How do you hear front/rear sound?

[Simxp]

Back in prep school Biology, I learnt that you can hear where a sound comes from by the fact that, if it originates to your left, your left ear hears it a fraction of a second before the right; and vice versa for sounds that originate from your right. If it comes from in front of or behind you, they hear it at approximately the same time. However, such an explanation would mean that you would not be able to tell whether a sound comes from in front of or behind you, only what angle left to right. And surely, any possible difference between in sound transmitted through your eardrums from in front of you or behind you would be eliminated when the sound is transmitted through the bones in your inner ear.

I have a surround sound speaker set for my computer (nothing terribly audiophilic, just a normal Creative one), that has a dial to adjust the balance between the front and rear speakers. I can definitely hear the difference when I change the dial (even though the front and rear speakers are at approximately the same angle and distance from me), and indeed, spent some time adjusting it so the system sounded balanced.

Once, as part of an ergonomics exercise that involved lying on the floor, I positioned the speakers around my head, and experienced the headphone effect - the sound sounded like it originated in the middle of my head. Moving the speakers on my chest and behind my head, I could move this 'point of origination' forwards and backwards.

However, I recently tried an experiment with the surround sound speakers, and discovered that, if I shut my eyes, I could convince myself that the sound was coming from the front two speakers when in actuality, I had moved the dial it all the way to the rear speakers.

As far as I can conclude, then, there are only two possible explanations. The first is that, as my first two experiences seem to bear out, you can tell whether a sound comes from in front of you or behind you. If this is true, could someone explain the mechanism by which you can, because, considering you have only two ears, I cannot think of one.

The second explanation, borne out by what I was taught and my third experience, is that you cannot tell whether a sound comes from in front of you or behind you through audible means, and thus rely only on context. This would mean that all audio systems (5.1, 7.1) which have more than two positional speakers (not including things like a subwoofer) are a complete rip-off and work entirely through the placebo effect.

Could anyone tell me which of these is correct?

(By the way, excellent and informative website!)

 

[MDS]

As far as I can conclude, then, there are only two possible explanations. The first is that, as my first two experiences seem to bear out, you can tell whether a sound comes from in front of you or behind you. If this is true, could someone explain the mechanism by which you can, because, considering you have only two ears, I cannot think of one.

The second explanation, borne out by what I was taught and my third experience, is that you cannot tell whether a sound comes from in front of you or behind you through audible means, and thus rely only on context. This would mean that all audio systems (5.1, 7.1) which have more than two positional speakers (not including things like a subwoofer) are a complete rip-off and work entirely through the placebo effect.

Could anyone tell me which of these is correct?

Obviously, explanation one is correct.

Sound is a wave and propogates in all directions. If you drop a pebble in a pond, the waves propogate outward from the center until they encounter an object which alters their path. Same principle for sound waves.

We hear the direct sound that reaches the ears first and we also hear reflected sound that has been reflected off other surfaces. The ear derives spatial clues from the timing of the sounds.

Have to cut this short: I just heard my meat thermometer that is BEHIND me beep because my steak has reached the desired temperature.

 

[Simxp]

MDS,
If the only way the ear can tell the difference between front and rear sounds is by post-processing of the time difference from reflected sounds (and context) (and I completely agree with you, in that I cannot think of any other way you could), then, with a sufficiently advanced audio processor, you could replace all surround sound systems with two speakers (or sets of speakers covering different frequencies), one on either side of you, which could be programmed to imitate the characteristics of the reflected sounds of any different kind of room (listening chamber, concert hall, recording studio) through a careful mixture of crossfeed, timimg, lower amplitude repetition after millisecond pauses etc etc.

Considering that any such spacial 'footprint' would be reasonably easy for a computer to consistantly apply, and compared to high end surround-sound speakers, computers are fairly cheap, all you would need is for someone to analyse the sound footprint and work out what processing is needed, and put whatever the modern equivalent is of eggboxes on the walls to eliminate further reflections on top of the 'virtual' ones already applied, and you've saved potentially housands of pounds. Unless there's some critical flaw that I haven't thought of. Which there probably is.

Enjoy your steak!

 

[BMXTRIX]

I would think that would work with a PC, for one person, for one ear, in a room.

The issue is that both ears hear things differently and reflections and hearing capability is different from person to person. A computer reproducing an accurate surround field would work, but it still has to deliver that sound in a real environment and space.

I think the possibility that your ear itself may help with the direction of the sound plays into it as well. The forward pointing cup shape captures sound and directs it down the canal. Just turn your head an inch either way and you notice how a certain sound can be louder, or quieter. The other ear plays in as well so you can localize the specific direction of a sound.

But, new speakers from companies like Yamaha use angled speakers to bounce surround audio off walls to achieve directionality from surrounds in a way that can work fairly well. Not perfect, but better than no surrounds sometimes.

I think there is a review on this site of them.

 

[mtrycrafts]

Back in prep school Biology, I learnt that you can hear where a sound comes from by the fact that, if it originates to your left, your left ear hears it a fraction of a second before the right; and vice versa for sounds that originate from your right. If it comes from in front of or behind you, they hear it at approximately the same time. However, such an explanation would mean that you would not be able to tell whether a sound comes from in front of or behind you, only what angle left to right. And surely, any possible difference between in sound transmitted through your eardrums from in front of you or behind you would be eliminated when the sound is transmitted through the bones in your inner ear.

I have a surround sound speaker set for my computer (nothing terribly audiophilic, just a normal Creative one), that has a dial to adjust the balance between the front and rear speakers. I can definitely hear the difference when I change the dial (even though the front and rear speakers are at approximately the same angle and distance from me), and indeed, spent some time adjusting it so the system sounded balanced.

Once, as part of an ergonomics exercise that involved lying on the floor, I positioned the speakers around my head, and experienced the headphone effect - the sound sounded like it originated in the middle of my head. Moving the speakers on my chest and behind my head, I could move this 'point of origination' forwards and backwards.

However, I recently tried an experiment with the surround sound speakers, and discovered that, if I shut my eyes, I could convince myself that the sound was coming from the front two speakers when in actuality, I had moved the dial it all the way to the rear speakers.

As far as I can conclude, then, there are only two possible explanations. The first is that, as my first two experiences seem to bear out, you can tell whether a sound comes from in front of you or behind you. If this is true, could someone explain the mechanism by which you can, because, considering you have only two ears, I cannot think of one.

The second explanation, borne out by what I was taught and my third experience, is that you cannot tell whether a sound comes from in front of you or behind you through audible means, and thus rely only on context. This would mean that all audio systems (5.1, 7.1) which have more than two positional speakers (not including things like a subwoofer) are a complete rip-off and work entirely through the placebo effect.

Could anyone tell me which of these is correct?

(By the way, excellent and informative website!)

http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html
Try this link and explore the acoustics and hearing sections.