Audiobook Recording For ACX

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Audiobook Recording For ACX

Post by Cassandra »


Can anyone inform me of some standard effects I should use on my files to help pass ACX quality requirements

What are the ACX Audio Submission Requirements?

Be a 192 kbps or higher MP3, constant bit rate (CBR) at 44.1 kHz
Contain only one chapter/section that is shorter than 120 minutes
Section header must be read aloud
Have room tone at the head and at the tail
Be free of extraneous sounds
Measure between -23dB and -18dB RMS
Have-3dB peak values
Have a maximum -60dB noise floor

Visit the ACX website for the complete Audio Submission Requirements ... /201456300

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Martin Hairer
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Re: Audiobook Recording For ACX

Post by Martin Hairer »

The first five items are quite self-explanatory. You can adjust the peak value by using the "Effects -> Normalize" function. You'll want to set "Maximal sample value" to something below -3dB, so just try -4dB for example. You can then check on RMS by selecting Analyze -> Waveform Statistics and looking at the "Average RMS Power" entry, which should be between -18 and -23. If it's lower, then you may want to apply some compression (Effects -> Audio Unit -> AUDynamicsProcessor) and normalise again.

The noise floor is basically the "Minimum RMS power" entry of the waveform statistics. If this is just above -60dB, you could try to lower it with suitable equalisation, but the only proper way to lower it is to improve your recording setup. (Make sure your room is sufficiently soundproof and your recording gear is high quality.)

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Jim Edgar
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Re: Audiobook Recording For ACX

Post by Jim Edgar »

Martin gives good salient advice - start with the space and the quality of the recording. If the core audio is bad, it's very, very difficult to fix afterwards.

I master and narrate audiobooks for a living. Here are some other points.

Raw recordings should be done in uncompressed audio file formats - either WAV or AIFF (native AP is fine as well). Do not work/edit in MP3. That is only for the final output to upload. Record with conservative levels (peaks in the -6 to -12 dB range are optimal - a bit lower is fine). 24 bit/44.1 khz in the WAV. You also want to make sure you are saving your MP3's at 192 kbps output bit rate (constant).

The most common issues are (1) environmental/background noises and (2) reflections/echoes within the space.

You generally cannot meet ACX spec without some sort of dynamics control. The AU Dynamics Processor which comes native in MacOS can be used to compress the peaks. It can also be set up as a "downward expander" which behaves a bit like a noise gate. Though - if your recordings are noisy, a gate/downward expander typically makes it much more noticeable. EQ can remove the low frequency "rumble" inherent in most home recordings. A Limiter will control the peaks and allow you to balance the Peaks value while increasing the RMS.

In most cases, ACX Mastering includes the following steps (with iterative saves along the way so you can always "go back" a step):
- Raw record (and save a master copy). With most recommended software, you are using "Punch & Roll" to correct errors while recording.
- Edit/QC recording against copy. Text and recording must match.
- EQ (High Pass at a minimum)
- Declick / Denoise (Izotope RX's Mouth Declick and Voice Denoise are really the gold standard here - they will show as plug-ins)
- Dynamics controls - Light Compression (can increase noise floor) to control peaks or a Limiter. Boost RMS and control peak. Loudmax from Thomas Mundt is an excellent transparent (no sound coloration) Limiter available for free.

Then 2nd Opinion by Steven Jay Cohen can be used to make sure all chapters meet spec.
(this is really an invaluable tool)

Then batch process to convert to MP3 (again - Highest Quality, Constant bit rate, 192kbps output).

Also - it's quite possible to meet ACX specs and have really bad sounding audio. Have someone give a listen to make sure there are no audio issues.

I actually offer this as a free service here -

- Jim

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