NBSIR 74-517

Methodology and Supporting Documentation

for the Measurement of Noise from

Medium and Heavy Trucks

W. A. Leasure, Jr.

and

T. L. Quindry

Applied Acoustics Section

Mechanics Division

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

June 1974

Final Report

Prepared for Office of Noise Abatement and Control

U S Environmental Protection Agency

Washington, D C. 20460

NBSIR 74-517

METHODOLOGY AND SUPPORTING DOCUMENTATION

FOR THE MEASUREMENT OF NOISE FROM

MEDIUM AND HEAVY TRUCKS ^

W. A. Leasure, Jr.

and

T. L. Quindry

Applied Acoustics Section

Mechanics Division

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

June 1974

Final Report

Prepared for Office of Noise Abatement and Control

U. S. Environmental Protection Agency

Washington. D. C. 20460

U. S. DEPARTMENT OF COMMERCE, Frederick B. Dent. Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts, Director

Table of Contents

Page*

Executive Summary ii

Acknowledgements ,. iii

1. Introduction 1

2. Technical Backup 1

2.1- Product Classification 1

2.2- Normal Use Conditions (and Range) 2

2.3- Effects of Noise and Parties Affected 3

2.4- Major Truck Noise Sources 4

2.5. Rationale for Methodology 4

2.5.1. Methodology Development 7

2.5.1.1. Acoustic Environment ... 3

2.5.1.2. Instrumentation 13

2.5.1.3. Vehicle Operation lA

2.5.1.4. Measurements 15

2.5.1.5. Measurement Uncertainty 16

3. Medium and Heavy Duty Truck Measurement Methodology 17

3.1. Purpose and Applicability 17

3.2. Acoustic Environment 17

3.2.1. Test Site 17

3.2.2 Weather Conditions 18

3.2.3. Background Noise 18

3.3. Instrumentation •. • 19

3.4. Vehicle Operation 19

3.5. Measurements ................. 20

3.6 Data to be Recorded 20

4. Appendix A. A Discussion of Truck Subsource Noise 21

5. Appendix B. Discussion of Alternative Low Speed Acceleration Measurement Procedures 24

6. Appendix C. Projection Method for Predicting Significant Fan Operating Time . . 26

7. Appendix D. Research Needs .............. 27

3. References 27
Executive Summary

Vehicular traffic noise continues to be a major source of complaint and many consider trucks to be the major offender. Truck noise can be categorized as the noise produced by the power plant [including the engine, exhaust, intake, cooling fan, etc.] and by the tire-road interaction. The noise

from the power plant increases as the engine speed (and power) increases while the noise from tires in- creases as the vehicle speed increases. Maximum power-plant noise usually occurs when the engine is delivering maximum power at its maximum operating speed. The percentage of time a vehicle spends at maximum noise conditions is determined by the vehicle power-to-weight ratio and operating conditions

such as acceleration, speed, load, grade and, to some extent, the manner in which the vehicle is driven.

Medium and heavy trucks [GVWR of 10,001 pounds or greater] operate near their horsepower limit a high

percentage of the time. The exact speed at which the tire-roadway noise starts to predominate power- plant-associated noise is a highly complicated function of tire characteristics, engine-exhaust characteristics, road surface, vehicle condition, etc.

In general, the truck manufacturer's product is a two- or three-axle prime mover equipped with

six or ten wheels. The "truck" is built up by the addition of van enclosures, platforms and stakes,

dump bodies, concrete mixer bodies, semi-trailers, multiple combinations of trailers and many other

types of equipment. The types and numbers of tires are dependent on the use and loads which the purchaser determines and the equipment which is later installed on the truck.

The EPA is charged with taking strong, comprehensive action to protect public health and welfare from increasing noise, thus ensuring a quieter environment. In the case of trucks, it is the consensus of the NBS and the members of its task force that the following regulation strategy, which

is based on a systems approach to the truck noise problem, will ensure a decrease in community noise.

•Identify trucks and tires separately as major noise sources.

•Establish a level, or set of levels, in conjunction with an appropriate measurement

methodology, to be met by the truck (prime mover) manufacturers.

•Establish a level, or set of levels, in conjunction with an appropriate measurement

methodology, to be met by the tire manufacturer.

•Label tires in order that the user can choose tires for his single-chassis

and combination vehicles which will maintain or reduce overall truck noise and

will meet Federal interstate motor carrier regulations or State regulations.

If such a strategy were followed, a truck could be certified for sale and a subsequent tire change

(maybe upon initial sale of the vehicle) or changes in the type ^of trailer pulled, would not result

in an increase in community noise levels.

This report develops a measurement methodology which defines the procedure by which the noise

from medium and heavy trucks, excluding tire noise, may be accurately and repeatably measured to ascertain whether the noise emitted is in compliance with noise-emission standards to be promulgated by

the EPA.* The methodology is based on a low-speed acceleration passby (high engine speed, low vehicle

speed) procedure similar to that specified in SAE Standard J366b - Exterior Sound Level for Heavy

Trucks and Buses. Numerous modifications and additions have been made in order to reduce variability

and improve repeatability of the test results. It should be pointed out, however, that this test procedure has some limitations. Since the test is performed outdoors, it is weather dependent. If large numbers of vehicles are to be tested, severe production holdups would result. Passby tests re- quire considerable space (which makes location of suitable test sites difficult for some manufacturers)

and cannot be easily performed at the end of an assembly line. The major advantage of this procedure

is, of course, that it is the only existing test method that is backed with an extensive data base.

Alternative test procedures such as the stationary run-up test, dynamometer test or indoor

tests — which would eliminate some of the difficulties, associated with the passby test — were considered; however due to the time constraints on EPA and the lack of a data base and facilities, these methods could not be considered further at this time. This is not to say that there should not be a concentrated effort in the future to develop a simple, repeatable, accurate test procedure that would

be independent of weather and site influences and would correlate with what the community hears as "truck" noise. Such a procedure, if properly developed and validated, would be welcomed by both

government and industry.

*A methodology that could be utilized for tire certification is presently being developed by DOT and NBS and will be available in final form in the very near future.

ii
Acknowledgements

The National Bureau of Standards (NBS) gratefully acknowledges the assistance of many individuals

and organizations whose contributions have helped make it possible to produce this report.

Of great assistance to the NBS was the task group of experts assembled by NBS to provide technical expertise and back-up data — both acoustic and vehicle. The task group included: Mr. John U. Damian (Ford Motor Company), Mr. Frederick W. Krey (GMC Truck and Coach), Dr. Ben H. Sharp (acoustical

consultant, Wyle Laboratories), and Mr. Richard L. Staadt (International Harvester Company).

In addition, between March 19, 1974 and April 2, 1974, a series of meetings was conducted by

NBS with representatives of truck manufacturers to obtain input prior to the completion of this report.

During these meetings the existing data base, typical operational modes and use statistics were reviewed and the problems associated with the technical capability of industry to utilize particular

measurement methodologies and the possible economic impact associated with alternative measurement re- quirements was discussed. Truck manufacturers visted included: F.W.D., Clintonville , Wisconsin;

Freightliner , Portland, Oregon; Pacific Car and Foundry (Kenworth and Peterbilt) Seattle, Washington;

White, Cleveland, Ohio; Ford, Dearborn, Michigan and Louisville, Kentucky; Chevrolet/GMC , Pontiac,

Michigan; and International Harvester, Fort Wayne, Indiana.

We thank all of these people for their cooperation and contributions.

ili

Methodology and Supporting Documentation for the Measurement of Noise from Medium and Heavy Trucks

W. A. Leasure, Jr.

T. L. Quindry

This report contains a measurement methodology which defines the procedures by which

the noise from medium and heavy duty trucks, excluding tire noise, may be measured to ascertain whether the noise emitted is in compliance with noise emission standards to be

promulgated by the U. S. Environmental Protection Agency pursuant to Section 6 of the Noise Control Act of 1972 (P.L. 92-574). Associated supporting technical documentation

for the methodology is included.

Key Words: Acoustics (sound); measurement methodology; noise emission standard;

noise measurement; truck.

1. Introduction

At the request of the U. S. Environmental Protection Agency (EPA), the National Bureau of Standards (NBS) , with the cooperation of industry, has developed a measurement methodology which

defines the procedure by which the noise from medium and heavy duty trucks, excluding tire noise, may be measured to ascertain whether the noise emitted is in compliance with noise emission standards

to be promulgated by the EPA pursuant to Section 6 of the Noise Control Act of 1972 (Public Law 92-

574) .

The method is applicable to complete and incomplete trucks and truck tractors (as defined in Section 2.0) having a gross vehicle weight rating (GVWR) of 10,001 pounds or greater. The measurement

method provides for the determination of the maximum A-weighted sound level output of a vehicle during a low speed acceleration passby (high engine speed, low vehicle speed) as measured in the freefield over a reflecting plane. It should be noted that tire noise is negligible at low vehicle speeds.

The methodology is based on a consideration of truck operational modes and the noise emission

characteristics associated with each mode. The methodology includes the method in which the vehicle is

to be operated during testing, a complete test site description and other necessary descriptors to ensure a reliable and repeatable test.

A detailed discussion of the rationale for selection of this particular measurement methodology

is presented in conjunction with associated supporting documentation.

2. Technical Backup

Webster's definition of the word "truck" is as follows: "An automotive vehicle for hauling loads

along highways, street, etc." The Federal Highway Administration, U. S. Department of Transportation,

Motor Safety Regulations [1]^/ make a distinction between "truck" and "truck tractor." A truck is defined

as "any self-propelled motor vehicle, except a truck tractor, designed and used, or exclusively used

whether or not so designed, for the transportation of property." A truck tractor, on the other hand,

is defined as "a self-propelled motor vehicle designed and used primarily for drawing other vehicles

and not so constructed as to carry a load other than a part of the weight of the vehicle so drawn."

It is also important to review the definition of "completed vehicle" according to the National

Highway Traffic Safety Administration, U. S. Department of Transportation[ 2] . A completed vehicle is defined as "a vehicle that requires no further manufacturing operations to perform its intended

function, other than the addition of readily attachable components, such as mirrors or tire and rim

assemblies, or minor finishing operations such as painting." Since most "truck" manufacturers do not manufacture truck bodies and since "truck tractor" manufacturers do not mount the fifth wheels (devices used principally to connect a truck tractor to a semitrailer) in all cases, the majority of "trucks" and "truck tractors" coming off the assembly line do not meet the criterion to be called

complete vehicles. Such vehicles are termed "incomplete vehicles" and are defined as "an assemblage

consisting as a minimum, of frame and chassis structure, power train, steering system, suspension

system, and braking system, to the extent that those systems are to be part of the completed vehicle,

that requires further manufacturing operations, other than the addition of readily attachable components, such as mirrors or tire and rim assemblies, or minor finishing operations such as painting to become a completed vehicle" [2]. For the purposes of this report, complete and incomplete trucks and

truck tractors will be considered and will be referred to simply as trucks.

2.1. Product Classification

Before we consider the problem further we must develop a reasonable product classification

breakdown, i.e., we must be able to define exactly what we mean when we speak of medium and heavy

trucks. Since trucks are designed to haul loads, a classification can conveniently be made according

to the load a truck is capable of handling. The logical breakpoint between light trucks and medium and

\J Numbers in brackets indicate the literature references at the end of this report.
heavy trucks appears to be a gross vehicle weight rating (GVWR)2./ of 10,001 pounds. Trucks with a GVWR

of 10,000 pounds or less (termed light trucks) include 1/2, 3/4 and 1 ton pick-up and van vehicles which

have recently become very popular for personal use. According to statistics of the Motor Vehicle

Manufacturers Association [4], about 74 percent of the light trucks in use today are personal vehicles.

