FINAL REPORT

FOR

FHWA WORK ORDER NO. DTFH71-83-3610-IA-12

Ef fec t iveness o f P a r a l l e l Noise B a r r i e r s

An Iowa Study

Ron Ridnour and Dale Vander Schaaf

5151239-1613

O f f i c e o f P r o j e c t Planning

Planning and Research D i v i s i o n

Iowa Department o f Transpor ta t ion

Ames, Iowa 50010

August 1987

SPONSORSHIP This report is based on work conducted by the lowa Department of Transportation and the Federal Highway Administration. This research project was funded through the Demonstration Projects Program of the U.S. Department of Transpor- tation. Federal Highway Administration

DISCLAIMER The contents of this report reflect the views of the author and do not necessarily reflect the official views of the lowa Department of Transportation or Federal Highway Administration. This report does not constitute a standard, specification or regulation.

ABSTRACT

Traf f ic noise monitoring using FHWA's Demonstration Projects Division

Mobile Noise Laboratory a t f r e e f i e l d , s ingle wall and para1 le l bar r ie r

s i t e on 1-380 i n Evansdale, Iowa i s described. Access t o 1-380 pr ior t o

i t s being open t o t r a f f i c afforded a controlled pass-by monitoring phase

involving d i f fe ren t vehicle types. A subsequent second phase entailed

identical measurement methodology t o monitor "real world" 1-380 t r a f f i c

noise. Phase I data indicated increases i n noise were s ign i f ican t under

the paral le l bar r ie r conditions fo r l i g h t duty vehicles operating in the

f a r lane. Phase I1 r e su l t s showed t h a t the actual 1-380 t r a f f i c mix

largely o f f s e t the e a r l i e r observed e f fec t , but minor increases in t r a f f i c

noise under the paral le l system were noted. These differences i n noise

bar r ie r system effectiveness a re judged t o be ins ignif icant a t t h i s par t i -

cular study location.

Effec t iveness of P a r a l l e l Noise B a r r i e r s - An Iowa Study

INTRODUCTION

The Iowa Department of Transpor ta t ion (Iowa DOT) is one of a number of

s t a t e highway agencies (SHA) which have cons t ruc ted p a r a l l e l no i se

b a r r i e r s . I n t h e f a l l of 1982, 2600 f e e t of p a r a l l e l s t e e l no i se b a r r i e r s

were cons t ruc ted adjacent t o I n t e r s t a t e 380 i n t h e City of Evansdale, Iowa.

It was determined dur ing t h e i n i t i a l no i se impact a n a l y s i s f o r t h i s p r o j e c t

t h a t some type of p a r a l l e l noise b a r r i e r s would have t o be cons t ruc ted .

The pre l iminary b a r r i e r des ign concept c a l l e d f o r t h e cons t ruc t ion of an

ear then b a r r i e r on one s i d e of t h e highway and a s o l i d wa l l on t h e oppos i te

s ide . It was f e l t t h a t t h e berm would not only reduce b a r r i e r c o s t s , but

v i r t u a l l y e l imina te any problems due t o r e f l e c t e d noise. However, because

of r e s t r i c t e d a v a i l a b l e r i g h t of way and o t h e r highway design

cons idera t ions t h e berm and wa l l concept had t o be el iminated i n the f i n a l

design. Using t h e b e s t p r e d i c t i o n models a v a i l a b l e a t t h e time (2,3), i t

was concluded t h a t a l though t h e i n s e r t i o n l o s s may not be a s h igh i f t h e

p a r a l l e l wa l l s were b u i l t , i n s t e a d of the o r i g i n a l berm and w a l l concept,

t h e e f f e c t i v e i n s e r t i o n l o s s would s t i l l be s i g n i f i c a n t enough t o be of

b e n e f i t t o t h e impacted r ece ive r s .

It was during t h e development of t h e 1-380 no i se b a r r i e r p r o j e c t t h a t

t h e Iowa DOT no i se a n a l y s i s s t a f f f i r s t became aware of t h e d i f f i c u l t i e s i n

analyzing t h e e f f e c t i v e n e s s of p a r a l l e l b a r r i e r s . Unlike f o r t h e s i n g l e

b a r r i e r a n a l y s i s , t h e r e were no computerized p red ic t ion models a v a i l a b l e

f o r p a r a l l e l b a r r i e r ana lys i s . The Federal Highway Administrat ion (FHWA)

had provided t h e SHA's with a simple " p a r a l l e l b a r r i e r nomograph" (3) f o r

t h e a n a l y s i s of p a r a l l e l b a r r i e r s . Because the s t a f f was not t o t a l l y

