International projectionist (Jan-Dec 1940)

RCA SOUND SERVICE TOOLS BY RCA PHOTOPHONE ENGINEERING STAFF (The second instalment of this article appeared in July) m. WHILE listening tests in theatres are final criteria for good sound reproduction, very often acoustic response readings taken in the auditorium will indicate where changes are necessary to improve quality. RCA has employed the General Radio 759-A sound level meter (Fig. 9) for this purpose since the early part of 1937. In addition, it is extremely useful for checking noise levels in projection rooms and the theatre proper, for checking extraneous noise produced by fans in air-conditioning systems, and the effectiveness of vibration insulation of power equipment. • Warble-Tone Freq. Reel For checking acoustic response of speaker systems, a warble-tone frequency reel is employed as a source of signal, to reduce as much as possible the effects of standing waves in the theatre. The microphone is set up at various points throughout the theatre, and then readings are taken from 30 cycles to 9400 cycles at each station. The plotted curves so obtained indicate approximately the acoustic response in various sections of the theatre, and this can usually be confirmed by listening tests. On the basis of the acoustic response so obtained, adjustments can be made for proper sound quality from the speaker system. This procedure has been used in numerous theatres and the results as compared to listening tests were very gratifying. • Associated Equipment This device is also very valuable in checking the sound distribution throughout the auditorium. Response curves run at several points quickly show up any deficiencies. The speakers can be accurately angled on the basis of such readings to give the optimum sound distribution. Other test instruments have been de veloped by various branches of RCA for specific applications. These are often used for theatre work in routine service or solving special problems. One of these is the RCA vibration pick-up (Fig. 10). This unit, with its associated equipment, is very useful in locating defective gears, bearings, or other moving parts. Such defects usually show up as "knocks" occurring at regular intervals, or as vibrations at an audio frequency. If the approximate location and the frequency FIGURE 9 RCA Sound-Level Meter of the "vibration" can be determined, the exact location of the defect is rather easy to find. The output of this vibration pick-up is fed into an amplifier and in turn to the vertical plates of a cathode-ray oscillograph. A prod is provided on the vibration pick-up for prodding around the sound-head until the approximate source of the vibration is located, as indicated by a maximum deflection on the oscillograph screen. If the frequency of this vibration or knock is known, the problem is still further simplified. The frequency can be determined by using the cathode-ray oscillograph externally synchronized by an audio beat-frequency oscillator or an RCA synchronizing generator. If the synchronizing generator is used, it is coupled to the projector crank shaft or to a sound-head sprocket shaft. It generates a synchronizing voltage at intervals which are directly related to the r.p.m. of the shaft. The synchronizer also provides a movable marker voltage which can be impressed on the signal under observation and gives a means of marking the oscillograph trace with respect to the angular position of the shaft. Knowing the r.p.m. of the motoi and sound-head sprockets, the problem is still further reduced and can be solved by associating the frequency of knock or vibration with the r.p.m. of the moving parts in the vicinity of the source of vibration. THE END SEES MODERATE BIZ GAIN! A "moderate gain" in film companies' net income in the fourth quarter is forecast by Standard Trade & Securities in its current survey of the film and amusement industries, published recently. Increase is predicated on lower production costs and the expected larger domestic theatre attendance and receipts. Third quarter profits, however, are expected to bs seasonally depressed and to reflect, as well the loss of British and other foreign business. "Domestic theatre attendance continues slightly ahead of a year earlier, and the imposition of additional taxes on theatre admissions has had almost no effect on attendance,'' says the survey. '"Theatre profits are also unaffected as, in over 90 per cent of all cases, the tax has been immediately passed on to the public. In a few instances, the opportunity was taken to increase admission prices by a few cents more than the additional tax. In general, however, there has been no increase in admission prices." DENY PATENT SUIT DISMISSAL Federal Judge John W. Clancy (N. Y.) has denied applications of Warner Bros.. Albert A. Radtke, Radtke Patents Corp. and Leonard Day to dismiss the patent infringement suit of Photometric Productions Corp. against them.' The suit covers a patent known as "Methods for and Means of Optically Recording and Reproducing Sound" without which it is probable sound films would not be in existence. I. A. Local 257, of Ottawa, Canada, has made a voluntary contribution to the Canadian government for use in financing the war by the donation of the sum of $5Q0 as an outright sift from its members. AUGUST 1940 13