Worship spaces require a very unique acoustical environment which provides both intelligibility for the spoken word and a high spatial impression for music appreaciation by both the congregation and performing organist, choral or orchestral ensembles. Clearly, the design of worship spaces at the present time, requires an electroacoustic approach. This discussion, however, wil focus only on the acoustic aspects of intelligibility, the quality of perceived music, or listener preference and the degree of ensemble experienced by performing musicians. We will illustrate the usefulness of a new broad-bandwidth wide-angle sound diffusing surface treatment, the reflection phase grating diffusor (RPG), in improving these three subjective parameters. The RPG is especially suited for use in worship spaces, because it can efficiently diffuse confusing late specular reflections, while simultaneously providing incoherent diffuse lateral reflections, which improve coverage, increase the spatial impression, and also provide a heightened sense of ensemble when used as an acoustical shell for performing groups. Methods to predict speech intelligibility using the Signal-to-Noise ratio, ALCONS, and the Speech Transmission Index, STI, will be reviewed. A new method to determiner the STI, using time-delay spectrometry (TDS), will be introduced. The STI determined from this approach will be referred to as the TEFSTI index. The ALCONS and the TEFSTI will be used to evaluate intelligibility accompanying application of the RPG. The RPG was found to improve speech intelligibility in a severe flutter situation. The energy-time curve (ETC) will be used as a criterion to evaluate sound quality. An ideal ETC is defined as one in which the direct sound is temporally separated from the indirect energy by an initial time delay gap which is related to the autocorrelation of the program material and the indirect energy is uniform, without any density gradients or discontinuities, with a linear decay. The ability of the RPGs to reduce isolated late reflections and flutter echos and provide a linear uniformly dense decay characteristic is impressive. This conclusion is based on a comparative analysis of the energy time curve curves at nine listening positions at the Evangel Assembly of God. Application of the RPGHs converted a room which ws marginally useable into a room which performed well by the accounts of all concerned, for the function for which it was intended.
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