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Author Topic: How A Shotgun Mic Works  (Read 1071 times)

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Offline Gutbucket

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Re: How A Shotgun Mic Works
« Reply #15 on: October 16, 2017, 03:25:03 PM »
It's the same principle that differentiates directional mics from omnis in general.

I don't follow, how so?

To my way of thinking interference tube (shotgun) mics are quite different because their directivity mechanism is not based on the same basic principle which governs all first-order patterns ranging from omni-directional through bi-directional.

One could mount an interference tube on a first-order capsule of any pattern (omni, subcard, cardioid, supercardioid, hypercardioid, bidirectional).  I could actually take one of the miniature interference tubes off my DPA 4098 super/hypercards and mount it on one of my 4060/61 omnis as the capsules and grids are the same diameter.  I've actually been meaning to do that for years and make some tests of the resulting miniature shotgun-omni but have never gotten around to it.
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Offline kindms

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Re: How A Shotgun Mic Works
« Reply #16 on: October 16, 2017, 03:25:50 PM »
was checking out the spec  sheet on those  AKG C747 s

the picture makes it look much larger than 5.4inches. thats a pretty compact shotgun mic. pretty cool
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Offline Gutbucket

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Re: How A Shotgun Mic Works
« Reply #17 on: October 16, 2017, 03:37:10 PM »
FWIW, most shotgun mics are based around a supercardioid capsule, and exhibit standard supercardioid directivity at frequencies below the range where the interference tube becomes effective.  What made the gargantuan EV 643 remarkable other than it's monstrous size was that it retained the tighter front-lobe shotgun pattern down to ~700Hz before the standard supercard directivity took over below that.  The tiny DPA 4098 interference tubes which are only a couple inches long would probably only make the omnidirectional 4060/61 capsule directional above something like 10Khz or more (guessing), and is probably intended to fine tuning the directivity pattern of just the topmost octaves.

The other method of achieving greater than first-order directivity is via DSP - combining and manipulating the signals from more than one capsule, which is how the Schoeps SuperCMIT achieves such radical directivity with minimal artifacts using a relatively short interference tube.  They start with a supercardioid, made more directional at high frequences via the interference tube in the standard shotgun way, but also more directional at lower frequencies than where the tube is effective via DSP manipulation with a second rear-facing capsule located a few inches behind the primary capsule.

Otherwise, any shotgun directivity beyond that of the native polar pattern of the capsule is largely determined by the length of the tube.
« Last Edit: October 16, 2017, 05:51:04 PM by Gutbucket »
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Offline opsopcopolis

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Re: How A Shotgun Mic Works
« Reply #18 on: October 16, 2017, 03:52:32 PM »
It's the same principle that differentiates directional mics from omnis in general.

I don't follow, how so?

To my way of thinking interference tube (shotgun) mics are quite different because their directivity mechanism is not based on the same basic principle which governs all first-order patterns ranging from omni-directional through bi-directional.

One could mount an interference tube on a first-order capsule of any pattern (omni, subcard, cardioid, supercardioid, hypercardioid, bidirectional).  I could actually take one of the miniature interference tubes off my DPA 4098 super/hypercards and mount it on one of my 4060/61 omnis as the capsules and grids are the same diameter.  I've actually been meaning to do that for years and make some tests of the resulting miniature shotgun-omni but have never gotten around to it.

Sorry, didn't qualify that even a little bit hah.

I had just read this part of the third link:

Quote
A shotgun microphone operates on the principle of waveform interference. If you put a diaphragm at the bottom of a solid tube you will not have a shotgun microphone because you need the slots. It's the slots that are the reason for the tube in the first place.

Each of the slot openings is a point source of audio; there is a different location for each slot, so there is a phase difference for each of these point sources, resulting in constructive and destructive interference, enahncing on-axis sound and attenuating off-axis.

I was referring to the 'slots' in modern shotguns
Mics: Berliner CM-33, CA-14 card, CA-11 card & omni, AT-853, Sony ECM-907
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Offline Gutbucket

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Re: How A Shotgun Mic Works
« Reply #19 on: October 16, 2017, 05:49:12 PM »
Quote
A shotgun microphone operates on the principle of waveform interference. If you put a diaphragm at the bottom of a solid tube you will not have a shotgun microphone because you need the slots. It's the slots that are the reason for the tube in the first place.

Each of the slot openings is a point source of audio; there is a different location for each slot, so there is a phase difference for each of these point sources, resulting in constructive and destructive interference, enahncing on-axis sound and attenuating off-axis.

The bold part is incorrect.  It's the other way around.  The slots aren't the reason for having the tube in the first place, it's a way of using just one tube instead of needing a whole bunch of tubes without any slots.  You get the effect from two non-slotted tubes only in an extremely narrow frequency range determined by the difference in dimension between the two tubes.  Using a whole bunch of tubes of progressive lengths averages the large number of narrow-band effects across a wide enough frequency range to make an effective enhanced directivity microphone based on this phenomena - see the image I posted above of the DIY version of Harry Olsen's original enhanced directivity microphone featuring a great number of tubes arranged in a spiral bundle with the capsule at one end (longest tube in the middle, shorter tubes packed around it, all of them in parallel with the mic capsule at the back end).  I wish I could locate a photo of the actual RCA version of the microphone from sometime around the mid 50's.

The ingenious follow up innovation was to use a single tube with either a series of holes, slots, a long slit, or some other form of acoustic resistance along it's length to replace the bulky bundle of individual tubes.  The tiny interference tubes on the 4098 are made from fine metal screening formed into a cylinder.  The whole tube surface provides a uniform acoustic resistance along it's entire length.
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Offline Gutbucket

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Re: How A Shotgun Mic Works
« Reply #20 on: October 16, 2017, 05:56:57 PM »
Paper on the innovations of Schoeps's SuperCMIT from the Schoeps website (how it uses a second capsule in combination with DSP processing to increase directivity below the range where the interference tube is effective)- http://www.schoeps.de/documents/Paper_SuperCMIT_Development.pdf
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