confident i n t h e r e s u l t s of a simple nomograph p red ic t ion , a l i t e r a t u r e

search was made for data related to parallel barrier analysis. It was

discovered that although most noise abatement specialists concede that some

reduction in the insertion loss does occur when parallel barriers are

built, there is no consensus over just how significant the reduction could

be. Most of the data relating to the degradation problem is based on

theoretical acoustical analysis or scale model studies. Many of the

"laboratory" studies show that the effective insertion loss of one barrier

can be significantly reduced or even eliminated by the presence of an

opposite parallel barrier (4,5,6). At the same time the limited number of

full scale field measurements which have been made (5,6,8) have provided no

clear cut data which can be used to predict the potential reduction in the

insertion loss when parallel barriers are built.

In early 1983 the Iowa DOT received copies of two papers (1,7) which

not only provided much needed information on the subject of parallel noise

barriers, but also rekindled the noise analysis staff's concern over just

how effective the recently completed 1-380 parallel barriers would be. The

Bowlby and Cohn paper (1) described the development of an algorithm and a

computer program called IMAGE-3 for the analysis and design of parallel

barriers. This paper emphasized however, that although models which are

developed for analyzing the effectiveness of parallel barriers may be

mathematically and acoustically sound, few if any, well documented field

validation studies have been performed.

Because the Iowa DOT is always interested in the performance of any

noise barriers constructed along Iowa highways "before" and "after" noise

level data is often obtained for analysis. This data is used to not only

determine overall barrier performance, but to also test the accuracy of the

model used to predict barrier effectiveness. Although no formal study was

originally being proposed, the noise analysis staff was preparing to

! I undertake a more extensive than normal noise monitoring effort after 1-380 I I

was opened to traffic.

In August, 1983, noise analysis staff members attended the annual

summer meeting of the Transportation Research Board's (TRB) Transportation I

! Noise Committee in Boston, Massachusetts. Although no formal discussions

were held concerning the problem of parallel noise barriers during the 1 course of the meeting, it was learned that there has still been very little

field data collected in the vicinity of parallel barriers. The FHWA

personne'l present at the meeting made it known that the FHWA was concerned

about the parallel barrier reflection problem and were in the process of I

funding some experimental field work in this area. Upon hearing of the

increased involvement of the FHWA, Iowa DOT staff inquired as to the

possibility of having the FHWA provide support in obtaining noise data

along the 1-380 parallel barrier segment. The FHWA indicated that

assistance for this type of work was available.

Shortly after returning from Boston, the noise analysis staff

submitted a Research Work Plan (Appendix A) to the FHWA for the proposed

1-380 barrier study. Acting expeditiously, the FHWA approved the work plan

in September 1983.

The following interim report describes the procedures used in the

initial controlled passby phase of the project and discusses the noise data

collected.

11. COORDINATION WITH THE FEDERAL HIGHWAY ADMINISTRATION

The work plan submitted to the FHWA Iowa Division office described a

two-phase study at the 1-380 site in Evansdale. The first phase would

entail noise measurements from controlled vehicle passbys at each of three

barrier conditions - free field (no barrier), single wall and parallel

walls. A second phase to be undertaken after the highway is open to

traffic would collect noise data from the normal traffic mix at the same

three locations.

Federal Highway Administration participation was to consist of

providing the Demonstration Projects Division's noise analysis trailer

along with a technician and a project manager to oversee the use of the

FHWA equipment. A $10,000 grant was requested to be administered through

The Demonstration Projects Division to cover costs incurred by the State.

A preliminary report was to be prepared by the State upon completion

of the first phase of the study and a final report was to be prepared upon

completion of the phase two monitoring.

Provisions were also agreed upon to provide the study site details and

noise measurement data to Vanderbilt University for application of the

IMAGE-3 parallel barrier model.

111. STUDY SITES

The t h r e e b a r r i e r condi t ions a r e loca ted i n a s i n g l e mile-long s e c t i o n

of 1-380 a s shown on Figure 1 and i n t h e fol lowing photos:

1) The f r e e f i e l d site was an open f i e l d loca t ion wi th no major

obs t ruc t ions (Figure 2)

2) The s i n g l e w a l l s i t e was a state-owned p a r c e l l y ing between

t h e highway right-of-way and r e s i d e n t i a l land use.

