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Author Topic: Microphone Sensitivity, Polar Patterns, and Myth busting  (Read 673 times)

Gary Bowling

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Microphone Sensitivity, Polar Patterns, and Myth busting
« on: November 18, 2019, 02:02:25 pm »

This is more just to share some general info and testing in case others find it useful.

I have a local group of audio guys that I do a lot of work with. FOH guys, studio engineers, musicians, etc.

We have been discussing how to reduce the bleed into various mics for live stage performances. The biggest problem being drum and cymbal bleed into other mics, particularly vocal mics. It's not that it's such a problem, just something we're trying to improve.

Various opinions and tales were shared, some good info, some old wives tales, and some head scratchers.

Long story short, we decided to take a number of mics and do some experiments to either verify or "myth bust" some of the ideas and information.

I'll give the bottom line results first, then some discussion about how we determined and what we did.

- Mic sensitivity among all the popular types of mics is basically linear. Maybe you could measure some non-linearity, but for practical purposes our tests showed them to be linear.

- Rejection of sounds from the sides and back are what changes based on the polar pattern of the mic. But not the sound from directly in front of the mic.

- If you're trying to reduce bleed from "back line" instruments into a vocal mic, you're only choice is to get the singer to be closer to the mic and/or sing louder. Back line instruments hit the front of the mic, no mic does any better (or worse) at rejecting background noise that hits the front of the mic. Singing louder and closer allows you to turn the gain down, reducing the back line volume and bleed. Nothing else!



The experiments. We have a venue that we know well and one of the guys is the FOH guy there, so we could use that venue for testing. Nothing special about the venue, a typical small stage venue. Not the worst acoustics, not the best. Some sound absorbing materials on the walls of the stage, reasonably high ceiling, and mid sized room. Basically the average stage.

We were able to use the venue during off times, they only are open Thu-Sat. We set up a rock band that plays classic rock and spent quite a bit of time testing. We had the band play the same song and try to sing/play at the same volume levels. Seasoned band and singers so pretty easy to do.

We would set up a mic, have the singer sing with lips touching the mic. Set the levels, and record to a DAW via the console (it's a console with direct USB recording capabilities).

Then we would have the singer back up to 4 inches from the mic and have the band play and sing again.

There is a section of the song with no vocals, which is where we captured the back line "bleed" level.

In trying 10 different mics, there was virtually no difference in volume at 4" or the volume of the back line bleed. Everything from cardiod to super cardiod to hypercardiod, dynamic and condenser mics.

Yes, the mics had different sensitivities, which required adjusting the gain with "lips touching" for each mic to be equal. But once that was done, the levels at 4" and the back line levels were so identical that any differences would be un-detectable.

Yes, volumes at 125 degrees, 180 degrees, etc. were all very different, following the polar patterns of whatever mic we were testing. But the volume of sound directly into the front of the mic from singer or back line drums (which were directly behind the singer) were virtually identical.


If your stage is big enough to put drums or other back line amps off to the side a bit. Then you can probably see a reduction in bleed based on the polar pattern of the mic. And certainly bleed and feedback from monitors can be reduced with tighter patterns. We purposely did not use monitors in this test just for this reason. But direct back line bleed and/or volume a few inches away from the mic was not affected at all by the type of mic used.

gabo
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Keith Broughton

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Re: Microphone Sensitivity, Polar Patterns, and Myth busting
« Reply #1 on: November 18, 2019, 02:35:52 pm »

An interesting test and fairly predictable results.
That said, I have not run into anyone (so far...)who suggested that polar patterns had any effect on "backline bleed"
For monitor wedge applications, polar patterns can be of help if used correctly.
As you noted, singing closer to the mic is about the only solution other than, perhaps,  some of the early "dual mic cancellation" setups.
For me, it's tall plexi shields around the drums :D
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Patrick Tracy

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Re: Microphone Sensitivity, Polar Patterns, and Myth busting
« Reply #2 on: November 18, 2019, 04:06:32 pm »

I never bought into that sensitivity @ distance thinking. The mic doesn't detect distance, it just reacts to pressure at the diaphragm (or velocity at the ribbon).

Gary Bowling

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Re: Microphone Sensitivity, Polar Patterns, and Myth busting
« Reply #3 on: November 18, 2019, 04:16:57 pm »

An interesting test and fairly predictable results.
That said, I have not run into anyone (so far...)who suggested that polar patterns had any effect on "backline bleed"
For monitor wedge applications, polar patterns can be of help if used correctly.
As you noted, singing closer to the mic is about the only solution other than, perhaps,  some of the early "dual mic cancellation" setups.
For me, it's tall plexi shields around the drums :D

I haven't run into anyone saying specifically "polar patterns" but I have had many people suggest to use "Mic X" or "Mic Y" to lessen the bleed.

