Suburban Noise Control with Plant Materials and Solid Barriers

by DAVID I. COOK and DAVID F. Van HAVERBEKE, respectively professor of engineering mechanics, University of Nebraska, Lin- coln; and silviculturist, USDA Forest Service, Rocky Mountain Forest and Range Experiment Station, Fort Collins, Colo.

ABSTRACT.-Studies were conducted in suburban settings with specially designed noise screens consisting of combinations of plant inaterials and solid barriers. The amount of reduction in sound level due to the presence of the plant materials and barriers is re- ported. Observations and conclusions for the measured phenomenae are offered, as well as tentative recommendations for the use of plant materials and solid barriers as noise screens.

YOUR$50,000 HOME IN THE SUB- relocated truck routes, and improved URBS may be the object of an in- engine muffling can be helpful. An al- vasion more insidious than termites, and ternative solution is to create some sort fully as damaging. The culprit is noise, of barrier between the noise source and especially traffic noise; and although it the property to be protected. In the will not structurally damage your house, Twin Cities, for instance, wooden walls it will cause value depreciation and dis- up to 16 feet tall have been built along comfort for you. The recent expansion Interstate Highways 35 and 94. Al- of our national highway systems, and though not esthetically pleasing, they the upgrading of arterial streets within have effectively reduced traffic noise, the city, have caused widespread traffic- and the response from property owners noise problems at residential properties. has been generally favorable. Blame for the problem rests with gov- Land forms consisting of earthen ernmental bodies at all levels, and with dikes, also known as acoustical berms, real-estate developers and the general have shown promise. Roadside develop- public in its demand for rapid auto- ments, strategically placed to take ad- mobile access and movement. Rather vantage of natural hills and other topo- than attempting to fix the responsibility, graphic features, reflect good planning however, our objective is to help reduce to reduce noise from passing traffic. the suburban traffic-noise problem, us- Trees, shrubs, grass, and other plant ing combinations of plant materials and materials can significantly reduce cer- solid barriers. tain noises ; and where conditions favor their use, no other means may be THE ALTERNATIVES required. Preliminary planning that includes noise considerations, adequate zoning, HOW MUCH REDUCTION IS NEEDED and setbacks offers the greatest assur- The amount of reduction in noise (at- ance of a relatively noise-free suburban tenuation) needed is determined by the environment. Where the noise problem level at the source and by requirements already exists, however, means are at the receiver. Source noise emitted available for reducing noise to an ac- by a large truck often exceeds 100 deci- ceptable level. bels (dBA) within a few feet of the Ideally, noise is controlled by reducing vehicle. Requirements at the receiver the source level. Lowered speed limits, vary considerably, depending on per- sonal sensitivity, but most. observers HOW MUCH REDUCTION think that noise levels exceeding 70 dBA IS POSSlBLE are objectionable for outdoor daytime With the means considered - plant activities, especially if conversation is materials axid solid barriers - the to be maintained. (A reduction of 10 amount of reduction possible depends dBA means noise is cut about in half.) largely on the height of the structure We favor levels in the 60- to 65-dBA and the density and extent of the plant range for daylight hours, and 50 to 55 materials. Plant materials, by them- dBA for evening hours, when back- selves, are capable of reducing noise ground sounds are quieter, Our ex- levels as much as 8 dBA, and occasion- perience indicates that the majority of ally more. A tall masonry wall or persons living adjacent to a 35-n~ile-per- earthen dike is capable of reducing noise hour arterial street do not