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Does diaphragm size impact GBF?

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Does diaphragm size impact GBF?

Postby katie.amelia » Sat Feb 23, 2019 11:32 pm

I've heard it said before, including on these forums, that small-diaphragm condenser mics have more gain before feedback in live settings, but I have yet to find an explanation why. To me it seems as though the opposite would be true as large diaphragm mics have less off-axis sensitivity at higher frequencies, making them more directional than small-diaphragm mics.
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Re: Does diaphragm size impact GBF?

Postby Sam Spoons » Sun Feb 24, 2019 10:46 am

Gain before feedback is a function of the relative position of the mic and speakers and the polar patterns of both. SDCs have polar patterns from Omni to super/hyper cardioid and shotgun, LDCs don't usually do the narrower patterns but many have figure 8 which gives deeper nulls than other types so, in theory, pointing the null at the speakers may give greater GBF than a SCD cardioid mic.
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Re: Does diaphragm size impact GBF?

Postby Hugh Robjohns » Sun Feb 24, 2019 11:34 am

katie.amelia wrote:I've heard it said before, including on these forums, that small-diaphragm condenser mics have more gain before feedback in live settings, but I have yet to find an explanation why. To me it seems as though the opposite would be true as large diaphragm mics have less off-axis sensitivity at higher frequencies, making them more directional than small-diaphragm mics.

It's a little more complicated than that...

When people talk of large-diaphragm capacitor mics, they're often referring to multi-pattern mics and/or studio fixed-pattern cardioid mics. The important feature of these is that they employ Weber-Braummuhl capsules which have dual-diaphragms -- one in the front, and another at the back.

It is the presence of the rear diaphragm that causes the problems in a live sound situation because its action serves to change the microphone's polar-pattern at low frequencies depending on the proximity of the sound source.

For a close source (like someone singing into it), the rear diaphragm helps to maintain a tight cardioid polar pattern at low frequencies and a moire consistent proximity effect, but for more distant sources the LF polar-pattern becomes much more omni-directional.

It is this unique characteristics that makes it more likely to pick up LF spill from the PA speakers/monitors (which also tend to radiate LF omni-directionally). This is not the case with a single-diaphragm capsule -- and most small-diaphragm capacitor mics have a single-diaphragm capsule, of course.

Consequently, dual-diaphragm mics tend to be more prone to low-frequency feedback in stage applications than single-diaphragm mics... and thus the slightly misleading but widespread assertion that all large-diaphragm capacitor mics are worse than small-diaphragm mics for gain-before-feedback problems...

There's an informative paper describing the interesting properties of dual-diaphragm capsules from Shure here:

https://522bb370f5443d4fe5b9-f62de27af599bb6703e11b472beadbcc.ssl.cf2.rackcdn.com/publication/upload/340/pdf_ea_dual_diaphragm_mics.pdf
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Re: Does diaphragm size impact GBF?

Postby Wonks » Sun Feb 24, 2019 11:56 am

katie.amelia wrote:I've heard it said before, including on these forums, that small-diaphragm condenser mics have more gain before feedback in live settings,

I don't think you have, because I can't find it mentioned. Where have you copied that from?
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Re: Does diaphragm size impact GBF?

Postby James Perrett » Sun Feb 24, 2019 3:14 pm

Wonks wrote: Where have you copied that from?

I thought the same as you but I actually did a quick search for a section of the text in the original post before approving it and nothing similar came up. It appears to be a genuine question.
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Re: Does diaphragm size impact GBF?

Postby Wonks » Sun Feb 24, 2019 3:33 pm

No context, no detail. And I couldn't find any post here where someone had stated that opinion.
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Re: Does diaphragm size impact GBF?

Postby Hugh Robjohns » Sun Feb 24, 2019 4:08 pm

The differences in GbF between dynamic and capacitor mics on stage has often been discussed, so perhaps it's a variation on that theme...

Regardless of the inspiration or motives in asking the question, it's an interesting and worthy topic of conversation.
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Re: Does diaphragm size impact GBF?

Postby katie.amelia » Sun Feb 24, 2019 4:48 pm

Wonks wrote:No context, no detail. And I couldn't find any post here where someone had stated that opinion.

I honestly can't find the thread it was mentioned in anymore. It was a brief mention on a pretty old thread on stereo miking of choirs.
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Re: Does diaphragm size impact GBF?

Postby Sam Spoons » Sun Feb 24, 2019 4:59 pm

IIRC many older threads were deleted when the new forum software came online.
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Re: Does diaphragm size impact GBF?

Postby Dave Rowles » Sun Feb 24, 2019 10:48 pm

Great post by Hugh on the technical aspect.

On a stage it depends on what you're trying to mic, and where it is. LCDs are very popular as drumkit overheads and percussion. If you're close micing an instrument they can be very effective, and if you have something that has a lot of low end they can provide a less lumpy response than a dynamic mic, which can be quite useful. Mostly you'll find them with the highpass/lowcut enabled.

I like to use LDCs on:
Kick
Snare - As side mic
OHs
Bass Cab
Guitar Cab
Percussion - if micing from a slight distance
Cello
Double Bass

Mostly I work on low volume or IEM only stages which helps a lot! If you're on a noisy stage and trying to mic up something quiet then they aren't your friend.
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Re: Does diaphragm size impact GBF?

Postby Wonks » Mon Feb 25, 2019 11:00 am

It may help to explain what the context is for using SDCs or LDC s on stage. Is it for a choir with FOH reinforcement, or an acoustic guitar, or a string section etc?

To me it seems as though the opposite would be true as large diaphragm mics have less off-axis sensitivity at higher frequencies, making them more directional than small-diaphragm mics.

As Hugh says , it's normally LF feedback that presents the main problem live, just because low frequencies radiate in all directions, so without any monitors, just the front of house rig pointing forwards and in line or ahead of the front of the stage, a there's a lot more LF present and so LF feedback will normally be the main problem.

There are of course SDCs designed (mainly) for handheld stage use, and these tend to have much overall tighter polar pattens than your standard LDCs and SDCs. Not to say that you can't get feedback with them, but no more so than with dynamic cardioids.

But add monitors into the picture, firing a full range of frequencies across the stage, and you have a very different situation. You can of course remove the low frequencies from the monitors so that there isn't an excess of LF from both the FOH and the monitors on stage to keep things balanced, but the possibilities for feedback have increased significantly.

So context is very important. As is the general choice of polar pattern and the specifics of the polar pattern(s) of a given microphone.
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