Which sort of implies that they either believe that polar patterns or mic sensitivity effects it. But neither actually do.

Agree with the wedge monitor and plexi shield observations. I haven't tried the dual mic cancellation set ups, but want to do an experiment with that just for fun. Not sure I would use it regardless of effectiveness, but you never know.  If it's really good and a band wants to make a live recording, it might be useful.

One thing that was a bit surprising. The dropoff from "lips touching mic" to 4" was about 30db! In other words, a 0db signal with lips touching becomes a -30db signal at 4".

So in a small club, the message to singers is. Work and practice to stay up on those mics.

gabo
« Last Edit: November 18, 2019, 04:32:48 pm by Gary Bowling »
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Keith Broughton

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Re: Microphone Sensitivity, Polar Patterns, and Myth busting
« Reply #4 on: November 19, 2019, 06:38:29 am »

So, given staying close to the mic is a good idea, perhaps a test to see what effect different mics or polar patterns have on proximity effect.
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Dave Garoutte

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Re: Microphone Sensitivity, Polar Patterns, and Myth busting
« Reply #5 on: November 20, 2019, 03:41:47 pm »

Generally speaking, the tighter the polar pattern, the more proximity effect.
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Chris Grimshaw

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Re: Microphone Sensitivity, Polar Patterns, and Myth busting
« Reply #6 on: November 21, 2019, 05:48:45 am »

I haven't run into anyone saying specifically "polar patterns" but I have had many people suggest to use "Mic X" or "Mic Y" to lessen the bleed.


Well, it ain't quite that simple.

Super/hyper/whatever-cardioid mics are rated such at a nominal frequency. It's like speaker impedance - 8ohm nominal means it might dip to 6ohm and then get up to over 30ohm. The mic might have a perfect pickup pattern at 500Hz, but what about the rest of the range?


FWIW, the EV N/D967 has the most GBF I've ever worked with, but I suspect that's little to do with the supercardioid pattern (although if you go through the maths, that does have the best rejection to the rear of the mic, integrated over all angles) and more to do with the fact that the outer grille is about 1/2" away from the diaphragm.


As a minor correction to Dave G's post, the pattern that's closest to dipole has the most proximity effect. In order from least to most: omni, sub/wide cardioid, cardioid, super, hyper, fig-8.

Chris
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brian maddox

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Re: Microphone Sensitivity, Polar Patterns, and Myth busting
« Reply #7 on: November 21, 2019, 05:44:17 pm »

....
One thing that was a bit surprising. The dropoff from "lips touching mic" to 4" was about 30db! In other words, a 0db signal with lips touching becomes a -30db signal at 4".



This is the counter intuitive thing that i have yet to find a way to adequately communicate to singers. 

I think on loud stages in small spaces, the old "foam windscreen and sing with your lips touching the foam" thing beats mic choice, placement, and processing by a WIDE margin....
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Andrew Broughton

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Re: Microphone Sensitivity, Polar Patterns, and Myth busting
« Reply #8 on: November 21, 2019, 06:19:54 pm »

Did you try this test with an OM7 or a Crown CM311? How about a KSM8?
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Andrew Broughton

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Re: Microphone Sensitivity, Polar Patterns, and Myth busting
« Reply #9 on: November 21, 2019, 06:28:43 pm »

This thread probably should be moved out of the AV section, but while I'm thinking of it, how many of you have heard the famous story of Robert Scoville and how he got Tom Petty to stop singing into a 57 and into a KMS105 (or some other Neumann)?
As he explains it, it wasn't so much that he hated the vocal sound he got from the 57, and level wasn't an issue, since Tom basically eats that mic, but it was the off-axis tone that was the big problem. Petty's backline was (is?) so loud that Tom's vocal mic would pick up tons of guitar and other instruments, but when they hit the 57 off-axis, they sounded really bad. With the Neumann, the off-axis tone was similar to the on-axis, just lower in level, so when Tom moved away from the mic the backline sound through his mic was at least tonally acceptable.
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Luke Geis

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Re: Microphone Sensitivity, Polar Patterns, and Myth busting
« Reply #10 on: November 21, 2019, 11:05:01 pm »

Mic selection is about two things.

1. Practical application. Is it being used for vocals, drums, guitar or.....

2. The subjective opinion of the sonic result for the use of that microphone in said application.

Simply, what are you using the mic for and how does it sound in use.