These trucks use gasoline engines (exclusively) very similar to those used for automobiles and operate

below their horsepower limit a vast majority of the time. Trucks with a GVWR of 10,001 pounds or greater (termed medium and heavy trucks) utilize both gasoline and diesel engines and because of their

characteristic weight/horsepower ratio, they operate near their horsepower limit a high percentage of the time. Trucks in this category are utilized nearly 100 percent for commercial purposes.

Medium-duty trucks are popular for short haul jobs as in agriculture, Intracity delivery, and

construction. These vehicles are predominantly powered by gasoline engines (see Table 1) . The heavyduty trucks are usually diesel powered and are generally designed to haul semitrailer, full trailer or combinations of trailers. Dump trucks and cement mixers are also included in this category. Despite

the vast amount of freight moved by trucks (motor trucks accounted for 78 percent of the 1971 freight

dollar) from manufacturers, only 30 percent of truck trips are between cities. The great majority are

local in nature.

Table 1. Truck sales as a function of weight category and type of fuel for 1973 [5]

.

MEDIUM HEAVY

CLASS

GVWR

(Pounds)

3 10,001-

14,000

4 14,001-

16,000

5 16,000-

19,500

6 19,501-

26,000

7 26,001-

33,000

8 Over

33,000

Gasoline

Diesel

Total

44,724

44,724

7,181

296

7,477

18,921

20

18,941

198,404

4,896

203,300

25,006

17,194

42,200

19,937

145,983

165,920

PERCENT

GASOLINE 98% 22%

PERCENT

DIESEL 2% 78%

2.2. Normal Use Conditions (and Range)

In order to develop an appropriate, reliable and repeatable test or series of tests, it is necessary to consider the range of possible truck operational modes. The general operations discussed

above such as delivery, long haul, etc., are not of sufficient detail for the purpose of defining

normal truck operation. What is needed is definition of the percentage of time which a truck typically

spends at idle, accelerating, decelerating, at low speed cruise, at high speed cruise, at wide open

throttle, etc. Such typical usage data do not presently exist for trucks.

A program known as the CAPE 21 Study (sponsored jointly by the U. S. Environmental Protection

Agency and the Coordinating Research Council, Inc.) is presently underway to define typical truck

operation. Although the primary objective of the study is to define typical truck cycles for air

emission purposes, the defined typical cycle might also be applicable in the noise arena. A knowledge

of the percent of time spent in each operational mode coupled with the noise level generated during

that mode would provide guidance as to the appropriate operational modes which the measurement

methodology should specify.

2/ The gross vehicle weight rating (GVWR) is defined [3] as the value specified by the manufacturer as the loaded weight of a single vehicle. This should not be confused with

the gross combination weight rating (GCWR) which is defined as the value specified by the manufacturer as the loaded weight of a combination vehicle.

2
Phase I of the CAPE 21 Study has been completed and one report [6] has been prepared which presents data collated from various sources which identify the composition, function and travel behavior

of urban truck travel in New York City. A similar report is presently in preparation for the Los Angeles Basin area. From these data, various stratifications have been organized to permit the characterization of mission, function and use patterns of trucks operating over urban streets.

On the basis of these data, a sample of 50 medium and heavy trucks (gross vehicle weight

rating of 10,001 pounds or greater), representative of the total truck population in these urban areas, has been selected. The New York sample includes 10 percent diesel ' powered vehicles and 90 percent

gasoline powered vehicles. In Los Angeles the mix is to be 40 percent diesel and 60 percent gasoline.

The trucks are to be instrumented and operated in normal fashion to collect detailed operating profile data of truck operation. The data will include recordings of engine torque, engine temperature,

throttle-valve closure and other parameters. Additionally, the onboard observer will be able to input into the instrumentation the road type — expressway, arterial roadway, local street — and

prevailing traffic conditions manually. One data scan of the measured variables is to be accomplished

each 0.8 second. The data is converted from analog sensor voltages to digital output in binary coded

decimal (BCD) form and recorded on magnetic tape for later data processing and statistical analysis.

The plan is for the instrumented vehicles to be put into normal service by its owner /opera tor and monitored for one to five days. A total of 172 truck-days of data are to be accumulated in each

area. The resultant data will provide truck operating profiles, or usage patterns, broken into

periods of truck operation such as acceleration, cruise, deceleration, idle, etc. Although the typical truck operational cycle resulting from this study will have been developed for air emmisions

purposes and is restricted to only urban areas, the progress of the CAPE 21 Study should be closely

monitored.

One could assume that medium duty trucks spend a different percentage of their time operating

at or near full power and at engine speeds near maximum than heavy duty trucks or that gasoline powered

trucks operate differently than diesel powered trucks and therefore different test procedures might

be considered appropriate to each class. Upon further investigation, however, a logical breakpoint

separating medium and heavy duty trucks on any basis other than operation is difficult, if not impossible to define. For example, the breakpoint based on GVWR and that based on engine type are different as summarized in the following chart. International Harvester [7] —' for example, offers

diesel engines in their 21,200 pound GVWR cab-over Cargostar model and 19,700 pound GVWR Loadstar

1750 model while also offering a gasoline engine in their 54,000 pound GVWR Fleetstar vehicle.

Diesel.

Gasoline

.

Medium i | i Heavy

10,001 pounds 26,000 pounds 33,000+ pounds GVWR GVWR GVWR

Since at the present time the data are not available to distinguish between medium and heavy

duty trucks according to normal operation considerations, the same measurement methodology should be

applied to all trucks of 10,001 pounds or greater. When the results of the CAPE 21 Study are available, it would be appropriate to see if there is any basis for modification of the present thinking.

2.3. Effects of Noise and Parties Affected

It is also important to identify the parties affected by truck noise and the nature of the effects. Operators and passengers are affected and the nature of the effects range from annoyance

and task interference to possible hearing loss. Although hearing loss is not a potential problem

for neighbors and bystanders near highways due to truck traffic, they certainly can have their tasks

interfered with or be annoyed.

The interior truck noise problem is, in general, covered under the Bureau of Kotor Carrier

Safety Regulations [8] which establish a maximum interior sound level for commercial trucks (trucks

of 10,001 pounds GVWR or greater are nearly 100 percent commercial) operated in interstate commerce.

For this reason, the methodology to be developed will be applicable only to the exterior noise of vehicles which contributes to community noise levels

.

_3/ Commercial vehicles are identified in order to adequately define the basis for the examples cited as part of the technical backup in this report.

3
2.4. Major Truck Noise Sources

It is important to identify the major sources of noise in trucks and the effect of normal

operation on the noise levels associated with each of these sources. Basically the sources are as follows:

• Engine (mechanical) Noise

• Fan Noise

is primarily produced by the combustion process which

produces rapid changes in pressure in the cylinder which in turn results in excited mechanical vibrations in the engine structure. Some of this vibrational energy is subsequently radiated to the atmosphere as acoustic energy.

is the result of air flow past the fan blades which

results in rotational (associated with pressure differences across the fan blades) and turbulent noise (associated with the eddy flows and vortices shed at the blade

edges) . Such noise is related to fan tip speed and air

distribution which is governed by the fan configuration

and environment.

'Exhaust Noise

•Air-Intake Noise -

•Tire-Road Interaction Noise

• Transmission Noise

•Noise From Accessories

is created when high pressure exhaust gases, released

through the engine exhaust valves, excite oscillations

in the exhaust system which are radiated to the atmosphere at the tail pipe, through the pipe surfaces

and through the muffler shell. The noise is a function of engine type, engine timing, valve duration time, intake

system type, muffler type, size and location, pipe diameter,

pipe bends, dual Or single system, etc.

is created by the opening and closing of the intake valve

which causes the volume of air in the system to pulsate.

Associated noise levels are dependent upon whether the engine is gasoline or diesel, turbocharged or naturally

aspirated, 2-cycle or A-cycle the number of cylinders, the engine displacement, engine rpm, engine load, etc.

is related to tire and tread configuration, highway road

surface, vehicle speed, load and state of tire wear. Vibration of the tire carcass and the entrapment and release

of air from tread pavement interstices (air pumping) appear to be the major mechanisms for tire noise generation.

arises from the meshing of gear teeth in transmission

systems and may be apparent under load, no load, or reverse load conditions. The noise may be aggravated

by resonance of the gearbox casing or by structure born

components that excite large body surfaces that form

efficient radiators.

Although subjectively noticeable to the pedestrian,

gear noise components are unlikely to contribute significantly to objective measurements of emitted noise at the present time; however, as noise levels of other sources

are reduced in the future, transmission noise could become

a problem in certain vehicles.

is created by blowers, brakes (squeal, air brake release),

generators, air conditioning equipment or miscellaneous

equipment such as stake racks, tie down chains, tailgates,

etc.

Appendix A contains a more detailed discussion of the component noise sources.

2.5. Rationale for Methodology

There is no such thing as a "typical" truck from a noise source standpoint. Many component

combinations, which can affect the noise output of a given vehicle, exist in trucks due to the need

to perform various tasks in different geographical and political climates. Such requirements as
maximum allowable truck width, truck length, truck height, axle weights, etc., which vary from state to state represent political constraints. Owner preference also has a significant impact on final truck

configurations since the owner specifies the options. To establish the complexity of the problem the following example is developed. The International Harvester Company[9] markets six basic models having

a GVWR of 10,001 pounds or greater — Fleetstar, Loadstar, Paystar, Conventional Transtar, Cab-Over

Transtar, and the Cargostar. With the exception of the Cargostar (for which data were not compiled),

4298 options which can have an effect on noise levels are purchasable in the remaining basic model lines.

To explain the basis for these numbers a discussion of the Fleetstar options is presented. For

example, 360 combinations are possible for the Fleetstar 2050 series. This series has four engine

choices [IH diesels - DV550 (200 HP) and DV550 (180 HP), Cummins V555 and Caterpillar 1160], two transmission options [manual or automatic], five exhaust configurations, a choice of two hood types

[fiberglass tilt hood or a steel butterfly hood], and can be purchased with or without shutters and a standard radiator or one designed to accommodate a power take-off. These options represent 320 combinations. In addition, two of these engines — the DV550 (200 HP) and the DV550 (180 HP) — are also available in eight special vehicles designed to haul steel; thus, an additional sixteen combinations exist. An additional engine (Detroit Diesel 6V53) is offered with three injector configurations,

two hood options, two exhaust options and the choice of a standard radiator or one designed to accommodate a power take-off. This adds another 24 combinations. Therefore, a total of 360 combinations exist for the Fleetstar 2050 series. The Fleetstar 2010 series has 293 combinations while the 2070 series has 140 combinations. Similar calculations for the other models yield the following

results: Loadstar, 2700 combinations; Paystar, 369 combinations; Conventional Transtar, 62 combinations;

and Cab-Over Transtar, 374 combinations. This is not to say that all of the 4298 combinations discussed

in this example are sold, but very few trucks built are identical.

Even though an enormous number of different trucks exist on the market, the major noise in trucks

can be categorized as the noise produced by the propulsion system (including the engine, exhaust,

intake, cooling fan, etc.) and by the tire-road interaction. The noise from tlie propulsion system

increases as the engine speed increases while the noise from tires increases as the vehicle speed

increases (see Figure 1)

.

On this basis, one must consider the possibility of (1) regulating the total truck, including

tires or (2) regulating the truck and tires separately. The proposed methodology in this report is a variation of the second of these alternatives.

•Identify trucks and tires separately as major noise sources.

•Establish a measurement methodology and a level or set of levels to be met by the truck (prime

mover) manufacturer.

•Establish a measurement methodology and a level or set of levels to be met by the tire

manufacturer

.

