiobook volume booster →
M
- M4A
- An MPEG-4 audio file, usually AAC. The same format as M4B with a different extension.
- M4B
- An audiobook file in the MPEG-4 format: usually AAC audio with chapters, a cover and tags in one file. What is an M4B file? →
- Mastering
- The last step of audio production: setting loudness, controlling peaks and noise, and exporting the delivery format. ACX mastering →
- Monoalso Stereo
- One audio channel. A single narrator is usually recorded and delivered in mono; ACX requires all files in a book to be mono, or all stereo.
- Mouth noisealso Mouth clicks
- Clicks, smacks and wet sounds from lips and tongue, worst in pauses. ACX asks for files free of excessive mouth noise; water and a relaxed mouth help, and clicks can be softened afterwards. Narration clean-up →
- MP3
- The most widely supported compressed audio format. ACX requires MP3 at 192 kbps or more, constant bit rate.
N
- Noise floor
- The level of background noise in a recording with nobody speaking. ACX requires −60 dB RMS or lower, measured in the quietest half-second. Noise floor meter →
O
- Opening credits
- The short spoken start of an audiobook. ACX asks for the title, author(s) and narrator(s), matching the cover and the book’s details. Credits generator →
- Opus
- An open, royalty-free codec that works from very low bitrates and handles speech well.
P
- Pay for Production
- An ACX deal where the author pays the narrator a one-time fee per finished hour and keeps the whole royalty. ACX deals compared →
- Peak
- The single loudest moment in a file. ACX requires peaks no higher than −3 dB. ACX checker →
- PFHalso Per finished hour
- Per finished hour: how narrators are paid, for each hour of the finished audiobook rather than hours of work. ACX estimates 6.2 hours of work per finished hour. Narrator cost calculator →
- Pitch correctionalso Time-stretching
- Changing playback speed without changing the voice’s pitch, so a narrator at 1.5× sounds faster, not higher. Simply playing the samples faster would raise the pitch. Audiobook speed changer →
- Plosive
- A burst of air from “p” and “b” sounds that thumps the microphone. A pop filter and good mic position prevent it; afterwards, the low thump can be lowered. Narration clean-up →
- Pop filter
- A screen in front of the microphone that softens plosives and helps keep your distance steady.
- Punch-and-roll
- A recording method: at a mistake, go back a little, let the software play a few seconds in, and read on from there, replacing the error. The industry standard for narration. Narration recorder →
R
- Read & Listenalso Whispersync for Voice
- Amazon’s feature (once called Whispersync for Voice) that keeps a Kindle ebook and its Audible audiobook at the same place, so you can switch between reading and listening. Audiobooks on Kindle →
- Retail sample
- The short preview stores play. ACX asks for 5 minutes or less, from the audiobook, ideally its first five minutes, with nothing explicit. Retail sample maker →
- RMS
- Root mean square: the average level of audio, close to how loud it sounds. ACX requires every file between −23 and −18 dB RMS. ACX checker →
- Room tone
- The sound of a recording space with nobody speaking. Used at the start and end of files (ACX: 1 to 5 seconds) and to fill gaps. Room tone explained →
S
- SAG-AFTRA
- The US performers’ union. On ACX, offers of at least $250 PFH (or $100 PFH with Royalty Share Plus) qualify for its health and retirement contributions. Audiobook narrators →
- Sample rate
- How many times a second audio is measured, in kHz. ACX requires 44.1 kHz.
- Sibilance
- The hiss of “s”, “sh”, “z” and “ch” sounds. Some voices and microphones exaggerate it; a de-esser tames it. Narration clean-up →
T
- Top-up
- A one-time purchase of 10 extra Spotify audiobook hours, valid for 12 months. Spotify audiobooks →
- True peakalso dBTP
- The highest level of the waveform including between samples, measured by oversampling, in dBTP. It can sit above the sample peak, and MP3 or AAC encoding can raise it, so masters leave a margin (Apple: −1 dBTP for podcasts). Audiobook volume booster →
V
- Virtual Voice
- Amazon KDP’s AI narration beta, which turns eligible Kindle ebooks into audiobooks sold on Audible and Amazon, clearly labelled as virtual voice narration. AI audiobooks →
- Voice replica
- An AI copy of a narrator’s own voice, offered in an ACX beta; titles are labelled with “[name]’s voice replica”.
- Voices by INaudioalso Findaway Voices
- An audiobook distributor (formerly Findaway Voices by Spotify) that reaches retail, subscription and library stores, paying authors 80% of what it receives. Publishing routes →
W
- WAV
- Uncompressed audio. The usual format for recording and editing before the final MP3 is made.
- Words per finished hour
- How many words fill an hour of audiobook: about 9,300 on average, according to ACX, or 155 a minute. How long is an audiobook? →