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Without Directionality, How does a Mic' Recreate the Sense of a Room?
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hduncan
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#1
27th June 2008
Old 27th June 2008
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Without Directionality, How does a Mic' Recreate the Sense of a Room?

This post is long, so my question has been summed up in this sentence:

"When I listen to a mic'd source through speakers, I am hearing a complete lack of directionality, yet I still "hear" the room... How does this work?"

-----Read on if confused by what I am asking--------
I've been wrestling with this notion for a while. I think what it comes down to is like looking at a cathedral and looking at a picture of a cathedral. One is fully 3dimensional and the other is a 2dimensional representation.

The Question:
When standing inside a Cathedral and listening to someone speak, the sound from their voice hits you directly (usually first to arrive) and then their voice, (which propagates outward in a spherical manner from their vocal chords) also reflects off of other surfaces and hits you from all directions (front back top bottom left right etc etc).

Put a single microphone where you were standing aimed at the sound source/speaker, and the microphone is going to capture the direct signal, the top reflections, bottom reflections, left and right reflections, etc but its going to pick it up and record it as if all of those reflections came from one direction- toward the front of the mic. (the mic has no way of giving us directionality).

Furthermore- we take this compounded, unidirectional signal and play it back to ourselves with speakers. To make things easier, lets say its just mono out of a single speaker aimed at our nose. All of those reflections that (in reality) would be arriving at our ears from all different angles/directions/etc, are all coming at us from dead ahead.

This means that the reflection that would have hit the back of our heads comes at us from the front which is incorrect (if attempting to recreate the source accurately).

Essentially what we are hearing is an impossible reality: say we are in a rectangular room, you are standing in the middle, the corresponding distances from the walls to you are; front of room: (you are facing this wall) 8', back of room: 8', ceiling: 10', side walls: 6'.

When the reflections from these walls arrive at the mic, their respective delays are ~8ms, 8ms, 10ms, and 6ms. when you play the mic'd signal back through a single monitor infront of your face, it is as if the direct source reflected off of multiple walls that are all infront of you.
--two walls that are 8' in front of you (front/back), 2 walls that are 6' in front of you (sidewalls), one wall that is 10' in front of you (ceiling) , and another wall that is 5ish' in front of you (floor, depending on your height).

To your ears, it would seem that somehow the reflections are uninhibited by the other walls in front of them, in other words, the sound that reflects off of the wall 10 feet in front of you (which was the ceiling and would naturally be reflecting down to the tops of your ears) had to pass through 5 walls before hitting the 6th wall and then reflecting back to you through those 5 walls again without being attenuated at all. This is what it sounds like with all of the reflections being recreated through a single speaker source.

This may all seem obvious and what not, but why is it that I can still hear the Cathedral's character imparted on the sound, even when I am hearing something that is impossible to recreate in the real world?

I am hearing a complete lack of directionality, yet I still "hear" the room... How does this work?
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#2
27th June 2008
Old 27th June 2008
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the microphone does not 'recreate' the sense of the room. Your brain does

If you leave your house and go to the cathedral, while a lot of time may elapse, you should notice the difference. Big time. The folds in the pinnae of your ear reflect sound differently as it comes from different directions. You can even determine that a sound comes from above or below- something you could not do with only two microphones.

If you listen to some well-made surround recordings on a good surround system, you would realize that a good deal of the sense of the room is indeed missing from the plain old stereo recording. Switching back to stereo can be a big letdown.

Most of what you do get in terms of 'space' is inferred by your brain using the arrival times of the sound.

I would compare it to similar visual inferences. You can watch something on TV, and infer depth and distance on two objects- once close to the camera, one far away. Even though your eyes are focusing on a flat plane that is 4 feet in front of you, you can use the remaining cues, distance, prior knowledge of the object's size, experience - to infer how far away they are.

It works, but that is not to say it works well, or holds a candle to reality. Which is why movies and TV shows have to cut back and forth with camera angles and wide shots and close-ups to give you as much information as you would have gotten had you 'been there'.
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hduncan
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27th June 2008
Old 27th June 2008
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You are one of the first people to agree with me on the fact that stereo and a single mic cannot reproduce reality.. thank you.....


But back to the topic-

So directionality and space should be treated as completely different things... your brain isnt really hearing space from the directions, only from the shape of your ear.

So does your brain basically give you a blurry cloudy idea of space based on reflections and the directionality is dealt with differently? Hence why we can hear the spacial characteristics of a room, but not feel like we are actually inside of it?

I mean, the brain HAS to be seperating each of those delayed signals out and can't be processing them as If they were actually coming from that single location, because that would be an impossible acoustic situation in reality.

I am assuming directionality is completely seperate from spatial recognition of the brain. Which is why we can hear multiple delays from a single point source and infer a sense of space from a 2 dimensional plane.
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27th June 2008
Old 27th June 2008
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chasman is offline
just a thought but you might want to check out:

moultonlabs.com.................on the left side under articles (not featured articles)..................click on hearing...............scroll down to the microphone vs. the ear..............................there might be a few other interesting articles for you there too..................I've barely scratched the surface.............but I like it alot..............
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27th June 2008
Old 27th June 2008
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This points up what is unique about our sense of hearing.

We have two eyes, yet we can only see what is in front of us. We have two ears, but we can hear everything around us.

The people who dismiss surround as a "gimmick" have never understood this. We always hear in surround; it's not possible to hear in stereo or mono.