•Label tires in order that the user can choose tires for his single-chassis and combination

vehicles which will reduce the overall truck noise and will meet Federal interstate motor

carrier regulations or State regulations.

The benefit from regulating the total truck, including tires, is simplicity; however, pursuit of this option will not ensure a decrease in overall noise levels since the high speed noise generated by

these vehicles depends on the number of tires on the road, the tread design and state of wear, the load,

vehicle speed and highway pavement surface — variables which are out of the control of the truck (prime

mover) manufacturer. If the entire truck were certified, the owner, under Section 10 requirements of the Noise Control Act of 1972, would be restricted to the purchase of identical tires or tires that were

known to produce lower noise levels. Without a labeling system, this knowledge will not exist. Also

it is the fleet owner who decides what tires are mounted on the tractor and trailer. These tires will

dictate the high speed noise levels produced by the combination vehicle if the truck is fairly well

maintained and has an adequate exhaust muffler. Without a knowledge of typical tire noise levels (which

would be known if tires were identified as a major noise source and were included in a labeling program)

the fleet owner must base his decision on experience and little data. An incorrect decision on his

part could result in an expensive citation. Also, on what basis does the individual tractor owner decide whether or not he will be in violation of Federal interstate motor carrier regulations or State regulations when he agrees to pull a given trailer? He must be provided tire noise information to make

such a decision since (1) he may pull a different trailer every trip and (2) he does not decide what

tires are placed on the trailer. In addition, the following reasons form- the basis for our recommendation that the tires be handled separately from the remainder of the truck's components:

•The purchaser may send the truck manufacturer new or used tires to be mounted on his vehicle or may specify the tires which should be purchased.

•If the manufacturer mounts "quiet" tires on the vehicle, at the time of delivery, he has no control over the purchaser who might immediately replace these tires with others more suited to his needs.

5
VEHICLE SPEED, mph

TIRE NOISE

STEERING AXLE DRIVE AXLE TRAILER AXLE

o

NEW RIBS 1/2 WORN

X-BARS

NEW POCKET RETREAD

NEW RIBS NEW

X-BARS

NEW RIB RETREAD

NEW RIBS NEW RIBS NEW RIB RETREAD

Figure 1. Engine related and tire noise for an 18-wheel tractor-trailer as measured

50-feet from the centerline of the lane of travel of the vehicle[10].

6
• The noise generation mechanisms are totally different for truck tires than they are for the propulsion system. Propulsion system noise is engine rpm dependent. Tire noise is road

surface and vehicle speed dependent.

•In the case of the truck tractor, the truck manufacturer does not sell trailers and he does

not know the type of trailer which his tractor will pull, the number of axles nor the number of tires

•In the case of single-chassis vehicles, the truck manufacturer may or may not know the type of body which the truck will have (which would define in general terms the typical usage) and cannot control the addition of tack-on axles, a practice which is quite common with single-chassis

vehicles (depending on state load/axle restrictions).

•Tire noise is load dependent and generally trucks cannot practically be loaded at the factory

since they have no bodies (in the case of single-chassis vehicles) and may not have fifth wheels

(in the case of truck tractors) by which the load can be carried or pulled, respectively.

•Historically, original equipment manufacturers equipment is utilized in replacement truck parts

with the exception of tires. An owner never returns to the truck manufacturer to buy tires or for a recommendation as to which tires to buy.

•The breakdown of shipments of new truck tires in 1973 indicates 36.7 percent were for original

equipment, 62 percent were for after-market replacement and 1.3 percent were for export[ll].

It should be noted that these figures include tires for trailer use. When one takes into con- sideration the number of retreads on the road, it is obvious that the number of original equip- ment tires on the road is small. So, regulating total truck noise would not be a very effective

means for controlling tire noise.

•It is possible to buy tires by specification in the safety area but the equivalent is not available in the noise area.

•There exists a practical restriction on tire noise testing in that quite a bit of room — on the order of 1.5-2 miles for a fully loaded combination vehicle — is necessary to allow a truck to attain speeds of 50-60 mph for a high speed certification test and then be able to stop the vehicle safely.

Therefore, the remainder of this report will concentrate on the development of measurement

methodologies appropriate for the determination of the noise produced by the propulsion system. A

methodology that would be appropriate for tire certification is presently being developed by DOT/NBS

and will be available in final form in the very near future. Appendix B contains a discussion of alternative methodologies that were evaluated prior to the selection of the low speed acceleration

test procedure discussed below.

2.5.1. Methodology Development

The existing Society of Automotive Engineers (SAE) test method for measuring the noise produced

by motor vehicles (SAE J366b[12] for medium and heavy trucks) was designed to extract the maximum

noise that can be produced by the propulsion system without considering the problems of tire noise.

Accordingly, the tests are conducted at high engine speed and fairly low vehicle speed. The noise

level produced during such a test represents the worst-case noise level for a truck operating under

urban driving conditions. Such a test would, in addition, represent the worst-case propulsion-system

noise contribution to the over-all noise levels produced by the vehicle at high speeds.

An extensive data base utilizing this test method exists today. A similar data base does not

exist at the present time for alternative methodologies that might be utilized for measuring the noise

of medium and heavy trucks. Alternative methods that specified stationary test modes rather than

passbys and allowed for measurements that were not weather dependent would certainly be attractive and

should be pursued in the future; however, the present legislative time constraints under which EPA must

operate coupled with the lack of data for these alternative methods does not allow for their consideration at this time.

Although the major concepts of SAE J366b have been adopted as the basis for the methodology

described in this report, modifications have been made in an effort to decrease the site-to-site

variability due to test site and environmental considerations and to increase the repeatability of the

test itself. Major emphasis will be placed on the modifications and the rationale for them.

The present SAE J366b procedure establishes the method for measuring the maximum exterior sound

level for highway motor trucks, truck tractors and buses. Test site requirements limit measurements

to a flat open space, free of large reflecting objects within 30 metres of either the vehicle path or

the microphone. The measuring microphone is located 15 metres from the centerline of the vehicle path.

The vehicle approaches the microphone in a gear ratio selected such that at a point (acceleration point)

7
15 metres before the microphone, the vehicle is at no more than two-thirds of maximum rated or governed

engine speed. At this point the vehicle is accelerated at wide open throttle such that maximum rated

or governed engine speed is achieved between 3 and 15 metres (the end zone) beyond the microphone

without exceeding 56 km/hr at the end point.

* 2.5.1.1. Acoustic Environment

2. 5. 1.1. a. Test Site

Modifications have been made to the existing test site requirements contained in SAE J366b. The

proposed modifications are aimed at decreasing the variability of the test results due to test site,

meteorological and operational factors without invalidating the existing data base.

Vehicle position within the present end zone — that area in which the vehicle is to reach maximum

rated or governed engine speed — is a significant variable which is affected by vehicle configuration,

driver technique, etc. To gain a clearer understanding of how the component noise sources contribute to total truck noise during a J366b test, consider the following illustrative example, which might be

considered typical for a fan dominated vehicle. For this particular truck, the exhaust and engine

noises are about equal and dominate the total truck noise for about the first 11 metres. The engine

noise levels off and remains essentially constant for the remainder of the passby. Exhaust noise

continues to increase, reaching a maximum about 6 metres beyond the microphone point. The exhaust is the predominant source during the central portion of the drive-by. The cooling fan speed has been

increasing throughout the test and near the end of the end zone it reaches maximum speed where it also

produces maximum noise. At this point, the contribution of the other sources has decreased, and the cooling fan is the predominant source. It is here where the total truck noise also reaches a maximum.

Thus, it is evident that variation in overall truck noise according to SAE J366b procedures can be

expected depending on whether maximum rated or governed engine speed is reached near the beginning or

end of the end zone

.

Mathematically, assuming a constant intensity omni-directional point source in a free field, one would predict a maximum variation in A-weighted sound level of 2.9 dB over the extremes of the present

12.2 metre end zone. Measurements [13] made with a Ford LNT-9000 truck equipped With a Cummins NTC-335

diesel engine showed a 2.3 dB variation in A-weighted sound level on the basis of whether the vehicle

reached governed engine speed at the beginning or end of the end zone. For this reason, the end zone

has been reduced to 4.6 metres. Practical reasons associated with a truck reaching maximum rated or governed engine speed in a limited end zone precluded shortening the end zone further. Since the end

zone is a fixed location and shorter than that specified in J366b, the acceleration point must now be

allowed to float within an acceleration zone located 11.4 to 19 metres before the microphone point

(rather than being fixed at 15 metres as in J366b) so that a beginning speed and proper gear ratio can

be determined which will ensure that a vehicle reaches' maximum rated or governed speed within the end zone.

The question still remains as to the proper location of the shortened end zone (4.6 metres in

length) within the limits of the end zone which is presently specified in J366b (12.2 metres in length).

The consensus of the NBS task force is that the shortened end zone should be located approximately in

the middle of the existing zone; in other words, the present end zone would shrink 3.8 metres on either

end. At first it was thought that the optimum location would be at the beginning of the existing end zone (closest to the microphone location); however, limited data indicated that the sound level measured

under this configuration would be lower than that measured according to J366b procedures. Obviously

comparative data between this newly developed procedure and J366b procedures are needed with special

emphasis paid to determining the optimum location of the shortened end zone to ensure the preservation

of the extensive existing data base.

At present a single microphone location is utilized and no readings are made after the truck has

left the end zone. Before accepting this concept two questions need to be addressed: (1) Is there a need to replace the single microphone with an array of microphones located parrallel to the path of the vehicle and simply read the maximum noise level that occurs during a passby regardless of microphone location? (2) If a single microphone is sufficient, is the presently specified location

appropriate?

In order to address the first question, data previously taken by NBS [14] according to J366a (the

changes made between J366a and J366b are mainly editorial in nature) procedures utilizing an array of microphones have been reevaluated. Figure 2 shows a view of the test section and placement of all

microphones in the array. In addition to the J366 location (microphone number 3), four additional

microphones were located along a line parallel to the vehicle path at locations 4.6 metres and

9.2 metres on either side of microphone number 3. As shown in Figure 2, the microphones are located on

the right side of the vehicle. A mirror image provides the proper view for left side measurements;

that is, microphone number 1 is nearest the acceleration point regardless of the direction of travel.

8
ACCELERATION

POINT

M 30.5 m

ZONE IN WHICH

TO REACH MAX

RATED RPM

END

POINT

• PHOTOSENSORS

A MICROPHONES

Figure 2.

4.6 m-PA

M4 M5

view of test section showing placement of microphones (M1-M6) , photosensors (P1-P9)

,

acceleration point, end point, microphone point, and the zone in which the maximum rpm

is to be reached[lA].

Table 2 shows the results of subtracting the sound level observed at the J366 microphone from those

observed at the remaining microphones in the array (excluding the microphone located 7.6 metres from the centerline of vehicle travel) . Minus and zero differences indicate that the maximum was read by the J366 microphone. Positive differences indicate that the maximum sound level was measured at a microphone other than the J366 microphone. These data show that 70 percent of the time the maximum sound

Table 2. A-weighted sound level differences observed between the J366 microphone and the remaining

15.2 metre microphones in the array shown in Figure 2.

Number of Occurrences

Microphones A-weighted Sound Level Differences

Involved -3 dB -2 dB -1 dB 0 dB 1 dB 2 dB 3 dB 1-3 3 4 20 12 12 5 2-3 1 16 19 18 1 1 4-3 3 12 23 15 3 5-3 2 10 11 21 9 2 1

level was measured at the J356 location and 94 percent of the time the reading at the J366 microphone

location was within 1 dB of the maximum sound level which was observed at one of the other microphones

in the array. On the basis of these 224 test runs, the use of an array of microphones does not appear

to be justified in view of the cost and complexity associated with them — especially when one considers

the small manufacturers.

