We use Optical Character Recognition (OCR) during our scanning and processing workflow to make the content of each page searchable. You can view the automatically generated text below as well as copy and paste individual pieces of text to quote in your own work.
Text recognition is never 100% accurate. Many parts of the scanned page may not be reflected in the OCR text output, including: images, page layout, certain fonts or handwriting.
76
owei frequencies. Materials recently developed are of such varied design and construction that it is possible to obtain the proper sound absorption characteristics in almost any type of auditorium, and thereby obtain the desired acoustic effects.
Sound Waves in the Auditorium
The unsatisfactory shape or size of an auditorium, and the focusing effect of wrongly shaped reflecting surfaces, produce echoes and objectionable concentrations of sound which are the basis of many of the major acoustic defects in present day. auditoria.
In Fig. 8 are shown the successive stages of the progress of a sound wave in a sectional model of a badly shaped auditorium. It will be seen in (1) that the initial sound wave is closely followed by another of almost equal intensitv due to the reflection from the horn-chamber. In the subsequent views are
Fig 7.
Absorption Characteristics of Various Materials.
K. Thatchboard covered with paper, hangers and canvas. (McNair.)
1. 2 in. of bare Thatchboard (Bureau of Standards.)
Z. Acoustic Rock Wool 4 in. thick behind wire net and muslin. (Knudsen.)
Thatchboard covered with p«rforated metal. (Building Research Station.)
Eelgrass Quilt 3-ply scrim covered (Sabine.)
Eelgrass Quilt 3-ply 2 layers 1$ in. from wall and Canvas Cover I in. distant.
AcousticCelotex BB type. (Sabine.) | in. double Insulite with t in Air space. (Building Research Station.) I in. Nashkote asbestos felt fabric painted AX finish. (McNair.)
1 in. Hair felt li in. from waH McNair.)
i in. Masonite fibre board on
2 in. x I in. studs at 12 in. c/c . (Sabine.)
7/16 in. Celotex standard board
on 4 in. x 2 in. studs. (Sabine.)
J in. Lino. (Building Research
Station.)
2 in.Wood (Pine). (Behm.)
Marble. (Behm.)
D.
FggqUENCV
shown the almost complete absence of direct sound in the rear stalls due to the bad design of the balcony front and the excessive balcony rake ; and the sound reflections from the roof dome and the rear wall of the balcony concentrating in the balcony area.
To obviate long path echoes in an auditorium the nature of the wall and ceiling surfaces should be such as to absorb the sound falling upon them, or preferably, the shape and position of the surfaces relative to the source of the sound should be such as thoroughly to diffuse the reflected sound.
A very high proportion of the sound reaching the ears of the audience should come by direct path, or by paths differing at most by 45 feet from the direct path, i.e., the maximum time interval between the reception of the direct and indirect sound wave is 1 25th seconds. It is possible that this permissible difference of sound path length will have to be rigidly enforced, and possibly reduced, for such future sound developments as stereophonic sound, where the apparent sound source has to move across and even off the screen.