Elements of acoustical engineering (1940) Elements of acoustical engineering elementsofacoust00olso Year: 1940 286 ARCHITECTURAL ACOUSTICS acoustic resistance of the holes. However, the attenuation in the high fre- quency range increases with frequency due to the acoustic reactance of the holes. The inertance increases with the thickness of the screen and de- creases as the ratio of the open to closed area of the screen increases. For example, for 3 db attenuation at 10,000 cycles the hole area is usually 15 to 20 per cent of the screen area. If the hole area is 7 to 15 per cent the at- tenua
Elements of acoustical engineering (1940) Elements of acoustical engineering elementsofacoust00olso Year: 1940 286 ARCHITECTURAL ACOUSTICS acoustic resistance of the holes. However, the attenuation in the high fre- quency range increases with frequency due to the acoustic reactance of the holes. The inertance increases with the thickness of the screen and de- creases as the ratio of the open to closed area of the screen increases. For example, for 3 db attenuation at 10,000 cycles the hole area is usually 15 to 20 per cent of the screen area. If the hole area is 7 to 15 per cent the at- tenuation is about 6 db at 10,000 cycles and about 3 db at 5000 cycles. These examples show that the screen is an important problem in wide range reproduction. LOUD MICROPHONES ^ 65 > 60 t45, T / ' ^x- o^-^ -' \ L ' ,^^''' \ POSITION ON C LISTENING B LEVEL Fig. Arrangement of the components of a sound re-enforcing system in a theatre. The graph shows the intensity level due to direct sound at the points indicated on the orchestra floor. Curve 0, the intensity due to the original source of sound. Curve L the intensity level due to the loud speaker. Curve T the resultant intensity level. C. Sound Re-enforcing Systems ^^. — A large theatre equipped with a sound re-enforcing system is shown in Fig. Microphones are con- cealed in the footlight trough for collecting the sound on the stage and others are placed in appropriate positions in the orchestra. The loud speakers are located above the stage in the proscenium arch. The volume ' Olson, H. F., RCA Review, Vol. 1, No. 1, p. 44, 1936.
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