In answer to the second question posed — namely the suitability of the J366 microphone location —

it appears, on the basis of these data, that the present location is better than any of the others

investigated during the test program under discussion.

Another question which deserves consideration is whether measurements should terminate when the vehicle leaves the end zone or should continue for a limited time thereafter. The basis of the question

is the speculated "peanut shell" directionality patterns to the front and rear for fan dominated

vehicles. Although the question cannot fully be answered on the basis of these data [no attempt was made in this program to identify the predominate component noise sources], the fact that the maximum

9
A-weighted sound level was not observed more often at any one single microphone located on either side

of the J366 microphone seems to indicate that there is no justification for continuing the meter reading

after the truck has left the end zone.

Tolerances have been added to the specifications of the height of the microphone above the ground

plane and the distance between the source and microphone. Acoustically these changes should make

negligible differences [15] but it provides a little flexibility to the person performing the test and ensures that the data will not be legally dismissed because a given dimension is not exactly what it should have been. In addition guidance is provided to allow for the proper orientation of the microphone with respect to the sound source. The microphone should be oriented with respect to the source

so that the sound strikes the diaphragm at the angle for which the microphone was calibrated to have

the flattest frequency response characteristic over the frequency range of interest.

A recent study[16] suggests locating the measuring microphone at ground level rather than at a height of 1.2 metres above the ground. However, experimental data are too limited to support such a change at the present time especially since (1) the present data base would be invalidated and

(2) problems encountered due to meteorologically induced variations (shadow zones) are possibly more

severe at ground level than at other heights.

Manufacturers, in conducting SAE J366b tests, have observed that the porosity of the surface of

the test pad is critical as well as the planar smoothness of the surface in the measurement area. Additional specifications have been added to this methodology to control these parameters as required

to limit the variability in test results.

For example. International Harvester Company observed that noise levels taken on their new test pad were significantly lower tha;n those taken on the surfaces previously used. (The two surfaces, both

asphalt, were thought to be identical.) An impedance tube— placed directly on the surface of the test

pad was utilized to acquire data such that the absorption characteristics of the two surfaces might be

compared. Figure 3 shows the results of this investigation. The discontinuity at 1000 Hz is due to a change in the diameter of the impedance tube utilized (from 10 cm to 3 cm) . A comparison of the datum

point on either side of the discontinuity provides some indication of the accuracy of the measurement. Although the measurements were rather crude and were for normal incidence (whereas almost grazing

incidence occurs in the truck passby measurements) , the percent absorption below 2000 Hz was clearly

different for the two asphalt surfaces. The new asphalt surface was 35-40% absorptive compared to the

old asphalt which was found to be 2% absorptive. The solution in this case was simply to seal the new

asphalt with commercial driveway sealer. Following the sealing operation, comparable sound levels were

obtained for identical vehicles regardless of the test pad utilized. The conclusion that can be drawn

from this experience is that, due to its porous nature fresh asphalt is highly absorptive. Old surfaces

that have borne much traffic have evidently been sealed by the wear pattern much as the driveway sealer

does on a new surface.

For practical reasons we do not limit the surface to concrete. Resurfacing is a problem with con- crete since the entire test pad has to be dug up and a new pad laid. With asphalt a new surface can

be laid fairly quickly and relatively inexpensively. We do not require the surfaces to meet given

absorption restrictions either (present in-situ measurement techniques are not well defined) , but simply

require the hard surface area to be of smooth concrete or smooth sealed asphalt.

The effect of test site topography is another important aspect of passby site noise test variability.

Researchers at General Motors [19] conclude that variations of a few inches in surface flatness (of the plane containing the vehicle path and the measuring microphone) may cause significant differences in

overall sound levels and spectral content. GM looked at this problem as a result of consistent differences in sound level of 0.5 to 2 dB observed for identical vehicles on two test sites. (The two sites were measurement areas on opposite sides of the same vehicle test path.) An investigation of the topography was made with the intent being to make whatever modifications were necessary to the site

geometry to force results of future vehicle passby noise tests to agree between sides within instrumentation accuracy.

The modifications to the pads were made in the form of a repaving to achieve uniform flatness over both test pads. Before repaving, the south side showed a "dished out" geometry while the north side

showed substantial crowning. The repaving was successful, the measurement differences, based on measured data, being reduced to typically 0.5 dB. Unfortunately it is not possible to separate the effect of the resurfacing from that of correcting the surface topography since both influencing

factors were changed as a result of the modification. However, it is felt that a practical flatness

restriction is needed to lessen site-to-site sound level measurement variations. Therefore, this

methodology requires that the plane containing the vehicle path (acceleration point to end point) and

the microphone location be flat within +0.05 metres.

— For a description of the impedance tube and its use reference should be made to ASTM Standard

C-384-1972 — Test for Impedance and Absorption of Acoustical Materials by the Tube Method[17].

10
80

60

o

a.

cco00

QQ<

T T T

A UNSEALED ASPHALT

VsAMPLE "A" SEALED ASPHALT -

- SOUND PAD A CONCRETE SURFACE - P.G. BLACKTOP SURFACE - P.G. SOUND PAD O UNSEALED ASPHALT - SOUND PAD SAMPLE "B'

500 1000

FREQUENCY, Hz

5000

Figure 3. Comparison of sound absorption characteristics of various surfaces meeting the present

requirements of SAE J366b[18]

.

2.5.1.1.b. Weather Conditions

The only modifications to the existing requirements contained in J366b are the following:

•Testing is restricted to times when the wind speed is less than 5 metres/second (acceptable

since only A-weighted measurements are being made).

•Corrections are required for barometric or altimetric changes (effects on calibration devices

that are ambient pressure dependent)

.

It is the consensus opinion of the NBS task force that all measurements should be corrected to a standard temperature and barometric pressure (e.g., 1 atm and 20°C) so that data taken in Michigan in

winter can be compared with that taken in Arizona in summer, for instance.

The Ford Motor Company routinely applies a temperature correction to its data of 1 dB/27.8°C

(1 dB/50°F) (see Figure 4) . The correction factor is based on theoretical considerations of the effect of temperature on noise generation by individual truck components.

General Motors Corporation has compiled data on six identical passenger cars. Although automobiles typically have different noise characteristics than trucks, it was felt by GM[21] that variations

in noise level due to temperature changes observed for these vehicles (air bag test vehicles) would

be similar to what would be expected for trucks. These vehicles were equipped with 350CID 4-BBC

engines, thermostatic clutch fans, automatic transmission, 3.42 to 1 rear axle ratios and dual exhaust

systems. Much scatter can be seen in the data (Figure 5) but the trend shows that the A-weighted

sound level decreases by 1 dB for an increase in temperature of 24.7°C (44.5°F).

Data [Figures 6 and 7] are also available to indicate that 20°C should be selected as the standard

temperature. Some SAE procedures presently utilize 2b°C as a reference temperature.

At this time, no consideration of a barometric pressure correction has been made by the truck

manufacturers. Theoretical considerations indicate that for a constant volume-velocity source (which may be representative of selected truck component noise sources but probably not total truck noise)

barometric pressure and temperature Influence pc and therefore affect the sound power measured. For instance a constant volume-velocity source will emit 2.2 dB less sound power at the top of Pike's Peak

than at sea level. In this case, changes in barometric pressure are more influential than changes in temperature. It is possible that some of the scatter thought to be a result of vehicle configuration

might possibly be partly attributable to barometric pressure variations.

11
X3

oLU

QC

O

C_9

<

QC

AMBIENT TEMPERATURE, °C

0 10 20 30 40

+0.5 -

Fig

0 10 20 30 40 50 60 70 80 90 100

AMBIENT TEMPERATURE, °F

ure 4. Ambient temperature correction factors for truck exterior noise

tests[20]

.

-10

AMBIENT TEMPERATURE, °C

10 20 30 T

40

T

80 - • • S

CO

S 79

aLU

I—CO

LU

78

77

20

• • •• « • • I t •%

m ® • •

,1 dB/44.5 °F

• •

J L

30 40 50 60 70 80 AMBIENT TEMPERATURE. °F

Figure 5. A-weighted sound level versus ambient temperature measured during air bag vehicle

variability study[21].

12
AMBIENT TEMPERATURE, °C

0 10 20

CO

I—CO

0 10 20 30 40 50 60 70 80 90 100

AMBIENT TEMPERATURE. °F

Figure 6. Temperature distribution for J366a noise tests conducted by

International Harvester during calendar year 1973[18].

At the present time the data base is such that correction factors for temperature and barometric

pressure cannot be defined. Such corrections are needed and research to provide definition of such

factors should be given a high priority.

2. 5.1. I.e. Background Noise

To be a valid test the existing background noise at the test site must be at least 10 dB below

the regulated level.

2.5.1.2. Instrumentation

The response of instrumentation to transient signals, e.g., vehicle passbys, is not well defined

in existing instrumentation standards. In an attempt to tighten the requirements as much as possible

at this time, the instrumentation to be used while making measurements according to this methodology

are required to meet the Type 1 requirements of American Standard Specifications for Sound Level Meters,

S1.4-1971[22] . In addition, pertiment sections of American National Standard Methods for the Measurement of Sound Pressure Levels, S1.13-1971[23] have been incorporated as part of this methodology.

Calibration requirements — both single point in the field and complete calibrations (to determine

compliance with precision. Type 1 Specifications) — are specified in detail and allowable tolerances

are defined for calibrators.

Requirements are given for the use of a windscreen and the allowable effect of the windscreen on

the sensitivity of the microphone.

Anemometers, thermometers and barometers are required as additional instrumentation and the required accuracy is spelled out in the methodology.

13
AMBIENT TEMPERATURE, °C

0 10 20

T

00

IXJ o

cc

QC

UO

o

LU

00

0 10 20 30 40 50 60 70 80

AMBIENT TEMPERATURE, °F

90 100

Figure 7. Test (J366a) temperature histogram for Ford Motor Company 1973 noise test program[20]

.

2.5.1.3. Vehicle Operation

The deceleration test procedure now specified in J366b was added to the original procedure to address the problems of engine brakes. These devices, when in use, essentially convert the engine into

an air compressor which causes the exhaust valve to open at the top of the compression stroke

allowing the compressed air to blow down through the exhaust system instead of working against the piston on the subsequent down stroke as per normal operations [ 24 ] . When one considers the total truck

population the numbers of such brakes are quite small; however, In certain geographical areas they are

quite popular. Although improvements have been made in the engine brakes themselves and in exhaust

mufflers, the engine brake noise may still be the limiting factor on lowering the overall noise level

for some vehicles. For this reason we retain this test procedure, but only require that it be performed for those vehicles equipped with engine brakes.

The recommendation to go to the unloaded configuration is a practical one. Tractors are typically

sold without fifth wheels; therefore, no load can be pulled and it is not practical to load such a vehicle in another manner. Single-chassis vehicles are typically sold without bodies so loading this

vehicle is also not practical. The J366b test procedure is such that the engine is inertially loaded

during the acceleration so that there is no apparent need for pulling and/or carrying a load.

The present J366b procedure implys that governed engines, be they gasoline or diesel, should be

tested at their governed speed and ungoverned gasoline engines at their maximum rated speed. This

methodology turns this implication into a requirement.

Today's trucks typically utilize direct drive fans designed for "worst case" cooling demands — heat

rejection at maximum engine power with little or no ram air. Because of this it is not surprising that

the cooling fan is not necessary to maintain engine cooling under many conditions. Available on the market today, but in limited use, are clutched fan drives which operate only when there is a cooling

demand. High initial cost, space and maintenance problems have limited more widespread use of these devices. In addition to being attractive from a noise reduction standpoint, the usage of the clutched

fan also saves the otherwise wasted power utilized to drive the fan when it actually is not needed.

