Why are monophonic files played at -6dB in Foobar2k. Shouldn't these files play at equivalent amplitude to stereo files?
They are playing to the "center" speaker mapping, and the system is downmixing them to stereo somehow. If you don't like this, add a DSP which upmixes mono to stereo, one is provided with the player.
That's excellent... Thanks. Been trying to understand why my mono WAV files were playing quieter than stereo. Looks like this is a foobar2k PC thing only. Foobar2k on Mac already interprets single channel WAVs as full amplitude on both channels without the DSP addon so I was thrown a bit there. Are there default multichannel WAV file amplitude mix values? Single channel WAVs are applied at -6dB to both left and right. Two channel files are at 0dB channel 1 to left and channel 2 to right. Do you know the default mix ampltudes for the WAV files with more than 2 channels when played on 2 channel systems? Is this a standard or just software preference?
It's not the player reducing the volume. It's outputting a single channel signal, and specifying that the single channel is the front center speaker. The OS is downmixing it using whichever algorithm it decides to use to do that. Apparently, on Windows 10, this can include reducing the volume of both left and right channels rather than just doubling the center speaker to both channels.