(Figure 3)

3) The p a r a l l e l wa l l s i t e was t h e midpoint of t h e p a r a l l e l wa l l

s e c t i o n of 1-380 i n an e s t ab l i shed r e s i d e n t i a l a r e a (Figures

4 and 5)

Remote Microphones from t h e FHWA Demo P r o j e c t s no i se a n a l y s i s system

were pos i t ioned a t t h e following loca t ions :

Free F ie ld

1. 40' From c e n t e r l i n e N.L. (Near Lane) 5' above roadway 2. 40' From c e n t e r l i n e N.L. 15' above roadway 3. 90' From c e n t e r l i n e N.L. 10' above roadway 4. 90' From c e n t e r l i n e N.L. same e l e v a t i o n a s roadway 5. 140' From c e n t e r l i n e N.L. 5' above ground, same e l e v a t i o n a s

roadway 6. 190' From c e n t e r l i n e N.L. 5' above ground, same e l eva t ion a s

roadway 7 . I n Middle of median 50' from c e n t e r l i n e of each f a r lane , 5 ' above

roadway

Figure 1. Study s i t e locations

Single wall Parallel walls s i t e s i t e Free field s i t e

Figure 2. Free f ield studv s i t e

Figure 3. Sing1 e wal l study s i t e

Figure 4. Parallel walls study site

Single Bar r i e r

40' From c e n t e r l i n e N.L. 1' Behind wa l l , 5 ' above roadway 40' From c e n t e r l i n e N.L. 1' Behind w a l l , 5' above top of w a l l , 17' above roadway 90' From c e n t e r l i n e N.L. 15' above roadway 90' From c e n t e r l i n e N.L. 6' above roadway 140' From c e n t e r l i n e N.L. 5' above ground, 5' below roadway 190' From c e n t e r l i n e N.L. 5' above ground, 5 ' below roadway 240' From c e n t e r l i n e N.L. 5' above ground, 5 ' below roadway I n Middle of median 50' from c e n t e r l i n e of each f a r l ane , 5' above roadway

P a r a l l e l Barriers

40' From c e n t e r l i n e N.L. 1' behind wa l l , 5 ' above roadway 40' From c e n t e r l i n e N.L. 1' behind w a l l , 5' above top of wa l l , 17' above roadway 90' From c e n t e r l i n e N.L. 15' above roadway 90' From c e n t e r l i n e N.L. 6' above roadway 140' From c e n t e r l i n e N.L. 5' above ground, 3' below roadway e l e v a t i o n 190' From c e n t e r l i n e N.L. 5' above ground, 3' below roadway e l eva t ion 240' From c e n t e r l i n e N.L. 5' above ground, 3' below roadway e l e v a t i o n 290' From c e n t e r l i n e N.L. 5' above ground. I ' below roadway e l e v a t i o n I n Middle of median, 5 ' above roadway On south s i d e of south b a r r i e r 140' From c e n t e r l i n e N.L. 5' above ground, 3' below roadway

Addi t ional ly , a n independent microphone was loca ted on t h e highway

c e n t e r l i n e i n t h e median a t a he ight of 5 f e e t above the roadways. The

c r o s s s e c t i o n s of t h e monitoring sites and microphone l o c a t i o n s a r e shown

on Figures 6 and 7. A l l s i t e s a r e considered "soft" with low growing

g ras ses being t h e primary ground cover.

IV. EQUIPMENT AND CONDITIONS

Five vehicle types were used as controlled noise sources:

1. 1971 International truck tractor, 6 cylinder diesel, 270

horsepower, 855 cubic inch, Fuller 13-speed transmission, pulling

trailer carrying JD 450 dozer weighing 8 tons. Total Loaded

Weight 48,000 lbs. (Figure 8)

2. 1981 Ford LT8000 tandem axle, 8 cylinder diesel, 210 horsepower,

Caterpiller 3208 engine, Fuller R66-13 transmission (13-speed).

Total Loaded Weight 52,000 lbs. (Figure 9) -

3. 1975 Ford F700 two axle, 8 cylinder gas, 5-speed Clark

transmission. Total Loaded Weight 28,000 lbs. (Figure 10)

4. 1980 Ford F150 % ton pickup, 6 cylinder gas, 300 cubic inch

engine, automatic transmission. Carried No Load. (Figure 11)

5. 1981 Chevrolet Impala Station Wagon, 8 cylinder gas, 267 cubic

inch engine, automatic transmission.

The tandem axle and two axle trucks were equipped with Firestone Super

All Traction tires on the rear axles. These have a cross bar type of tread

design. (Figure 12) The semi tractor had Goodyear Super High Miler tires

on the rear which are also cross bar type. The remainder of the test

vehicle tires were of a conventional rib design. (Figure 13) The vehicle

drivers were instructed to cruise past the microphone site in normal

open-road gear at a steady speed of 50-60 mph. Actual passby speeds were

measured with a portable radar gun.

