Experiments on the Reduction of Noise Radiated by Subway- Car Rails

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Experiments on the Reduction of Noise Radiated by Subway- Car Rails Northwood, T. D.

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DIVISION OF BUILDING RESEARCH

NATIONAL RESEARCH COUNCIL OF CANADA

'f

E

C

JH[

N II CAlL

NOTlE

No.

109

NOT FOR PUBLICATION

PREPARED BY T.D. Northwood

PREPARED FOR

CHECKED BY

FOR INTERNAL USE

APPROVED BY R.F.L.

DATE March 6, 1952.

SUBJECT: Experiments on the Reduction of Noise Radiated by Subway-car Rails

This note is an addendum to Technical Note No. 102: "Noise Control in Rapid Transit System, Toronto Transportation Commission". The following few sentences, summarizing the previous note, will serve as an introduction to the work' reported here:

Although the cars specified for the Toronto SUbway are of the improved PCC design, which is relatively quiet, they may be expected to produce an uncomfortably loud noise in the confined

reverberant space of a SUbway tunnel. In fact, this has been

observed in a Chicago subway which uses the PCC cars. At the

request of Deleuw, Cather and Co., consultants to the T.T.C., a series of measurements were made by Armour Research Foundation

on the Chicago subway. At a meeting in Chicago, in which I

participated, a sound-absorption treatment was prescribed for the walls, which, it was calculated, would reduce the noise to a tolerable level.

The previous Note deals with one way of achieving a

reduction in noise. The noise level in an enclosure is determined

mainly by the intensity of the noise source and by the sound

absorption at the boundaries. Hence the noise may be reduced by

applying absorbent material to the walls. Bare concrete walls

are highly reflective acoustically, and consequently an untreated

SUbway absorbs the noise very slowly. In these circumstances the

introduction of a little absorbent material has a striking effect. The proposed treatment covers only four square feet per foot of the subway tunnel, yet it reduces the noise level by about half the reduction which was observed when the Chicago trains move from the underground to elevated sections of the line.

An alternative method of reducing noise is to reduce

the intensity of the noise source. The present note describes

- 2

-While in Chicago I had the opportunity of listening to the Chicago sUbway, and re ached some conclusions about the nature

of the noise source. The noise has a definite frequency range,

independent of speed, which appears to be associated with a wheel

or rail resonance. It seemed just possible that if the noise

were radiated by the rails it could be reduced significantly by

some treatment of the rails. This idea was proposed earlier by

Mr.

Tryhorn, of the T.T.C., and in fact he set up a demonstration showing that the "ringing" of a freely-suspended rail could be reduced by coating it with automobile undercoating.

In the following remarks, a more quantitative version

of Mr. Tryhornts experiment is described.

An

effort was made

to produce in a rail the sort of noise produced by the new PCC SUbway-trains, and measurements were made of the reduction in noise radiation obtained with various procedures.

2. Experimental Arrangement

Three "I" be ams, 6" high by Jilt wide were borrowed from the Plant Engineering Section of the N.R.C. and a discarded cast-iron pulley, 12" in diameter with a

4"

rim, was found. The be ams

were assorted lengths, 7 ft., 9 ft., and 20 ft. Noise reduction

experiments were carried out on the two longer beams; the short one was left untreated, since there was some doubt initially about the repeatability of the noise-producing mechanism.

The pulley was found to have no prominent resonance, even when freely suspended, and this was considered a suitable approxi-mation to the wheel of a modern PCC car, in which the rim is bonded

to the hub by a layer of rubber. The pulley was mounted on a

wooden handle, so that it could be rolled back and forth by hand along the be am.

The site of the experiments was the N.R.C. reverberation chamber, a large bare room which constitutes a good acoustical simulation of a subway tunnel, and which is equipped for making noise analyses.

The beams were laid on the concrete floor of the chamber,

with various mounting pads between the beams and the floor. The

following mountings were used, spaced at

4-

to 5-ft. centres: Mounting (1):

Mounting (2):

Mounting (3):

An "acoustic tile", made of wood fibre, 12" x 12"

x

3/4"

thick. (Hereafter labelled "12 inch padU

) . A

3"

strip of acoustic tile was placed between mounting (1) and the beam. (Labelled "3 inch pad" )

A piece of 1" steel strap was placed between the 12" tile-and the beam. (Labelled "1 ineh steel cleat")

3

-3.

Procedure

The pulley was rolled by hand back and forth over about

six feet in the middle of the rail. It was found, jUdging by a

number of runs on the control beam, that this procedure was reproducible enough from day to day to give adequate accuracy

for the present experiments. The operator settled into a very

stable rhythm, which he could maintain within very close limits from day to day (Fig. 2)

The two longer rails, on various mounting pads, were tested before and after a thick layer of automobile undercoating

had been applied. The coating was applied to the web and to the

non-bearing surfaces of the top and bottom flanges of the rails.

4.

Observations

Some preliminary experiments were made with the three mountings and with the effect of coating the inside of the pulley

with plastic automobile undercoating. The chief effect of the

latter was to increase the wheel load - a process which

accentu-ated the low-frequency rumble slightly. Each rail and each

mounting sounded slightly different, owing to rail resonances associated with length. Qualitatively, the

3"

and 12" pads sounded like a better simulation of subway-train noise than the 1" steel cleat. The 3" pad sounded best for the 9' beam, the 12" pad for the 20' beam.

Results of four tests are summarized in Fig. 1. These

data were obtained by measuring the sound level in each of five octaves covering the important range of sound frequencies and

converting to loudness units (Sones). Thus the results may be

compared with those reported by rmour Research Foundation for

the Chicago subway.

