Harrisburg telegraph. [volume] (Harrisburg, Pa.) 1879-1948, 1916-04-10
ENGINEERS DETERMINE ACOUSTIC
PROPERTIES OF BIG GAUDITORIUMS 4
BEFORE THEY ARE CONSTRUCTED

each other. When this interference
/ occurs, sound distribution is altered:
some sounds are reinforced while
others are neutralized, much as waves
| beating against breakwater in
storm are reflected and tend to en-
large or quell the succeeding waves.
Care must be employed in selection
of Materials and hangings of the walls
of an auditorium, else certain partially
absorbed sounds will set them in vi
bration, as, for example, certain piano
notes cause objects in a. music room to
resound. This quality of resonance"
permits a distortion of sound which is
undesirable.
! Closely associated with resonance is
"consonance." the process of strength
ening the voice by proper arrangement
of rejecting walls. 1t is significant in
this connection to note that so little
/ is known about this one positive factor
/ in acoustics that we do not even pos
/ sess an English word for it. The an
cient Greeks advocated the use of re-
sonant vases in their theaters. claim
/ ing that the acoustics were improved
/ by these bronze objects "set in incon-
spicuous places about the room" 1t
is because the invisible air waves ra-
diate with the incredible speed of
| LOO feet per second. that they are so
difficult to control.
| Sound Absorption
This whir of sound is known as the
reverberation or residual sound, and
the time elapsing between the forma-
ton Of the original sound and the in
slant when the last echo has been
stilled, is called the time of reverber-
alton.
To determine the time of reverbera-
tion in a room after it has been built
is merely n matter of observation, but
how can this time be fixed when
room exists only as a few lines on
piece OF paper? A simple formula has
/ been derived by Professor Wallace c.
Sabine of Harvard University. a world
authority on acoustics, by which. when
we know the volume of air contained
in the room, the material with which
the wall, floor and ceiling are covered,
and the size of the audience the dur
alton of any given sound may be de-
termined.
Considering that an open window is
100 per cent. efficient in absorbing
sound waves-for no sound is ever re-
/ flected from it-by repeated expert
ments extending over a period of
years, he found values of absorption
I for concrete, wood tile, felt. cloth
and host of other building materials.
These values. together with the sound
wave absorbing power of men and
open window unit so that now it be-
comes an easy matter to find the total
sound wave absorbing power in an oc-
cupied room of any dimensions.
Photographing Echoes
There are numerous spots scattered
about the country where one may talk
o" even whisper, wait a few seconds,
and then hear the very same syllables
repeated with startling distinctness as
/ from great distance. This phenome-
non, known as the echo. is merely a
reflection of sound waves without dis
tortion, and in enclosures may be
nearly as troublesome as reVerber-
alton. As il conforms to. simple laws
of wave reflection. however. it is OBJ
waited by altering the form of the
interior, or breaking an expanse of
solid echoing wall by projections or
doors.
In this study of the relation of form
of room to sound reflection, a method
has been devised by which the echo
itself is photographed. A model on
about a quarter of an inch scale of a
section of the proposed hall or theater
is made, and enclosed between two
glass plates, one of which is sensitized.
An electric spark is made in that part
of the model representing the stage,
and as the sound wave induced by the
spark travels outward, a second spark
is off from the side. The light
from this spark traces on the sensi-
tized plate the exact form of the
sound waves at that instant, the air
composing the wave being of different
density from the rest of the confined
air. BY varying the time of the sec
ond spark the wave may be photo
graphed in its various positions and a
complete study of its progress may be
made. This plan of study is followed
in designing New York theaters to-
day, and because ii may be done be-
fore construction, like the determina-
tion of time of reverberation. it is an
effectual aid to the prevention of
acoustic failures.