Much speculation has been made on the actual percentage of time that a fan is needed to maintain

engine cooling for trucks in normal operation. International Harvester Company is currently conducting

a study to determine actual fan-on time. The study is a part of the DOT "quiet truck" program. At the present time 23 vehicles have been equipped with either on-off or modulating fan clutch systems. The

on-off unit is either full-on or full-off while the speed of the modulating fan is variable and is determined by the cooling demand. The 23 vehicles are equipped with diesel engines ranging from

250-350 horsepower, are of conventional and cab-over designs, and represent 8 fleets which operate

14
throughout the United States. Each vehicle Is Instrumented so that the following data result: (1) engine

speed versus time, (2) fan engagement, (3) total engine revolutions, (A) hours of engine-on time and

(5) hours of fan-on time. A log is also kept indicating the fleet, truck model, engine type as well as

a compilation of times and destinations.

On the basis of present data[25,26], resulting from 700,000 miles and 18,800 hours of engine

operation, the projected fan operation (total fan-on percent) would be 4 percent of the annual vehicle

operation time. During only 2.6 percent of the annual vehicle operation would the fan speed by high

enough (during fan-on time) to be significant from a noise standpoint. Significant fan-on time is defined as that time the fan is engaged while the engine is above 1600 rpm. 1600 rpm is the speed at which the fan noise would be 8-10 dB less than the noise at governed engine speed for the majority of the trucks involved in the program. The fan would not contribute to vehicle noise at engine speeds

less than 1600 rpm. Details of the projection method for predicting significant fan operating time

for a year are presented in Appendix C. The present projection is based on data from 8 of the 23 vehicles. Data acquisition with all 23 vehicles is continuing at present and the projections will be

updated as more data become available.

It should be pointed out that the existing data (measured) are for diesel engines only and cover operation over an ambient temperature range from -4°C to 27°C. Also the present projections do not

include any modulating fan data since the modulating fans in the study have not operated, as yet,

above 1600 rpm. Even though the percentages may increase slightly for other engine types or when

additional hot weather data are acquired, the net benefit provided by such fans outweighs the small

portion of the time when they contribute significantly to overall noise levels. The economic

feasibility of these devices hinges on the acceptance of higher noise levels during that small

percentage of the time when the heat rejection demand on the cooling system is high in order to achieve

lower noise emissions the remainder of the time. To provide some inducement for the utilization of such devices (assuming mechanical reliability can be demonstrated), this methodology proposes that the fan be declutched during tests for those vehicles equipped with on-off or modulating fan units. In addition to noise reduction, they also offer such advantages as fuel savings due to the fan not being

on all the time — an important consideration in light of the fuel shortage, less noise in the cab and more power.

Many trucks today are equipped with radiator shutters for temperature control. The J366b procedure requires the measurement of maximum noise; therefore, in general, the test is run with fan-on and

shutters closed. Since this may not be the typical condition occurring during actual service, this

methodology simply requires that the test be run with the shutters in the position producing the highest noise level that occurs when the vehicle is brought to its normal operating temperature.

2.5.1.4. Measurements

All things considered, it is proposed for practical reasons to utilize the A-weighted sound level

as the basic measure for truck noise. The choice of the fast meter response is made to allow accurate

readings to be made on the rapidly changing sound level associated with acceleration tests.

Selection of a single-number descriptor of the loudness or the noisiness of a sound is complicated

by the relatively complex system by which humans hear and discriminate sounds. The frequency response

of the hearing mechanism and the ability of humans to extract varying characteristics of noise have

generated extensive research into the human reaction to noise and the means for specifying this reaction.

Present research provides two methods for determining subjective reaction to noise — direct

psychological judgements such as paired comparisons or jury (group) decisions and indirect measures

such as the use of simple weighting networks in direct measuring instrumentation which are based on data from psycho-acoustic measurements. Such methods provide numerous measures for subjective reaction

to noise including loudness, loudness level by the Stevens or Zwicker methods, perceived noise level,

speech interference level and C- and A-weighted sound level.

The subjective response of subjects to vehicle noise has been evaluated during two comprehensive

studies [27, 28 ] . A comparison has been made between the results utilizing the psychologically derived

and physical measures of sound. The Armour Research Foundation (now the Illinois Institute of Technology Research Institute) study utilized diesel truck spectra as the stimulus while the Motor Industry

Research Association study included the noise of gasoline and diesel trucks, automobiles and motorcycles. The results of these studies, summarized in Figures 8 and 9, show that with the exception of the C-weighted sound level and the speech interference level the remaining psychologically derived and

physical measures of sound studied could reasonably serve as predictors of human response to motor

vehicle noise, in particular trucks.

Since the A-weighted sound level is the only measure having high correlation with subjective reaction that can also be read directly on a commercially available meter having standardized performance,

it is proposed that the A-weighted sound level be selected as the basic measure for truck noise at this

time. Should new types of propulsion systems become practical for widescale use in trucks, such as the

gas turbine engine, it may be necessary to reconsider the present proposal in light of the different

frequency characteristics of the sound produced.

15
T 1 1 SONES (LOUDNESS)

SPEECH INTERFERENCE LEVEL

LOUDNESS LEVEL (STEVENS)

PERCEIVED NOISINESS LEVEL

A-WEIGHTED SOUND LEVEL

^C-WEIGHTED SOUND LEVEL

1 1 1

0.6 0.7 0.8 0.9 1.0 CORRELATION COEFFICIENT

Figure 8. Correlation coefficients for various measures of subjective reaction to diesel truck noise. At the time this

study was conducted, sound level meters meeting the requirements of American

Standard ASA Z24. 3-1944 were used. These meters had relatively wide tol- erances for microphones and circuitry.

The A-weighted sound levels reported

' '~" in this figure were computed from the original tapes utilizing Instruments meeting American Standard ASA S1.4-

1961 [29].

I

I T

SPEECH INTERFERENCE LEVEL

LOUDNESS LEVEL (ZWICKER)

LOUDNESS LEVEL (STEVENS)

PERCEIVED NOISINESS LEVEL

A-WEIGHTED SOUND LEVEL

C-WEIGHTED SOUND LEVEL |

'

' 0.6 0.7 0.8 0.9 1.0 CORRELATION COEFFICIENT

Figure 9. Correlation coefficients for various measures of subjective reaction to motor

vehicles. The test included automobiles,

light trucks, diesel trucks and motorcycles [29] .

2.5.1.5. Measurement Uncertainty

SAE Standard J366b describes a test procedure which places restrictions on many variables — such

as limits on windspeed, reflecting surfaces, etc. — so that the measured variable, the sound level of

the test vehicle, can be accurately determined. In spite of these restrictions, it has been the experience of many individuals that there are still variabilities in the test method that result In different

noise levels being measured with the same vehicle on different days (or even on the same day) and at different sites. Many attempts have been made to quantify the variabilities and/or develop correction

factors [13,15,16,19,30, and 31], but very little success has been achieved to date.

The probable sources of measurement variance, which affect the generation, radiation and/or

propagation of motor vehicle noise, are not difficult to identify. They include:

• Meteorological Effects

- air temperature

- humidity

' - atmospheric pressure

- wind velocity and direction

- temperature gradients

- wind gradients

• Instrumentation Effects

- Calibration and instrument variances (Including transient response)

- Microphone orientation

- Windscreens

• Test Site Effects

- Vehicle position within the end zone

~ - Absorption of the surface of the test pad

- Planar smoothness of that portion of the test site containing the microphone location and

the vehicle path

• Vehicle

- Driver technique

- Truck-to-truck assembly differences (nominally identical vehicles)

16
These are just some of the known influencing factors. J366b recommends a 2 dB allowance over the sound

level limit to provide for variations in test site, temperature gradients, test equipment, and Inherent

differences in nominally identical vehicles. Studies[13,30,31] indicate that the 2 dB allowance is not

large enough to account for all the variables.

What is required is either a test that is independent of test site and environmental variables or

a means of accounting for changes in the measured noise levels resulting from such variables, e.g., through site calibration techniques or the application of correction factors. At present, neither of these options is available; therefore, the attempt has been made in this methodology to tighten the

test restrictions as much as possible in those areas which are controllable. It is imperative, however,

that one be aware of the possible magnitude of uncertainty associated with a given test procedure so that the compliance procedures are practical.

3. Medium and Heavy Duty Truck Measurement Methodology

3.1. Purpose and Applicability

This methodology defines an acceptable procedure by which the noise from medium and heavy duty

trucks (gross vehicle weight rating of 10,001 pounds or greater) may be measured to ascertain whether

the noise emitted is in compliance with the noise emission standards promulgated by the U.S. Environmental Protection Agency pursuant to Section 6 of the Noise Control Act of 1972 (Public Law 92-574)

For the purpose of this methodology, trucks include complete and incomplete trucks and truck tractors

as defined in the Code of Federal Regulations, Title 49, Transportation (U.S. Department of Transportation, Federal Highway Administration, Part 390 — Motor Carrier Safety Regulations and National

Traffic Safety Administration, Part 568 — Vehicles Manufactured in Two or More Stages)

3.2. Acoustic Environment

3.2.1. Test Site

3.2.1.1. The test site shall be such that the truck radiates sound into a free field over a reflecting

plane. This condition may be considered fulfilled if the test site consists of an open

space free of large reflecting surfaces, such as parked vehicles, signboards, buildings or hillsides, located within 30 metres of either the vehicle path or the microphone (see

Figure 10)

3.2.1.2. The microphone shall be located 15.2 + 0.3 metres from the centerline of truck travel and

1.2 + 0.1 metres above the ground plane. The microphone shall be oriented with respect

to the source so that the sound strikes the diaphragm at the angle for which the microphone

was calibrated to have the flattest frequency response characteristic over the frequency

range 100 Hz to 10 kHz.

3.2.1.3. In order to establish the critical dimensions of the test site the following reference points

are established (see Figure 10).

3. 2. 1.3. a. The microphone location is that point on the ground plane established by the normal from the microphone to the ground plane.

3.2.1.3.b. The normal from the microphone location to the centerline of vehicle travel shall

establish the datum point.

3. 2.1. 3. c. The end zone begins 6.8 metres past the datum point and ends 11.4 metres past the datum point.

3. 2.1. 3. d. An acceleration zone shall be established on the vehicle path 11.4 to 19.0 metres before the datum point. The final choice for the acceleration point, within the acceleration zone, should be such that the vehicle (vehicle reference point is defined in Section 3.2.1.3.f.) under test reaches maximum rated speed (for

ungoverned engines) or governed speed (for governed engines) within the end zone.

3. 2.1. 3. e. An end line shall be established on the vehicle path 19 metres past the microphone point

3.2.1.3.f. The reference point on the vehicle, to indicate when the vehicle is at any of the points on the vehicle path, shall be the front of the vehicle except as follows.

If the engine is located rearward of the center of the chassis, the rear of the vehicle shall be used as the reference point. If the horizontal distance from the reference point of the vehicle to the exhaust outlet is more than 5.1 metres,

tests shall be run using both the front and rear of the vehicle as reference

points.

17
ZONE IN WHICH FULL THROTTLE ACCELERATION

IS TO BE ESTABLISHED

ZONE IN WHICH TO

r— REACH GOVERNED

OR MAXIMUM RATED

ENGINE SPEED

Figure 10. Minimum test site.

3.2.1.4. The road surface within the test site upon which the vehicle travels, and, at a minimum, the measurement area (BCD in Figure 10) shall be smooth concrete or smooth sealed asphalt, free

of extraneous material such as gravel.

3.2.1.5. The plane containing the vehicle path and the microphone location (plane ABCDE in Figure 10)

shall be flat within +0.05 metres.

