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ARCHITECTURAL ACOUSTICS 309
in determining the acoustic properties of a room. However, reverberation is not the only factor affecting the acoustic properties of an enclosure. Thus, the size and shape of the room, and the presence of extraneous noise, all contribute to the resulting acoustic merit of a room.
It is not a simple matter to give a quantitative rating to a room which is to be used for music, since so much depends upon the musical taste and disposition of the listeners. It is, however, a relatively simple matter to give a quantitative rating to a room which is to be used for speaking, since our primary concern is how well we hear the spoken words of the speaker. The most feasible scheme for such a rating is probably the one used by telephone engineers for testing speech-transmission over telephone equipment, which goes by the name of "articulation tests". The method of conducting articulation tests was described in the lecture on "Speech and Hearing". This method has proved to be very useful for investigating the effects of noise and reverberation upon speech reception in auditoriums.
The "percentage articulation" in any room signifies what percentage of typical speech-sounds can be heard correctly by an average listener in that room. A speaker calls out typical monosyllabic speech-sounds, in groups of three, at a rate of three syllables in two seconds. Observers stationed in representative positions in the room write down what they think they hear. If, on the average, they hear correctly four-fifths of the total number of called speech-sounds, the articulation for this room is rated at eighty per cent. It would seem that such a scheme as this offers a satisfactory means for rating the acoustic quality of a room which is to be used primarily for speaking.
It is obvious that the percentage articulation in an auditorium will depend upon (1) the size of the room, (2) the reverberation characteristics of the room, (3) the amount of disturbing noise in the room, and (4) the shape of the room. It is apparent that, for speaking purposes only, the ideal auditorium is a small room free from all noise, and bounded by perfectly absorbing surfaces. In such a small room the listener will be near the speaker and therefore the speaker's voice will be heard with adequate loudness. Further, there will be no interfering noise, reverberation or delayed reflections. Actual tests con