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Test 5: Microphone Reach
This comparative field test was done to show the directivity factor γ and the distance factor of each of the microphones used in dialogue recording on location. The REE = Random Energy Efficiency is a ratio of a microphone’s response to diffuse energy relative to a response from on-axis. In other words: One can find out about the amount of noise reduction of a directional microphone when a directional microphone and a non-directional microphone are exposed to the same sound power while microphone sensitivities are equal. For omnidirectional microphones this factor equals 1, because there simply is no existent directivity. The table shows the REE factors for other polar patterns:
|Polar Pattern||REE (1/γ)||Directivity Index in dB||Distance Factor|
So for example a cardioid receives only ⅓ of the sound power compared to the omnidirectional microphone. The effect of this is that in a diffuse sound field
the sound pressure level of a cardioid is 4.8dB below the SPL of the omnidirectional microphone in the same diffuse field. With the increase of the directivity factor γ
(decrease of REE) the microphone becomes less responsive to diffuse sound. This in turn means that the unidirectional microphone can be placed further away from the sound
source while the ratio of diffuse and direct sound stays the same at a certain distance. This distance is calculated by the distance factor √γ. If the distance between sound
source and omnidirectional microphone is 1m, the distance between sound source and cardioid can be 1.44m. For shorter distances, the portion of direct sound to diffuse sound
will increase, for greater distances it will decrease.
In film sound this is called the ‘reach’ of a microphone.
In this test the reach of different microphones was examined. The location was a small church with a wooden ceiling and
wooden pews. This is why the reverberation time of 1.3s is fairly short. The omnidirectional microphone was always placed right above frame line. The determined distance
between the sound source and the omnidirectional microphone was then to be multiplied by the different distance factors
in order to set up all directional microphones. It is not easy to figure out the correct distance factors for shotgun microphones, as they differ in tube length and original polar
pattern (supercardioid vs. hypercardioid). For this test a distance factor of 2.1 was chosen.
All shots happened inside the critical distance of 1.06m (considering the CCM 2).