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Full text of "Correction of echoes and reverberation in the Auditorium, University of Illinois"



tL/dtsonanci UDhiie 

Correctioh of 

echoes and reverbera 
tion in "the duditonu 
University of Illinois 



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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, 
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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. 
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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. 
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*Bulletin No. 65. The Steam Consumption of Locomotive Engines from the Indicator 
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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- 
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*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 
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*Bulletin No. 77. The Effect of Boron upon the Magnetic and Other Properties of 
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*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 
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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. 
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* 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 
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*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