Projection engineering (Sept 1929-Nov 1930)

Page 20 Projection Engineering, September, 1929 10" lo re2 Fig. 4. The masking of a 1500-cycle tone by a 300-cyc!e tone. in comparison with those of the lower frequencies, which are unimportant for articulation. When, then, enough porous material is introduced to reduce the reverberation of those tones in which speech is rich to the point where it is no longer objectionable, the intensities of the important hight frequency tones may be below the threshold of audibility at remote parts of the room. At these points it will be difficult to interpret the speaker's words. Masking of Tones Aside from the distracting effect of the reverberation of low frequency tones, their presence in excessive amounts reduces the ability of the auditor to hear those of high frequency. It has for a long time been known qualitatively that the threshold of audibility for a given tone is raised by the presence of another tone, especially if this tone be one of lower frequency. A quantitative study of this masking of one tine by another has been made by Wegel and Lane,3 from wrhose paper the curve of Fig. 4 is taken. An ordinate of this curve gives the relative power of a 1500-cycle tone that is just audible in the presence of a masking tone of three hundred cycles having the power indicated by the abscissa. Unity power for either tone is taken as its power at the threshold of audibility when no other tone is sounded simultaneously. For example, when a tone of three hundred cycles is sounded at an intensity of 10° times its value at the threshold of audibility, a fifteen hundred cycle tone to be heard must be 104 times as intense as when the three hundred cycle tone is absent. These values of intensities for the respective frequencies are quite possible for speech in a room, which has its acoustic characteristics controlled by the absorption of ordinary porous materials. The phenomenon of masking further emphasizes the importance of keeping down the average intensity of the lower tones, which for articulation are quite unimportant and yet are delivered in large amounts by the speaker. Increasing Absorption Characteristics The conclusions here reached, while resting primarily on indirect data, seem to be borne out by experience. Auditoriums having the most desirable acoustic properties are found to be those which are sufficiently dead but are free from hangings and other porous materials. Many of the best auditoriums have a large amount of wood paneling. Fig. 5 gives the absorption curve for -such paneling as de termined by W. C.' Sabine. This is seen to be somewhat selective in favor of the lower frequency tones. For a room that is too reverberant we could thus hardly do better than to increase the absorption by covering certain of the walls by some kind of wooden panels. Unfortunately, as the absorption coefficient for such panels is low, a very large amount of wall space would usually have to be covered in order to bring the reverberation down to a proper value. This the fire laws will not permit in many cases. It thus appears that there is a need for a material which is fire-proof, fits into the decorative scheme of interiors, and has absorption characteristics that are high and selective for low frequency tones. Having available materials of this type in addition to the various porous absorbers now on the market we could place the problem of acoustic treatment of rooms on a better engineering basis. 11 10 9 8 UJ O 6 a UJ c a. D 4 3 2 / / / 1 0 c, 64 c2 128 C3 C„ C5 MUSICAL SCALE 256 512 1024 FREQUENCY c. 2046 Ct 4096 ; Physical Review, XXIII, February, 1924. Fig. 5. The per cent absorption of wood paneling at different frequencies. THE PROJECTIONIST'S NEW ROLE {Continued from page 13) records and was operated from the orchestra pit. This non-sync device was coupled through the fader on the left-hand side in parallel with the lefthand machine through the same amplifier and speakers which were used on synchronized sound-on-film. Naturally it was in series with a switch for cutting out the non-sync device when it was not in use. On the fade-over from the silent comedy, which was being run on the left-hand machine, to the sound feature on the right-hand machine, the switch was not thrown to cut out the non-sync device by the afternoon crew. This switch had no effect on the right-hand machine as it was only in parallel with the left-hand side. Therefore, sound on this machine was O. K. and the evening crew took it for granted that everything was in running order. However, when they faded over to the left-hand machine for reel two, this switch was still on and acted as a short circuit, killing the sound on this machine. You can attach a dozen morals to the story and fix the blame where you will, but things like this do happen and will continue to happen just as long as untrained men are in charge who lose their heads in an emergency. Another Angle It would be no more than proper to relate here a story that will contrast with the above in which the operator used his head in an emergency and saved the day, although this particular operator had very little experience in the operation of sound. The spring in the sound gate broke and the operator held the film in place with his finger until the fade-over to the other machine. His attempts to fix the spring while the other machine was in operation were futile, so when it came back to the machine on which the sound gate was broken, he removed the sound gate from the good machine and put it in the other. As the sound gates are interchangeable and can easily and quickly be removed for cleaning purposes, it caused less than a minute's delay and the audience was hardly conscious that anything had happened. This gave him the necessary additional time to continue his repairing on this broken sound gate and then the show continued on serenely without a single call-dojvn by the manager, as it is doubtful if the manager even knew anything had happened'. The author is familiar with a great many similar cases as the above but in no instance dtoes he know of a single case that would 'he serious enough to stop the show, if the projectionist in charge had been familiar with the fundamental principles of sound and used his head. To sum up, it might be well to say that no matter how far the research laboratories develop sound pictures or how good the reproducing apparatus is, or how well the sound is recorded, the ultimate results and the heights to which synchronized sound pictures will soar, will still depend largely on the projectionist himself, who must advance himself to the place of engineer, as operating, within itself, has become a secondary consideration.