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Author Topic: Curious Omni Polar Pattern At 90░, 270░  (Read 3708 times)

Offline emulation

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Curious Omni Polar Pattern At 90░, 270░
« on: April 07, 2011, 08:07:07 am »
Why is a double-membran mic more sensitive for sound from the side (90░ and 270░, 4-6kHz)? For instance the polarpattern blows up to +3dB between 4kHz-6kHz.
« Last Edit: April 08, 2011, 05:40:52 pm by klaus »

Offline klaus

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Re: Polarpattern (90░,270░) double membrane capsule?
« Reply #1 on: April 07, 2011, 03:14:28 pm »
Your +3dB figure for 90║ sound entry is curious. Can you post a manufacturer's polar pattern graph?
« Last Edit: April 07, 2011, 05:11:33 pm by klaus »
Klaus Heyne
German Masterworks«
www.GermanMasterworks.com

Offline emulation

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Re: Polarpattern (90░,270░) double membrane capsule?
« Reply #2 on: April 07, 2011, 04:40:16 pm »
If I look at the omni-polarpattern of the U87Ai or U89, when the level is not 0dB (omni, 4kHz, 90░ and 270░). The relative level increase from 0dB (0░,on axis) to +3dB (90░, off axis). Other mics with double membrane capsule mics show this effect (in omni-mode). I want to know why this happened or what is the technical background?

Offline klaus

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Re: Polarpattern (90░,270░) double membrane capsule?
« Reply #3 on: April 07, 2011, 05:31:34 pm »
Ahh.. Omni. I misunderstood.

My speculation for the 3dB rise @only around 4kHz in these two mics: diffraction or standing waves between the only two parallel surfaces encountered by the capsule: back plate flank and basket frame bar.

I say "speculation" because, you would think that the same boost would appear in the U47 (same frame-bar construction) but it does not. Possibly because the distance between capsule side and frame bar is twice as large and therefore the wavelengths bouncing around vary.

I was probing whether the capsule construction differences between dual and single backplate could account for this, but could not come up with an explanation, as any influence of the gradient design would probably not show up as a boost but as a (slight) cut in omni response at 90║ as it does in the U47.
Klaus Heyne
German Masterworks«
www.GermanMasterworks.com

Offline emulation

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Re: Polarpattern (90░,270░) double membrane capsule?
« Reply #4 on: April 07, 2011, 08:06:29 pm »
The omnipattern of the U47 shows only very few frequency points, so the effect could occur there too. The free-field frequency response shows a small valley around 8kHz (U89 omni). The position of the valley varies from mic to mic, but in this region the omni-pattern (90░,270░) will increase a little bit. Is there a relationship with the summation of the two cardiods?

@Diffraction or standing waves: I think the dimensions of the frame-bar are to small against the wavelength to build up standing wave.
« Last Edit: April 08, 2011, 05:10:15 am by emulation »

Online Kai

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Re: Polarpattern (90░,270░) double membrane capsule?
« Reply #5 on: April 08, 2011, 07:53:23 am »
Why is a double-membran mic more sensitive for sound from the side (90░ and 270░, 4-6kHz)? For instance the polarpattern blows up to +3dB between 4kHz-6kHz.
A double-membran mic is in fact two cardiods back to back.
So let's look at a single cardiod.

The polar pattern of every cardiod mic is made up from two different effects:

In the lower frequency range:
Sound is mixed from those hitting the diaprags front directly and those going though the rear entry to the back of the diaphrag witch causes an opposite phase signal.
The rear entry damping elements lower the level of the sound to about 50%.

This way the cardiod is a 50% mix of a pressure gradient mic (fig. of eight) and a pressure mic (omni).

In the higher frequency range:
The rear entry acts like a low pass filter, few sound passes through the rear entry of the mic.
The mic acts like a pressure mic (omni).
If the size of the mic is the same or bigger as one quarter of the wavelength of the signal (high f.'s have a short wavelenght), diffraction causes a directivity to the front, like with any omni.

So in fact you have a "two way mic" with a frequency dividing network.
The x-over frequency lies in the range of 2-8 kHz, depending on size and construction.

The art of the designer of such a mic is, to couple the two ranges as smooth as possible.
It's impossible to achieve this absolutely perfect (same problem like with a 2-way loudspeaker).

Therefore most irregularities of cardioid mic's responses, specially in the polar pattern, happen to be in the range between 2-8 kHz.

For the double cardiod there is one more problem:
the diaprags are some millimeters apart, so signals aren't in phase, specially in the critical f. range.
This can lead to more problems in the capsule design and is a reason for the different frequency responses of the patterns of switchable double diaphragm mic's.

Regards
Kai

Offline klaus

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Re: Polarpattern (90░,270░) double membrane capsule?
« Reply #6 on: April 08, 2011, 12:33:22 pm »
Kai,
Thanks for your nice explanation, in lay-language to boot!
But realistically, only your last paragraph could then be the reason for the extended midrange around 4kHz in the U87 capsule, because, like the U87 and U89 capsules, the K47 capsule is also a (same sized) dual-membrane cardioid of the same diameter, but does not exhibit the boost.



Klaus Heyne
German Masterworks«
www.GermanMasterworks.com

Online Kai

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Re: Polarpattern (90░,270░) double membrane capsule?
« Reply #7 on: April 08, 2011, 02:48:45 pm »
It's not only the diameter, but every detail in the backplate design that affects the coupling of the frequency ranges.
The transfer ports (little holes in the back of the backplate), together with the coupled air volume(s) in the cavities behind the diaphrag, act as a LP filter for sound from the back.
If this filter is tuned different, rear sound mixes different with the front sound.

One more parameter - capsule resonances:
there are several build into the mic's by purpose (usually for HF extension) and some that are inevitable by the priciple like the diaphrag resonance.
Although the latter might be dampened, it still can change the freq. response.
The designer might even decide that a boost around 4K is what he wants to improve intelligibility.

As we see, there are a lot of possibilities to change the response of a mic.

The only thing that cannot be built:
a mic with a mathematically perfect uniform response usable for music recording.
Designing a mic is always a compromise, made for a certain purpose.

Regards
Kai

Offline emulation

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Re: Curious Omni Polar Pattern At 90░, 270░
« Reply #8 on: April 08, 2011, 06:52:23 pm »
Many thanks to Klaus and Kay for the quick and competent answers. It is surprising that the technical literature gives no hint. The capsule design seems to be more complicated than expected when the capsule is composed of two membranes. So the effect is dependent on the distance between the membranes and the inner acoustic tuning.

Best wishes
emulation