Figure 9. Tandem a x l e t ruck 1

Fisure 10. Two a x l e truck

Figure 11. Pickup truck

Figure 12. Cross bar type tread

I Figure 13. Rib desiqn tread i

Noise measurement equipment was provided by the FHWA Demonstration

Projects Division's Noise trailer. General Radio $ inch microphones were

connected to the Hewlitt-Packard data analysis system. A BBN Model 614

noise analyzer was located in the median and set on the threshold mode to

record peak noise levels for each passby. A sample of 70 seconds duration

was collected at each of the remote microphones during each passby.

Favorable meteorological conditions were noted for each test period:

free field - 0% cloud cover, wind 0-5 mph from the south humidity

50-60%, temperature 64-68 F.

single wall - 0% cloud cover, wind 3-8 mph from the southeast,

humidity 40-45%, temperature 60-65 F .

parallel walls - 100% cloud cover, wind 0-5 mph from the south,

humidity 45-50%, temperature 55-60 F.

Pavement conditions were dry for all passbys. Pavement texture was

118 inch transverse grooves at 518 - 16 inch spacing. (Figure 1 4 )

V. PROCEDURE

I Test vehicle passby sequencing was maintained through hand-held radio

contact with one of the vehicle drivers w h then would instruct the other I

I

vehicle drivers to begin their runs. The radios also permitted contact

between the roadside test area and the noise analysis trailer. Personnel 1 in the trailer were alerted at the beginning of each passby so that the 1

I passby noise curve would be included within the 70-second monitoring period

required by the computer. The roadside "coordinator" also measured passby 1

speed with the battery powered radar gun and made a notation of the

specific passby description on the BBN printout tape. In the trailer hard 1 1

copies of the Leq and Ln values measured at each microphone were printed.

Additionally a frequency analysis printout was prepared for microphone 1 number 6 located 190 feet from near lane (150 feet from wall) at each test

I site. The four truck types were used at all three sites. The automobile

was used at the free field site and the single wall site but was eliminated I 1

from the test during the parallel wall portion of the study. It was

observed that the automobile passby data were nearly identical to that of I I

the pickup and it was decided that the time spent at the parallel site

could be better used obtaining data from the other vehicle types.

VI. DATA OBSERVATIONS

A total of 102 vehicle passbys were recorded and were broken down as

follows: 28 free field passbys, 35 single barrier passbys, and 39 parallel

barrier passbys. The Leq and L1 values measured at each microphone are

shown on Tables 1, 2, and 3. After a preliminary review of the passby

data, the initial single barrier passhy was eliminated. As can be seen on

Table 2, although values were recorded for the initial passby, they were

much lower than would have normally occurred with a tandem axle truck

passby. In addition the underlined values shown on Table 3 were also

considered invalid. The occasional traffic on a nearby street influenced

the noise levels at these microphones to a greater degree than was

originally expected. The remaining data is summarized by the mean values

shown on Table 4, 5 and 6. These data represent noise levels at the

microphones located 100 feet and 150 feet from the wall and also at the

reference microphone and the median microphones respectively.

Upon reduction and review of the noise data definite trends could be

identified; however the limited number of passbys and the variance in

individual passby speeds and noise levels make the value of a larger data

base clear. From the data collected the following observations were made:

1. Noise data from all microphones show a tendency towards increased

noise levels under the parallel wall condition as compared to the

single wall.

2. This tendency for higher noise levels with parallel walls is

generally more apparent in the far lane passbys than the near

lane passbys with this tendency very obvious at the reference

microphone (Mic. 2) location.

Ta

ble

1.

Ve

hic

le

pa

ssb

y

da

ta-f

ree

fi

eld

F

ree

Fie

ld S

ite

T-T

Tra

cto

r T

rail

er.

T-

A

Tan

dem

A

xle

, 2-

A

Two

Ax

le 6

-Tir

e P -

Pic

ku

p,

A -

Au

to,

Wes

t=N

ear

Lan

e,

Eas

t=F

ar L

ane

Pas

sby

V

eh

icle

D

ire

cti

on

T

IME

SP

EE

B

Mic

.1

Mic

.2

Mic

.3

Mic

.4

Mic

.5

Mic

.6

mc

.7

Mic

.8

lied

fan

S

o.

Wal

l .. no.

Typ

e P

assb

y

MPH

L

eq

L1

Leq

L

l L

eq

L1

Leq

L1

Leq

L

1

Leq

L1

Leq

L

1 L

eq

L1

Leq

L

max

M

ic.

