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8.2 - DIRECTIVITY

8.2.5 - DIRECTIVITY VERSUS FREQUENCY RESPONSE

Omnidirectional microphones have in general a good response in the bass frequencies. In the case of omnidirectional condenser microphones, the onset of low frequency roll- off can be as low as a few Hz. However this is not the case for pressure gradient microphones, a bi-directional or figure-of-eight microphone has a 6db/octave roll-off in the bass frequencies. The exact frequency at which the roll-off starts is determined by the manufacturer, depending upon the required sensitivity and signal to noise ratio. The variation in roll-off amplitude is determined by the degree of pressure to pressure-gradient acoustic coupling, or in other words the proportion of omni to bi-directional response in the directivity model. Figure 49 shows the variation in roll-off as one passes from omnidirectional through to bi-directional or figure-of-eight directivity. This series of frequency response curves related to directivity assumes that all other frequency dependent factors are equal, especially sensitivity. The start of roll-off in the frequency response diagrams has been set at 200Hz purely for the sake of illustration.

Figure 49 – Directivity and Bass Frequency Response
Figure 49 is a comparison based on theoretical calculation. To see how this applies to the real world, Figure 50 shows the low frequency response for five different directivity patterns from the same manufacturer. The ‘ripple’ in the low frequency response curves is due small imperfections in the measurement process, however the profile of each response curve gives us the information we need concerning comparison between the frequency response relative to each of the different directivity patterns. In Figure 49 the hypocardioid response is for a directivity with a back attenuation of 6dB, whereas the real pattern in Figure 50 is a hypocardioid (MK21) with 10dB back attenuation – this explains the difference between the two response curves. Similarly Figure 49 shows a hypercardioid with 8dB of back attenuation, whereas the Supercardioid in Figure 50 (MK41) has a back attenuation of about 12dB. The theoretical bi-directional in Figure 49 and the real bi-directional (MK8) in Figure 50 show the same low frequency attenuation slope of 6dB/octave.


Figure 50 – low frequency response versus directivity for five microphones
from the same manufacturer
© frequency response curves published by courtesy of Schoeps
In stereophonic sound recording any practical Stereophonic Recording Angle (between ± 90° and ± 30°) can be obtained with any chosen directivity pattern. It is therefore quite possible to take advantage of the better bass response of a hypocardioid microphone as against a cardioid, or on the contrary, to attenuate bass response with a hypercardioid directivity response. This choice is generally determined by the acoustic environment of the recording (traffic or ventilation noise, structure borne vibration, etc...), or simply by one's personal preferences. There are obviously other reasons for choosing different directivity patterns which will be considered in later sections, for example the response of the microphone system to the surrounding acoustic environment such as reverberation or early reflections.