W A R N I N G !

W A R N I N G !

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Friday, April 22, 2011

I just discovered I can tell the difference of 3cm of head positioning

I've been feeling that my left speaker's a bit louder than the right. Thinking it could be either due to room acoustic, supported by the fact that the perceived bass is stronger on the left even though the subwoofer is on the right, I ignored it.

Today I was thinking that the left/right volume can be altered by the positioning of the head, so I decided to shift my head towards the right to get the perfect balance. When I got the balance I want, I looked at my head's position...

...to find it right in the middle of the two satellites.

You see, due to some strange situation of my table, my monitor is placed 6cm closer to the left satellite than the right. And I always make sure my head is at the middle of the screen. Which resulted in my head being 3cm off the sweet spot.

But what's a concept without testing.

Test methodology

Music is played through the speakers. Subwoofer volume turned to zero to prevent it from influencing the balance. The room lighting and computer screen are turned off (it's night btw). Eyes remained closed throughout each test until each measurement is taken.

1. Stand up from the chair, pulling it a short distance away from the original position.
2. Walk a metre or two away and back, pulling back the chair and sitting down at a random position. The direction to face is decided by the chest feeling against the table edge (my table is fairly large enough to do this).
3. The positioning of the head is shifted left right forward backward while facing the same direction as much as possible as the center position is attempted to be found.
4. After deciding position of head, a torchlight, one end placed against the chin and the other end pointing at and close to the table is switched on. There is a piece of paper fixed onto the table and where the torchlight is pointing at (the brightest part at the center) is marked.
5. Repeat from step 1 for next measurement.

A total of 6 readings were taken with the speakers at the original positions untouched. Then the left speaker is shifted 6cm to the left such that the distance between each satellite and the screen is both approximately 9.5±0.5cm. (Originally the left speaker is approximately 3.5cm away) This results in the sweet spot shifting left by 3cm. Another set of 7 readings was taken.

The screen is 53cm wide, so the original distance between the speakers is 66cm while the new distance is 72cm - an increase of 9.1%. The speakers are approximately 65cm from the edge of the table. The resulting change in distance between speaker and "sweet spot" - taken to be the edge of the table, equidistance from each speaker, is 1.9%, and resulting increase in angle is 7.7%.


The first set of readings are those directly above the horizontal line while that of the second set are indicated by a circle.

It is not a lot of readings and variance is fairly huge. However, two important observations - the average distance (from the left side of the paper) of each set of readings differed by approximately 3cm, while the differences between both maximums and both minimums are both 3.5cm. The difference between max and min values of each set is 4cm, resulting in a 0.5cm overlap.

Which means that it is fairly possible for the human ear(s) to detect changes in sound due to the sweet spot shifting by 3cm. 5cm would be a safe estimate.

Note that this little test does not reveal which position is the more accurate one, since there are other factors that affect balance of both channels - channel imbalance of equipment, room acoustics, hearing loss etc, but the main objective is to see if audible differences in perceived sound can result from small shifts in the positioning of the head relative to the speakers.

Granted, I used some sort of averaging technique to increase my precision - I shifted my head left and right to find the positions at which one channel is a certain degree louder than the other, and took the center. Like the way you tune the TV and focus the camera. But my head only moved by a few cm in the process (although it feels like a lot). This adds more weight to the idea that our hearing is precise to a few cm.

I'm not really surprised by the result - animals are able to pinpoint the location of predators and preys that are far away by just hearing, and some are even able to use them to navigate and eat prey. This would require even better precision. Way, way better. If I go and search I believe I would find better technical articles on the precision of human hearing.

This has implications on speaker positioning and how subjective listening tests are being carried out. I already know that some instruments in a particular song sound relatively louder and softer depending on the position and angle of my head, but having numbers puts things in perspective. We would need to be accurate to a few cm and a few degrees when positioning our speakers, and in subjective listening tests you may be hearing differences that are not actually there due to the head not being in the same position. And how many people actually make sure their heads are in the same position throughout the test? It also questions the credibility of auditions carried out simultaneously by a group of people, for no two heads can be in the same position at the same time, although subjective bias caused by the first guy to open his big mouth is more likely to cause everybody to "hear the same thing".

Looks like we may need, as it is jokingly mentioned on some forums, a vice clamp for our heads.

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