Figure 1 can be interpreted by recalling the basis of the sound absorption treatment which was designed for the Toronto·

subway (Tech. ote 102). Armour Research Foundation furnished

noise measurements made on underground and elevated sections of the same Chicago line, and it was assumed that the difference in noise betwe en the se two loc ations repre sented the maximum

reduction which could be obtained by absorption treatment. An

absorption treatment was prescribed which will accomplish

approxi-mately half this reduction. The data at the bottom of Fig. I

show the two noise levels for the Chicago line and the predicted level in, Toronto with the prescribed treatment. (Actually, the Chicago measurements were made over a slightly wider frequency range, and a small adjustment has been made here to make them comparable with the rail-noise experiments.)

4

-In the first experiment a significant reduction in loudness was obtained by the rail treatment - a reduction of the same order as will be achieved by the proposed wall

absorption treatment. The other three ･ｸｰｾｲｩｭ･ｮｴｳ show a much more drastic reduction - greater than the difference between SUbway

and Elevated noise in Chicago. It is probable that the obser-vations of experiment No. 1 differ from the other three because of an unfortunate combination of the fairly large extraneous factors in these.experiments. If this is so, the effectiveness of the rail treatment would best be represented by the average of the four experiments (also shown in Fig. 1).

There seemed no point in refining the present experi-ment, since the process studied may be far from the real

process of a PCC car moving on a properly mounted rail.

The complete set of results is given in Table I, and various details of the observations are illustrated in the

subsequent figures. Figure 2 shows the degree of repeatability maintained, in the series of runs on the control beam. This

should be compared with Fig. 3, which shows a typical "before" and "after" experiment. Figure

4

shows the noise spectra obtained with the 9-ft. beam on each of the three mountings, and this may be compared with Fig.

5,

which shows subway noise spectra.

5.

Conclusions

An attempt has been made to simulate the noise produced by a PCC car rolling on rails which are mounted on resilient pads. In the experiment s it was found that loading the web and other available portions of the rails with automobile undercoating effected a great reduction in radiated noise. The reduction appeared at least as effective as the proposed wall absorption treatment.

The results indicate that a more precise experiment is warranted. Similar tests might be carried out with a PCC surface car travelling over open rails. Ideally the test should be made on rails which are mounted precisely as is proposed for the subway. However, the effectiveness of the rail treatment would probably be demonstrated as long as the rails are not

actually imbedded in concrete or stone ballast. Ordinary railroad construction, of wood ties on rock ballast, would probably

provide an adequate test.

So far only the one coating material has been considered. If the street-car tests ｳｵｧｾ･ｳｴ･､ above confirm the good results of the preliminary work then the next stage might be a laboratory study of coating materials, effect of various thicknesses, ·etc.

.'

-TABLE I

DATA: RAIL NOISE EXPERIMENTS

Frequency Band Total

150- 300- 600- 1200- 2400-

(Sones)

300

600

1200 2400

4800

TEST

#1

20'

Beam on

12

ft _{pads Untreated}

₂₈

43

75

33

19

198

(spaced at

5'

c. c. ) Treated

₃₃

₄₅

₅₅

16

10

159

TEST

#2

20'

Beam on

3"

pads Untreated

₃₃

₆₃

₉₀

60

20

266

(spaced at

5'

c.c.) Treated

28

48

40

12

8

136

TEST

#3

9'

Beam on

3

1t _{pads Untreated}

₃₀

₅₈

₈₅

₅₇

₂₂

_2.52

(Spaced at

_4'

c. c. ) Treated

18

₃₄

₄₃

12

7

114

TEST

#4

9'

Be am on lit steel cleats Untreated

₃₈

60

7.5

87

53

313

e

(spaced at

4'

c.c.) Treated

₁₉

₃₇

46

12

8

122

Untreated

9'

Beam on

12"

pads (for

23

38

53

32

13

159

comparison with

#3

and

#4

above)

7'

Beam on

12

ft _{pads (Control runs) Run}

_#4

8

14

22

42

49

59

27

40

52

62

57

34

48

58

37

38

50

58

35

13

15

177

211

177

181

Subway Section of Chicago· Line

₄₅

70

70

40

15

240

Elevated Section of Chicago Line

32

52

42

22

8

156

Toronto Subway with Wall Absorption

38

59

49

27

10

183

o ｌｏｕｄｎｅＮＮｃ［ｾ 100 (SONE-S)'200 .300

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NOISE:. SPE.CTRA ｆｏｾ 9' BE.AM

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NOISE.. SPECTRA. of UNTRE.A.TE..t'

t

T RE..A.TE.t' B E.AM (9I BEAM ON 3" PAPS)

/' / ' 'STEE.L CLEATS 100 r-.. ｾ Z 0 (fl V (fl

5°

{fl uJ Z t> :J 0 -l 0 150 - 300

--300-4000 000-1'200 1'200-'2400 ｆｒｅＮｾｕｅＮＮｎｇｙ BANe:' '2400-4800

F1Go4

OISE. SPE.G R

ON ｔｈｾｅＮｅＮ MOUNTI BE.AM 100 r " ' ({l uJ Z 0 (/l

---

_{(rl 5°} ｾ z c::. :J 0 -.J 0

FIGo:;

ＳＰＰＭｾｏｏ ,",00-1'200 1'200 - '2400 '2400-4800 FRE.QUE.Nc.,y BA.Nt7 NO ISEo SPE.0TRA.

or

(a.) C H ICA60 5UBIVAY (uNTR.E..AT£l7)

(b) TOR,ONTO SU81VAY lVITH IVALL ABSOR,PTIONＨｃａｌｃｕｌａｔｾ

(c) EL£VAT£l7 SECT/ON of CHICAGO LINE...