How long is my audiobook? Three numbers, and only one measured the audio

the wanting mind is a free book I wrote about why people want. It runs to 36 chapters and about 147,000 words, across neuroscience, economics, philosophy and anthropology. The question started when I lived in meritocratic Singapore and then came back to Japan, a country that runs on community and, in places, on a very clear hierarchy.

Every chapter has English narration in AI voices. The app and its store listing said that narration was about 12 hours long. When I checked in October, the files disagreed, and so did the word-timing data the reader uses.

12 hours: the copy

The in-app purchase screen said "Full 12-hour narrated audiobook with word-sync". Both App Store descriptions said "about 12 hours" (約12時間 in Japanese).

20.5 hours: the files

There are 36 MP3 files, one per chapter, 1,181,948,756 bytes in total. The first file's frame header says 128 kbps. If every file uses the same bitrate:

1,181,948,756 bytes × 8 ÷ 128,000 bit/s ≈ 73,872 s ≈ 20.5 h

That assumes a constant bitrate, so I checked it with ffprobe on every live file. It gave 20.52 hours. This is the number that measures the audio.

16.6 hours: the word timings

Each chapter has an alignment file with a start and end time for every word. The reader uses it to highlight the word being spoken. If the timings covered the whole file, the last word would end near the end of each file. Summed over all 36, the last end times came to 16.56 hours.

So I compared them chapter by chapter on 16 October. In 31 of the 36 chapters, the alignment file ended more than a minute before the audio did. The worst was the preface: its last aligned word ended at 2.9 minutes, in a 9.7-minute file. The other five ended within a minute of their audio.

The highlight is built from these files, so in those 31 chapters it had nothing to follow after the last aligned word. The fix was to regenerate them, and the check was the same sum: it had to come out within a few minutes of 20.5 hours. I re-timed them on 16 October, and on 17 October the sum came to 20.51 hours, with every chapter ending within a minute of its audio. In one chapter and the appendices, though, the audio still held more words than the word list (about 45% and 17% more), and the sum can't see that.

And 11.1 hours, which is something else

The chapters also carry a reading-time estimate. Summed, it is 666 minutes, about 11 hours. That is how long the text takes to read, not to hear.

What it says now

The purchase screen in the current code says "Full 20-hour narrated audiobook", and my posts say "about 20 hours". On 16 October both App Store descriptions still said 12. On 17 October, version 3.13.0 changed both to about 20 hours (約20時間).

The book, and the other ways into it (the concept map, the simulations, the references you can open from the line that cites them), is here: the wanting mind.

I wrote this post with help from an AI assistant. The figures come from the project's repository and the live audio files.