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Audio mixing: Headphones vs loudspeakers

Posted: 20 Nov 2013     Print Version  Bookmark and Share

Keywords:audio mixing  headphones  loudspeakers  hypothesis testing 

When we think audio mixing, the vision that comes to mind is one of an engineer with twenty fingers (or more) wound around a large ensemble of knobs, wearing a pair of [large] headphones that preclude the possibility of any earthly influences on the mixing process. Now consider that last artifact – the headphones – and ask yourself whether there is a possibility that there is a mismatch of some sort with the fact that in the real wold, the music being mixed will be played back more often not on headphones but on loudspeakers, which offer a substantially different acoustic experience. So should one mix with headphones or on loudspeakers, considering the fact that the latter often makes for a more expensive and elaborate proposition, since headphones allow for easier elimination of extraneous noise? Or is there a difference at all that is worth considering?

This question has been explored a few times. Richard L. King, Brett Leonard, and Grzegorz Sikora recently did in a paper titled "Loudspeakers and headphones: The effects of playback systems on listening test subjects" in the Proceedings of the Meetings on Acoustics, June 2013, published by the Acoustical Society of America. What is interesting about this paper is that the authors apply no-nonsense methods of statistical hypothesis testing to determine if there were genuine statistical differences between the results of mixing different types of music using headphones versus loudspeakers.

Hypothesis testing 101
To understand the authors' results, it's useful to have a basic understanding of statistical hypothesis testing. Suppose I had a visitor called Raju who claimed he was a native of the Indian state of Kashmir. Now I live in the South Indian state of Karnataka and the people of this state look quite different from those from Kashmir. I see Raju is distinctively fair, as I would expect a Kashmiri to be, but not quite as tall as I would expect. So I resort to a test. I dig out two pieces of demographic data: the average heightµ of people in Kashmir, and its standard deviation σ. Knowing that heights tend to follow a Gaussian distribution, I reason to myself that there is less than a 1% chance that a person from Kashmir would have a height of less thanµ-3σ. In Raju's case I find that he is indeed less tall than that, so I have reason to suspect his assertion. I think it is very unlikely that he is from Kashmir (though he may be an exception). In this case we made the assumption of a Gaussian distribution but in general where you cannot make such assumptions, so-called non-parametric methods can be used.

Test setup
The test setup used by the authors was curiously simple. Subjects were asked to complete a basic – almost childish – mixing task: each subject is instructed to set the level of a stereo stem (group or submix) containing a lead instrument or vocal, as presented along with a stereo stem containing the instrumental accompaniment to the excerpt. The stems were extracted from the full mix – not the raw tracks. That's it – just one level to be adjusted for an already equalised and mixed recording, and the result to be compared to the actual level in the original recording. This was to be done with headphones and with loudspeakers. Couldn't be simpler. Do expert recording engineers get it consistent?

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