Two Bachs at a Time

I happen to have a radio receiver both in the living room and in my own room. Often both of these are turned on at the same time to the same station (the countrywide classical station, obviously), so they play exactly the same thing. I noticed a peculiar effect when the two radios were turned on. Whenever I am in the proximity of one of them, I ignore the other, pretty much not hearing it at all – at least, that’s how I perceive it.
I might sit and read in my room, listening to Beethoven’s seventh symphony, being distinctly aware that it is emitted from the speakers behind me. Then I go to the kitchen to take a glass of water, and on the way, a phase transition occurs. At some point during the long and arduous trip to the water tap, I stop perceiving the music as coming from my own room, and notice that it is instead being played by the system in the living room. There is no trace whatsoever of any sound coming from my bedroom. Try as I might to observe otherwise, the facts are there: there is only one orchestra playing, and it is sitting squarely behind the couch in the living room.
What, did I all of a sudden grow half deaf and ignore half the music? Nope, this is just how my brain interprets the input from the ears based on sound intensity and echoes. I know for certain that the music is still playing on both radios, and that I am physically able to hear it: when I stand near one receiver and mute it, I can still hear the other playing. So what makes this happen?
One could try to explain this phenomenon by blaming interference between the two sound waves – that the two waves negate each other – but there is so much echo inside my house that this hardly seems to be the case. What’s more, when standing directly between the rooms, each ear is supposed to get data without interference. No, interference will not save the day, but a bit of cognitive science will.
It seems that our brains were not built to handle situations where two exactly identical sounds come from two different directions. Instead of taking the auditory signals and translating them into two different perceived objects, we combine the signals, and generate just a single object in our mental map of the world.
Ok, so we had some big words in that last paragraph. Let’s ask an interesting question: when a vase is dropped and we hear it shattering behind us, how do we know that it is really behind us? Our brain holds a “mental image” of the world. Look around you – you see all sorts of objects – a computer screen, a table, a window, maybe a picture on the wall. Now close your eyes, and imagine how the room looks like. You can still do that, even though there is no input coming into your eyes. Sure, you might be missing in on the little details (what’s the shape of the leftmost button on your computer monitor?), but nonetheless, you see that your brain can hold an image of your surroundings. Much of this image is constructed using visual data, meaning, what we see. However, almost all of our senses contribute to its making – our sense of smell, our sense of touch, and even some of the lesser known senses, such as our sense of spatial orientation. Also participating is our sense of hearing.
So an important task the brain is required to perform, is to create a mental image of the world from sounds. How does it do this? Much like how you take raw photon data from your two eyes and turn them into a three dimensional model, so do you do with hearing. Differences in pitch and volume exist between the inputs that reach the ears, and these help you unconsciously calculate from which direction the sound came from.
Apparently, our brain has not evolved to take into consideration the possibility that two sources will emit the same sound at the same time. It is quite reasonable to assume that our hearing and auditory perception looks as it does because of two main factors: communication, and hunting / survival. When dealing and speaking with other people, there are no two people who sound alike, so it is relatively safe to say that we did not encounter identical sounds when interacting with our living relatives. And when hunting or surviving in the wild, it is of crucial importance to know exactly from where a sound came from, and this sound had just a single source – the snapping of a twig by a tiger looming behind us, for example. So our brain evolved with this assumption: a single pattern of sound is created by a single physical entity.
Of course, this stopped being true once we could replicate sounds with good enough precision, which I reckon started with antique musical instruments. However, there both wasn’t enough time nor evolutionary reason for us to adapt to this fact, and we still only conjure one mental object when we hear a musical pattern.
So what happens when we hear the same music from several sources at once? We merge them together and treat them as one. However, it is possible to trick the brain in this case. You can place two speakers to the left and right of a man, and have them play the same thing but at different volumes, so that that man perceives the source of the sound to be somewhere in the middle of the line connecting the two speakers – but not any individual speaker.
I tested this out myself today. Putting Pergolesi’s Stabat Mater in the CD player, I sat in front of the two speakers, and played around with the balance knob on the amp. I could cause all the music to emanate from just one speaker, and I perceived this perfectly well – looking at the speaker, I just “knew” that it was the one playing the music. This effect continued also when both speakers where working, but where very uneven in their intensities. When the left one was at 90% and the right one at 10%, then I heard the music as emerging only from the left – this is the same phenomenon as I described at the beginning of this paper. If the left one had been at 0%, and the right at 10% – I would have heard the music as coming from the right, but the fact that the left is so much stronger completely eliminated this possibility from my brain (in general, almost all of our senses are relatively calibrated – we rarely judge absolute magnitudes when giving detail to our internal 3d model of the world). Given the distances between the two speakers (several meters), my brain probably interpreted the situation with the 10% part as just an echo.
The nicest part indeed was that for some balance ratios, the apparent source was neither of the speakers, but somewhere in the middle. That same feeling of “knowing” that you are looking at the source occurred only when I looked in between them. If you have ever been at an advanced IMAX cinema, this is how they emulate the sound of moving footsteps, using an advanced surround system.
All in all, much as there are visual tricks concerning colors, straight lines, and ducks peeking from bunnies’ heads, there are also auditory illusions which trick the mental image generated from sounds. I suppose that these are less common since they are harder to distribute than regular pictures, but it’s definitely a field worthy of more investigation.


One comment

  1. Interesting reading! Initially I thought there might be a point in the end. Vi ♥ has a few relatively new videos about sounds, highly recommended.

    If you really want to hear surround sound maybe we should go back to attending real concerts, you know, in the concert hall (as opposed to your living room).

    Jolly week to you too.

    P.S. the link is broken (add those HTTPs back in, honey).

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