object to levels by 15 clBA, when properly placed, noise levels in the 60- to 65-dBA range However, practical considerations such in their backyards. as local ordinances, expense, and ap- To ftxrtlier illustrate the a~lloulltof pearance rarely permit the constr~zc%ion reduction needed, consider a Iiypotheti- of a solid barrier of suscient height cal situation in which tlne noise level of (12 to 20 feet) in residential areas. a large truck is 90 dBA at a point 10 Our earlier studies in rural ayeas, meters from $he center of the tragc where we used con~binationsof 12-foot- lane, We would like to reduce the noise high solid earthen dikes and large dense level to an acceptable 65 dBA at 30 trees, showed tlna"Eeductions of 12 to meters from the same point. Some re- 18 dBA are possible, Results of cur- d~xctionoccurs naturally with distance. rent experiments in suburban areas are Assuming a point source and spherical not yet complete, but it appears that divergence, we would find the level at con~binations of lower solid barriers 30 meters to be abotri 80 dBA. The re- and moderate-size plankings are also mnaining 15-dBA reduction would have effective. to come from some form of barrier in- terposed between source and receiver. SlTE SELECTlON AND PROCEDURE However, neither a belt of tall devise Ten residential properties in an up- trees, nor a solid wall of reasonable per-nniddle-class neighborhood were se- height, nor any combination of the two, lected for study. Backyards of the will provide su%cient attenuation, Thus properties faced a heavily traveled ar- this situation represents a collditjon that terial street with a 35-mile-per-hour can be remedied only by source-level re- speed limit. Screen plantings and duction. It would therefore seem that grouxld-surface configurations varied heavy truela traffc, at its present high considerably from site to site. Three noise-emission level, should be excltzded sites that illustrate the n~osisignificant from suburban residential areas. features, for noise-control purposes, are Consider a second illustration in described here (figs. 1, 2, and 3), which the noise level. of an automobile is A sound-level meter was used to meas- 75 dBA at a point 10 meters from the ure the noise of a vehicle as it passed by center of the traffic lane, The level at a test site, and a second meter was used 30 alieters is then only 65 dBA, which is to measure noise from tlze same vehicle acceptable to many persons for daytime as it passed by an open (control) area outofdoor environmen"c, although un- immediately thereafter. The difference satisfactory for evening hours. We need in the two readings (attenuation) repre- only to reduce the level by as nluch as sented the amount of reduction that 5 to 10 dBA to provide a satisfactory could be attributed to trees and sl~resbs, acotzstical environment in this case. solid barriers, and ground-profile effect. Figure I.--Site 105 as viewed from the street, The procedure was ~epeatedat distances tow, dense, cotoneaster shrubs were backed of 5, 10, 15, and 20 meters filnorn the by +all ponderosa pines, curb. A special site, with relauvely young trees on BTnfversity of Nebraska prop- erty, was also selected to provide addi- tiorla]. flexibility and control fo~L11e experiments, 11el.e a power lawnmower was used as a noise source, and both microphone and source positions wgre varied* A concrete block wall (figs. 4 and 5), the height of wl~ichcould be changed by adding courses, was also eased in cox~~binatioriwith the t~ces.One segn~enhofthe wall was placed in an open area, and an identical segment was placed within the tree belt, Sound levels were nreasured by using three dieerent barrier treatn~elatsand M control. In one case trees alo~aewere located between the noise source and receiver; in a second case the wall alone was used; in the third both trees and wall were used. Source and receiver distances from the wall were varied f~*om5 to 20 metelso Readi~agswere taken at various positions l,o deteranine the effects of wall, trees, and distance in reducing noise levels.

