Radio age research, manufacturing, communications, broadcasting, television (1941)

left: dr. H. F. OLSON ARRANGES APPARATUS FOR ACCURATE TESTS OF AN ARTIFICIAL VOICE IN RCA LABORA- TORIES' FREE-FIELD SOUND ROOM. RIGHT: IN THIS ROOM ENGINEBHtS WATCH INSTRUMENTS WHICH RECORD THE QUALITY CHARACTERISTICS OF A LOUDSPEAKER UNDER TEST. urements showed that less than three tenths of one percent of the sound energy striking the baffled and padded walls is reflected. This trace of reflected energy, while un- desirable, represents such a small part of the total that any error it might introduce into actual tests would be less than the error of good electrical measurements. Measuring equipment used in conjunction with the free field sound room is located in the adjoining lab- oratory. The apparatus includes means for measuring the response frequency, power frequency, and directional characteristics of elec- tro-acoustic transducers. Signal lines connect the free field sound room with the adjacent laboratory, as well as all other rooms in the Acoustic Laboratory. Noise Absent at Night Spurious noises reaching the room from the outside are only slightly above the threshold of hearing even when the laboratories are in normal operation, but at night, with the shops closed down, the noise level decreases to 0 db, the absolute threshold of the nor- mal ear. The room is heated by hot air forced through openings in the floor but the thermal insulation is so ccmplete that even on the cold- est days, the heating system must be operated only one third of the time to hold the temperature with- in a 3 degree variation. Many complex factors influence the collection of sounds by micro- phones and the dispersion of sound by loudspeakers. Therefore, after either form of these electro-trans- ducers has been developed, the next step is a test under actual operating conditions. These tests require a sound stage with acoustic charac- teristics that can be altered as de- sired. The RCA Laboratories include such a stage, 48 feet long, 36 feet wide and 24 feet high. Two large monitoring rooms on the side give an unimpeded view of the stage through sound proof windows. Walls and Ceilings Padded The walls and ceiling of the sound stage are finished in one-inch Ab- sorbex, backed by four inches of rock wool blanket. The stage floor, for practical reasons, is covered with asphalt tile with practically no absorption. An abuse-resisting wainscot extends up four feet from the floor. The reverberation time of 0.4 second is quite low and is about one-half that recommended for a normal broadcast studio of this volume (40,000 cubic feet). The reverberation time is made low so that flexible acoustic char- acteristics may be obtained. For example, in sound motion pictures and television, the standard prac- tice is to build sets upon a sound stage with very low reverberation time. Under these conditions, the acoustics of the collected sound is determined by the set. Added real- ism is attained by this expedient because the acoustics of the col- lected sound corresponds to the pic- ture depicted on the screen. For tests under normal broadcast acous- tic conditions, the reverberation time of the sound stage may be in- creased to optimum values by in- troducing reflecting surfaces on the walls. In this case "V'ed", polycy- lindrical or other dispersing sur- faces may be used. This sound stage provides a means for testing the acoustical properties of these surfaces. A low noise level is another es- sential requirement of a sound stage. The noise level in the sound stage when the laboratories are in normal operation is 20 db. The sound stage is designed so that it may be employed as a small theater for the reproduction of sound. For these tests, the room may be "livened" by increasing the reverberation time. This is done by introducing reflecting surfaces along the sides and in the ceiling. Radio receivers and phonographs, as used in the home, constitute the largest number of complete sound- reproducing systems. For this rea- son, the performance of a loud speaker in a relatively small room is an extremely important problem. Duplicates Living Room The living room laboratory is de- signed to be the acoustical equal of the average living room. It is 24 feet long, 20 feet wide, and 8 feet 6 inches high. Audio frequency signal lines connect the living room with measuring equipment perma- nently installed in other parts of the Acoustic Laboratory. These lines are essential when it is desired to obtain objective data on response frequency, non-linear distortion, spatial distribution, intensity level and transient characteristics of sound reproducers operating in the living room. Dust-free rooms are highly de- sirable for the assembly of the [RADIO AGE 13