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Correctioh of
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UNIVERSITY OF ILLINOIS BULLETIN
PUBLISHED WEEKLY BY THE UNIVERSITY
VOL. XIII MAY 29, 1916 . NO. 39
(Entered as secoud-class matter at the Postoffice at Urbana, III., under Act of August 24, 1912)
BULLETIN No. 87
BY
F. R. WATSON
And
JAMES M. WHITE
UNIVERSITY OF ILLINOIS
ENGINEERING EXPERIMENT STATION
URBANA, ILLINOIS
PRICE: FIFTEEN CENTS
EUROPEAN AGENT
CHAPMAN AND HALL, LTD., LONDON
THE Engineering Experiment Station was established by act of
the Board of Trustees, December 8, 1903. It is the purpose of
the Station to carry on investigations along various lines of engi-
neering and to study problems of importance to professional engineers
and to the manufacturing, railway, mining, constructional, and indus-
trial interests of the State.
The control of the Engineering Experiment Station is vested in
the heads of the several departments of the College of Engineering.
These constitute the Station Staff, and, with the Director, determine
the character of the investigations to be undertaken. The work is
carried on under the supervision of the staff, sometimes by research
fellows as graduate work, sometimes by members of the instructional
staff of the College of Engineering, but more frequently by investi-
gators belonging to the Station corps.
The results of these investigations arejyiblished in the form of
bulletins, which record mostly the experiments of the Station's own
staff of investigators. There will also be issued from time to time, in
the form of circulars, compilations giving the results of the experi-
ments of engineers, industrial works, technical institutions, and
governmental testing departments.
The volume and number at the top of the title page of the cover
are merely arbitrary numbers and refer to the general publications of
the University of Illinois; either above the title or below the seal is
given the number of the Engineering Experiment Station "bulletin or
circular which should be used in referring to these publications.
For copies of bulletins, circulars, or other information address the
Engineering Experiment Station, Urbana, Illinois.
UNIVERSITY OF ILLINOIS
ENGINEERING EXPERIMENT STATION
BULLETIN No. 87 MAY, 1 ( H6
THE CORRECTION OF ECHOES AND REVERBERATION IN
THE AUDITORIUM AT THE UNIVERSITY OF
ILLINOIS
BY
F. R. WATSON, Associate Professor of Experimental Physics, at the
University of Illinois, and
PROFESSOR J. M. WHITE, Supervising Architect
CONTENTS
Page
I. INTRODUCTION 1
1. Preliminary 1
2. Object of Bulletin 2
II. PRELIMINARY ACOUSTICAL INVESTIGATION 2
III. MODIFICATION OF INTERIOR OF AUDITORIUM 4
3. Installation of Pipe Organ 4
4. Method of Mounting Hair Felt 5
5. The Decoration and the Lighting System 7
IV. FINAL ACOUSTICAL INVESTIGATION 8
G. Investigation of Echoes 10
7. Investigation of Reverberation 12
V. DISCUSSION AND CONCLUSION 14
VI. BIBLIOGRAPHY.. ,.16
LIST OF FIGUEES
Pag
1. Floor Plan Showing Interior of the Auditorium at the University of
Illinois 3
2. View toward the Stage Showing the Grill Work for Free Passage of
Sound from the Concealed Organ 5
3. Photograph Showing the Mounting of the Hair Felt in Vertical Strips. . G
4. Photograph Showing the Side Wall of Fig. 2 when Completed 7
5. Diagram Showing the Dome Surface with Wooden Ribs Separating the
Hair Felt in Radial Strips 8
6. Photograph Showing Auditorium before Changes Were Made 9
7. Photograph Showing New Interior of Auditorium 10
8. Photograph of Stage Setting Showing Experimental House with Mega-
phones Used for Testing Echoes It
9. Diagram Showing the Reflection of Sound from the Unpadded Pendentive
in the Rear Wall 12
10. Plan of Auditorium Showing the Concentration of Sound by the Walls
under the Balcony 13
THE CORRECTION OF ECHOES AND REVER-
BERATION IN THE AUDITORIUM AT
THE UNIVERSITY OF ILLINOIS
I. INTRODUCTION.
1. PRELIMINARY. The work described in this bulletin may be
considered as a continuation of an earlier investigation on * ' Acoustics
of Auditoriums. ' '*
Figure 1 shows the floor plans of the auditorium under investiga-
tion. The interior approximates a sphere cut off on the lower surface
by the sloping floor of the room. There is a balcony, but no gallery.
The balcony projects 12 feet over the main floor at the sides and 34
feet in the rear. The stage is built out into the room instead of being
set back behind a proscenium arch as originally designed, the stage
house having been omitted to reduce the cost of the building.
The domed ceiling is supported on four equal arches, and the side
walls above the gallery are double curved surfaces. The limited ap-
propriation for the building made it impossible to embellish the sur-
faces of the walls and ceiling, and therefore, they were left practically
plain, which increased their power to reflect sound and cause echoes.
There are no windows in the room, the daylight lighting being exclu-
sively through a ceiling light 30 feet in diameter in the center of the
dome.
The results set forth in the previous bulletin are briefly as fol-
lows. A systematic investigation of the acoustical properties of the
Auditorium at the University of Illinois was carried on for several
years. ' ' Cut and try ' ' methods of cure were avoided. It was shown
by theory and experiment that the usual acoustical faults in a room are
due first, to a reverberation, or undue prolongation of sound, and sec-
ond, to echoes ; both of these defects being caused by the reflection of
sound from the walls. Various methods of cure were considered, the
effect of padding and paneling the walls, the possible advantage of
installing wiresf and sounding boards, t and finally, the action of
'Bulletin No. 73 of the Engineering Experiment Station, University of
Illinois.
t" Inefficiency of Wires as a Means of Curing Defective Acoustics of Audi-
toriums. ' ' Science, Vol. 35, p. 833. 1912.
i"The Use of Sounding Boards in an Auditorium." Physical Review, Vol.
