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Audiobook Volume Booster

Make a quiet audiobook louder, and even out quiet passages if you like, without distortion. Your M4B or MP3 keeps its chapters, cover and tags.

  • Free
  • No upload
  • No sign-up

M4B, MP3, M4A and more · one file or a folder of chapters · chapters, cover and tags kept

Where it runs

Works in a current Chrome, Edge, Firefox or Safari, on Windows, Mac, Linux and Chromebook computers, Android phones, iPhones and iPads. Nothing to install, no account.

The first file loads the audio engine (ffmpeg, about 31 MB) once, and your browser keeps it. Very long books are easiest on a computer, which has more memory than a phone.

How to use it

4 simple steps.

  1. 01

    Drop the quiet book

    An M4B, an MP3, or a folder of chapter files. Each is measured where it sits on the loudness scale.

  2. 02

    Pick how loud

    A little louder, podcast level or music level, and whether to even out quiet passages. Twenty seconds play before and after.

  3. 03

    Make it louder

    The gain is tested on stretches of the book first, then the whole file is re-encoded once, with a limiter guarding the peaks.

  4. 04

    Download and listen

    Chapters, cover and tags come through. The result is measured again, so you see the new level, not a promise.

Why audiobooks are quieter than everything else

Loudness is measured in LUFS (loudness units relative to full scale): the closer to zero, the louder. Different worlds aim at different levels:

WhereLevelWho says so
Music on Spotify−14 LUFSSpotify normalises playback to it1
Podcasts on Apple Podcasts−16 LUFS ±1, true peak −1 dBApple’s audio requirements2
TV and radio in Europe−23 LUFSEBU R 1283
Audiobooks on Audible (via ACX)−23 to −18 dB RMSACX’s submission requirements4
Two LibriVox chapters we measured−27 and −28 LUFSOur measurement, October 2026

ACX measures RMS rather than LUFS, but the gist is plain: audiobooks are allowed to be much quieter than music and podcasts, and volunteer and older recordings often are. Switch from a song to a chapter and you reach for the volume; in a car or on a train, the top of your phone’s range may not be enough.

What the booster does

  1. Measures the book’s integrated loudness (EBU R 128, as ffmpeg’s meter computes it) on twelve 30-second stretches spread through it. On our test chapters that came within 0.1 dB of measuring every second, in about two seconds instead of minutes.
  2. Tests the gain. The gain to your target is tried on the same stretches through the limiter, and corrected (up to three times), because a limiter takes a little loudness from lively narration.
  3. Limits the peaks at −2.5 dBFS, so true peaks stay under −1 dB even after encoding, Apple’s ceiling for spoken audio.2
  4. Re-encodes once, at the source’s own bitrate, with its chapters, cover and tags.
  5. Measures the result the same way, so the number you see is the file you get.

Tested

BookBeforeTargetAfter (whole file)True peak
LibriVox, 20-minute chapter−27.0 LUFS−16−16.0 LUFS−1.4 dB
LibriVox, 17-minute chapter, evened out−28.3 LUFS−16−16.1 LUFS−1.5 dB
Uneven test file (passages 6 dB apart), evened out−36.8 LUFS−16−15.9 LUFS−1.5 dB

Measured with ffmpeg’s EBU R 128 meter over the whole processed file, before encoding; the M4B fixture in our browser test came out at −16.2 LUFS with a −1.2 dB true peak after AAC encoding. The tool was also measured against ffmpeg’s own loudnorm filter: in a single pass it stopped 3 dB short of the target and took more than twice as long, so it isn’t used.

Even out, or just louder?

“Just louder” turns the whole book up by the same amount: the narrator’s performance, loud and soft, is kept exactly. “Even out” also narrows the gap between quiet and loud stretches, over a window of about 15 seconds, so a whispered chapter or a narrator who drifts away from the microphone stays audible. It doesn’t squash individual words. On our uneven test file, passages 6 dB apart came out within 2 dB, and the loudness range fell from 17 to 6 LU; the pauses between sentences rose about 3 dB against the voice. For a car or a noisy commute, even out; for headphones at home, louder is usually enough.

Books you can’t boost

Audible’s AAX files, Libby’s and Spotify’s audiobooks are protected: they play only in their own apps, and changing them would mean removing DRM, which this site never does. Turn them up with your phone or car instead. Everything DRM-free works: LibriVox, Libro.fm and Downpour downloads, ripped CDs, your own recordings. Not sure what you have? The audiobook inspector tells you, and the Audible AAX guide explains the protected formats.

Making an audiobook rather than listening to one? Use ACX mastering, which hits ACX’s own levels and format.

Sources

  1. Loudness normalization on Spotify, Spotify for Artists, checked 23 October 2026
  2. Audio requirements, Apple Podcasts for Creators, checked 23 October 2026
  3. EBU R 128: Loudness normalisation and permitted maximum level of audio signals, European Broadcasting Union (v5.0, 2023)
  4. What are the ACX audio submission requirements?, ACX Help

FAQ

Questions, answered.

Why is my audiobook so quiet?

Audiobooks are mastered quieter than music. ACX, which supplies Audible, accepts files from −23 to −18 dB RMS, while Spotify plays music at −14 LUFS and Apple asks podcasts for −16. Older and volunteer recordings are often quieter still: two LibriVox chapters we measured sat at −27 and −28 LUFS. On a phone in a car or on a train, that’s too quiet.

How do I make an audiobook louder?

Drop the file here, choose podcast level (−16 LUFS) and press the button. The tool measures the book, works out the gain that reaches that level through a limiter (so nothing distorts), and re-encodes it at its original bitrate with its chapters, cover and tags.

Why is Audible so quiet, and can this boost Audible books?

Audible’s books come through ACX’s rules, which allow files as quiet as −23 dB RMS. Audible’s AAX files are DRM-protected and can only play in Audible’s own apps, so no tool should change them; use your device’s volume or its own loudness settings. This booster is for DRM-free files: Libro.fm and Downpour downloads, LibriVox, CDs you ripped, files you made.

What LUFS should an audiobook be?

There is no single listener standard. Apple recommends −16 LUFS (±1 dB) for spoken podcasts with peaks no higher than −1 dB true peak; Spotify plays music at −14 LUFS; broadcast TV in Europe is −23 LUFS (EBU R 128). For listening on a phone, −16 is a good level; −14 if your surroundings are loud.

Will boosting the volume distort it?

Not with this tool: a limiter holds every peak under −2.5 dBFS (under −1 dB true peak after encoding), and the gain is tested first so the limiter only touches rare peaks. If a book has unusually loud peaks for its level, it stops short of the target rather than squashing them, and tells you.

What does “Even out loud and quiet passages” do?

It turns quiet stretches up and loud ones down over a window of about 15 seconds, so a whispered scene or a quiet narrator doesn’t vanish under road noise. On a test book whose passages differed by 6 dB, they came out within 2 dB. The cost: the pauses between sentences come up a little too.

Does it keep chapters and the cover?

Yes. Chapters, the cover and tags such as title, author and narrator are copied to the new file. An M4B stays an M4B and an MP3 stays an MP3, at the original bitrate (64 to 128 kbps for M4B, up to 192 kbps for MP3).

Is my audiobook uploaded?

No. It is measured and re-encoded inside your browser on your device. Nothing is sent anywhere.

How is this different from ACX mastering?

ACX mastering is for people making audiobooks: it hits ACX’s RMS range, peak limit and noise rules and outputs ACX’s MP3 format. The booster is for listening: it makes a finished book louder to a level of your choosing and keeps its own format, chapters and cover.

Keep going

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