Track Volume is a number in a media file that says how loudly to play one track. A track is one part of a movie, such as the picture, a sound version, or subtitles.
Track Volume does not measure how loud the recording is. The value sits in the track header, apart from the sound data, and works as a playback instruction.
Which files carry it
Track Volume belongs to the QuickTime file format and the formats related to it. You find the field in MOV and MP4 videos, and in M4A audio files. ExifTool, a common metadata reader, lists Audible's AAX audiobooks in the same format family.
Every track in these files needs its own track header, so every track has its own value. A movie with sound tracks in English, French, and Japanese has 3 sound values, one per track.
A track is not the same as a channel. A stereo track holds 2 channels and still has only one Track Volume value.
Not every media format has this field. The Matroska specification, which defines MKV files, lists no volume element for a track.
What the values look like
The track header, stored under the name tkhd, keeps Track Volume as a 16-bit fixed-point number. A report may print that number in a different form. ExifTool, for example, divides the stored number by 256 and prints a percentage with 2 decimals.
These are the values you normally see:
0x0100in the file, or1.0as a decimal, means normal volume. ExifTool prints this value as 100.00 percent.0in the file, or0.0as a decimal, normally appears on a track with no audible media, such as a video track. ExifTool prints this value as 0.00 percent.
QuickTime software also allows other numbers. Apple's older QuickTime developer documentation says the tracks in a movie can have different volumes. It gives a range from -1.0 to 1.0, and a negative value plays no sound.
Where the value comes from
The program that writes the file sets Track Volume. FFmpeg, an open-source media program, writes 0x0100 for every audio track and 0 for every other track. FFmpeg writes the same value whether the recording is quiet or loud.
How Track Volume differs from other volume values
Other values also describe volume, and they have different purposes:
- Preferred Volume sits in the movie header and says how loud to play the whole movie. A value of
1.0means full volume. - The volume control in a media player belongs to the player, not to the track header.
- An MP3 file can use the ID3v2 frame
RVA2to adjust the volume of each channel during playback.
Why it matters for your file
A value of 0 does not prove that your file is silent. That value normally belongs to a track with no sound, such as the picture track. Check the Codec Type of each track to see which one holds the sound.
A report can also show fewer values than the file holds. By default, ExifTool keeps the first Track Volume it finds and leaves out the rest.
If the picture track comes first, the report shows 0.00 percent even when an audio track holds 1.0. The ExifTool option for duplicate tags shows the other values.
You also cannot assume that every player follows the number. The FFmpeg reader for these files skips the stored value and does not use it. Track Volume holds no personal information.