Radio Broadcast (May 1928-Apr 1929)

7 BROADCAST V <(\ ENGINEERING >>>>> \ BY CARL DREHER • JIF Pertinent Information on Broadcast Monitoring BROADCAST monitoring generally consists in putting some sort of loud speaker in a control room and leaving the rest in the hands of an exwireless operator who has got tired of going to sea. This gives sufficiently satisfactory results in some cases, but as the programs get bigger and costlier, corresponding to a similar progression in the advertisers who pay for them, and as receiving sets improve and field strengths go up, judgment of what leaves the station becomes more critical. Some of the scientific aspects of the problem are worth considering. Two papers by Irving Wolff and Abraham Ringel, primarily on the subject of loud speakers, should be read by the up-to-date broadcaster in this connection. These articles (" Loud Speaker Testing Methods," Proceedings I. R. E., Vol. 15, No. 5, May, 1927, and "Sound Measurements and Loud Speaker Characteristics," Proceedings I.R.E., Vol. 16, No. 12, Dec, 1928.)throw considerable light on problems of soimd reproduction in general. One possible defect in acoustic judgment, which is not mentioned by these authors, is that of anomalies in hearing. Naturally anyone who has anything to do with judging the output of a broadcasting station should have normal ears, with adequate response within the usual frequency limits of human audition. I mention this because I am told that at one station the technical staff has an unhappy time with a very elderly orchestra leader whose ears have become insensitive to high notes. He comes out into the control room and Listens to the orchestra, with an assistant conducting, and insists on getting a better balance in the higher octaves. Nobody wants to tell him that the trouble is in his own head, and he remains an unsolved problem. Monitoring should be done in a quiet room with average acoustic properties. Noise from motors or other sources is very objectionable. It leads to auditory fatigue and carelessness, and may mask defects in transmission. The best plan is to put the gain control into a small, acoustically treated room within sight of the studio through a double glass window and well isolated acoustically, with a good loud speaker near the operator, and a musician to advise him. The room should be free from unusual resonances and the period of reverberation should be low. Wolff and Ringel point out this pertinent fact: "A person who has been in a room, which is not average, for a rather long time, becomes accustomed to it. and is no longer struck by its acoustic peculiarities." A monitoring chamber about 12 by 9 by 8 feet, with a hard floor, but the walls and ceiling lined with material absorbing about 25 per cent, at 512 cycles and free from extraordinary absorption at any frequency, is a good compromise. Such a room will have a period of 0.4 second and will not show the striking resonance effects of small rooms finished in hard plaster and not treated for acoustic improvement. SELECTION OF LOUD SPEAKER The differences between loud speakers are such that the selection of one for a monitoring room is a moot problem. The old horn types, with their single high-resonance peaks and a rapid fall to nothing on either side, may be left out of consideration. One solution is to use a moving-iron loud speaker and a moving-coil type, or several moving-coil speakers in different sized baffles, and to set up the orchestra and the performers on a basis of compromise between them. A more practical procedure is to select one speaker which is fairly typical for the period and to judge on the basis of its output. It should be a good loud speaker in the interest of the progress of the art generally, and because it is hopeless to try to adapt transmission to the defects of all kinds of receivers in the homes of listeners. Ringel and Wolff found, in this connection, that "the loud speaker which has the best looking characteristic (most free from peaks) will, in spite of the defects of existing broadcasting transmitters and receivers, generally sound best when tried on radio." Where conditions favor it, radio monitoring as a supplement to audio is desirable. W hen a relatively noise-free signal is available from the air a good radio receiver to which the loud speaker can be switched from the audio amplifier output of the station should be provided. If there were enough diff erence it would be worth while to simulate radio transmission during rehearsals by modulating a baby transmitter with the same characteristics as the radio plant of the station. On important programs the preparation of a cue sheet is worth while. This may include the gain settings for various portions of the program, as determined during rehearsals. ANOTHER SYSTEM It is quite possible that more elaborate methods of broadcast monitoring will be developed in the future as standards of performance rise. The sound movies may be pointing the way in this field. Some of the large producers have gone to the trouble of placing the microphone mixing and gain controls on a "bridge" or platform in one corner of a room about fifty feet on a side, otherwise empty except for a few men and pieces of apparatus, and with monitoring loud speakers of the theatre projection type, operating at theatre volume, in the diagonal corner. The speakers are about 70 feet from the operators. The room is acoustically treated and gives a good imitation of theatre conditions during the recording. There is nothing else in the place; this volume of 125,000 cubic feet is entirely turned over to the monitoring staff. The actual recording on film or disc takes place elsewhere, and the studio where the action is photographed and picked up acoustically is on the other side of a sound proof wall, although it is within sight through double windows. Interphone connections are of course provided between the separate units. This scheme may not improve the quality of the product enough to justify the expense; if a cheaper system will give almost equally good results, it will naturally prevail. It is, however, a bold attempt to fit the means to the end, and the attitude, if not the mechanism, is worthy of imitation. The Truth is Mighty and Must Prevail THE lament is frequently raised that once an error is spread abroad it is impossible for the truth to catch up with it. This is a sad fact, even in engineering circles. Specialization adds to the confusion. The highly skilled technicians in some little corner of applied science can't be fooled when it comes to their own specialty, but they take more or less on hearsay material from other fields, and thus they learn many things which are not so. The fallacy about remedying defective acoustics in auditoriums by the use of stretched cords and wires is known to all acoustic engineers. Its absurdity was first exposed by Wallace Clement Sabine. It can be shown theoretically that the scheme cannot work, and practice bears out the theory. When it seems to work the usual reason is that a large audience has been crowded into the hall, and the resulting increase in acoustic absorption has partially remedied the trouble. To an acoustic engineer, or any university student in the physics of sound, or to broadcasters who know the elements of their business, all this is well known, but among other learned men the romantic device of stretching wires for acoustic correction still holds its own. I present in evidence an excerpt from the discussion on Prof. F. R. Watson's "Acoustics of Motion Picture Theatres," taken from the Transactions of the Society of Motion Picture Engineers, Vol. XI, No. 32, 1927, p. 650: Mr. S. : You probably remember when the Century Theatre in New York was opened. The acoustics made it almost impossible to give a play, so they intermeshed the ceiling with fine wire and it made a great improvement. Mr. R.: I should like to add that I know a case in Chicago where a plain wall building gave a bad echo. They stretched wires across the hall and kept adding them until they broke up the echo. Mr. C: I believe that is a common method. I know the Denver Auditorium suffered that way and the cross wires corrected the effect, but of course they are unsightly. Mr. Watson (communicated) : Wires in an auditorium are practically useless. If an auditorium with wires has good acoustics, this must be due to other features — carpet, upholstered seats, or other absorbents. The Denver Civic Auditorium, for example, had a considerable amount of absorbent installed. • february, 1929 . . . page 246 •