Audacity is not a mixing desk, it is a multi-track recorder. Multi-track recorders and mixing desks are essentially different in that a mixing desk
mixes multiple
channels of sound into
one (mono or stereo) channel, whereas a multi-track recorder records multiple audio
tracks. Where a correlation does exist between a multi-track recorder and a mixing desk is that when the multiple tracks are played back they are
summed (mixed) to one (mono or stereo) output stream that is played through the sound card in much the same way as a mixer sums (mixes) multiple channels into the master output.
"Mixing" is an additive process for both mixing desks and for the playback of multi-track recordings.
http://en.wikipedia.org/wiki/Audio_mixi ... %29#Mixers
You've not said what make/model of mixer you use, but on all the mixers that I have ever used (and I use mixing desks every day) the level on the master meter
does increase if additional audio signals are added to the mix. For example, if I have an 8 channel mix of instruments that are all playing and it registers on the master meter as -3 dB (peak), if I then start muting channels (mute channel 1 > 2 > 3 > ...) then the level on the master meter will go down until finally the level will be -inf when all channels are muted. Note that this is
not the same as
soloing one or more channels. Typically a soloed channel will show the pre-fader level of that specific channel and not the post fader "mix" level of that channel.
Ianpb wrote: I have worked with real mixing desks and the output level is NEVER higher than the highest input.
The behaviour that you are referring to is a peculiarity of mixing desks. Typically a mixing desk will be designed such that the sum of all channels, when carrying an
average / normal type of music signal where each channel has a peak level of 0 dB and all faders are set at 0 dB, the mix level will be somewhere around 0 dB. This is achieved in the hardware design by reducing the gain of each channel prior to mixing according to the number of channels that the mixing desk has. So for example, a 0 dB signal on one channel will register considerably lower than 0 dB on the master meter, but if you have music on all channels, and all channels are "typical" music channels (such as voices or single instruments), then the master output will be somewhere around 0 dB. Note that it will only be "somewhere around" and not "exactly" as the actual output level will depend on the actual audio signals being fed into each channel. (if same phase and same frequency sine waves, each at 0 dB, are fed into each channel of the mixer then the mix level will be considerably higher than 0 dB, but of course one is unlikely to ever do that in real life situations).
While this approach is very useful on mixing desks it is less desirable (and more problematic) for a multi-track recorder. A mixing desk has a set number of channels, where as the the number of audio tracks in Audacity is not predetermined - there could be just one mono track, or 50 stereo tracks, so it would be impractical to automatically reduce the level of each track by a fixed amount as occurs with hardware mixers. You may suggest that perhaps it would be better to automatically reduce the level of each track according to the number of tracks in the mix, but that is still problematic - if you decide that you want to mute one or more tracks, should the level reduction then be altered to reflect the lower track count in the mix? If you don't do that then you will be applying too much automatic level reduction to the mix, but if you
do do that, then the levels of individual tracks within the mix will be going up and down depending on which tracks are muted and you will have no idea what the
actual level of each track is.
Another problem with attempting to compensate for sum level of all tracks is that with a multi-track recorder, some tracks may be exact duplicates of other tracks. In this case the sum of the two signals is 6 dB higher than the level of one signal alone. This is a lot more than the increase that you would get from unrelated signals (such as a vocal track and a guitar track). On the other hand, one track could identical to another but with the phase inverted. In this case the mix from the two tracks will be silence (and there's no way that you could compensate for that situation).
The "solution" that is adopted by multi-track recorders (such as Pro-Tools, Sonar, Ardour, Reaper, Cubase et al) is for the master meter to show the actual level of the sum (mix) of all channels, and that is what Audacity does.