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Noise Control, 2, 5, pp. 28-32, 62, 1956-11-01
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Noise control in Toronto's new subway
Paterson, W. H.; Northwood, T. D.
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Ser
TH1
N21r2
no. 28
c.2
BhDG N A T I O N A L R E S E A R C H C O U N C I L C A N A D ANOISE CONTROL
IN TORONT07S
NEW SUBWAY
by
W. H. PATERSON Toronto Transit Commission
and
T. D. NORTHWOOD
Division of Building Research, National Research Council
REPRINTED FROM NOISE CONTROL VOL. 2, NO. 5, SEPTEMBER 1956
RESEARCH PAPER NO. 28 OF THE
DIVISION OF BUILDING RESEARCH
NRC 4125
Ottawa
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Rcprintcd froxn NOIS COSTI<OL, Vol. 2, NO. 5, 28-32, 62, Scptcmbcr, 1956
Printed in U. S. A
F O R many ycilrs o n e of the classic examples of obnoxious noise has been that produced by the subway train. Perhaps its place has been threatened recently by such things as bigger a n d noisier nilcl,tlt, b u t lor inany people the noisie5t o l evclyday rioises is still likely to be the s ~ l l ~ w a y train. P u b - lic tl'tnsit authorities a l e ilot u n - aware of this a n d have tried many tinlcs over the years to reduce noise a n d vil,ration to m o r e acceptable levels. I n the e;uly clays the pur- pose 01 such efforts was usually to placate c o ~ n p l a i n i n g neighbors rather t h a n to improve the passen- gers' lot. Since the traditional h n c - tion 01 public transit systems is to p1ovide low-cost transportation, the authorities have always been dubi- ous a b o u t spending rnoriey o n what
FIG. 1. Interior view of rapid transit car.
Noise Contro
in
Toronto's
New
Sub
W . H. PATERSON
I O K O N 1 0 1 R A N S I T COi\.IhIISSION
AND
T. D. NORTHWOOD
1)1V1910N OI' I3llII.L)lhG RLSL rZKC.H, N A I I O N \I, R l STARCII COUNCIT. OF C A N A D A
some 1,)eol)lc iiiiglit call l u s ~ t r y . -1.0-
tiny, however, it is recognized tliat improvements i n service ancl in ~);tssauger conllort are long overdue a n d possibly may pay for tlieni- selves i n increased revenue.
T h i s trend has been e~nphasized in the new rapid transit line i n .l'oron~o. T h e new line is b u t one elenleiit i n a drastic renioclelling o l
~ h c ~ r l i o l e T o r o n t o system designer1
LO effect a substantial increase i11
tile use of p ~ t b l i c transportation. 1 ' 0 this e n d every efrort has been ~ i ~ a d e to ensure Lhat the passengers' ride will be pleasant 21s tvell as 1.apic1. Noise control tvas, therelore, an inlportant consideration, espe- cially i n the underground portions o l the line.
Little has been pul~lisliecl o n sub- way noise control; in fact, only o n e paper has been fouiitl, dealing with tlie use of absorption treatment in Lonclon T~1bes.l Most public transit orgaiiizations, lio~vever, have a file of reports a n d memoranda concerning noise problems ancl the sicps taken to solve them. W e are indebted to some of tliese organi- zations f o r allowing us to benefit by their experience. Additional noise d a t a o n existing subways were obtained by Armour Research Founcla~ion, u.110 also assisted i n the completion of a n acoustical de- sign for tlie T o r o n t o system.
T h e source o l subway noise, broadly speaking, is tlie subway car itself. 111 modern cars with properly designed suspensions, cou- plings, a n d drive mechanisms tlie principal remaining noise is that ~wo(lucecl by the rolling contact of m e ~ a l ~vheels o n rails. T l i e vibra- tions thus set u p in wheels a n d rails are radiated directly as airborne noise ~ v i t h i n the subway enclosure a n d are greatly accentuated b ) ~ tun- nel reverberation. T h e y inay also
be transniitted via tlie floor slab into he ground and into adjacent s t r ~ ~ c t ~ i i - c s . So there are two prob- lems: noise produced ~ ~ i t l i i n the s u l ~ ~ v a y a n d noise proclucetl in buildings atljacent to the suIxr;ly.