1 (2), p. 241, 1913. Also The Brick Builder, June, 1913.
4 ILLINOIS ENGINEERING EXPERIMENT STATION
the ventilating system.* The conclusion was drawn that the most ef-
fective cure lay in padding the walls with materials which absorb
sound.
An experimental diagnosis of the acoustical properties of the Audi-
torium was made. This was done by tracing the path pursued by a
small bundle of sound when it was sent in a definite direction and not-
ing what became of it after reflection. Several methods of tracing
sound were tried before a suitable one was found. A ticking watch
backed by a reflector, or a metronome enclosed in a box having a di-
rected horn gave definite data. However, a hissing arc light Avith
a parabolic reflector was much more satisfactory and gave conclusive
results. Enough data were secured in this way to show the general
behavior of the sound in the room and also to indicate how the chief
echoes were set up.f Attempts were then made to secure satisfactory
acoustics by hanging curtains and draperies at critical points suggested
by the diagnosis. This result was finally secured by suspending four
large pieces of canvas in the dome.
From the acoustical standpoint, the Auditorium was then in a
much improved condition. The canvas, however, was very unsightly
and did not accord with the architectural features of the room. It was
therefore proposed that the materials used to correct the acoustics be
installed in such manner as to remedy this fault. It was also proposed
at this time to install a pipe organ, to decorate the interior of the room,
and to change the lighting system.
2. Object of tJie Bulletin. The object of this bulletin is to de-
scribe the changes that were made in the Auditorium to carry out the
proposals just mentioned, and especially to show how the acoustical
properties were modified.
II. PRELIMINARY ACOUSTICAL INVESTIGATION.
It was desired that the materials used to correct the acoustics be
installed in such manner as to conform with the architectural fea-
tures of the Auditorium. This introduced a new problem since in the
provisional cure the canvas sheets in the dome hung with very little
conformity to the curvature of the walls. A further complication ap-
peared when it was found by calculation that the amount of material
necessary to correct the reverberation was insufficient to pad all the
*"Air Currents and Acoustics of Auditoriums." Engineering Record. Vol.
67, p. 265, 1913.
t" Echoes in an Auditorium." Physical Review, Vol. 32, p. 231, 1911.
BALCONY FLOOR
PLAN
SCALE >i> v-<7"
AUDITORIUM
UNIVERSITY
OF ILLINOIS.
FIRST
PLAN
SCALE &." \-a
AUDITORfUM
UNIVERSITY
OF ILLINOIS.
FIG. 1. FLOOR PLANS SHOWING INTERIOR OF THE AUDITORIUM AT THE UNIVEB-
SITY OP ILLINOIS WHICH WAS CORRECTED FOB ECHOES AND REVERBERATION.
6 ILLINOIS ENGINEERING EXPERIMENT STATION
walls that produced echoes. It was desirable to eliminate the echoes,
but it was regarded as risky to install too much sound absorbing mate-
rial, owing to the danger of making the Auditorium too dead for sound.
Because of these difficulties it was decided to carry on further ex-
periments and to secure more data before deciding on the final cure.
Accordingly, one large curved wall was covered with strips of one-inch
hair felt, 30 inches wide, placed vertically and 30 inches apart so as to
leave bare spaces between them. This arrangement was satisfactory
for several reasons ; it did not change the curvature of the wall ; it used
only half the amount of material, necessary to cover the entire wall ;
and because of diffraction and interference effects, it was theoretically
more efficient in breaking up the reflected sound than if the same
material were spread continuously over the whole surface. Although
encouraging, the results were not so marked as expected in diminish-
ing the echoes.
On the basis of this experiment, plans were made for covering
other walls in a similar way, except that the hair felt was to be
mounted on wooden ribs built out from the wall surface. Such an
installation seemed more likely to break up the incident sound than
the first plan of mounting the hair felt snugly against the wall. The
sound wave on striking these outer felt strips would suffer partial
reflection and change of phase, while the remaining portion of the
sound would pass through the open spaces and be spread out by dif-
fraction and reflection from the walls. The hair felt strips would op-
pose the incident and reflected waves, thus breaking up the original
sound and diminishing its intensity and possibility of producing
echoes.
Because the scaffolding erected for the use of the workmen inter-
fered with the passage of sound waves, the efficiency of this method of
placing the felt could not be tested step by step as the material was
mounted. The test was deferred, therefore, until the installation was
completed. In the meantime the pipe organ was installed, the interior
was redecorated, and the lighting system changed, so that only the com-
bined effect of all these factors on the acoustics could be investigated.
III. MODIFICATIONS OF THE INTERIOR OF THE AUDITORIUM.
3. Installation of the Pipe Organ. The organ was mounted in a
unique way by dividing it into two parts and placing them in lofts
24 feet above the ends of the stage with a distance of 75 feet between
centers. This arrangement placed the organ at a considerable distance
CORRECTION OF ECHOES AND REVERBERATION 7
above the audience. The absence of any vertical surface between the
lofts and the audience room prevented any visible arrangement of the
organ pipes, but the necessary free exit of the sound was provided
for by the construction of ornamental plaster grills covering the pen-
dentives on either side of the stage. (See Fig. 2.)
FlG. 2. VIEW TOWARD THE STAGE SHOWING THE GRILL WORK FOR FREE PAS-
SAGE OF SOUND FROM THE CONCEALED ORGAN. THE ORGAN CONSUL IS SHOWN TO THE
LEFT. CARPET is REMOVED FROM STAGE IN PREPARATION FOR AN ORCHESTRA CONCERT
4. Method of Mounting Hair Felt. The hair felt was mounted
on thin furring strips which were bent to fit the curvature of the sur-
faces. The dome above the arches and the double curved side walls and
single curved rear wall above the balcony were padded in this way.
