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Old July 2nd, 2012, 01:34 AM   #1
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Line Gradient advantages over dual transducer pressure gradient?

I understand that there are essentially two ways of making a condenser microphone more directional:

1. Line Gradient (used in most "boom" or "shotgun" mics): a tube in front of the transducer with a slit or similar delays the sounds that come from off axis more than the direct sounds, causing cancellation (hence directionality).

2. Pressure gradient (used in most directional mics which are super or hypercardioid): Conceptually two transducers fore and aft receive the direct and indirect pressure waves respectively, and phase reversal on the aft transducer causes it to tend to cancel sounds from the aft direction. This makes for a cardioid pattern, and super or hypercardioid directionality is achieved by varying the ratio of the direct vs indirect transducer

My question is why bother with a line gradient design (as many if not most boom mics do) when a pressure gradient design (no interference tube) can achieve the same super or hyper directivity, and without the issues that the interference tube design causes in indoor situations where sound reflects also off the walls. Is is just an economic choice, that the interference tube is cheaper than making a transducer with the 2:1 direct/indirect ratio that John's post refers to? Or is there some actual directivity advantage that cannot be achieved with a non-line-gradient super or hypercardioid mic? Most line gradient mics seem to have super or hyper directivity anyway, but worse indoor performance.
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Old July 2nd, 2012, 02:17 AM   #2
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Re: Line Gradient advantages over dual transducer pressure gradient?

I have been poring over specs and I think I can now answer my own question: The line gradient will have more directional high frequency response than the equivalent pressure gradient only mic, at the expense of more frequency dependence and greater proximity effect. That extra high frequency directionality increases the "reach" outdoors, at the expense of causing problems indoors with reflected sound.

A very pure way to see the difference is to compare the Sennheiser MKH8050 (supercardioid without line gradient) versus the MKH8060 (supercardioid line gradient). Here are the manuals with polar plots towards the end:

http://www.sennheiserusa.com/media/p...ionsforuse.pdf
http://www.sennheiserusa.com/media/p...n%20manual.pdf

Even though boith mics might have about the same supercardioid polar response for low frequencies, as the frequency gets higher the line gradient 8060 gets more directional, and the response more ragged, likely inducing more indoor comb filtering effects.

Last edited by Tom Morrow; July 3rd, 2012 at 01:22 AM.
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Old July 24th, 2012, 08:43 PM   #3
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Re: Line Gradient advantages over dual transducer pressure gradient?

Shotgun mikes were designed for use outdoors, they were never intended to be used in inside spaces; per Douglass Spotted Eagle.
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Old July 25th, 2012, 06:11 AM   #4
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Re: Line Gradient advantages over dual transducer pressure gradient?

Quote:
Originally Posted by Tom Morrow View Post
I understand that there are essentially two ways of making a condenser microphone more directional:

1. Line Gradient (used in most "boom" or "shotgun" mics): a tube in front of the transducer with a slit or similar delays the sounds that come from off axis more than the direct sounds, causing cancellation (hence directionality).

2. Pressure gradient (used in most directional mics which are super or hypercardioid): Conceptually two transducers fore and aft receive the direct and indirect pressure waves respectively, and phase reversal on the aft transducer causes it to tend to cancel sounds from the aft direction. This makes for a cardioid pattern, and super or hypercardioid directionality is achieved by varying the ratio of the direct vs indirect transducer

My question is why bother with a line gradient design (as many if not most boom mics do) when a pressure gradient design (no interference tube) can achieve the same super or hyper directivity, and without the issues that the interference tube design causes in indoor situations where sound reflects also off the walls. Is is just an economic choice, that the interference tube is cheaper than making a transducer with the 2:1 direct/indirect ratio that John's post refers to? Or is there some actual directivity advantage that cannot be achieved with a non-line-gradient super or hypercardioid mic? Most line gradient mics seem to have super or hyper directivity anyway, but worse indoor performance.
An interference tube microphone *is* a pressure-gradient microphone.

The capsule is normally a super-cardioid or hyper-cardiod which is about the most directional you can get without going to a fig-8 with a huge rear lobe.

The interference tube then goes on the front of this capsule to make it more directional at higher frequencies.

The longer the tube the lower the frequency at which the interference tube starts working and the more directional it gets at high frequencies.

You just cannot get the directivity required without using the tube - it's nothing to do with cost and everything to do with the physics of acoustics and microphone design.
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Old July 25th, 2012, 02:09 PM   #5
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Re: Line Gradient advantages over dual transducer pressure gradient?

The Sanken CS-3e is an interesting mic in that it has an interference tube as well as three transducers. From the Sanken website...

"The CS-3e delivers sharp directional sound even in the lower frequencies. Sanken's original design results in a low proximity effect which facilitates working in a scene close-up and wide, while producing little change in sound character. In the CS-3e, three directional capsules are arranged in a front-back array to combine line microphone performance and second-order pressure gradient response in a single system. With this unique design, the CS-3e achieves phenomenal supercardioid directivity in the lowest frequencies and throughout the full range in a microphone only 27cm (10 inches) in length"

The Schoeps SuperCMIT 2 U uses an interference tube, multiple capsules, and advanced digital processing to separate front from off-axis sounds.
Shotgun microphone SuperCMIT 2 U - Overview - Schoeps Digital

In any case, directionality is achieved by combining sounds at various distances and possibly electronic time shifts and phase inversions from the front of the mic. Done with an interference tube alone, you get bumpy lobes, comb filter effects, proximity effects, quite different frequency responses at different angles, and a strong rear lobe. With more complex designs, like the Sanken CS-3e and the Schoeps SuperCMIT, these effects are reduced dramatically, though not eliminated.

