Third dither test (With Youtube Loudness penalty) of Acustica Audio's "LACE" and Reaper's internal dithering at 16bit.

Be aware there is a sine-wave pinging sound in this video that may appear loud.

Test-procedure steps:

01: Created a 96kHz/32bit sine-wave 'ping' (Using Izotope RX9) that fades from 0 dB FS to absolute silence (Followed by a short insertion of absolute silence to make sure there is a period with zero normalized signal amplitude)
This file has zero harmonics by itself.

02: Loaded the ping-clip into Reaper and lowered the channel-gain of the ping to -80 dB FS using the channel-fader in Reaper.

03: Feeding the -80 dB ping into "LACE" while going through some of LACE's dither-settings at 16bit and rendering this from Reaper with the master-fader at nominal gain (With Reaper's dithering turned OFF)

03a: Rendering the -80 dB ping from Reaper while going through both of Reaper's internal dither-settings at 16bit with the master-fader at nominal gain (Also rendered one with all dithering turned OFF)

04: Loaded all the rendered clips into individual channels on Reaper and used an Airwindows bit-shift-gain set to +11 bits (Should raise the volume without creating artifacts) and once again played each clip by itself (The clips placed first on the time-line in Reaper)

05: Loaded all 6 dithered and non-dithered versions (Of the -80 dB ping-clips rendered from Reaper into 96kHz/16bit files) into Izotope RX9 and created a -6 dB and -12 dB gain-reduced version at 96kHz/16bit (Simulating Youtube reducing the volume of an uploaded file to meet their loudness-target)

06: Loaded all the rendered clips into individual channels on Reaper and used an Airwindows bit-shift-gain set to +11 bits (Should raise the volume without creating artifacts) and once again played each clip by itself (The clips placed second, on the time-line in Reaper, are the -6 dB versions and those placed third are the -12 dB versions)

Conclusion:

In terms of subjectively perceived distortion; the noise-shaped versions survived the gain-reduction better than those only using TPDF without noise-shaping (And the non-dithered versions were the worst in all cases)

In terms of loudness; see the measured values below.

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Relative comparisons with numbers:

Files that were NOT gain-reduced (So that would be those that were exported from Reaper before Youtube's loudness-penalty. The original source-files with optimum quality if you will. The songs you would put on a CD)

No dither TruePeak maximum: -13.7 dB FS / integrated -17.0 LUFS / Crest Factor: 3.3 dB
Reaper dither with NO noise-shaping TruePeak maximum: -10.3 dB FS / integrated -18.6 LUFS / Crest Factor: 8.3 dB
Reaper dither WITH noise-shaping TruePeak maximum: -6.4 dB FS / integrated -14.2 LUFS / Crest Factor: 7.8 dB
LACE dither TPDF TruePeak maximum: -10.6 dB FS / integrated -17.7 LUFS / Crest Factor: 7.1 dB
LACE dither Psychoacoustic TruePeak maximum: -7.1 dB FS / integrated -15.1 LUFS / Crest Factor: 8 dB
LACE dither Entropic TruePeak maximum: -2.7 dB FS / integrated -11.1 LUFS / Crest Factor: 8.4 dB

Files that WERE gain-reduced -6 dB (So that would be those that were struck lightly by Youtube's loudness-penalty and lowered in volume)

No dither TruePeak maximum: -16.9 dB FS / integrated -20.3 LUFS / Crest Factor: 3.4 dB
Reaper dither with NO noise-shaping TruePeak maximum: -15.3 dB FS / integrated -21.3 LUFS / Crest Factor: 6 dB
Reaper dither WITH noise-shaping TruePeak maximum: -11.1 dB FS / integrated -18.1 LUFS / Crest Factor: 7 dB
LACE dither TPDF TruePeak maximum: -15.3 dB FS / integrated -20.1 LUFS / Crest Factor: 4.8 dB
LACE dither Psychoacoustic TruePeak maximum: -12.0 dB FS / integrated -18.2 LUFS / Crest Factor: 6.2 dB
LACE dither Entropic TruePeak maximum: -7.9 dB FS / integrated -15.4 LUFS / Crest Factor: 7.5 dB

Files that WERE gain-reduced -12 dB (So that would be those that were struck hard by Youtube's loudness-penalty and lowered even more in volume)

No dither TruePeak maximum: -22.0 dB FS / integrated -23.5 LUFS / Crest Factor: 1.5 dB
Reaper dither with NO noise-shaping TruePeak maximum: -19.2 dB FS / integrated -23.9 LUFS / Crest Factor: 4.7 dB
Reaper dither WITH noise-shaping TruePeak maximum: -16.3 dB FS / integrated -23.1 LUFS / Crest Factor: 6.8 dB
LACE dither TPDF TruePeak maximum: -19.2 dB FS / integrated -23.6 LUFS / Crest Factor: 4.4 dB
LACE dither Psychoacoustic TruePeak maximum: -17.2 dB FS / integrated -24.0 LUFS / Crest Factor: 6.8 dB
LACE dither Entropic TruePeak maximum: -14.1 dB FS / integrated -20.6 LUFS / Crest Factor: 6.5 dB