If we listen to stereo or mono sources, they're still in surround by the time we add in the characteristics of the space we're listening in.

And now back to our regularly scheduled programming.

3rd&4thT
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28th June 2008
Old 28th June 2008
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Quote:
Originally Posted by 3rd&4thT View Post
This points up what is unique about our sense of hearing.

We have two eyes, yet we can only see what is in front of us. We have two ears, but we can hear everything around us.

The people who dismiss surround as a "gimmick" have never understood this. We always hear in surround; it's not possible to hear in stereo or mono.

If we listen to stereo or mono sources, they're still in surround by the time we add in the characteristics of the space we're listening in.

And now back to our regularly scheduled programming.

3rd&4thT
Each semester I do a demonstration for my students. I ask them to close their eyes and point to me when I say "now". I go to the right side of the room, say "now" they all point right at me. I repeat for the left side of the room and the center. Then I stand on the desk and say "now" and they all point up.

I explain that if I had given each one of them a pair of mics and they had been monitoring through headphones, they would have pointed to the same place for the both the center on the ground and center standing on the desk.

Not only do your ears collect sounds from all these different angles and directions, but your brain can sort them out to give you more information than you "ought" to be able to get from just two receptors.
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28th June 2008
Old 28th June 2008
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Quote:
Originally Posted by DarkEcho View Post
"When I listen to a mic'd source through speakers, I am hearing a complete lack of directionality, yet I still "hear" the room... How does this work?"
Soundstage depth.

Not everything about image localization is about lateral positioning, you know
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28th June 2008
Old 28th June 2008
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While the OP stipulates he would like to know the answer to his question "without directionality" being taken into account, I don't think you can do so. IOW, just because a single mic can impart a sense of the room, doesn't mean you can eliminate directionality from the equation... let's say the mic is centered in a well-balanced room or "cathedral" as he mentions... well, if there's no major differences between the left side & right side of the cathedral, then the mic going to still give you a sense of the room in a kind of "equibalance" (OK, i'm trademarking that term now, LOL). Now, depending on the pattern & the "reach" of different mics, the sound may differ from one to the other, hence directionality CAN make a difference; but there's no technical or perceptual reason that you shouldn't get a sense of the room just because you're only using one mic, and one mic vs two ears = just both ears "summed to mono", essentially.

So, am I missing something here???
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28th June 2008
Old 28th June 2008
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Quote:
Originally Posted by Peacock View Post
Eargles book on sound recording covers the reasons why we hear room effects from stereo and mono sources in great detail.

In fact, to take this a step further (for example)

Warmth occurring in a room that is associated with high reverberation at low frequencies (125-250Hz) is the hardest attribute to add to a recording by signal processing- according to Eargle.
This is not a surprising conclusion from John Eargle, as his work on both the Mercury and Delos labels is conspicuously lacking in that resonance. The man is superbly gifted, but his taste is unmistakable. Meanwhile, for signal processing the Waves Shuffler might be a good place to start.

Meanwhile, on a practical basis, for classical, jazz and such like acoustic music, it is far superior to have two room mics rather than one.

The reason is this: many listeners will be hearing your work on their home theater surround systems, using Dolby Digital or some other ambience extraction program. Two-channel audiophile systems are in full retreat.

With a single room mic, it is infuriating to have hall or church decay follow the end of the music to a solid point in front of you, centered between and distanced behind the main speakers, when it should more realistically diffuse around and behind you.

Even if it's just a studio ambience around a small group, it is nonsensical and a waste to have the room itself in a hard knot dead in front of you and seemingly further away than the musicians. In life the room is all around you. The recording will not reproduce this, however, if your single room mic supplies only vicious mono.

Rock, of course, plays by another set of rules.

3rd&4thT
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28th June 2008
Old 28th June 2008
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Quote:
Originally Posted by 3rd&4thT View Post
This is not a surprising conclusion from John Eargle, as his work on both the Mercury and Delos labels is conspicuously lacking in that resonance. The man is superbly gifted, but his taste is unmistakable. Meanwhile, for signal processing the Waves Shuffler might be a good place to start.

Meanwhile, on a practical basis, for classical, jazz and such like acoustic music, it is far superior to have two room mics rather than one.

The reason is this: many listeners will be hearing your work on their home theater surround systems, using Dolby Digital or some other ambience extraction program. Two-channel audiophile systems are in full retreat.

With a single room mic, it is infuriating to have hall or church decay follow the end of the music to a solid point in front of you, centered between and distanced behind the main speakers, when it should more realistically diffuse around and behind you.

Even if it's just a studio ambience around a small group, it is nonsensical and a waste to have the room itself in a hard knot dead in front of you and seemingly further away than the musicians. In life the room is all around you. The recording will not reproduce this, however, if your single room mic supplies only vicious mono.

Rock, of course, plays by another set of rules.

3rd&4thT
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28th June 2008
Old 28th June 2008
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Quote:
Originally Posted by chrisj View Post
Soundstage depth.

Not everything about image localization is about lateral positioning, you know
I take the term 'depth' to be essentially a function of masking, where we can also include spatial 'cues'.

When we hear reflections (in a recording or in life), they are:

later in time of arrival
lower in volume
lower in bandwidth (at the extremes)

than the direct sound and so are masked or partially masked by it, which gives us 'depth'.

If the reflections were as loud, as early & as bright as the direct sound, we would have no depth.

Andy
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