It's an interesting science!

Of course, don't forget the practical aspects like size, weight, cost, and ruggedness. I don't know that I'd want to take a SuperCMIT on a swamp expedition. It's too valuable!
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Old July 29th, 2012, 08:54 PM   #6
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Re: Line Gradient advantages over dual transducer pressure gradient?

I wonder about pressure gradient mics like the supercardioid Schoeps CMC641 or the supercardioid Sennheiser 8050. In particular I wonder how their pickup pattern would compare to my supercardioid cs-3e.

Going to microphone-data.com I was able to see their polar patterns side by side and it's clear that the cs-3e has a more directive pattern. But since the graphs are by different manufacturers it's hard to tell how the high frequency compares between the mics.

I'm thinking about getting another mic to supplement my cs-3e in situations where the cs-3e is overly directive, e.g. when I want to capture sound from two actors near each other without a second mic. It seems like the Schoeps 641 has a wider pattern than the 8050 for this purpose. Given that I normally shoot indoors I might lean towards the more narrow pattern of the 8050 to reject wall-bounce, but the 641 has a great reputation for indoor use.
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Old July 29th, 2012, 09:14 PM   #7
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Re: Line Gradient advantages over dual transducer pressure gradient?

The difference between the Sennheiser 8050 (supercardioid, not line gradient) and the Sennheiser 8060 (supercardioid, line gradient) seems to be a good illustration of the difference. Here are the instruction manuals which have the polar patterns, if you want to follow along at home:

http://en-de.sennheiser.com/download...eda89d5d2f.pdf
http://en-de.sennheiser.com/download...232e5677af.pdf

The 8060 is just a little more directive at low frequencies, but above frequencies approx 1-2k the 8060 is much more directive, and the 8060 polar plot is much more ragged showing all the phasing/comb filtering.

Given this, I wonder if the 8050 might be a better microphone for many if not most "film/video" dialog recording situations, even outdoors.

The one thing that I hate about interference tube shotgun mics is the proximity effect where the tone gets lower as you move the mic closer to the actor. Even outdoors I really prefer the cs-3e which has essentially no proximity effect so I don't have to worry about changing the tone if I have to be closer in one scene than another; with mics that have strong proximity effects I tend to back them away further from the actor so that I can capture everything in the scene with the same tone.

This is why I'm looking to supplement my indoor-focussed cs-3e mic with the 641 (or perhaps 8050) which most people use indoors, but wondering if I'm missing something by not instead getting an outdoor focused mic like the 8060 or 416 or NTG8 to supplement my cs-3e.

Strangely the polar plot for the Rode NTG8 (very directive line gradient mic) doesn't show any of that combing at high frequencies that the you see on the Sennheiser 8060 plot.

http://www.rodemic.com/mics/ntg8

But I suppose that's probably due to how they measured or massaged their data than any breakthrough in the NTG8 mic itself.

Last edited by Tom Morrow; July 29th, 2012 at 10:11 PM.
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Old July 30th, 2012, 07:03 AM   #8
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Re: Line Gradient advantages over dual transducer pressure gradient?

Quote:
Originally Posted by Tom Morrow View Post
...
The one thing that I hate about interference tube shotgun mics is the proximity effect where the tone gets lower as you move the mic closer to the actor. Even outdoors I really prefer the cs-3e which has essentially no proximity effect so I don't have to worry about changing the tone if I have to be closer in one scene than another; with mics that have strong proximity effects I tend to back them away further from the actor so that I can capture everything in the scene with the same tone.
....
FYI, any directional mic, even a plain old cardioid, exhibits the proximity effect. The more directional the pattern, the farther away from the sound source the effect becomes noticeable.
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Old July 31st, 2012, 06:08 AM   #9
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Re: Line Gradient advantages over dual transducer pressure gradient?

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FYI, any directional mic, even a plain old cardioid, exhibits the proximity effect. The more directional the pattern, the farther away from the sound source the effect becomes noticeable.
Actually it's the *closer* it gets to the sound source the more noticeable it gets.
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Old July 31st, 2012, 06:41 AM   #10
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Re: Line Gradient advantages over dual transducer pressure gradient?

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Actually it's the *closer* it gets to the sound source the more noticeable it gets.
Yes, for a given mic the closer the mic is to the source the more pronounced the proximity effect will be. But when you compare the behaviors of different types of mic, as you move a mic in from a distance in towards the source, the more directional the mic is the sooner the proximity effect will start to kick in. You can get a cardioid closer to the subject before you hear the proximity effect than you can a hyper while a shotgun will start to exhibit the effect even farther out than the hyper.
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Old August 2nd, 2012, 03:38 AM   #11
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Re: Line Gradient advantages over dual transducer pressure gradient?

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Originally Posted by Steve House View Post
Yes, for a given mic the closer the mic is to the source the more pronounced the proximity effect will be. But when you compare the behaviour of different types of mic, as you move a mic in from a distance in towards the source, the more directional the mic is the sooner the proximity effect will start to kick in. You can get a cardioid closer to the subject before you hear the proximity effect than you can a hyper while a shotgun will start to exhibit the effect even farther out than the hyper.
Yes, this is correct.

An interesting note is that if the sound is 90° off-axis, there is no proximity effect at all.
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