The felt was mounted in vertical strips on the west side wall as shown
in Fig. 3. Fig. 4 shows the wall after the material was installed and
decorated.
ILLINOIS ENGINEERING EXPERIMENT STATION
PIG. 3. PHOTOGRAPH SHOWING THE MOUNTING OF THE HAIR FELT IN VERTICAL
STRIPS. THE MATERIAL WAS FASTENED TO THIN FURRING STRIPS WHICH COULD BE
BENT! TO CONFORM TO THE CURVATURE OF THE SURFACE
On the east balcony wall the felt was mounted on wooden ribs so
that it stood concentric with the plaster surface at a distance of one
foot. Eighteen inches below the edge of the skylight in the dome radial
strips of felt which approached the wall until they touched at the
crown of the arches, were mounted on wooden ribs. (Fig. 5.) The hair
felt used was the Akustikos Felt developed especially for correction of
acoustical faults by the H. W. Johns-Manville Company under the
direction of Professor Sabine.
Before the changes were made in the Auditorium, Professor Sabine
visited the building at the invitation of President James. After this
visit, he wrote to President James as follows: "If such confirmation
of the results of Professor Watson's investigation is necessary, please
permit me to assure you that you will obtain an excellent effect from
following out his suggestions in all detail." The final installation was
CORRECTION OF ECHOES AND REVERBERATION
FIG. 4. PHOTOGRAPH SHOWING THE SIDE WALL OF FIG. 2 WHEN COMPLETED. A
REP OF SUITABLE COLOR WAS STRETCHED OVER THE ENTIRE SURFACE AND DECORATED.
IT WAS NECESSARY FOR THE FREE PASSAGE OF THE SOUND THAT THE MATERIAL USED
IN DECORATING SHOULD NOT CLOSE THE PORES OF THE REP
modified somewhat from the original plans, but the general features
were maintained.
5. The Decoration and the Lighting System. The modification
of the lighting system involved the elimination of the suspended fix-
tures. The wall brackets were retained, but the main lighting was
changed to a semi-indirect system with reflectors above the arches and
around the skylight. An ivory tone was selected for the basic color in
the redecoration. Ornamentation was stenciled and painted on the
various walls and surfaces to give a unified effect. With the excep-
tion of the ornamental borders the rep covering the padded surfaces
was left its natural color. The difference between the old and new in-
teriors is shown in Figs. 6 and 7. The modifications relieved the audi-
torium of its cheerless, barn-like interior.
10
ILLINOIS ENGINEERING EXPERIMENT STATION
FIG. 5. DIAGRAM SHOWING THE DOME SURFACE WITH WOODEN RIBS SEPARATING
THE HAIR FELT IN RADIAL STRIPS. THIS FALSE CEILING ARRANGMENT WAS THOUGHT
MORE EFFECTIVE FOR ABSORPTION OF SOUND THAN IF THE MATERIAL WERE MOUNTED
SNUGLY AGAINST THE SURFACE
IV. FINAL ACOUSTICAL INVESTIGATION.
The remodeled Auditorium has been tested under varied conditions
for music and speaking, and popular opinion has pronounced the
acoustics satisfactory. A speaker with a moderate voice can be heard
distinctly by auditors in the most distant seats. The music of the new
pipe organ, according to experts, is satisfactorily rendered. The room
is also suited for orchestra music, though for this case, it has been
found advantageous to follow the usual custom of leaving the wooden
floor of the stage bare of carpet so as to reenforce the sound from the
instruments.
CORRECTION OF ECHOES AND REVERBERATION
While the Auditorium has proved to be generally satisfactory, a
detailed investigation of the acoustical effects secured by the modifica-
tion of the room was thought desirable. A request was made, accord-
ingly, that auditors report any echoes or acoustical disturbances how-
ever slight they might be. About a dozen replies were received, and
on the basis of these and other considerations, a systematic investiga-
tion was undertaken.
FIG. 6. PHOTOGRAPH SHOWING AUDITORIUM BEFORE CHANGES WERE MADE
The acoustical results, beneficial and otherwise, may be anticipated
by considering the changes made. According to Sabine, the hair felt
installed would reduce the reverberation. This would also eliminate
echoes if installed on certain surfaces in accordance with the analysis ;
but, since the amount of material used to correct the reverberation
was insufficient to cover all the walls, acoustical defects might still
be set up by the unpadded surfaces, especially by the pendentives.
The pipe organ, by generating musical sounds that emerged through
the pendentives in the dome, might introduce new acoustical dis-
turbances. The openings made in the surfaces of two of the penden-
tives for the passage of the organ music would reduce the general
12
ILLINOIS ENGINEERING EXPERIMENT STATION
FIG. 7. PHOTOGRAPH SHOWING NEW INTERIOR OP AUDITORIUM. THE SUSPENDED
LIGHTING FIXTURES WERE REMOVED, THE INTERIOR REDECORATED, AND THE REAR WALL
IN THE ALCOVE PADDED
reverberation and would also diminish echoes. The changes in the
decoration and in the lighting system would produce little effect.
6. Investigation of Echoes. Tests were made in several ways to
determine the presence of echoes. The opinion offered by auditors that
the echoes had generally disappeared was, of course, the most satisfac-
tory evidence. One test was made by talking through a megaphone
toward different walls (Fig. 8). The sound was generated inside a
small house and its direction of propagation controlled by two mega-
phones, one being pointed toward an observer and the other toward a
wall which previously gave echoes. No distinct echo could be obtained
by speaking simultaneously into the two megaphones. The ticks of
a metronome produced very little additional effect, but when a sharp
intense metallic sound was tried, echoes were obtained from the un-
padded walls but only faint responses from the padded walls. The
intense hissing sound of an arc light backed by a parabolic reflector
gave more pronounced results. It showed that the padded walls pro-
duced a marked effect in reducing the intensity of the sound.