Microphones are essentially a perfect tool. They have no bias and no difference in results will occur given exact circumstances. The difference between mic A and mic B is what makes one microphone better suited for any given application. The factors that determine ultimately what a mic will likely be used for are things like proximity effect, off-axis coloration, and off-axis rejection or polar pattern control.

Some mics have a better off-axis sound quality but may have a wider pattern that also allows more sound to enter into the mic at any given angle. Some mics may have better/tighter pattern control that may not sound as good off-axis, but what comes in on-axis is louder and sounds subjectively better. There are a million variables and in the end, it all boils down to application and the subjective opinion of the result.

Certain mic types tend to be better suited for certain applications. A hyper-cardioid mic is often utilized for drums to reduce bleed. Cardioid mics are often selected for vocals because proximity effect and quality of sound is often very good. There are no hard rules for that though. The Sennheiser MD421 is an example of a mic that gets used very often for drums because it sounds great and works really well, but it is a cardioid mic. You don't often see the MD421 used on vocals though. The Neuman KMS105 is most often used on vocals and is rarely seen used on anything else. The KMS105 is a super-cardioid mic for reference. The long and short of it is that there is no magic bullet when it comes to microphones.

For me, the only real considerations I make for microphone selection is what my monitor setup will be and ultimately how does it sound in use. If I have a particular monitor setup ( texas headphones vs. a single monitor for instance ) I will pick a mic that best suits that first and then after listening to the artist I can make a final determination if I should choose another option and deal with the compromises. In the end, it all comes down to listen first, asses, correct and repeat.
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Art Welter

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Re: Microphone Sensitivity, Polar Patterns, and Myth busting
« Reply #11 on: November 22, 2019, 03:32:19 pm »

One thing that was a bit surprising. The dropoff from "lips touching mic" to 4" was about 30db! In other words, a 0db signal with lips touching becomes a -30db signal at 4".
Gary,

That level change seems a bit extreme for most vocal mics, but good on you for the test!

The inverse distance law, -6dB for doubling of distance works just fine for microphones as well as speakers, however (as Chris mentioned) the distance with from the windscreen varies with microphones, so "lips touching" might be anywhere from .25" to as more than an inch using a external foam screen.
On top of that is the cardioid microphone proximity effect, an increase in bass output at near distances.

Measure the distance between screen and diaphragm of microphones noted for more "gain before feedback", and you find they are usually less than the "old standard" SM58. In fact, an easy 6 dB can be gained from any old SM58 by simply squashing the grill flat.

0dB .25"
-6dB .5"
-12dB 1"
-18dB 2"
-24dB 4"

0dB .5"
-6dB 1"
-12dB 2"
-18dB 4"

For what it's worth, just measured a drop of about 14 dB from singing (G#, 208Hz) at the screen of a Shure 565 SD to 4", had to go to about 9" to drop to -30 dB. The 565 diaphragm is about 3/4" from the screen. Removing the ball, singing in to the diaphragm's inner screen, the -30 dB distance dropped to only 3", with about 10dB more gain before feedback available.
Of course, that gain is only available before the windscreen saturates...

Art


« Last Edit: November 22, 2019, 03:40:55 pm by Art Welter »
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Tim McCulloch

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Re: Microphone Sensitivity, Polar Patterns, and Myth busting
« Reply #12 on: November 22, 2019, 11:16:51 pm »

This thread probably should be moved out of the AV section, but while I'm thinking of it, how many of you have heard the famous story of Robert Scoville and how he got Tom Petty to stop singing into a 57 and into a KMS105 (or some other Neumann)?
As he explains it, it wasn't so much that he hated the vocal sound he got from the 57, and level wasn't an issue, since Tom basically eats that mic, but it was the off-axis tone that was the big problem. Petty's backline was (is?) so loud that Tom's vocal mic would pick up tons of guitar and other instruments, but when they hit the 57 off-axis, they sounded really bad. With the Neumann, the off-axis tone was similar to the on-axis, just lower in level, so when Tom moved away from the mic the backline sound through his mic was at least tonally acceptable.

I recall him saying something to the effect of "if it's going to pick up the whole stage it needs to sound good."

Having a coherent phase response let him align the sound-making things inputs with their acoustic arrival at Petty's vocal mic. Since the Heartbreakers set up the same every night, once done it was done.  Brilliantly simple in concept.
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Re: Microphone Sensitivity, Polar Patterns, and Myth busting
« Reply #12 on: November 22, 2019, 11:16:51 pm »


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