3.2.2. Weather Conditions

3.2.2.1. Measurements shall not be made when the wind velocity exceeds 5 metres/second.

3.2.2.2. If calibration devices are utilized which are not independent of ambient pressure (e.g., a pistonphone) corrections must be made for barometric or altimetric changes according to the

recommendation of the instrument manufacturer.

3.2.2.3. Measurements shall not be made when the road surface is wet, covered with snow, or during

precipitation.

3.2.3. Background Noise

3.2.3.1. The maximum A-weighted fast response sound level observed at the test site immediately before

and after the test shall be at least 10 dB below the regulated level.

18
3.3. Instrumentation

3.3.1. The measurement system utilized shall conform to the requirements of American National

Standard SI. 4-1971 "Specifications for Sound Level Meters" for a Type 1 instrument over the frequency range 50 Hz-10 kHz.

3.3.2 A windscreen shall be used when the average wind noise is within 20 dB of the regulated level,

provided that the sensitivity change does not exceed +0.5 dB to 5 kHz and +2.0 dB to 10 kHz.

3.3.3. The sound level meter, or equivalent system, shall be calibrated (e.g., by means of an acoustical calibrator) at one or more frequencies in the range 250 to 1000 Hz, at the beginning and

end of each series of measurements. The calibrator shall produce a sound pressure level, at

the microphone diaphragm, that is known to within an accuracy of +0.5 dB. The calibrator

shall be checked annually to verify that its output has not changed.

3.3.4. A complete calibration of the instrumentation over the entire frequency range of interest

shall be performed at least annually using methodology of sufficient precision and accuracy to determine compliance with the standards cited in Section 3.3.1.

3.3.5. Additional Instrumentation Requirements

3.3.5.1. An anemometer or other device for measurement of ambient wind speed accurate within

+10% at 5 m/sec.

3.3.5.2. A thermometer for measurement of ambient temperature accurate within +1°C.

3.3.5.3. A barometer for measurement of ambient pressure accurate within +1%.

3.4. Vehicle Operation

3.4.1. A full throttle acceleration test is to be utilized. A beginning speed, proper gear ratio

and the location of the acceleration point within the acceleration zone shall be determined

for each vehicle for use during measurements.

3.4.2. All vehicles tested shall be unloaded.

3.4.3. Trucks equipped with on-off or modulating fans shall be tested with the fan off.

3.4.4. The truck shall be brought to its normal operating temperature prior to commencement of testing.

3.4.5. For those trucks equipped with radiator shutters, tests shall be conducted with shutters in

that position producing the highest noise level that occurs when the vehicle is brought to its

normal operating temperature.

3.4.6. The highest rear axle and/or transmission gear ("highest gear" is used in the usual sense: it

is synonymous to the lowest numerical ratio) and an initial vehicle speed such that at wide

open throttle the vehicle will accelerate from the acceleration point (which must be previously established within the acceleration zone for the particular test vehicle) under the following conditions: (a) starting at two thirds of maximum rated (for ungoverned engines) or governed engine speed, (b) reaching maximum rated or governed engine speed within the end zone,

(c) without exceeding 15.7 metres/second before leaving the end zone.

3.4.7. Should maximum rated or governed engine speed be attained before reaching the end zone, alter

the location of the acceleration point within the acceleration zone maintaining an approach

rpm of two thirds of maximum rated or governed engine speed. Should maximum rated or governed

engine speed still be attained before reaching the end zone, decrease the approach rpm in 100 rpm increments until maximum rated or governed engine speed is attained within the end zone.

3.4.8. Should maximum rated or governed engine speed not be attained until beyond the end zone, alter

the location of the acceleration point within the acceleration zone maintaining an approach

rpm of two thirds of maximum rated or governed engine speed. Should maximum rated or governed

engine speed still not be attained until beyond the end zone, select the next lower gear until

maximiim rated or governed engine speed is attained within the end zone. Should the lowest

gear still result in reaching maximum rated or governed engine speed beyond the end zone, increase the approach rpm in 100 rpm increments until the maximum rated or governed engine

speed is reached within the end zone.

19
3.4.9. The vehicle should approach the acceleration point using the engine speed and gear ratio

selected according to the above procedure (Sections 3.4.6 - 3.4.8) and at the acceleration

point wide-open throttle should be rapidly established. Acceleration shall continue until

maximum rated or governed speed is reached.

3.4.10. Wheel slip which affects the maximum sound level must be avoided.

3.4.11. For trucks equipped with an engine brake the following additional tests shall be run. The

vehicle shall approach the microphone point at maximum rated or governed engine speed in the gear selected for the acceleration test. At the microphone point, close the throttle and

allow the vehicle to decelerate to one-half of maximum rated or of governed engine speed. The

engine brake shall be full on immediately following closing of the throttle.

3.5. Measurements

3.5.1. The quantity to be measured, under the test conditions described above, is the A-weighted

sound level with fast meter response as defined in the American National Standard SI. 4-1971.

3.5.2. Bystanders have an appreciable influence on meter response when they are in the vicinity of

the vehicle or microphone; therefore not more than one person, other than the observer reading

the meter, shall be within 15.2 metres of the vehicle path or instrument and the person shall be directly behind the observer reading the meter, on a line through the microphone and observer.

To minimize the influence of the observer on the measurements, a cable should be used between

the microphone and the sound level meter.

3.5.3. A minimum of two test runs shall be made for each side of the vehicle. The meter shall be

observed during the period while the vehicle is accelerating or decelerating prior to leaving

the end zone. The applicable reading for each run shall be the maximum sound level to the nearest decibel obtained for the run. Tests shall be rerun if unrelated peaks occur due to extraneous ambient noises.

3.5.4. The sound level for each side of the vehicle shall be the average of the two highest readings

which are within 2 dB of each other. The noise level for the side of the vehicle with the highest reading shall be utilized to determine compliance with this regulation.

3.5.5. An additional 2 dB allowance over the sound level limit is recommended to provide for variations in test site, meteorological effects, test equipment and Inherent differences in nominally identical vehicles.

3.5.6. The wind velocity, barometric pressure and ambient temperature shall be measured at the microphone height and within at least 3 metres of the microphone location.

3.6. Data to be Recorded

The maximum A-weighted sound level, using the fast response characteristic, shall be recorded for each measurement.

The following additional data shall also be recorded.

•General Data

- Date of test

- Test report number (manufacturer)

- Other

•Truck Data

- Manufacturer

- Model

- Serial number

- Other

•Test Conditions

- Manufacturer's test site identification

- Ambient temperature, °C

- Ambient wind velocity during sound level measurement, m/sec

- Atmosphere pressure, mm Hg

- Other

20
•Instrumentation

- A calibration history of all equipment shall be maintained.

- Microphone /serial number

- Sound level meter/serial number

- Calibrator/serial number

- Other/serial number

- Windscreen on/off

•Sound Level Data

4. Appendix A. A Discussion of Truck Subsource Noise~^

Gasoline engines power 65 percent (based on 1973 new truck sales) of the trucks in operation with a GVWR of 10,001 pounds or more. The remaining 35 percent are powered by diesel engines. The major con- tributing subsources of truck noise include engine (mechanical) and transmission, exhaust, cooling system, intake and tires.

• Engine (mechanical) "

The engine and transmission form a complex acoustical system composed of a large number of sources,

the most important of which result from the increase in pressure within the cylinder during combustion

of the fuel. This rapid rise of cylinder pressure excites mechanical vibrations in the engine structure.

Some of this vibrational energy is subsequently radiated into the atmosphere as acoustic energy. The

gas forces are also transmitted through the piston and crank mechanism to the main bearings and hence

excite flexible or mobile structures coupled to the crankshaft. Most of the noise generated by these

mechanisms is radiated by the crankcase, oil pan, clutch housing and valve covers. The noise level produced by both gasoline and diesel engines depends upon engine size, speed and load. The noise level

depends more on engine speed than on size, so that for a given horsepower, the quieter engines are associated with a larger displacement and slower engine speed. In practice there is a significant range

in engine (mechanical) noise levels for a given engine size. Calculating the effect of engine noise on

the overall vehicle noise level is difficult because the noise produced by a given engine depends upon

the way in which it is mounted so that the level varies with the type of truck in which it is installed.

Gasoline engines initiate combustion with a flame which smoothly spreads throughout the cylinder

until the fuel-air mixture is burned. Diesel engines, on the other hand, rely on much higher compression ratios to produce spontaneous combustion which rapidly oxidizes the fuel. This causes a more rapid

change in pressure in the cylinder which in turn results in increased engine vibration and hence higher

noise levels.

The primary combustion forces occur at the relatively low fundamental firing frequencies but the structure responds to the higher harmonics as well. Fuel injection equipment, pistons, valve train,

gearing and accessories, all integral with or attached to the engine frame, contribute at various fre- quencies to deflect or vibrate the engine structure.

The engine itself contains several areas which vibrate and radiate noise. The blocks of V engines

act as tuning forks while in-line engine blocks vibrate as beams. The motion of these relatively thin

case block surfaces as well as sheet-metal components such as the oil pan, rocker arm cover, oil sumps,

etc., which are attached to the vibrating structure, all increase the noise radiated to the atmosphere.

The character of the combustion forces can be changed advantageously by turbocharging. Turbocharging, which increases the pressure of the inlet air above atmospheric pressure, reduces the combustion force impact by making the pressure rise more gradual; thus, decreasing the noise radiated. Sheetmetal parts may be vibration isolated from the movement of the engine by use of soft gaskets and rubber

washers at the mounting bolts. Engine covers using high density barrier materials lined with absorbent

materials are the most effective way of reducing engine noise but have the disadvantages of reduced

accessibility to the engine, reduced cooling efficiency, added weight, and potential fire hazards due to leaking oil being absorbed by the acoustical liners (which also destroys the noise absorption characteristics) . Shielding under the engine can be effective in blocking the reflected path of noise from the engine to the receiver by way of a paved surface.

— The material in this appendix was mainly derived from Wyle Laboratory reports to EPA [32,33] on transportation noise and a cost-effectiveness study of medium and heavy trucks and material presented

in the SAE Twentieth L. Ray Buckendale Lecture on truck noise control [34].

21
• Exhaust System Noise

Exhaust noise is generated by the rapid release of high pressure combustion gases when the exhaust

valve opens. In addition to the discharge noise — noise radiated to the atmosphere at the tail pipe

outlet — this rapid release of pressure excites oscillations of the muffler shell and exhaust and tail

pipe surfaces which are radiated in the atmosphere as noise. Transmission loss through the pipes and

flexible connections and gas leaks are additional noise sources within the exhaust system. In the past,

the outlet noise has overshadowed the other exhaust system sources; however, as the outlet noise is reduced by use of improved mufflers, the pipe and shell noise become more significant contributors.

The primary frequency components of exhaust noise are dependent upon engine speed, number of cylinders, exhaust system configuration (single or dual exhausts) and whether the engine is a 2-cycle or 4-cycle engine. Its amplitude is dependent upon ignition timing, throttle setting and engine load

(pressure in the cylinder when the valve opens).

Mufflers or resonators are typically utilized to control exhaust discharge noise. The performance

of these devices is very dependent upon other acoustic elements within the system such as the engine and

the length and diameter of both the exhaust and tail pipes. In addition to providing effective noise

reduction, mufflers must be designed to produce a tolerable back pressure on the engine.

Muffler shell noise and pipe-radiated noise are similar in nature. General approaches to pipe and

shell noise reduction include reducing the distance between the muffler and the manifold by utilizing a shorter exhaust pipe, by concealing pipes with shielding or by routing the pipes behind other truck components, or by utilizing damping materials to reduce muffler and pipe vibration.