CORRECTION OF ECHOES AND REVERBERATION
13
TIG. 8. PHOTOGRAPH OF STAGE SETTING SHOWING EXPERIMENTAL HOUSE WITH
MEGAPHONES USED FOB TESTING ECHOES
The effect of the unpadded pendentives in the rear dome surface is
shown in Fig. 9. The cone of incident sound received by each pen-
dentive is small and, after reflection, spreads over a large area. It
was therefore anticipated that little disturbance would result. This
prediction was not entirely correct since the echoes reported by audi-
tors, so far as could be ascertained, came from these two walls. An
echo was perceptible when the speaker faced directly toward one of
these pendentives so that the profile of his face was seen by an
auditor seated at one side of the auditorium. The direct sound coming
to the auditor was then diminished while the reflected sound was aug-
mented, thus producing an echo.
Other unpadded walls, notably the side walls under the balcony,
still set up concentrations of sound. Thus, an observer at A, Fig. 10,
can hear not only the direct sound from the speaker, but also the por-
tion that is concentrated by reflection from B. He does not hear an
echo because the time interval between the direct and reflected sounds
is too short to enable his ear to detect them separately. The result is
14 ILLINOIS ENGINEERING EXPERIMENT STATION
FIG. 9. DIAGRAM SHOWING THE REFLECTION OF SOUND FROM THE UNPADDED
PENDENTIVE IN THE REAR WALL. ECHOES SET UP BY THIS WALL CAN OCCASIONALLY
BE NOTED
much the same as if his neighbor on the side toward the wall were to
say the words of the speaker in his ear at the same time that he re-
ceived them from the speaker. The auditor realizes that something
is peculiar about the sound but usually does not understand the cause
of the trouble. An auditor at C, however, may get an echo when the
speaker faces the point D.
7. Investigation of the Reverberation. By means of Sabine's for-
mula and coefficients of absorption* the time of reverberation of the
Auditorium was found and a calculation was made to determine the
amount of sound absorbing material necessary to correct the fault.
The following tabulation shows the method employed :
American Architect, 1900.
CORRECTION OF ECHOES AND REVERBERATION
15
SPEAKER.
\
FLOOR. PLAN
FIG. 10. i LAN OF AUDITORIUM SHOWING CONCENTRATION OF SOUND BY THE
WALLS UNDER THE BALCONY
Material Exposed Area
in sq. meters
Plaster on lath 2000
Plaster on tile 510
Wood work 1630
Glass 83
Cocoa matting 145
Wood seats . . . 2150 seats
Average audience 1200 people
Volume of room . . . . 12000 cubic meters.
Coeff. of
Absorption
0.0330
Total
Absorption
66.0
0.0250
13.0
0.0610
99.0
0.0270
2.3
0.0200
2.9
0.0082
17.7
201
0.44
527
Total 728
16 ILLINOIS ENGINEERING EXPERIMENT STATION
Substituting these data in the formula t =0.1 64 V-^-a, in which t is the
time of reverberation, V the volume of the room and a the total ab-
sorbing power, the following equation for the empty room is obtained :
*=0.164X12000-^201=9.8 seconds.
When an audience of 1200 people is present,
*=0.164Xl2000-f-728=2.7 seconds.
This value isi too great for good acoustics and a reverberation re-
sults. To correct the fault, absorbing material should be added until
the time of reverberation is reduced to about 1.8 seconds; this value
having been found satisfactory for halls as large as the Auditorium
when used for both music and speaking.
The amount of Akustikos Felt needed to carry out the plans al-
ready described was 3315 square feet. This was less than the area
necessary for felt mounted snug against the wall since the coefficient
of absorption is greater when the felt is mounted out from the
wall.* Calculations, which allowed for the sound absorbing power of
the felt and the other alterations in the Auditorium indicated that
the time of reverberation would be reduced to about 1.90 seconds with
1200 people present.
V. DISCUSSION AND CONCLUSIONS.
The Auditorium fulfilled the theory held many years ago by Lord
Rayleigh* that a large room with hard, non-porous walls and with
few windows has a prolonged resonance, and that the best chance of
improvement lies in padding the walls and ceiling with sound absorb-
ing materials. Thus, the installation of hair felt in the Auditorium
reduced the reverberation; the amount of reduction being calculated
in advance by Sabine'sf formula and constants of absorption.
The amount of hairfelt necessary to correct the reverberation was
insufficient to cover all the walls, and it was found that some of these
unpadded surfaces still produced echoes. This action was anticipated
in part from the general considerations discussed by Rayleigh:}: in
which the possibility of reflection of sound was shown to depend on the
positions of the source and receiver of sound, and also upon the size
and form of the wall compared with the wave length of the incident
sound.
*Sabine. Architectural Quarterly of Harvard University, p. 22, March, 1912.
Theory of Sound, Vol. 2, pp. 287 and 351.
tAmerican Architect, 1900.
t Theory of Sound, Vol. 2, p. 28.
CORRECTION OF ECHOES AND REVERBERATION 17
The installation in an auditorium of considerable sound absorbing
material eliminates the objectional condition of satisfactory reverbera-
tion being wholly dependent on the sound absorbing power furnished
by an audience. This means that rehearsals without an audience can
be conducted satisfactorily and that a speaker addressing a small
audience is not obliged to contend with a distressing reverberation.
