Mastering
The final audio processing stage before delivery, where recorded chapters are adjusted for loudness, peak levels, noise floor, and format to meet distributor specifications.
Mastering is the last step in audiobook production before files are delivered to the publisher or distributor. It involves processing the recorded audio to meet specific technical specifications: adjusting the overall loudness (RMS or LUFS), ensuring peaks do not exceed the maximum allowed level, verifying the noise floor is low enough, and exporting in the required file format and bit rate.
For ACX submissions, mastering means hitting precise targets: RMS between -23dB and -18dB, peak level no higher than -3dB, noise floor at -60dB or below, and final export as 192 kbps CBR MP3 at 44.1 kHz, 16-bit. The typical mastering chain for audiobook narration includes noise reduction (if needed), equalization to clean up the frequency spectrum, compression to even out volume differences between loud and quiet passages, limiting to catch peaks, and finally loudness normalization to hit the target RMS.
Many narrators use preset mastering chains in their DAW or standalone tools that apply these processing steps in sequence. The key principle is that mastering should be transparent: it brings the audio into technical compliance without noticeably changing how the narration sounds. If listeners can hear that mastering was applied, such as pumping from over-compression or artifacts from aggressive noise reduction, the processing was too heavy. The best mastering starts with a well-recorded source, which is why consistent microphone technique and a quiet recording environment matter more than any processing chain.