Figure 2.-Si+e 108 as viewed frcm the street: Figure 3,-Site 107 as viewed from +he skeet: medium-heighf psanfings, a woven board .&a11evergreens hido a 3- ts 6-foe+ brick wall, fonee, end a downward-sloping ground progle and an upwsrd-sloping ground profle from from streef +o residence, Figure 4*--Site I I I: Schematic diagram of we!! within bells of trees, SlTE 5 PLATTSMOUTH BELT AND WALL

ROW NO NAME OF TREE I AUSTRIAN PPsNE ALTERNATED WITH ROCKY MT. JUNBPER 2 PONDEROSA PINE ALTERMATED WBTH EASTERN RtDCmAR 3 SCOTCH PINE 4 AUSTRIAN PINE ALTERNATED WITH ORIENTAL ARBORVITAE (TREE SPAC1M6 1N ALL ROWS -- 2 M)

Figure 8,-Si#e I I I: to , Qe sound-barrier walJ; bo++om, the Lelk o%trees. Figure &.---Site? 105: profile of site, Figure 8.-Sih 1107: pro6le of site, noise level, and attenuation, noise level, and at9enuation.

SITE 105 CROSS SECTION SITE !07 CROSS SECTION

AUSTRIAN PINE SCOTS PiNE AN0 AUSTRIAN PIME COTONEASTE I .x ", g : 8i $=

15 20 *---C----i- CURB 10 MICROPHONE POSITION, METERS 10 15 20 '

0 5 I0 15 20 25 D1STANCC PROM CURB, METERS = 0 10 25

MaloMI-B1 0 3 10 15 20 25 DISTANCE FROM CURB, METERS DISTANCE FROM CURB, METERS

Figure 7.-Sife l08: profile of site, noise level, and aHsnuafion,

SITE 108 CROSS SECTION

RESULTS OF EXPERIMENTS Suburban Sites Noise reduction characteristics of the 0 5 10 15 20 three sites are sl~owrz graphically in CURB TO MICROPHONE POSITION, METERS figures 6,7, and 8. Curves are positioned directly bebow the schema"cc cross-sec- tion drawings soAthe points denoting sound level and attenuation line up with the corresponding microphone positions, All readings are in dBA units.

Tree-Farm Site

0 5 lo 15 2 0 2 5 Variation of sound level with distance DISTANCE FROM CURB, METERS and attenuation for the diff erent surface treatments is shown in %gelre 9, Tbe fu11 height (1.6 rnete~)wall and the 10- meter wall-to-microphone distance were 0 5 10 16 20 25 used in this instance, DISTANCE FROM CURB, YETERS The effect of different wall l~lacements --between source and receiver-orr the Figure 9.-Sound level and attenuation with different surfaces.

80 - I I I COMPARATIVE NOISE REDUCTION OF WALL, TREES, AND TREE- WALL COMBINATION SOUND LEVEL VERSUS DISTANCE 70 FROM NOISE SOURCE I

a71 60 > W n 0 Z TREES & WALL 50-

SITE 5 10 M. NOISE SOURCE DISTANCE FROM WALL TEST 268-72 1.5 M. MICROPHONE HEIGHT JULY 13, 1972 1.8 M. WALL 40- ,

REES & WA

DISTANCE FROM WALL, METERS

Figure 10.-Effect of wall placement.

20 I I I WALL PLACEMENT EFFECT ON NOISE REDUCTION TREE-WALL COMBINATION a m RELATIVE TO GRASS SURFACE i 15. 0 aF 1.6 M. WALL HEIGHT z3 lo-( z lo-( b 2 SITE 5 ) TESTS 27-A,8, & C $ JULY, 1972 du I I 01 I I I 0 5 10 15 20 RECEIVER DISTANCE FROM WALL, METERS Figure I I.-Effect of wall height. - I I I WALL HEIGHT EFFECT ON NOISE REDUCTION TREE-WALL COMBINATION I RELATIVE TO GRASS SURFACE I

SITE 5 TESTS 27,28, a 2s JULY, 1972

TREES ONLY

5 10 15 20 RECEIVER DISTANCE FROM WALL, METERS attenuation obtainable is shown in figure close to the traffic lane, additional pro- 10. tection in the form of a higher solid The effect of walls of different height barrier, such as an earthen dike, solid combined with trees is illustrated in masonry wall, or tight wooden fence figure 11. A trees-only curve has been may be necessary. included for comparison. It also appears that there is no prac- tical means for reducing noise from OBSERVATIONS AND CONCLUSIONS large trucks to a level acceptable in Results of the current series of sub- suburban areas. The source itself must urban experiments are not fully an- be brought under control. alyzed at this time, and further experiments under controlled conditions RECOMMENDATIONS are scheduled for next summer. Al- 1. To reduce noise from suburban auto- though our conclusions must be some- mobiles and light trucks to an accept- what tentative, the general pattern of able level where the residence is at results is consistent and reliable. least 25 meters from the centerline At some of the sites, it appears that of the roadway, plant one or two con- existing natural barriers, combined with tinuous rows of dense shrubs as close additional plantings and some form of a to the curb as possible, and one or low solid barrier, are adequate for the two continuous rows of dense trees control of traffic noise from 35-mile-per- behind the shrubs. One or both plant- hour automobiles (figs. 6,7, and 8). For ing~should be of evergreens for year- similar suburban sites, continuous plant- round protection. ing~of dense shrubs backed up by taller 2. Where immediate relief from traffic trees, as in the case of site 5 (figs. 1 and noise is desired, erect an earthen 6) are capable of providing sufficient dike, masonry wall, or solid wooden protection from automobile and light fence. The height should be sufficient truck traffic noise. to screen the noise source from view Where individuals are more sensitive at the location to be protected. Land- to noise, or where the residence is rather scaping should be included to provide additional protection, when the trees 3. Where the residence is less than become larger, and to decrease the about 20 meters from the centerline reflection from the hard wall surface of the roadway, both trees and a solid back across the street. barrier are necessary.