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Author Topic: Why does the stereo image narrow as the angle between mics widens?  (Read 1171 times)

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Online ycoop

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I’ve been playing around with the stereo configuration visualization tool at http://www.sengpielaudio.com/Visualization-NOS-E.htm

I can’t wrap my head around why the stereo image narrows as the angle between mics widens. Can anyone explain this to me?
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Offline heathen

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Re: Why does the stereo image narrow as the angle between mics widens?
« Reply #1 on: June 05, 2018, 03:58:38 PM »
You're going to get recommendations to read this: http://microphone-data.com/media/filestore/articles/Stereo%20zoom-10.pdf

Best to digest that first...not only may it answer your question, but it may also raise new questions!
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Online ycoop

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Re: Why does the stereo image narrow as the angle between mics widens?
« Reply #2 on: June 05, 2018, 04:30:56 PM »
Guess it’s time to do some of what us Jewish folk call chazarah.

(Apologies for the bageling https://www.urbandictionary.com/define.php?term=bageling)
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Offline heathen

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Re: Why does the stereo image narrow as the angle between mics widens?
« Reply #3 on: June 05, 2018, 04:44:28 PM »
Is there a term for when gentiles use Yiddish words?  Because I find myself using some Yiddish words pretty often!
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Online ycoop

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Re: Why does the stereo image narrow as the angle between mics widens?
« Reply #4 on: June 05, 2018, 05:00:09 PM »
That’s kinda like those rolls-with-holes they sell in supermarkets.

On page 5 and I already have the answer. I was misinterpreting the diagram. Now I see that the darker portion of the diagram indicates when sounds are shared between channels while sounds outside of this area are heard only (primarily?) on one of the two channels.
« Last Edit: June 05, 2018, 05:07:51 PM by ycoop »
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Offline Gutbucket

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Re: Why does the stereo image narrow as the angle between mics widens?
« Reply #5 on: June 05, 2018, 05:09:58 PM »
As heathen suggests, for more insight into the often counterintuitive 3-way relationship between microphone pickup pattern, angle and spacing with regards to a stereo pair of microphones, read Michael William's Stereo Zoom paper a few times.  It tends to take a few reads to really sink in.

As for a basic explanation to your question, the image doesn't narrow as the mics are angled further apart, it grows wider.  At the same time the Stereo Recording Angle (SRA) narrows.  The confusion stems from the difference in definition of stereo image and Stereo Recording Angle.

As you increase the angle between a pair of directional microphones, you increase the difference signal between channels, which results in a wider playback image- it sounds more spacious, and you hear a greater difference between left and right channels.  This is analogous to zooming out with a camera lens- the cropped limit of the overall scene grows wider as you collect light from a wider "seen" area.  But at the same time, the subjects themselves within the scene appear smaller.  So if you're photographing say, your brother and his wife standing on the beach, and zoom way out, the overall scene displayed in the resulting photograph grows wider, but the portion of interest (your brother and his wife standing center frame) appear smaller, and take up a narrower portion of the photo.  If you zoom in, that portion of interest effectively "expands" to fill the frame, even though, rather because the overall scenic perspective has grown more narrow.
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Offline Life In Rewind

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Re: Why does the stereo image narrow as the angle between mics widens?
« Reply #6 on: June 05, 2018, 11:47:29 PM »
Is there a term for when gentiles use Yiddish words?  Because I find myself using some Yiddish words pretty often!

I hope so - I've been called the "token goy" at times!
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Offline opsopcopolis

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Re: Why does the stereo image narrow as the angle between mics widens?
« Reply #7 on: June 06, 2018, 08:38:23 AM »
Is there a term for when gentiles use Yiddish words?  Because I find myself using some Yiddish words pretty often!

I hope so - I've been called the "token goy" at times!

It’s a term of endearment
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Offline Gutbucket

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Re: Why does the stereo image narrow as the angle between mics widens?
« Reply #8 on: June 06, 2018, 09:23:25 AM »
..when sounds are shared between channels while sounds outside of this area are heard only (primarily?) on one of the two channels.