Lm

ax

0-1

T-T

0-2

T-T

0-2

T- A

0-2

2-A

0-2

P

0-3

T-A

0-3

2-A

0-4

T-A

N

0-4

2-A

0

0-4

P

0-4

A

0-5

T-T

0-5

T- A

0-5

2-A

0-6

T-A

0-6

2-A

Wes

t

Ea

st

Ea

st

Ea

st

Ea

st

wes

t

Wes

t

Ea

st

Ea

st

Ea

st

Fa

st

Wes

t

wes

t

wes

t

Ea

st

Ea

st

h .o Vi VI **--c--mmmmm ~~~IIIMIIIIII PZOOOOOOOOUOOO

Tab

le

3.

Veh

icle

p

ass

by

Passby

Vehicle

- Direction

KO.

Type

Passby

*2-1

T-

A

West

2-1

2-A

2-1

P

2-2

T-T

2-2

T-A

2-2

2-A

2-2

P

2-3

T-T

2-3

T-A

2-3

2-A

2-4

T-T

N

f-

2-4

T-A

2-4

2-A

2-4

P

2- 5

T-

T

2-5

T-A

2-5

2-A

2-5

P

2-6

T-T

West

West

East

East

East

East

West

West

West

East

East

East

East

West

West

West

West

East

da

ta-p

ara

llel

b

arrie

rs

Parallel Barrier Site

TIME

SPEED

Mic.1

Mic.2

Mic.3

Mic.4

Mic.5

Mic.6

Mic.7

Mic.8

Median

So.

Wall

Leq Ll

Leq L1

Leq L1

Leq L1

Leq L1

Leq L1

Leq L1

Leq L1

Leq Lmax Mic.

Lmax

13:Z

O

55

57 62

69

78

61

68

57

63

56

62

56

61

57

66 -

62 -

75

76

83

63

*Underlined Values Affected By Extraneous Noise Source

Ta

ble

Passby NO.

2-6

2-6

2-6

2-7

2-7

2-8

2-8

2-8

Vehicle Direction

Type Passby

T-A East

2-A East

P East

T-A West

P

West

T-T East

T-A EasC

2-A East

P

East

T-T West

T-A West

2-A West

P

West

T-T East

T-T West

2-8 West

T-T East

T-T West

2-A West

P West

TIHE SPEED

Mic. 1 Leq

L1

56 62

59 67

53 59

69 78

53 62

56 63

56 67

60 68

50 55

60 71

68 77

62 72

56 64

58 68

62 72

62 72

56 63

59 69

63 73

53 65

uic. 2

Leq L1

66 78

71 81

59 69

73 85

63 76

68 78

68 81

71 81

58 69

72 87

73 87

72 89

69 78

70 81

71 87

74 88

68 79

73 88

75 89

63 77

Mic. 3

Leq L1

59 70

63 71

53 60

62 73

54 62

60 68

60 71

63 71

51 59

60 71

63 75

59 70

60 68

62 71

62 71

61 70

60 69

60 69

62 71

55 68

Mic.4 Leq

L1

55 64

59 66

51 57

58 68

52 61

56 63

56 66

60 68

49 54

58 69

59 68

56 65

55 63

57 67

59 66

57 65

55 62

56 64

58 67

53 66

Mic.5 Leq

L1

53 60

56 63

50 57

55 64

52 64

55 64

53 59

58 68

48 53

56 67

55 64

53 61

53 59

55 62

57 65

54 62

53 59

54 61

56 64

53 66

Mic.6 Leq

L1

54 60

55 61

51 57

54 63

55 56

57 67

-

53 59

60 73

--

50 55

60 2

-

55 63

52 59

54 59

58 8

-

60

-

54 60

53 59

54 61

55 61

53 63

Mic. 7

Leq L1

57 66

-

56 63

52 59

55 5

-

60 72

-

61 2

-

53 61

64

-

53 58

63 2

-

55 64

53 59

56 63

60 8

-

65 -

54 60

53 59

55 63

57 63

53 60

MiC.8 Leq

L1

63 14

-

60 72

-

56 59

59 2

-

67 -

67 81

-

58 7

-

72 86

-

60 7

-

68 80

-

58 68

-

56 61

62 72

-

66 2

-

70 83

-

56 60

56 60

58 65

-

62 13 -

55 58

Xedian Leq

Lmax

75 82

78 86

71 74

77 86

71 74

77 83

76 83

77 86

72 75

77 84

77 87

77 86

71 74

78 85

79 86

78 86

77 83

77 84

78 86

71 74

So. Wall

Mic. Lm

ax

62

65

-- 64

-- 63

62

65

-- 62

65

64

-- 65

- -- 63

64

64

-

Tab

le

4.