The theoretical advantages in absorbing and breaking up sound
waves when hair felt is mounted out from a wall instead of placed
snugly against the surface do not appear to be so great as expected.
Observers listened to sounds reflected from both types of surface and
concluded that a surface having the hair felt mounted out from the
wall was more efficient. The conclusions, however, should be checked
by quantitative, instrumental measurements since the ear is inaccurate
in its estimation of the comparative intensities of different sounds.*
It appears that the felt is more effective when mounted out from the
wall, but there is some question whether or not the advantages se-
cured justify the additional expense of installation and the greater
risk of fire.
The music of the pipe organ emerging in large volume from the
pendentives in the dome introduced concentrations of sound different
from those set up when the source of sound was on the stage. This
made it desirable to pad other walls in addition to those requiring
padding for the single source of sound.
The effect of the organ music confirmed one conclusion set forth
by Jagerf ; namely, that the strength of the source of sound for good
acoustics should be inj correct proportion to the volume of the room.
It appears that the Auditorium is too small for loud organ music since
the sound in this case becomes unpleasantly intense. On the other
hand, it appears that the volume is fairly well suited for softer organ
music and for a weak source of sound, such as a speaker with a mod-
erate voice. In this connection Jager contends that an auditorium
is limited in its acoustical possibilities; that if a room is too large,
it is impossible to make it satisfactory for weak sources of sound. He
points out also that the problem of correcting faulty acoustics must
include a consideration of intensity of sound as well as of reverbera-
tion; that is, the variable factors at command, the volume and ab-
sorbing power of the room and the source of sound, must be so propor-
*Kayleigh, Scientific Papers, Vol. II, p. 132.
\"Zur Theorie des Nachhals, " Sitzungsberichten der Kaiserl. Akademie der
Wissenschaften in Wien. Matem-naturw. Klasse; Bd. CXX, Abt. Ila, Mai,
1911.
18 ILLINOIS ENGINEERING EXPERIMENT STATION
tioned as to give not only a suitable reverberation but also an accepta-
ble intensity of sound. He discusses the limitations in obtaining this
desired result.
Another deduction made by Jager which applies rather directly to
the Auditorium is that the ratio S/W should be large for good acous-
tics, in which S is the total surface of walls, furniture, and fixtures
struck by the sound and W is the volume of the interior. Theoretically,
this ratio is smallest for a sphere, and, since the Auditorium approx-
imates a hemisphere, the excessive reverberation might have been pre-
dicted.
Reverberations and echoes were corrected simultaneously by in-
stalling a suitable amount of hair felt on the walls which produced
echoes. To locate these walls, a new method was developed in which
the source of sound was an arc light as explained earlier in this bul-
letin.
The investigation showed that curved walls are worse acoustically
than plane walls since they produce undesirable concentrations of
sound and echoes. It also appears that the openings in the penden-
tives for the organ music and the ventilation openings act similarly
to open windows and thus reduce reverberation and diminish echoes.
One acoustical disturbance which was not corrected was that due
to talking and walking in the foyer and on the stairs immediately out-
side the Auditorium. The sounds of footsteps and the reverberation
caused by loud talking and accidental noises in the foyer could be
reduced by covering the stairs and foyer with a yielding material,
such as cork and by padding some of the walls.
It is apparent from this discussion that the means employed to
correct the acoustics, as exemplified by this complex problem, were
based upon established scientific principles and this investigation and
others of like nature have served, to a large extent, to dispel the mys-
tery surrounding the action of sound in auditoriums.
BIBLIOGRAPHY ON ACOUSTICS OF AUDITORIUMS.
(This is an addition to the bibliography published with Bulletin
No. 73 on "Acoustics of Auditoriums.")
Aigner, Franz " Experimentella Studie iiber den Nachall" Sit-
zungsberichte der Kaisl. Akad. der Wissenschaften, in Wien,
Math - Naturwiss, Klasse. Vol. CXXIII, Abt. Ha, July, 1914. An
experimental investigation of the reverberation in rooms in which
instruments were used instead of the ear to measure the damping
CORRECTION OF ECHOES AND REVERBERATION 19
of sound. This work verified Jager's formula and also the em-
pirical formula and absorption coefficients found by Sabine.
Franklin, W. S. ' ' Acoustics of the Auditorium. ' ' Text-book on ' ' Light
and Sound." Franklin and MacNutt, pp. 282-290.
McGinnis, C. S. and Harkins, M. R. "The Transmission of Sound
Through Porous and Non-porous Materials." Physical Review,
Vol. 33, p. 128, 1911.
Miller, D. . ' ' The Science of Musical Sounds. ' ' The Macmillan Co.,
1916. A set of lectures giving the results of years of painstaking
investigation of the characteristics of tones produced by the voice
and by different musical instruments.
Moulton, R. H. "A New Memorial to Washington." Independent,
July 5, 1915. Describes a proposed auditorium designed acousti-
cally by Professor Sabine.
Rood, 0. N. "On a Method of Studying the Reflexion of Sound-
Waves." Amer. Jour, of Science, (3), Vol. 19, p. 133, 1880.
Sabine, Wallace C. "Architectural Acoustics: The Insulation of
Sound." The Brick Builder, Vol. 24, p. 31, February, 1915.
Sabine, Wallace C. ' ' Architectural Acoustics. ' ' Jour. Franklin Inst.
Vol. CLXXIX, p. 1, January, 1915. A resume of his investigations
in this subject.
Watson, F. R. "Acoustics of Auditoriums." Bulletin No. 73, Univer-
sity of Illinois Engineering Experiment Station, University of
Illinois Bulletin, Vol. XI, No. 29, March, 1914. Reprinted in full
in the Scientific American Supplement, Vol. LXXVIII, p. 358,
December 5, and p. 380, December 12, 1914.
