R/E/P > Klaus Heyne's Mic Lab
Curious Omni Polar Pattern At 90°, 270°
emulation:
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.
klaus:
Your +3dB figure for 90º sound entry is curious. Can you post a manufacturer's polar pattern graph?
emulation:
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?
klaus:
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 90º as it does in the U47.
emulation:
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.
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