Mea

n p

ass

by

no

ise

da

ta

1-3

80

Ev

an

sdale

Pa

rall

el

Ba

rrie

r S

tud

y

- T

yp

ica

l H

ouse

S

et-B

ack

D

ista

nc

e

Mic

rop

ho

ne

No.

5

100 F

ee

t F

rom

W

all

Mea

n P

assb

y

ve

hic

le s

pee

d a

nd

L

eq

I.

Wes

tbound

Ve

hic

le

Mea

n F

ree

M

ean

Sin

gle

M

ean

Pa

rall

el

(Nea

r L

ane)

T

ype

Spe

ed

Fie

ld

Sp

eed

-

Wal

l Spe

ed

Wal

ls

Mea

n P

assb

y

L1

Fre

e

Sin

gle

P

ara

lle

l F

ield

W

all

Wal

ls

11.

Eas

tbo

un

d

TT

60m

ph

59dB

A

54m

ph

53dB

A

56m

ph

55dB

A

70dB

A

58dB

A

6ldB

A

(Fa

r L

ane)

T A

5l

mph

59

dBA

49

mph

55

dBA

5l

mph

54

dBA

69

dBA

6l

dBA

60

dBA

Tab

le 5. M

ean p

assby

no

ise d

ata

1-380 E

van

sdale - P

ara

llel B

arrie

r S

tud

y

No

ise Le

ve

ls At T

yp

ica

l House

Set-B

ack D

istan

ce

Micro

ph

on

e No.

6 150 F

ee

t From W

all

I. W

estbound (N

ear Lan

e)

11. E

astbo

un

d

(Far L

ane)

Mean

Passb

y v

eh

icle

speed

and L

eq

Ve

hic

le

Mean

Fre

e

Mean

Sin

gle

M

ean P

ara

llel

Type

Fie

ld

Sp

eed

Speed

Wall

Speed

Walls

Mean

Passb

y L

1

Fre

e

Sin

gle

P

ara

llel

Fie

ld

Wall

Wall

Tab

le

6.

Mea

n p

ass

by

no

ise

da

ta

1-3

80

Ev

ansd

ale

Pa

rall

el

Ba

rrie

r S

tud

y

No

ise

Lev

els

A

t R

efe

ren

ce

Mic

rop

ho

ne

(Mic

.2)

And

A

t T

he M

edia

n M

icro

ph

on

e

Mea

n P

assb

y v

eh

icle

sp

eed

an

d

L1

Re

fere

nc

e M

icro

ph

on

e M

ean

Pas

sby

L

Med

ian ~

ic

ro

~%

$e

Ve

hic

le

Mea

n F

ree

M

ean

Sin

gle

M

ean

Pa

rall

el

Fre

e

Sin

gle

P

ara

lle

l I.

W

estb

ound

Type

Spee

d

Fie

ld

Sp

eed

W

all

Spe

ed

Wal

ls

Fie

ld

Wal

l W

alls

(N

ear

Lan

e)

TT

55m

ph

85dB

A

58m

ph

86dB

A

59m

ph

87dB

A

84dB

A

85dB

A

85dB

A

11.

Eas

tbo

un

d

TT

60m

ph

79dB

A

54m

ph

75dB

A

56m

ph

79dB

A

87dB

A

8ldB

A

84dB

A

(Far

Lan

e)

T A

5lm

ph

78dB

A

49m

ph

76dB

A

5lm

ph

8ldB

A

82dB

A

82dB

A

84dB

A

2A

5lm

ph

77dB

A

50m

ph

77dB

A

53m

ph

8ldB

A

83dB

A

84dB

A

86dB

A

3. The parallel barrier effect is generally more identifiable in the

higher frequency noise sources such as the pickup truck and in

the higher frequency components of the other noise source

vehicles.

4. Increases in passby noise levels from the single wall to the

parallel wall condition are generally in the 1-3 dBA range but

range up to 6dBA for the far lane pickup passby. This

observation is based on mean L1 values at microphone 6 which

represents the typical house setback distance.

VII. PHASE TWO RESEARCH

Measurements which will be taken at the same locations under normal

traffic conditions should reveal the extent to which reflected noise

actually reduces barrier effectiveness at the parallel walls. It will be

interesting to observe whether or not far lane noise in combination with

the near lane source causes comparable reductions in insertion loss.

Additionally we may be able to identify varying degrees of multiple

reflections depending on the percentage of automobile and other light duty

vehicles in the "real world" conditions. It might be expected, for

example, that 100% autos and other high frequency noise sources may result

in a significant reduction in insertion loss. This phenomenon would have

noteworthy implications in larger metropolitan areas where high automobile

volumes constitute the major traffic noise source.