Frc. 2. E ~ t e r i o r view of tvo-car 1;lpitl t ~ a n s i t subway train a t Davisville Yartl, shoxving file nlonitor toxuels ill I~ack- ground.
'['lie ]alter l)rol)le~n is a lxtrticu- larly tlifficult one, since it is iin- possible to establish a noise limit acceptal~le to all parties concerned. l ' h e lyamiliar example oE the drip- ping tap i l l ~ ~ s t r a t e s tlie fact that noises almost iml)erccptible iilay he anno>-ing in sonle circumstances. T l i e occupant o l a gro~tncl-floor a p a r t ~ i ~ e n t adjacent to the subway [nay ilot be contciit with a n y ~ h i i i g s l ~ o r t of i n a ~ ~ c l i b i l i t y , n ~ h i c h is dif- ficult to achieve in such a location.
X reasonable policy seemed LO be
to avoid increasing the anibient noise level i n any octave lrequency b a n d . T h e T o r o n t o subway system replaces a rather noisy streetcar line a n d , l o r the inost part, is lo- cated directly below the old line. T h u s , compared ~\.itli the old line tlie subtvay system is mucli quieter i n 1110s~ l o c a t i o ~ ~ s . T h e most serious a r o u n d vibration problem existed
?
i n a section ulliere tlie ne\v line nloves east o l the .street, passing u n d e r several buildings e n route. I11 tliis section special care was taken to minimize structural con- tinuity between the s u b ~ v a y ancl the b u i l c l i ~ ~ g foundations. H O W - ever, in tlie basements of tliese buildings subavay noise approaches tlic level lormerly ~ ~ r o d u c e c l by sur- lace cars.
Noise within the subway is best consicleretl in ternis of its effect o n spcecli con~muiiication. I t is impor- tant that a passenger on a s u b ~ v a y train be able to cominunicate with his nearest neighbor. Quite a p a r t lrom a specif c c o m m ~ ~ n i c a t i o n re- q ~ ~ i r e m e n t , inability to coinmuni- catc ~ r o c l ~ ~ c e s a feeling oC isolation t l ~ a t is most objectionable. O n tlie otlier l~~iiicl, communication beyond a Sew leet is ~ i s u a l l y unnecessary; in Tact, a little masking rioisc is son~etinles dcsira1)le.
Eal-ly estimates o l acceptable noise levels \\:ere made by studies on otlier veliiclcs, s ~ ~ c l i as stl.eetcars, tliat s ~ ~ b j c c t i v e l y appeared close to the tlesign criterion. I n the nleall- time, lieranek
'
a n d others devcl- o ~ e d a imore quantitative metliocl ol: dealing ~ r i t l i this type oC prob- lem basctl 011 articulatioil tests coil-tlucted in tlic presence o l brond- I ~ a n t l noise. I t was fount1 hat for
;I ~vitle variety o l iloise spectra i t
is suflicieiit for c o ~ n m ~ ~ n i c a t i o n 11~1r-
the 5oci)ces
of
,~ot.\e
in
.Ilil)-illny sysfe,,ls nlrd uzelliotl.\
o f
dealtng zuttl~ ,~oi.\e
/)el ceived by slil)zo(iy /)cc.\~enge)s
a,rtl
by
occtiptilct~
ofrrtljnce,lt Otiildtng~.
