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8.2 - DIRECTIVITY

8.2.7 - RATIO OF DIRECT TO REVERBERANT SOUND

It would seem logical to expect to hear a variation of direct to reverberant sound at different angles round a unidirectional or bi-directional microphone. In fact the situation is not quite so simple – although the theoretical ratio is actually varying continuously, we are only conscious of a change once the ratio has reached or passed a certain critical threshold value. In order to verify this phenomena it is necessary to try a very simple experiment. Place the microphone at the centre of a circle of about 1.5 metres in radius, and record someone talking whilst moving along the circumference of the circle - they must indicate their position every 10°. Either by listening ‘live’, or to the recording, after one complete circle, one must try and determine the angle at which a change in reverberation begins to be apparent (with respect to the ratio of direct to reverberant sound on axis). Obviously the direct sound is changing continuously according to the particular directivity used, but we perceive this change as a change in the ratio of direct to reverberant sound. However we do not hear the change in this ratio as we move off-axis – in fact it is not until we have reached a certain threshold value that this change begins to become apparent. This threshold value varies somewhat from person to person but in general it corresponds to an attenuation of 2.5dB of the direct sound level. In a cardioid microphone this corresponds to an angle of 60° to the front axis as shown in Figure 54. In other words from about 60° onwards we begin to hear the decrease in the ratio of direct to reverberant sound, but not before.

Figure 54 - Reverberation Thresholds for Hypocardioid, Cardioid
and Supercardioid Microphone Directivities