Yes, and this is where the photographic lens analogy breaks down.  Unlike photography, there is no hard-cropped edge defining the limits of the reproduced audio image.  The Stereo Recording Angle limits, where sounds within that region are sufficiently shared between channels so as to be heard as originating from the space between the two speakers forming a defined acoustic image, have been psychoacoustically determined by extensive listening tests using speakers arranged 60 degrees apart in front of the listener.

At a fundamental level, stereo is correctly managed acoustic crosstalk between channels, neither too much (mono), nor too little (no relationship between channels).  The key is finding an appropriate balance between those two extremes. Stereo Zoom explains how there are numerous ways to achieve that, helps explain a couple of the differences between them (although ignoring other important differences), and how to achieve them.
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Offline wforwumbo

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Re: Why does the stereo image narrow as the angle between mics widens?
« Reply #9 on: June 06, 2018, 09:56:34 AM »
The key distinction to me, is that the stereo image you see narrowing isn’t the ““overall” “image”” (cant put those in enough quotes, audio is NOT subject to the same properties visual stimuli are!!!). It’s the overlap in image that both microphones are seeing. So narrowing the on-axis angle will cause more overlap between the coherent wavefront both caps see. THAT is what “widens” the stereo image by narrowing the angle. It’s not the total stereo field, but rather the coherent overlap between the caps.

At least, that’s how I think of it.
« Last Edit: June 13, 2018, 02:54:13 PM by wforwumbo »
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Online ycoop

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Re: Why does the stereo image narrow as the angle between mics widens?
« Reply #10 on: June 06, 2018, 12:24:16 PM »
using speakers arranged 60 degrees apart in front of the listener.

How does this change our scenarios where the stacks are regularly parallel or some combination of parallel and angled in or out.
« Last Edit: June 06, 2018, 01:38:56 PM by ycoop »
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Offline Gutbucket

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Re: Why does the stereo image narrow as the angle between mics widens?
« Reply #11 on: June 06, 2018, 01:36:16 PM »
I like that. "Coherent overlap" < is a better phrase for describing this than "managed acoustic crosstalk"

Quote
using speakers arranged 60 degrees apart in front of the listener.
How does this change our scenarios where the stacks are regularly parallel or some combination of parallel and angled in or out.

It doesn't, it only affects the playback side of things.  It means that the microphone setup configurations suggested by Stereo Zoom apply to how listeners experience playback via a standard 60 degree triangle speaker arrangement.  If the speakers are arranged differently, or headphones are used it no longer necessarily applies directly, yet in the real-world sense it's robust enough that it still works fine.

Stereo Zoom does not address the radiation pattern or directionality of the sound sources being recorded, and does not address direct/reverberant-sound ratio issues at all (which IMO is much more fundamental to an acceptable recording than good imaging).  I've posted about this in detail in a number of threads discussing the Stereo Zoom at TS.
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Better recording made easy - >>Improved PAS table<< | Made excellent- >>click here to download the Oddball Microphone Technique illustrated PDF booklet<<

Offline DATBRAD

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Re: Why does the stereo image narrow as the angle between mics widens?
« Reply #12 on: June 07, 2018, 01:58:41 PM »
Another thing worth noting is that polar patterns and tools that display what different stereo setups look like are rendered in 2 dimensions. The patterns we see represent the pattern radius at it's widest point, as a cross section. This means that in order to realize what is shown on paper, each mic in a pair must be aligned somewhat parallel in the horizontal plane, and that plane must be set to align approximately with the center of the desired source. For example, when recording from a PA with the stand raised to 8-9' it's fine to have the mics level with the floor. However, if the stand is at 5-6', angling both mics up towards the main stacks will usually sound much better than keeping the mics level with the floor. If you visualize the polar patterns in 3 dimensions when you  are setting up it will make more sense. When I was learning this many many moons ago in my early taping days, I found pretending you're holding 2 apples together, stem side representing the rear of the mic, and looking for where your chosen target would align with the point the apples touch, it will guide you perfectly to decide the upward angle the pair needs to be set to. Bass response is closer to omni with cards and hypers, its the mid to high range sounds that fall into the tightest lines drawn on a polar pattern, and that is where the intelligibility and detail is captured.
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