Notse co,llrol ,,len.s7c7 es o n the
T O T O ) T ~ O
Iinpi(l
T
rnr~stt S y s t e ~ n
are t1escl.i Oetl.
poses to measure sound levels ill tlie three octave baiicls from 600 to 4800 cycles p e r second. T h e average 01 these three measureinents (in decibels) provides a single figure, the Speech I n t e ~ f e ~ e n c e L e l ~ e l , tliat is simply related to voice communi- cation a t various clistances. I n par- ticular, a speech interference level o l 70 d b will perinit coiiin~unica- tion in raised voices a t ;I c1ist;ince
01
2
or 3 ft. T h i s correspoiids closely to the design limit aclop~ecl f o r noise levels in tlic T o r o n t o sub- way.Frc. 3. Standartl tangent track T y p e 111 construction o n concrete, i ~ ~ c l u t l i n g vibrn-
tion isolation c q u i p ~ n e n t .
FIG. 4. I'lan a n d section of T o l m ~ ~ t o T r a n s i t C o ~ ~ ~ n ~ i s s i o ~ ~ rapitl transit T y p e B tic plate assclnbly ~ ~ s e c l on T y p e 111 Lrnck (superelevatetl concrete in sub'ivay).
FIG. 5. l'lan and section of the Type X rubber pad used under Type B tie plate.
In dealing with a noise problem one iliay atteinpt to eliminate the source, to introduce a barrier be- ttveen source ancl listener, or sinl- ply to absorb as much of the noise as possible. I t might be worthwhile to review the possibilities of each of the three approaches in dealing with subway noise.
T h e source will be taken to in- clude the wheels and rails, which constitute the primary vibrating bodies, as well as the metal-to-metal impact, which excites the vibration. An important first step is to pro- vide and maintain smooth surfaces on rails and wheels. One of the factors contributing to noise on older subway lines is the series of i ~ i ~ ~ ~ a c t ~ 1 ) r o c 1 i ~ c e c l by open rail joints. T h e modern practice of welding rail joints has eliminated this problem. Experiments were uildertaken on the Paris Metro a few years ago to eliiuinate the metal-to-metal contact by using r ~ t b b e r tires.3 T h e acoustical re- sults were apparently quite good, but the project is still in the ex- perimental stages. O n this conti- nent the modern PCC streetcars have wheels ~ d l i c l l incorporatc a rubber pad between the wheel rim and the hub. This design is very efEective in reducing the heavy vi- bration produced by rail discon- tinuities such as s\\ritcll points. It also rcduces the noise radiated by the wheel, which constitutes the main source of airborne noise when tracks are imbedded in pavement. However, in inodern subway serv- ice rail discontinuities are few, and a sul~stantial amount of airborne noise is radiated by the exposed rails. I11 these circumstances the superiority of the PCC wheel is not so striking.
Most transit organizations have stl~clied various types of rail mount- irigs and their effect on both ground-tra~~smitted vibration and airborne noise. T o nlinimize gi-ound vibration the solution is clearly to provide as much struc- tural discontinuity as possible be- tween rails and the floor slab. T o
Table I. Sound Absorption of Absorbent Material
Frequency 125 250 500 1000 2000 4000 Coefficient .30 .60 .95 .95 .85 .70
Table II. Average Speech Interference Levels in Toronto Subway and Surface Cars
Speech Interference Level (db) Inside moving train, windows closed
(a) I n open-cut section
(b) I n tunnel, normal maximum speed (c) I n tunnel, high speed
Inside moving train, windows open (a) In open-cut section
(b) Tunnel, low speed
(c) Tunnel, normal maximum speed (d) Tunnel, high speed
O n station platform, trains arriving 70-75 Inside modern PCC streetcar
(a) Car moving at normal speeds 60-70 (b) Car crossing intersection 78
ininimize the racliation of airborne noise the solution might be to con- trol the rail vibration by clamping or by a rigid clainping arrange- ment, or by enclosing the rail as much as possible ~vitll sound-ab- sorbing material. T h e matter is diflicult to study in the laboratory because the loatling effect of the car itself plays a major part. Re- cent experiments at Toronto and at the National Rese;irch Clouncil were carried only far enough to confir111 the complexity of the prob- lem; a thorough s t ~ ~ d y might even- tually yield valuable results. In the meantime the Toronto system has retained a fairly conventional de- sign, employing solid .rvheels on the cars and exposed rails \\.it11 a resil- ient mounting pat1 between rails ancl the concrete floor slab.
