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Author Topic: Is this U87 capsule too dirty?  (Read 6077 times)

Telefunk

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Is this U87 capsule too dirty?
« on: August 08, 2011, 10:05:32 am »

I just survived the nerve-wrecking experience of opening the U87 head basket for photographing.

So, my question is: Does this capsule need cleaning?

The back side is completely clean and sounds bit brighter. Could this amount of dirt in the front side be enough to cause this slight difference?
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radiovinhet

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Re: Is this U87 capsule too dirty?
« Reply #1 on: August 09, 2011, 07:19:42 am »

This capsule is not much dirty. I can't believe that this little contamination could affect the sound.
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klaus

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Re: Is this U87 capsule too dirty?
« Reply #2 on: August 09, 2011, 01:41:39 pm »

You would need an awfully strong accumulation of grime, dust or dried spit to slow down the membrane's movement enough to affect high frequency response. This diaphragm's dirt load is close to zero.

The perceived frequency response differences between front and rear need to be checked carefully, and cannot easily be confirmed by switching the pattern to omni or figure eight to compare.

The electric and acoustic interaction between both capsule systems makes that hard. The only true way to compare the cardioid performance of both sides is reversing leads. Not that hard on a U87A, a bit harder on the older U87.
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Klaus Heyne
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radiovinhet

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Re: Is this U87 capsule too dirty?
« Reply #3 on: August 10, 2011, 08:51:58 am »

A little "little" difference between the 2 sides of a k67/k87 it's normal...
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klaus

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Re: Is this U87 capsule too dirty?
« Reply #4 on: August 10, 2011, 12:23:47 pm »

A ± 2dB tolerance at any given spot on the U87's frequency response (new model and old) is actually not all that little. That can add up to a real-world differential of 4dB between two mics at any given frequency point.

Few U87 capsules ever spread that much, but it has occurred, and would still be within the manufacturing spec for that mic.
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Klaus Heyne
German Masterworks®
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Telefunk

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Re: Is this U87 capsule too dirty?
« Reply #5 on: August 13, 2011, 08:26:08 am »

Ok, got back to the studio and tested it again ("only" with ears..).

The thing is there is a slight difference between the capsule sides only in fig-8 but not with omni. The back side has a bit more bass/low-mid. Treble is identical.

How is THAT possible?

well, at least it tells me that it not about the capsule being dirty or not.
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Tim Campbell

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Re: Is this U87 capsule too dirty?
« Reply #6 on: August 13, 2011, 10:06:06 am »

As Klaus has stated here correctly many times, you cannot judge the differences between capsule halves by listening to them in fig.8 .
The polarization and phase are not the same and this could lead to perceived differences between the halves.

The difference you describe could be caused by phase differences between the halves and your monitoring setup or perhaps a slightly higher polarization voltage or capacitance on the back half of the capsule causing it to be slightly louder. A slight difference in volume can be perceived as increased proximity effect instead of volume.
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David Satz

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Re: Is this U87 capsule too dirty?
« Reply #7 on: August 13, 2011, 11:37:50 am »

Telefunk, a microphone that is called a "figure-8" can have less-than-ideal symmetry, either by design or by accident. It is definitely worth testing this. But if you are testing by ear, you must be sure that you can't hear the original sound while you are hearing the reproduced signal from the microphone--because they will combine in a way that is very different for the front vs. the back of the microphone, and that will lead you to false impressions.

Any figure-8 microphone picks up sound from the front and rear, but not (or not so much) from the sides. The front vs. rear lobes produce signals in opposite polarity to one another. This fact becomes very important in a test like yours. When a sound source is in front of a figure-8 microphone, the output signal will reach its highest positive voltage at the moment of the highest positive sound pressure. However, if the sound source is behind the microphone, the output signal will reach its greatest negative voltage at the moment of the highest positive sound pressure.

If you are testing the symmetry of a figure-8 microphone by listening to it, you must be where you can't hear the actual sound that the microphone is picking up. Otherwise, when you are listening to the front of the microphone, the original sound (especially at low and low-mid frequencies) will generally add to the signal from the microphone, while when you are listening to the back of the microphone, the signal from the microphone will generally subtract from it.

You noted the opposite effect: the back of the microphone seemed to have greater low- and low-mid response. That could actually be true of the microphone, but perhaps the signal polarity is inverted somewhere in your monitoring setup. If you can block out the original sound so that you don't hear any of it directly while you are testing, then the polarity of your monitoring setup will probably become insignificant; most people don't hear "absolute" polarity, particularly with continuous signals.

--best regards
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Kai

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Re: Is this U87 capsule too dirty?
« Reply #8 on: August 16, 2011, 04:14:20 am »

... there is a slight difference ... but not with omni
This is a strong evidence that both sides match closely.
In omni the front and back is coupled, but still work as individual cardioids.
This means you can listen to (almost) one side separately in omni, if the sound direction is on-axis.

My advice: leave the capsule alone and enjoy the U87 sound.

Regards
Kai
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mwurfl

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Re: Is this U87 capsule too dirty?
« Reply #9 on: September 11, 2011, 05:55:21 pm »

Are you listening to the mic through headphones, and using your own voice while you test?  Years ago I discovered the same phenomenon, where one side of figure-eight sounded fuller and with better bass, and the other sounded thin.  I finally realized that what was happening was that I was hearing phase cancellation through my own skull, where some of the sound from my voice got to my ears through my skull, and the rest through the mic/amp/headphone system.  Recall that fig 8 patterns have opposite polarity on either side of the capsule, so through one side you'll get constructive inteference, and destructive interference through the other side.  You can confirm this by flipping the polarity-reverse switch on your preamp if it has one, or by using an inline polarity-reverser.  If you then notice that the side that sounds thin is now the other side, you've just experienced this phenomenon, and the mic is just fine.

Mark W
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