V I I I . Phase Two Methods

The microphone loca t ions used i n the con t ro l l ed pass-by p o r t i o n o f

the study were rep l i ca ted a t each f i e l d s i t e t o measure actual 1-380

t r a f f i c no ise i n the f a l l o f 1985. This moni tor ing work was accom-

p l i shed w i t h the assistance o f s t a f f members from FHWA's O f f i c e o f

Environmental Po l i cy i n Washington, D.C. I t i s f e l t t h a t s u f f i c i e n t

data were co l l ec ted dur ing t h i s i n i t i a l " rea l world" sampling e f f o r t

t o character ize the ef fect iveness of the s ing le and p a r a l l e l b a r r i e r

systems under study. More extensive moni tor ing a c t i v i t y was

prevented by computer system mal funct ion i n the noise laboratory,

d i f f i c u l t y i n scheduling fol low-up monitor ing, and a reduct ion i n

Iowa DOT s t a f f i n g . The data reduced and reported here w i l l complete

the contractual study.

I X . Phase Two Results

The data co l l ec ted from actual 1-380 t r a f f i c can be analyzed from

three perspectives:

1) The three f i e l d s i t e s can be compared using noise data

co l l ec ted dur ing periods o f s i m i l a r t r a f f i c condit ions.

2) The s ing le and p a r a l l e l wa l l cond i t ions can be analyzed using

simultaneous and continuous moni tor ing o f hour ly noise l e v e l s

a t a given distance from each b a r r i e r system type.

3) The three f i e l d s i t es can be compared based on the frequency

spectra obtained dur ing sampling periods o f s i m i l a r t r a f f i c

condi t ions and comparable ove ra l l t r a f f i c noise levels .

Table 7. Mean Leq a t Each Microphone Pos i t ion

Mean Leq, dBA

Number of Microphone Pos i t ion

S i t e - Samples 40' h igh 40' low 90' High 90' Low 140' Low 190' Low

Free F i e l d 6 72.7 72.4 68.2 65.3 62.4 58.6

Single Wall 20 73.2 58.7 63.1 56.9 57.1 55.7

Para1 1 e l Wall s 17 73.2 58.9 62.2 58.9 56.4 55.1

A t the f r e e f i e l d s i t e s i x 15-minute samples were taken, a l l dur ing

mid t o l a t e morning off-peak hours. A t o t a l o f twenty 15-minute

samples were taken a t the s ing le wa l l s i t e and seventeen 15-minute

samples were obtained a t the p a r a l l e l wal ls s i t e . The mean Leq

computed f o r each'microphone loca t i on a t each f i e l d s i t e i s shown i n

Table 7. These data show a f a i r consistency between the s ing le wa l l

s i t e and the p a r a l l e l wal ls s i t e a t distances near the wal l and a lso

a t the more remote distances. However, a t the 90' low microphone

the noise l e v e l averages 2 dBA higher a t the p a r a l l e l wa l ls s i t e .

This would suggest t h a t mu1 t i p l e r e f l e c t i o n s might be in f luenc ing an

intermediate zone on the res iden t i a l s ide o f the p a r a l l e l wal ls . To

f u r t h e r examine t h i s mu1 t i p l e microphone data, i nd i v idua l sampling

periods w i t h s i m i l a r t r a f f i c condi t ions and reference mike

(40' high) noise levels were selected. Table 8 presents this I

comparison for off-peak traffic conditions. Again residential side I

noise levels are lower at the parallel site except at the 90' low I

location which indicates a tendency toward reduced barrier I

effectiveness in this intermediate zone, possibly due to mu1 tiple I ! ,

reflections. A similar tendency can be identified during peak hour

sampl ing, but unfortunately no corroborative traffic counts were I I

obtained during the periods which best demonstrated this effect. I

Additionally, continuous hourly samples were obtained simultaneously I I

at both the single and parallel sites at a distance of 100 feet from

the wall. Seventy-two such parallel samples were collected using a 1 I

BBN 614 Community Noise Analyzer and a Digital Acoustics Community

Noise Analyzer. For a typical hour the parallel walls site was I 1

0.7 dBA higher than the Leq measured 100 feet from the single wall. I

Nine hours of data were collected at the distance of 50 feet from I

the wall (which corresponds to the 90' from near lane low microphone I location discussed previously). The para1 lel site averaged 1.5 dBA 1

higher than the single wall site at this distance. These data also 1 I

support a finding of some reduction in barrier effectiveness under I

the para1 lel walls condition. 1 I

A frequency spectrum histogram for a selected microphone location 1 was obtained during each 15-minute monitoring period. Most of the I

I

frequency information was from the microphones near the noise wall I

(24 of 42 periods) and no significant differences could be identi- I I

fied at these locations among the frequency spectra printouts from

Table 8.