Airborne noise produced in the subway enclosure may be con- trolled by applying sound-absorb- ing material as close to the sound source as possible. I n the Toronto subway a strip
4
It u.icle of highlyabsorbent material was mounted along the tunnel \valls at wheel level. .Absorption coeficients of tlle inaterial used are shown in T a - ble I. Calculations based on data lrom other subway systems indi- cated that this amount of absorp- tion treatment sl~oulcl meet the de-
sign requirements. i\n alternative considered lvas the application oC sound-ahsorbing inaterial to the unclersicle of the cars, but there was no tiinc to investigate the prob- lem oC finding a suitably durable, \veathei--resistant material.
It was diflicult to predict noise levels for station platlornls, since they depencl o n the operating pro- cedures lor the trains during brak- ing ;tnd accelerating. Absorption treatinent similar to that used in the tunnel was applied in the sta- tion areas i ~ n d e r the platform over- hang. It \\,as llopetl that this would be sufficient L O llold the maximum
noise l)elolv the design limit. Finally, for the noise levels in- side the trains one can consider the effectiveness of the cars as bar- riers to the sound produced in the tunnel. T h e cars, ~vhich are of Rrit- is11 manufacture, are uilusually el- fective in this respect. Wall panels are coated with damping material ancl floors are of double construc- tion, incorporating a resilient layer .r\ihicl~ minilnizes the transmission of vibration. Windows are designed to seal properly when closed, and the inechanical ventilation system is efficient to the extent that the windows are openetl oniy in the hottest aveather.
Speech interlerence levels for typical conditions in the completed
subway are sho.rvn in T a b l e 11. Lev- els in PCC streetcars are also ill- eluded Lor comparison. I t will be noted that when car tvindo\\.s are closed, the subway cars are as quiet as the inodern PCC stieetcars. When all the .rvindo\vs are open, the illaxiinuin levels reach, ancl oc- casionally exceed, the clesign level. Since these occasions are r;ue and oC only a Cew seconds' duration, they clo not constitute a serious problem.
Noise levels on station platforms are higher than desirable, although, again, the inasim~uin level lasts only a feur seconds cll~ring the ar- rival oT a tiain. ,\dditional absorp- tion tieatinent is being contem- plated lor this area. When the trains were first put into service, they produced an intense brake squeal. Eventually this was elim- inated 11y changing to a different brake-shoe mate1 ial, a cast iron harder than that originall) used. T h e major problem of wheel noise has been disposed of to the point tvheie various ininor nui- sances beconle prominent. Air com- pressors and motor generators were lound to be excessively noisy on some cars as originally installed. Tllese ale gradually being brought under contiol. Similarly, there was
( C o n t i n f t e d o i l Page 62)
Noise Control in Toronto's New
Subway
(Cotittnl~ed ~ I O I I L knge 32)
an initial prevalence o l flat wheels, ~vlzicli has diminished w i ~ h iriore experienced handling of brakes.
Long beiole the subsvay was com- pletcd, ~ l l e Public Relatiolls De- paitment \{.as clailliing that this
would be "the quietest subway i n the world." T h e acoustical design- ers, plagued by comproillises with other requirements and worried about such uilknow~l quantities as the subway car itself, were less op- timistic; b u t results iilclicate that a111011g those for ~lllic11 v7e have data, it is the quietest subway.
References
1 J o h r ~ S. Trevor, "Sile~lcirig Lontlon's
,
.
lul)cs," Bull. Int. Railway Congress As- soc., pp. 4 7 5 4 7 9 (May 1939).
-'I.co L . I3eranck, "Specification of ac- ccplable noise levels," T ~ w s . Am. Soc. XIcch. Engss. 69, 97-100 (1917).
s " E s p c l . i u l ~ e ~ ~ t a l rail coaches on the Paris Metro," T h e E11gi11ecr 193, 5012, 259 (I'cbrt~ary 25. 1952).
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