Watson, F. R. "Acoustics of Theatres." The Play-Book, Madison,
Wisconsin, Vol. 2, p. 19, August, 1914.
Watson, F. R. "Acoustics of Auditoriums." Investigation of the
Acoustical Properties of the Armory at the University of Illinois.
The Brickbuilder, Vol. 24, p. 257, October, 1915.
Watson, F. R. "Acoustics of Auditoriums. Concerning the Theory
of Reverberation." The American Architect, Vol. CVIII, pp.
369-374, December 8, 1915. A translation of an important paper
by G. Jager on the theory of reverberation showing the limitations
of halls in regard to acoustics.
Watson, F. R. " An Investigation of the Transmission, Reflection and
Absorption of Sound by Different Materials." Physical Review,
Vol. 7, (2), pp. 125-132, January, 1916.
Whitman, F. P. " On the Acoustical Efficiency of a Sounding Board. ' '
Science, Vol. 38, p. 707, November 14, 1913. A short account of
20 ILLINOIS ENGINEERING EXPERIMENT STATION
experiments made to determine the effect of a sounding board on the
acoustical qualities of the chapel.
Whitman, F. P. "On the Acoustics of the Chapel of Adelbert Col-
lege." Science, Vol. XLII, p. 191, August 6, 1915. A continua-
tion of the previous work giving description of tests of acoustics
after the chapel had been corrected for reverberation.
PUBLICATIONS OF THE ENGINEERING EXPERIMENT STATION
Bulletin No. 1. Tests of Reinforced Concrete Beams, by Arthur N. Talbot. 1804.
None available.
Circular No. 1. High-Speed Tool Steels, by L. P. Breckenridge. 1905. None available.
Bulletin No. 2. Tests of High-Speed Tool Steels on Cast Iron, by L. P. Breckenridge
and Henry B. Dirks. 1905. None available.
Circular No. 2. Drainage of Earth Roads, by Ira O. Baker. 1906. None available.
Circular No. 3. Fuel Tests with Illinois Coal (Compiled from tests made by the Tech-
nologic Branch of the U. S. G. S., at the St. Louis, Mo., Fuel Testing Plant, 1904-1907),
)>y L. P. Breckenridge and Paul Diserens. 1909. None arailable.
Bulletin No. 8. The Engineering Experiment Station of the University of Illinois, by
L. P. Breckenridge. 1906. None available.
Bulletin No. 4. Tests of Reinforced Concrete Beams, Series of 1905, by Arthur N.
Talbot. 1906. Forty-five cents.
Bulletin No. B. Resistance of Tubes to Collapse, by Albert P. Carman and M. L. Carr.
1906. None arailable.
Bulletin No. 6. Holding Power of Railroad Spikes, by Roy I. Webber. 1906. Noni
arailable.
Bulletin No. 7. Fuel Tests with Illinois Coals, by L. P. Breckenridge, S. W. Parr, and
Henry B. Dirks. 1906. None available.
Bulletin No. 8. Tests of Concrete: I, Shear; II, Bond, by Arthur N. Talbot. 1906.
None arailable.
Bulletin No. 9. An Extension of the Dewey Decimal System of Classification Applied
to the Engineering Industries, by L. P. Breckenridge and G. A. Goodenough. 1906. Revised
Edition 1912. Fifty cents.
Bulletin No. 10. Tests of Concrete and Reinforced Concrete Columns. Series of 1906
by Arthur N. Talbot. 1907. None available.
Bulletin No. 11. The Effect of Scale on the Transmission of Heat Through Locomotive
Boiler Tubes, by Edward C. Schmidt and John M. Snodgrass. 1907. None available.
Bulletin No. IS. Tests of Reinforced Concrete T-Beams, Series of 1906, by Arthur N.
Talbot. 1907. None available.
Bulletin No. 13. An Extension of the Dewey Decimal System of Classification Applied
to Architecture and Building, by N. Clifford Ricker. 1907. None available.
Bulletin No. 14. Tests of Reinforced Concrete Beams, Series of 1906, by Arthur N.
Talbot. 1907. None available.
Bulletin No. 15. How to Burn Illinois Coal Without Smoke, by L. P. Breckenridge.
1908. Twenty-fire cent*.
Bulletin No. 16. A Study of Roof Trusses, by N. Clifford Ricker. 1908. Fifteen cents.
Bulletin No. 17. The Weathering of Coal, by S. W. Parr, N. D. Hamilton, and W. F.
Wheeler. 1908. None available.
Bulletin No. 18. The Strength of Chain Links, by G. A. Goodenough and L. E. Moore.
1908. Forty cents.
Bulletin No. 19. Comparative Tests of Carbon, Metallized Carbon and Tantalum Fila-
ment Lamps, by T. H. Amrine. 1908. None available.
Bulletin No. 20. Tests of Concrete and Reinforced Concrete Columns, Series of 1907,
by Arthur N. Talbot. 1908. None available.
Bulletin No. 21. Tests of a Liquid Air Plant, by C. S. Hudson and C. M. Garland.
1908. Fifteen cents.
Bulletin No. 22. Tests of Cast-Iron and Reinforced Concrete Culvert Pipe, by Arthur
N. Talbot. 1908. None available.
Bulletin No. 23. Voids, Settlement, and Weight of Crushed Stone, by Ira O. Baker.
1908. Fifteen cents.
*Bulletin No. 24. The Modification of Illinois Coal by Low Temperature Distillation,
by S. W. Parr and C. K. Francis. 1908. Thirty cents.
Bulletin No. 26. Lighting Country Homes by Private Electric Plants, by T. H. Amrine,
1908. Twenty cents.
Bulletin No. 26. High Steam Pressures in Locomotive Service. A Review of a Report
to the Carnegie Institution of Washington, by W. F. M. Goss, 1908. Twenty-five centt.
