The .mp4 export function is designed to provide an mp4 file that can playback on a consumer device like a Blu-Ray player/Atmos enabled AVR or an Atmos enabled soundbar.
The .mp4 contains a Dolby Digital Plus with Atmos Content bitstream (.ec3) and matching duration black video at 1280x720 resolution at the framerate of the loaded Atmos master file (.h264).
The audio is encoded with a user selectable profile: Film or Music
Film encodes the audio to Dolby Digital Plus with Atmos Content:
Dynamic Range Control Profile: Film Light
Dialnorm metadata value: set per measurement with ITU 1770-4 with Dialog Intelligence with a speech threshold of 15%
Music encodes the audio to Dolby Digital Plus with Atmos Content:
Dynamic Range Control Profile: Music Light
Dialnorm metadata value: set per measurement with ITU 1770-4
Note: the dialnorm metadata value is set in the .ec3 elementary stream as part of the encoding process. Loudness measurement to confirm compliance with delivery specifications should continue to use the 5.1 re-render for loudness measurement with product from Nugen, Izotope etc.