A Computer Investigation of the Performance of Virtual Reflectors: Part 1 - Development of the Method

In a companion paper [1] a technique for improving acoustic conditions in open stage auditoria by means of virtual reflectors has been proposed. Possible methods of testing the application of a virtual reflector system were discussed and it was concluded that computer simulation was the most appropriate. However, before employing a commercially available package it was necessary to devise suitable procedures.

The need for both a directional loudspeaker and a directional microphone is an essential requirement for the virtual reflector system but most computer simulation packages only allow for source directivity. Hence a method of incorporating the effect of microphone directivity using the principle of reciprocity was developed. The basis of establishing the gain factor has been devised and it is proposed that the convolution of echograms rather than the convolution of true impulse responses can be employed to predict system performance.

The action of the virtual reflector system has been investigated using a model of an open stage theatre. Both system microphone and system loudspeaker were selected from a catalogue of available modern commercial sound equipment. The resulting STI contours demonstrated that the effect of the virtual reflector over the audience area to the rear of a human speaker was to raise the speech intelligibility to be approximately equal to that to the front of the speaker.

It has been shown that the STI at the rear of a speaker with the virtual reflector system in operation does not increase significantly with gain. This is due to the Modulation Transfer Functions which determine the STI value being determined by the convolution of two impulse responses multiplied by a gain factor and added to a third impulse response. The resulting impulse response therefore consists of many ‘reflections’ and hence the MTF's and STI values will be adversely affected if excess gain is applied.