Com

parison of T

hree Field S

ites Under

Sim

ilar Traffic C

onditions

Traffic

Leq,

dBA

Autos - MT - HT

Microphone D

istance, P

ositio

n

Site

-

Date -

Time -

WB EB

40' H

igh 40'

Low 90'

High

90' Low

140' Low

190' Low

Free F

ield 11/8/85

11:05-11:20 40-2-11

57-1-10 73.9

73.1 69.2

66.4 63.4

59.4

Single W

all 11/5/85

1:50-2:05 50-2-8

61-0-6 72.0

58.1 64.4

55.6 57.7

56.7

Parallel W

all s 11/6/85

1:20-1:35 61-1-3

64-2-7 72.1

58.2 61.6

58.4 55.4

54.2 W

the three f ie ld study s i t e s . Frequency breakdown information from a

limited number of other microphone locations also proved t o be

inconclusive in attempting to compare the single wall against the

parallel wall si tuation. Lack of frequency component data from the

intermediate receiver zone identified previously can be considered

an oversight of t h i s research e f for t . Further longer term research

in t h i s subject area might serve t o smooth out t r a f f i c and meteoro-

logical variations and focus more directly on the frequency spectra

aspects of the parallel walls condition.

X. Summary and Conclusion

Both controlled pass-by and "real world" t r a f f i c noise was measured,

characterized, and interpreted in an e f for t to assess the acoustical

effectiveness differences between a single noise wall and parallel

noise walls on 1-380 in Evansdale, Iowa. Both phases of the study

resulted i n data which suggests minor reduction in noise wall

effectiveness as a resul t of mu1 t i p l e reflections within the paral - l e l barrier canyon. More extensive monitoring would serve t o remove

the influence of minor meteorological and t r a f f i c mix variations

which have probable b u t limited influence on th i s short-term study.

From a practical standpoint, the study suggests t h a t the degree t o

which barrier effectiveness i s compromised by the paral l e l si tuation

in t h i s part icular noise abatement system i s insignificant. The

steel barrier material and configuration, the s i t e geometrics

including barrier height and interbarrier distance, the use of

earthen berms and the transverse groove surface texturing a l l no

doubt influence the resul t ing t r a f f i c noise leve ls w i t h i n the study area.

I t would appear from this experience t ha t a s ign i f ican t influence of a

paral le l wall s i tua t ion would require very high walls of very re f lec t ive

material , a r e l a t i ve ly small in te rbar r ie r distance, and a higher proportion

of peak noise of the high frequency type being generated a t the

tirelroadway interface.

References i

1. Bowlby, W., and Cohn, L.F. "IMAGE-3: Computer Aided Design For

Parallel Noise Barriers." A Paper presented at the 1983 Annual

Meeting of the Transportation Research Board, Washington, D.C.,

January 1983.

1 2. Bowlby, W., SNAP 1.1 - A Revised Program and User's Manual for the

FHWA Level I Highway Traffic Noise Prediction Computer Program, Report I I

No. FHWA-DP-45-4, Federal Highway Administration, Arlington, Virginia, i

1980. 1

3. Federal Highway Administration, Demonstration Project No. 45: Highway I

I Noise Analysis (workshop notebook), Report No. FHWA-DP-45-2, Federal 1

Highway Administration, Arlington, Virginia, 1978.

4. Hajek, J.J., "Effects of Parallel Highway Noise Barriers," Proceedings

of the 1980 International Conference on Noise Control Engineering, I

Poughkeepsie, New York, 1980. i

5. Hajek, J.J., Noise Barrier Attenuation: Highway 401 North Side East

of Avenue Road, Toronto, Report No. 80-AC-01, Ontario Ministry of

Transportation and Communications, 1980.

6. Hajek, J.J., Performance of Parallel Highway Noise Barriers: Yonge

Street to Bayview Avenue, Toronto, Report No. 80-AC-01, Ontario

Ministry of Transportation and Communications, 1980.

7. Hajek, J.J., "The E f f e c t s of P a r a l l e l Highway Noise Bar r i e r s , " Draf t

Paper Offered f o r P resen ta t ion a t t h e Annual Meeting of t h e

Transpor ta t ion Research Board, Washington, D.C., January 1983.

8. Nelson, P.M., P.G. Abbott and A.C. Salvidge, Acoustic Performance of

t h e M-6 Noise B a r r i e r s , TRRL Laboratory Report 731, Transport and Road

Research Laboratory, Crowthorne, England, 1976.