Noise also can be produced by engine brakes which, when in use essentially convert a power producing diesel engine into a power absorbing air compressor. This is accomplished through a master-slave

piston arrangement which opens cylinder exhaust valves near the top of the normal compression stroke releasing the compressed air charge to the exhaust manifold. The blowdown of compressed air to atmospheric pressure prevents the return of energy to the engine piston on the expansion stroke, the effect

being a net energy loss since the work done in compressing the cylinder charge is not returned during

the expansion process. Since the diesel engine is turned over by the driving wheels of a vehicle

descending a hill through the vehicle drive line and transmission, application of the engine brake retards the "free-wheeling" effect of the driving wheels and a braking action results. Advantages such as elimination of continual braking with the vehicle service brakes, ability to maintain engine operating

temperatures on down grades, increased life of brake components and tires and added safety make this

device a popular option in certain geographical areas. The noise produced by the blowdown of the com- pressed air to the atmosphere is generally controlled by making improvements to the engine brake itself

or through the use of improved mufflers.

• Cooling System Noise

The cooling system is a major noise source on trucks due to the severity of the cooling requirements at full engine power and low road speeds. The fan must move enough air to maintain the required

coolant temperature when ram air is not available.

The fan is the predominant noise source in the cooling system. The fan, as an air moving device, creates lift forces to cause air motion. Since the lift force is the result of a pressure difference

across the fan blade, the lift force actually rotates with the blades and relative to a fixed point,

fluctuates each time a blade passes setting up pulsations in the surrounding area. Such pulsations

result in the generation of a fundamental tone, associated with the blade passage frequency, and higher

order harmonics. Non-uniform air flow across the face of the fan (due to improper distribution of the air from the radiator) and obstructions near the fan such as hoses, alternators, etc., which create air

flow irregularities intensify the fan noise problem. In addition, random pressure fluctuations, re- sulting in broadband noise, are caused by the non-coherent shedding of vortices from trailing edges of the blades and by turbulence generated upstream of tii^ fan arising from the passage of the cooling air

over the tube and fin surfaces of the radiator.

Fan noise can further be aggravated for trucks utilizing radiator shutters for engine temperature

control. By blocking the air flow through the radiator, shutters cause the pressure rise across the fan blade to increase resulting in an increase in noise level.

An obvious approach to reduce cooling system noise is to increase cooling efficiency and thus allow

for decreased fan speed. A larger fan turning at a slower speed which can be swept over a larger radiator core area with possibly a lower tip speed has obvious advantages. Fan shrouds can also be used to prevent air recirculation through the fan thereby ensuring that all of the air pumped by the fan has

passed through the radiator to provide cooling. The clearance between the outer edges of the fan blades

and the shroud (tip clearance) can also be a major factor. To ensure optimum noise reduction the tip

clearance should be as small as practical.

22
Cooling systems are designed for "worst case" situations which seldom occur. In actual service

conditions, the heat load is such that full fan speed is not required much of the time. The critical

condition for trucks, from a cooling demand standpoint, is when the vehicle is pulling a heavy load up

a long grade at full engine power but low road speed. Fans operating at full speed are necessary during

this period to provide necessary cooling in the absence of the ram air effect. Additionally, fans are usually operating when the vehicle is idling; however, the fan is not operating at its maximum rpm and

therefore the noise generation is not significant. At fan speeds of less than 1600 rpm the fan noise is sufficiently less than the fan noise at governed engine speed (usually about 2100 rpm) and therefore the contribution to the overall noise level of the vehicle is not significant.

On the basis of the previous discussion, it appears that the obvious approach to the fan noise problem would be the utilization of a demand fan in place of a direct drive fan. Clutched fan drives —

either on-off or modulating — do exist on the market today (in limited quantities); however, they are

not widely utilized in trucks at the present time. Such fans are thermostatically controlled and

operate only when needed. On-off fans are either full-on or full-off, in other words, when cooling is needed the fan comes full-on and remains on until the cooling demand has been satisfied. The speed of the modulating fan varies to compensate for the heat rejection demands of the engine and seldom is at "full-on" condition.

Present limited data (discussed earlier in this report) utilizing trucks equipped with demand fans

indicate that actual fan "on time" is on the order of a few percent of engine on time under actual over- the-road conditions. Such fan clutches are not prevalent at the present time; however, the promulgation

of new noise regulations which accept the higher noise levels during that small percentage of the time

when the heat rejection demand on the cooling system is high in order to achieve lower noise emissions

the remainder of the time should alter this situation.

• Air Intake Noise

Engine air-intake noise is produced by the intake pulsations on 4-cycle naturally aspirated engines,

by blower blades on a turbocharged engine, and by blower lobe passage disturbances on pressure scavenged

2-cycle engines

Intake systems are similar to exhaust systems in that the components and problems are much the same; however, each intake system must be treated individually since both the frequency and amplitude of intake

noise varies with engine type. Low frequency intake noise is generally controlled by the use of properly

designed and located large volume air-cleaners matched to the intake pipe. Tuned silencers are usually

required to reduce the high frequency components of intake noise. Location of the air intake openings,

e.g., toward the front of the vehicle rather than to the side, can also be effective in controlling intake noise as observed by a bystander along the road.

At present, the air intake system is not a major contributor to overall truck noise levels for most

manufacturers; however, as overall noise levels are reduced the intake system will have to be re- evaluated.

• Tire-Road Interaction Noise

The physical mechanisms of the production of noise by the tire-road interaction process are not

well understood. The general consensus is that air pumping (entrapment and release of air from the tire

tread cavities) and vibration of the tire carcass are the major mechanisms contributing to the generation and radiation of acoustic energy. However, the effect of tread design (rib, crossbar), effect of road surface, effect of wear, and the effect of load on the noise levels produced are known. The existing data do not provide the basis for the design of quieter tires.

Truck tread designs for highway usage can be classified basically into two categories — rib and cross-bar. Rib type tires, with the major tread elements oriented in the circumferential direction,

possess characteristics that are suitable for all wheel locations. These tires are noted for their lateral traction and uniform wear characteristics. Although they can be used on all axle positions, rib

tires typically are seen on steering and trailer axles. Cross-bar tires are used primarily on the drive

axles. This design, with the major tread elements oriented in the lateral direction, gives maximum

driving traction and provides a much more rigid tread structure plus added original design tread depth.

The final choice of which type of tire is utilized depends on many factors, some of which include the original cost of the tire, a concept of the safety provided by a given tire and a knowledge of the company's policy on retreading. The true cost of a tire (cost/mile) is dependent, for instance, on the number of times a carcass — be it a rib or cross-bar tread design originally — will be retreaded.

Although the mechanisms of tire noise generation are not well understood, influencing variables

such as wear, load, speed and pavement surface are known to affect tire noise levels.

23
Cross-bar tires, as a generic group, are noisier than the rib-tread tires. The difference between

the quietest cross-bar tire and the noisiest rib tire is typically A-8 dB. Also of interest is the fact

that only a 1 to 2 dB decrease is observed between the quietest rib tire and a completely smooth tread. Therefore, a breakthrough in basic tire design appears to be needed before tires exhibiting significantly

lower noise levels could be manufactured. Such a breakthrough does not appear to be imminent.

Road surface conditions do influence tire noise levels; however, the effect depends on the tire

type, for instance, cross-bar tires generally produce higher noise levels when running on a rougher surface but in any case the influence is not very significant. Rib tires tend to get noisier as the surface gets rougher. It should be noted at this point that no method now exists for quantitatively characterizing the surface roughness or texture of pavements typical on today's roads. Until the surface

texture can be physically characterized, little can be known about the effect of surface roughness on

the generation of tire noise.

It is known that sound levels rise with increasing speed for all tires, but at slightly different

rates. Empirically, tires are characterized by an increase of 6 to 12 dB for a doubling of vehicle

speed in the range of 30 to 60 mph. Theoretical considerations supporting relationships in which sound

level increases as either 30 log-j^gV or 40 log-j^(jV, or where V is speed, have been developed. This is within the range of the empirical data.

Load is also an important parameter. The increase in sound level that exists between the unloaded

and loaded condition is significant for cross-bar tires. As the load is increased for tread designs of

the cross-bar type, more of the tire load is carried on the outer sections of the tread where the most

drastic interruptions in tread pattern exist.

Rib tires are relatively unaffected by load. In this case the rib pattern is the same across the width of the tire and, hence, it is not important how much load is carried on the outer edges of the tire.

Tread wear is another variable that can greatly affect the sound level generated by tires. Tire

noise generally increases between the new and half -worn states and depending on the tire type either

increases or decreases with additional wear. It appears that the factor controlling the change in noise

level with wear is the change in tread curvature.

Other factors — inflation pressure, tire temperature, etc. — also can influence tire noise but

their effect is not thought to be as great as those parameters discussed above.

5. Appendix B. Discussion of Alternative Low Speed Acceleration Measurement Procedures

The low speed acceleration test procedure discussed in this report is a very detailed and repeatable test procedure. A wealth of data now exist utilizing this type of procedure and it has been shown

that the test does produce the maximum noise level which a truck is capable of making at low speeds. It

is basically a test of propulsion system noise. Because of the space, weather and time constraints of this procedure, it seems appropriate to investigate alternative procedures which would be simpler yet

correlatable with the more detailed test procedure discussed in this report. The correlation as we

speak of it here is not whether the simple test gives the same answers as the more detailed test but

rather that vehicles which fail the simple test would also fail the detailed test. In other words, a double jeopardy situation would not be created for the manufacturer to cope with.

5.1. Stationary Run-Up Test

One immediate candidate for consideration is the stationary run-up test utilized as one of the tests in the present draft of the Federal noise certification procedures for interstate motor carriers.

The basic components of a stationary run-up procedure are as follows:

1. Park the vehicle at a location so that no large reflecting surfaces such as other vehicles,

signboards, buildings, or hills, are within 24.4 metres of the vehicle under test.

2. Locate the microphone 12 metres above the ground plane and 15.2 metres from the fore to aft centerline of the truck in such a manner that the measuring microphone is on a line

with the exhaust outlet.

3. With the v-ehicle transmission in neutral gear, accelerate the engine with wide open throttle to its governed engine speed as rapidly as possible.

4. A minimum of two measurements shall be made for each side of the truck. The sound level

for each side of the vehicle shall be the average of the two highest readings which are within 2 dB of each other. The noise level for the side of the vehicle with the highest

readings shall be utilized to determine compliance.

24
-5 -4 -2 -1 0 +1

(STATIONARY TEST) - (SAE J366a), dB

+2 +3

Figure B-1. Comparison of stationary run-up and vehicle acceleration (SAE J366) test results [35] .

Only the highlights of the stationary run-up test procedure are discussed here. All of the meteor- ological and background noise constraints of the previously discussed measurement methodology would be

applicable as well as the sections on purpose and applicability, quantities to be measured and measurement instrumentation. The test site specifications and the detailed vehicle operation of course would

have to be modified.

Some data exist utilizing this procedure. Figure B-1 shows the results of a study of over 800 different trucks in which a comparison was made between the sound level obtained for the J366a procedure

and that for the stationary run-up procedure. The results are promising; however, due to difficulties

associated with the test, it has not been considered further as a candidate measurement methodology for new trucks at this time.

The two main difficulties with the test procedure is that gear train associated noise is not evaluated in a stationary test and the procedure is only applicable to governed engines. The test sample

shown in Figure B-1 included governed engines only — gasoline engines and turbocharged and naturally

aspirated diesel engines. The number of ungoverned engines sold by each individual manufacturer varies

but a large proportion, estimated to be about 40-50 percent, are sold ungoverned. Since the governor,

or lack of a governor, is a problem only for gasoline engines — for safety reasons all diesel engines

are governed — the adoption of the procedure as is would result in a distinction according to engine

type. As discussed earlier, this separation may have no merit and may be a false and unrealistic division criterion.