Bulletin No. 27. Tests of Brick Columns and Terra Cotta Block Columns, by Arthur
N. Talbot and Duff A. Abrams. 1909. Twenty-five cents.
Bulletin Nn. 28. A Test of Three Large Reinforced Concrete Beams, by Arthur N.
Talbot. 1909. Fifteen cents.
Bulletin No. 29. Tests of Reinforced Concrete Beams: Resistance to Web Stresses,
Series of 1907 and 1908, by Arthur N. Talbot. 1909. Forty-five cents.
*Bulletin No. 30. On the Rate of Formation of Carbon Monoxide in Gas Producers, by
,T. K. Clement, L. H. Adams, and C. N. Haskins. 1909. Twenty-five cents.
* Bulletin No. SI. Fuel Tests with House-heating Boilers, by J. M. Snodgrass. 1909.
Fifty-five cents.
Bulletin No. 32. The Occluded Gases in Coal, by S. W. Parr and Perry Barker. 1909.
Fifteen cents.
Bulletin No. 33. Tests of Tungsten Lamps, by T. H. Amrine and A. Guell. 1909.
Twenty cents.
* Bulletin No. 34. Teats of Two Types of Tile-Roof Furnaces , under a Water-Tube
Boiler, by .T. M. Snodgrass. 1909. Fifteen cents.
Bulletin No. S5. A Study of Base and Bearing Plates for Columns and Beams, by
N. Clifford Ricker. 1909. Twenty cents.
Bulletin JVo. 36. The Thermal Conductivity of Fire-Clay at Hif?h Temperatures, by
.T. K. Clement and W. L. Egy. 1909. Twenty rents.
Bulletin No. 37. Unit Coal and the Composition of Coal Ash, by S. W. Parr and W. F.
Wheeler. 1909. Thirty-fire cents.
*Bulletin No. 38. The Weathering of Coal, by S. W. Parr and W. F. Wheeler. 1909.
Twenty-five cents.
*Bulletin No. 39. Tests of Washed Grades of Illinois Coal, by C. S. McGovney. 1909.
Seventy-five cents.
Bulletin No. 40. \ Study in Heat Transmission, by J. K. Clement and C. M. Garland.
1910. Ten cents.
*Bulletin No. 41. Tests of Timber Beams, by Arthur N. Talbot. 1910. Twenty cents.
*Bulletin No. 42. The Effect of Keyways on the Strength of Shafts, by Herbert F.
Moore. 1910. Ten cents.
PUBLICATIONS OP THE ENGINEERING EXPERIMENT STATION
Bulletin No. 48. Freight Train Resistance, by Edward 0. Schmidt. 1910. Seventy-
lire cents.
Bulletin No. 44. An Investigation of Built-up Columns Under Load, by Arthur N.
Talbot and Herbert F. Moore. 1911. Thirty-five cents.
*Bulletin No. 45. The Strength of Oxyacetylene Welds in Steel, by Herbert L. Whitte-
more. 1911. Thirty-five cents.
*Bulletin No. 46. The Spontaneous Combustion of Coal, by S. W. Parr and F. W.
Kressmann. 1911. Forty-five cents.
*Bulletin No. 47. Magnetic Properties of Heusler Alloys, by Edward B. Stephenson.
1911. Twenty-five cents.
*Bulletin No. 48. Resistance to Flow Through Locomotive Water Columns, by Arthur
N. Talbot and Melvin L. Enger. 1911. Forty cents.
*Bulletin No. 49. Tests of Nickel-Steel Riveted Joints, by Arthur N. Talbot and Her-
bert F. Moore. 1911. Thirty cents.
*Bulletin No. SO. Tests of a Suction Gas Producer, by C. M. Garland and A. P. Kratx.
1912. Fifty cents.
Bulletin No. 51. Street Lighting by J. M. Bryant and H. G. Hake. 1912. Thirh,
fire cents.
*Bulletin No. 62. An Investigation of the Strength of Rolled Zinc, by Herbert F.
Moore. 1912. Fifteen cents.
*Bulletin No. S3. Inductance of Coils, by Morgan Brooks and H. M. Turner. 1912.
Forty cents.
*Bulletin No. 64. Mechanical Stresses in Transmission Lines, by A. Guell. 1912.
Twenty cents.
*BuUetin No. 66. Starting Currents of Transformers, with Special Reference to Trans-
formers with Silicon Steel Cores, by Trygve D. Yensen. 1912. Twenty cents.
*Bulletin No. 66. Tests of Columns: An Investigation of the Value of Concrete as
Reinforcement for Structural Steel Columns, by Arthur N. Talbot and Arthur R. Lord. 1912.
Twenty-five cents.
*BuUetin No. 57. Superheated Steam in Locomotive Service. A Review of Publication
No. 127 of the Carnegie Institution of Washington, by W. F. M. Goss. 1912. Forty cents.
*BuUetin No. 58. A New Analysis of the Cylinder Performance of Reciprocating En-
gines, by J. Paul Clayton. 1912. Sixty cents.
*Bulletin No. 59. The Effect of Cold Weather Upon Train Resistance and Tonnage
Rating, by Edward 0. Schmidt and F. W. Marquis. 1912. Twenty cents.
*Bullftin No. 60. The Coking of Coal at Low Temperatures, with a Preliminary Study
of the By-Products, by S. W. Parr and H. L. Olin. 1912. Twenty-five cents.
*Bulletin No. 81. Characteristics and Limitations of the Series Transformer, by A. R.
Anderson and H. R. Woodrow. 1913. Twenty-five cents.
Bulletin No. 68. The Electron Theory of Magnetism, by Elmer H. Williams. 1913.
Thirty-five cents.
