Motion Picture Herald (Dec 1932 - Mar 1933)

16 Better Theatres Section February 11, 19 3 S MODERN PROJECTION PROJECTION • SOUND REPRODUCTION • ACOUSTICS MUSICAL ACOUSTICS OF AUDITORIUMS [^The following article offers the unique suggestion that on account of the variation in the time between beats in music, more effective rendition of music could be accomplished by varying the time of reverberation of the auditorium or room in which the music is played. The discussion is reprinted from the Journal of the Society of Motion Picture Engineers, February issue, to which it was contributed by Peter Caporale of the Electro-Acoustical Engineering Company of America, of Philadelphia.] THE IMPORTANCE of increasing the usefulness of enclosures such as sound motion picture studios, recording studios, broadcasting studios, theatres, music halls, etc., by controlling the time of reverberation is being recognized more and more. In all these types of rooms music of some form is performed, and the control of reverberation may add considerably to the artistic presentation of such music. Thus, the effects of large tone or fine definition or articulation may both be achieved by suitable control. The following is a brief discussion of some of the important factors to be considered in controlling reverberation for musical purposes. Music is a unique art in that a third person (or group of persons) is necessary to convey the composer's thought to his audience. In particular cases it may happen that the third person and the composer are one, as, for example, when Kreisler plays his own compositions. But even in these cases Kreisler the violinist is not the same as Kreisler the composer. In other words, composition and expression are neither the same nor are they simultaneous. To speak of this situation in more familiar engineering terms, we may think of the history of a musical composition as divided into four stages. The first, which we shall call A, is the conception of the composition in the mind of the composer. The second, B, is the transcribing of this concept into a form known as the score. The third, C, is the transference of the concept from the score to the mind of the player, or interpreter; and the fourth, D, the transmission of the concept, by means of sound, to the audience. It must be obvious that in such a complex transition it is rare that a listener will sense the same musical thought {i.e., the same physical sound as conceived originally) that the composer had in mind, and we may therefore speak (rather loosely, of course) of the efficiencies of the various stages of the transition. For example, there are some effects that can not be indicated by the usual musical notation ; hence the efficiency of transition B is less than unity. Similarly, the score may be ambiguous in certain parts, or in some respects; therefore, the efficiency of transition C is also less than unity. Very little has been done to increase these various efficiencies. Reverberation control, however, offers the possibility of increasing the efficiencies of both transitions B and D. The combinations of sound that reach the audience are determined not only by the nature of the source of the sound (orchestra, organ, violin, etc.) but also by the character of the enclosure within which the sound occurs and the audience is located. This fact has been known from the most ancient times, but no direct use Avas made of the knowledge. For instance, when Bach wrote the Mass in B-minor he was acquainted quite intimately with the acoustical properties of the Leipzig Thomaskirsche and could foresee the approximate effect of the music. It was obviously impossible for him to foretell the effect in some other church or auditorium. Recent progress in acoustics, particularly as regards reverberation, has made it possible to control the acoustical properties of an auditorium so that a musical composition may be rendered in such a manner as to accord very closely with the wishes of the interpreter. Furthermore, the control of the auditorium might be placed in the hands of the composer, so that not only is efficiency D increased, but also that of B; or, more specifically, the composer might indicate on the score just what the acoustical conditions should be for any particular passage. The technical problem of controlling reverberation is this: given an auditorium having a certain volume and exposed wall ■a 1 -re P. Fig. I. — Chart showing duration of musical beats for metronome settings. surface, to vary the total sound absorption of the room so as to vary its period of reverberation. Several times the idea has been suggested, and in fact, a definite system has actually been proposed, whereby various surfaces of different coefficients of absorption could be exposed. None of these systems, however, could have been readily adaptable to the kind of control required for musical purposes. The realization of such a scheme, of course, would involve not only the designing of appropriate equipment by the engineer, but as well the training of the musician so that he might understand the full possibilities of the system. The latter problem is, of course, not of interest to us here. This discussion will be limited to the musical requirements from an engineering standpoint. It must be noted that in many cases it might not be practicable to vary the time of reverberation during the performance of a piece of music; but it might even then be practicable to vary it between successsive pieces. In general, musical compositions may be very roughly divisible into two classes — solo and ensemble, each of which is further divisible into slow and fast music. The crudity of this classification is obvious, but it is at least indicative of the range of types of music. It IS rare to find rapid, brilliant passages for one instrument in orchestral works (with the possible exception of solos for the first violin). Music is, moreover, characterized partly by rhythm, of which the elementary component is the beat. [The word "beat" must be distinguished from the common acoustical beat. As here used it has the more common musical meaning, indicating the instant of beginning a certain time interval in music. The rapidity or slowness of a passage depends upon the lapse of time between beats.] Figure 1 shows a curve indicating the duration in seconds, betAveen beats produced by a standard metronome. For convenience the corresponding musical terms are also given. Now, since the deleterious effect of reverberation is to cause overlapping of successive sounds, the problem becomes that of reducing the time of reverberation sufficiently to avoid undesirable overlapping. But it must be remembered that not in all cases is overlapping to be completely avoided. If successive sounds pertain to the same harmony, some overlapping is in fact desirable. This, however, is a problem for the composer rather than the engineer. It will be evident from Figure 1 that.