Monday Sep 25

Hearing study supports need for mono switching

In The Astonishing Hypothesis (1994) Nobel laureate Francis Crick, one of the DNA guys, describes a study on selective attention in hearing that makes clear the need for a stereo/mono switch for playback of single channel source material.   The main sources this affects are mono LPs, 45s, and 78s, but I have a few CDs were mastered from mono LP sources in which the production engineers did not take the obvious and necessary step of mixing down the left and right channels.

When playing a mono LP on a well balanced system, the music will be heard as coming from a phantom source centered between the left and right channel speakers. You don't need a switch to make that happen. Your neurons take care of that quite nicely when they pick up on the identical left and right signals. But even with the most carefully cared for LPs, differences between the two signals will exist: a snap here, a pop there, with crackles hither and yon.

If you have a switch you can throw to mix the left and right channels, the snaps, crackles, and pops get thrown to the middle along with the music. The music is generally going to be louder than the noises and will tend to mask them.

"But can't I simply focus on the music and not pay attention to the occasional odd noises coming from the other directions?"

No.  That doesn't work. Here's Crick's account of what happens when we try to do that.

"An example of selective attention in hearing is a subject concentrating on sounds coming from earphones in one ear while trying to ignore different sounds coming into the other ear. Many of the sounds coming in the unattended ear do not reach consciousness but it can be shown that they may leave some trace in the brain and may sometimes influence what is heard in the attended ear [my emphasis]."

Extending this to listening to monaural sound with electrically separate left and right speakers, the study would suggest that our perception of the centered up music can be compromised by the presence of low level noises from other directions that we're supposedly ignoring. "At some level," Crick adds, "they are being registered by the brain."