Bulletin No. 68. Entropy-Temperature and Transmission Diagrams for Air, by C. R.
Richards. 1913. Twenty-five cents.
* Bulletin No. 64 Tests of Reinforced Concrete Buildings Under Load, by Arthur N.
Talbot and Willis A. Slater. 1913. Fifty cents.
*Bulletin No. 65. The Steam Consumption of Locomotive Engines from the Indicator
Diagrams, by J. Paul Clayton. 1913. Forty cents.
Bulletin No. 66. The Properties of Saturated and Superheated Ammonia Vapor, by G.
A. Goodenough and William Earl Mosher. 1913. Fifty cents.
Bulletin No. 67. Reinforced Concrete Wall Footings and Column Footings, by Arthur
N. Talbot. 1913. Fifty cents.
*Bulletin No. 63. Strength of I Beams in Flexure, by Herbert F. Moore, 1913. Twenty-
cents.
*Bulletin No. 69. Coal Washing in Illinois, by F. 0. Lincoln. 1913. Fifty cents.
Bulletin No. 70. The Mortar-Making Qualities of Illinois Sands, by C. C. Wiley. 1918.
Twenty cents.
Bulletin No. 71. Tests of Bond between Concrete and Steel, by Duff A. Abrams. 1914.
One dollar.
*Bulletin No. 72. Magnetic and Other Properties of Electrolytic Iron Melted in Vacuo,
by Trygve D. Yensen. 1914. Forty cents.
*Buttetin No. 73. Acoustics of Auditoriums, by F. R. Watson. 1914. Twenty cents.
* Bulletin No. 74. The Tractive Resistance of a 28-Ton Electric Car, by Harold H.
Dunn. 1914. Twenty-five cents.
Bulletin No. 75. Thermal Properties of Steam, by G. A. Goodenough. 1914. Thirty-
five cents.
*Bulletin No. 76. The Analysis of Coal with Phenol as a Solvent, by S. W. Parr and
H. F. Hadley. 1914. Twenty-fire cents.
*Bulletin No. 77. The Effect of Boron upon the Magnetic and Other Properties of
Electrolytic Iron Melted in Vacuo, by Trygve D. Yensen. 1915. Ten cents.
*Bulletin No. 78. A Study of Boiler Losses, by A. P. Kratz. 1915. Thirty-five cents.
*Bulletin No. 79 The Coking of Coal at Low Temperatures With Special Reference to
the Properties and Composition of the Products, by S. W. Parr and H. L. Olin. 1915.
Twenty-five cents.
*Btdletin No. 80. Wind Stresses in the Steel Frames of Office Buildinsrs, by W. M.
Wilson and G. A. Maney. 1915. Fifty cents.
* Bulletin No. 81. Influence of Temperature on the Strength of Concrete, by A. B.
McDaniel. 1915. Fifteen cents.
* Bulletin No. 82. Laboratory Tests of a Consolidation Locomotive, by E. C. Schmidt,
J. M. Snodgrass, and R. B. Keller. 1915. Sixty-five cents.
*Bulletin No. 88. Magnetic and Other Properties of Iron-Silicon Alloys Melted in
Vacuo, by Trygve D. Yensen. 1915. Thirty-fire ccntx.
*Bulletin No. S4. Tests of Reinforced Concrete Flat Slab Structure, by A. N. Tallin!
and W. A. Slater. 1916. Fifty cent*.
*Biilletin Nn. 85. Strength and Stiffness of Steel Under Biaxial Loading, by A. J.
Becker. 1916. Thirty-fire cents.
*B\dletin No. 86. The Strength of I-Beams and Girders, by H. F. Moore and W. W.
Wilson IQIfi Thirtti ro-nts
THE UNIVERSITY OF ILLINOIS
THE STATE UNIVERSITY"
TJRBANA
EDMUND J. JAMES, Ph. D., LL.D., President
THE UNIVERSITY INCLUDES THE FOLLOWING DEPARTMENTS:
The Graduate School
The^'College of Liberal Arts and Sciences (Ancient and Mod&s*- Language*
nr.d Literatures; History, Economies, Political Science, Sociology; Philoso-
phy, Psychology, Education; Mathematics; Astronomy; Geology; Physics;
Chemistry; Botany, Zoology, Entomology; Physiology; Art and Design)
The College of Commerce and Business Administration (General Business,
Banking, Insurance, Accountancy, Railway Administration, Foreign Com-
merce; Courses for Commercial Teachers and Commercial and Civic,
Secretaries)
The College of Engineering (Architecture; Architectural, Ceramic, Civil, Elec-
trical, Mechanical, Mining, Municipal and Sanitary, and Railway Engi-
neering)
The College of Agriculture (Agronomy; Animal Husbandry; Dairy Husbandry;
Horticulture and Landscape Gardening; Agricultural Extension; Teachers'
Course; Household Science)
The College of Law (three-years' course)
The School of Education
The Course in Journalism
The Courses in Chemistry and Chemical Engineering
The School of Railway Engineering and Administration
The School of Music (four years ' course)
The School of Library Science (two years' course)
The College of Medicine (in Chicago)
The College of Dentistry (in Chicago)
The School of Pharmacy (in Chicago; Ph.G. and Ph.C. courses)
The Summer Session (eight weeks)
Experiment Stations and Scientific Bureaus: U. S. Agricultural Experiment Sta-
tion; Engineering Experiment Station; State Laboratory of Natural His-
tory; State Entomologist's Office; Biological Experiment Station on Illi-
nois River; State Water Survey; State Geological Survey; Mine Rescue
Station.
The library collections contain (December 1, 1915) 348,607 volumes and 83,566
pamphlets.
For catalogs and information address
THE REGISTRAR
Urbana, Illinois