This procedure offers little in the way of improvement over the passby procedure discussed earlier.

It would probably take less time to run the test and the amount of space necessary to conduct the test would be slightly reduced, however, all of the weather constraints which limit testing when the passby

method is utilized would also be a problem with the stationary run-up test.

Another question which must be addressed is whether one tests vehicles with this procedure or merely the accuracy or inaccuracy of the particular governor. The maximum noise level recorded during the stationary run-up test corresponds to that point where the engine hits the governor, overshoots and then

25
settles dovm to the governed engine speed (or high idle) . Because of the safety factor with diesel

engines the speculation is that diesel governors are usually more accurate than those utilized on gasoline engines.

5.2. Dynamometer or Inertial Loading

On the basis of the difficulties associated with the low speed acceleration test and the stationary

run-up test the next logical step would be to look for a means to load an engine externally.

It has been shown that it is possible to disconnect the drive shaft of the truck at the universal

joint and connect it directly to a shaft/flywheel assembly which if properly sized can simulate the inertial load on the engine that would be observed during a J366b acceleration test. At present only

limited experience exists with such a test setup.

Dynamometers, on the other hand, do exist and are in use in some parts of the motor vehicle industry. They tend to be noisy devices in themselves; however, the operating machinery could be remotely

located. Finally, tire noise, of possibly greater magnitude than normal for a J366b test, would be

introduced into the measurement. Tires on dynamometer rolls tend to heat up and deform in a manner

which is different than when in normal operation. In addition, the tires are in a different situation

on a dynamometer roll (with a radius) than they are in actual service on a highway (with a flat surface)

The noise levels in or near existing facilities on the grounds of a truck manufacturing plant, where

dynamometers are typically located, preclude the measurements being made in existing facilities.

, . 5.3. Indoor Tests

Since the weather is a major limiting factor for all outdoor tests, it would be logical to investigate the possibility of indoor testing.

If indoor testing were to be chosen as part of the standard requirements, a testing facility would

have to be specified in great detail and identical facilities would have to be constructed by each manufacturer — be he small or large. It would appear to be prohibitively expensive to construct a facility

to allow for 15.2 metre measurements; yet, measurements any closer than 15. 2 metre would be questionable

without extensive research to provide insight into the near field/far field effects and to establish the correlation between the results obtained according to such procedures with what the community experiences when a truck passes by on a nearby highway. Also, the practical problem of removing the heat and

exhaust fumes from the operating vehicle would have to be dealt with.

The legislative time constraints which EPA is working under do not allow for consideration of such

alternative methodologies at the present time. They each have pros and cons associated with them; but,

they have almost no data base or experience base to back them up. Much research would be necessary

before such procedures could be utilized as the measurement methodology for the new truck regulation.

6. Appendix C. Projection Method for Predicting Significant Fan Operating Time [26]

Step I - Fan-On Yearly Projection

A. Plot monthly normal temperatures from World Almanac for area of truck operation

B. Plot percent of fan-on time data for each truck versus the monthly normal tem- perature curve from A above. Draw best-fit line.

C. Establish projected percent fan-on time for each month by selecting normal tem- perature at mid-point of each month, going to curve in B, and reading back the percent of fan-on time for the monthly normal temperature.

Step II - Significant Fan-On Time

A. Establish average engagement time per truck by counting number of engagements

over two randomly selected one week data periods. Divide the fan-on time data

, by the number of engagements to give the average time per engagement. The mean

of the two data periods is used.

B. Count the number of engagements for all data periods where the engine speed is above 1600 rpm. Where time frame includes variable and constant engine speed

operation, count period as above 1600 rpm if more than 50% of the time is at constant speed and below 1600 rpm if more than 50% of the time is not at constant speed.

C. To establish the amount of time for fan operation greater than 1600 rpm multiply A above x B above.

26
D. The percent fan-on time greater than 1600 rpm is C above divided by the total

fan-on time for the data period. The average for all reporting periods available is considered representative for the year. No trend could be seen versus

temperature.

E. To establish the significant fan-on time for the year, multiply the projected

percent fan-on time (I;C) by the percent fan-on time greater than 1600 rpm

(II ;D) for each month, then determine the yearly average for each truck.

7. Appendix D. Research Needs

The following is a list of projects which are needed to fill the gaps in existing knowledge as they

pertain to the accurate and repeatable characterization of the noise levels of medium- and heavy trucks. The fact that uncertainties are associated with the test method has exceedingly important consequences

on the manufacturer of motor vehicles. If a manufacturer is required to certify that the vehicles sold

by him comply with certain noise "emission regulations, then uncertainty in the measurement procedure can be costly — increasingly so as the regulations become more exacting. — There exists a need to develop comparative data utilizing J366b procedures and the newly

developed procedure to evaluate the effect of the proposed modifications on the existing

data base. Of utmost concern is the proper location of the shortened end zone (4.6 metres

In length) within the present end zone (12.2 metres in length) specified in J366b. That

is, should the shortened end zone be located at the beginning, middle, or end of the present end zone to ensure that the existing data base is not invalidated? — There exists a need to attempt to define the barometric pressure and temperature correction factors to be applied to the measured results so that all data will be reported at a standard reference temperature and barometric pressure (e.g., 1 atm and 20°C). Present

data (noise level versus temperature) exhibit a great deal of scatter due to differences

in vehicle components such as engine, fan, exhaust, etc.

There exists a need to measure the response of existing instrumentation (sound level

meters, real-time analyzers, etc.) to actual transient signals in order to establish the interrelationships among the various precision instruments, to supply data to strengthen

existing standards, and to establish a data base so that the technical community, manufacturers, lawmakers and enforcement agencies will have a common basis for comparison of results obtained using supposedly comparable equipment.

There is a need for the various environmental and test site effects on noise measurements

to be systematically investigated and correction factors developed so that measurements made under any conditions may be corrected to a single standard set of conditions. If correction factors are not feasible then there is a need for a site calibration procedure

or definition of limiting test conditions. " There exists a need to develop a test procedure that is simpler to perform and is less dependent on weather and test site variables. Correlation must also be proven between the results obtained utilizing such a test and human response to motor vehicle noise.

8. References

[1] Code of Federal Regulations , 49CFR 390.4-390.5, Federal Motor Carrier Safety Regulations: Definitions, U. S. Government Printing Office, Washington, D.C., October 1973, p. 232.

[2] Code of Federal Regulations , 49CFR 568.3, Vehicles Manufactured in Two or More Stages: Definitions,

U. S. Government Printing Office, Washington, D.C., October 1973, p. 375.

[3] Code of Federal Regulations , 49CFR 571.3, Federal Motor Carrier Safety Standards: Definitions,

U. S. Government Printing Office, Washington, D.C., October 1973, p. 383.

[4] Motor Truck Facts, Motor Vehicle Manufacturers Association of the U. S., Inc., Washington, D.C.,

1973.

[5] Wasko, R. J., Staff Engineer, Motor Vehicle Manufacturers Association of the U. S., Inc., telephone conversation, March 1974.

[6] Cosby, J. C, Heavy Duty Vehicle Driving Pattern and Use Survey: Final Report, Part I - New York

City, Report APT.D-1523, U.S. Environmental Protection Agency, Ann Arbor, Michigan and The Coordinating Research Council, Inc., New York, New York, May 1973. (Available from the National Technical Information Service, Springfield, Virginia, PB-221-656.)

27
[7] Staadt, R. L., Staff Engineer Vehicle Sound, International Harvester Company, private correspondence, January 11, 1974.

[8] U. S. Federal Register , Chap. Ill, Subchapter B, Part 393, Vehicle Interior Noise Levels, 38, No. 215, November 8, 1973, pp. 30880-30882.

[9] Sales Data Book, International Harvester Company, Fort Wayne, Indiana, June 1973.

[10] Statement of C. R. Foster, Director, Joint DOT/NASA Office of Noise Abatement, Submitted to the Office of Noise Abatement and Control, U.S. Environmental Protection Agency, Public Hearings on the Interstate Motor Carrier Noise Emission Standards (40CFR Part 202), Washington, D.C.,

March 20-21, 1974.

Adams, N. , Rubber Manufacturers Association, telephone conversation, April 30, 1974.

Society of Automotive Engineers, Inc., J366b-Exterior Sound Level for Heavy Trucks and Buses

SAE Recommended Practice (SAE, New York, New York, 1974).

Noise Control for Heavy Duty Trucks, Ford Motor Company, May 1973.

Leasure, W.A., et al. , Interior/Exterior Noise Levels of Over-the-Road Trucks: Report of Tests,

NBS Technical Note 737, U.S. Department of Commerce, National Bureau of Standards, Washington, D.C.,

September 1972. '

Sharp, B. H., Research on Highway Noise Measurement Sites, Report WCR 72-1, Wyle Laboratories, El Segundo, California, March 1972. (Available from California Highway Patrol, Sacramento, California).

Piercy, J. E. and Embleton, T. F. W. , Effect of Ground on Near-Horizontal Sound Propagation, SAE Paper 740211 (Society of Automotive Engineers, New York, New York, 1974)

.

American Society for Testing and Materials, C-384-72, Standard Test for Impedance and Absorption of Acoustical Materials by the Tube Method in Part 14, Book of ASTM Standards, 1972 (ASTM, Philadelphia, Pennsylvania).

Staadt, R. L. , Staff Engineer Vehicle Sound, International Harvester Company, private correspondence, January 24, 1974.

Bettis, R. A. and Sexton, M. Z., The Effect of Test Site Topography in Vehicle Noise Measurement,

presented at the 85th meeting of the Acoustical Society of America, Boston, Massachusetts

(April 1973).

Damlan, J. U. , Manager Vehicle Noise Programs, Ford Motor Company, private correspondence,

April 30, 1974.

Dolon, T. E. , Executive Engineer with the Advanced Product Engineering Division, General Motors

Corporation, private correspondence, March 27, 1974.

American Standard Specifications for Sound Level Meters, SI. 4-1971, American National Standards

Institute, New York, New York (April 1971).

American National Standard Methods for the Measurement of Sound Pressure Levels, SI. 13-1971,

American National Standards Institute, New Yor, New York (July 1971).

Cummins, D. D., The Jacobs Engine Brake Application on Performance, SAE Paper 660740 (Society of Automotive Engineers, New York, New York, 1966).

Close, W. H. , Chief, Research Division, Office of Noise Abatement, U.S. Department of Transportation, private correspondence, February 28, 1974.

Landis- E. E., International Harvester Company, private correspondence, April 19, 1974.

Mills, C. H. G. and Robinson, D. W, , The Subjective Rating of Motor Vehicle Noise, The Engineer,

211 , No. 5501, June 1961, pp. 1070-1074.

Callaway, D. B. and Hall, H. H., Laboratory Evaluation of Field Measurements of the Loudness of Truck Exhaust Noise, J. Acous. Soc. Am., 26, No. 2, 1954, pp. 216-220.

Galloway, W. J., et al.. Highway Noise: Measurement, Simulation and Mixed Reactions, NCHRP Report

78, Highway Research Board (NAS-NRC) , Washington, D. C. 1969.

28
[30] Proceedings of the Conference on Motor Vehicle Noise, General Motors Corporation, June 1973.

[31] Hillquist, R. K. and Bettis, R. A., Measurement of Automotive Passby Noise, SAE Paper 720275

(Society of Automotive Engineers, New York, New York, 1972).

[32] Transportation Noise and Noise from Equipment Powered by Internal Combustion Engines, Report No. NTID 300.13, December 1971, U.S. Environmental Protection Agency, Washington, D. C.

[33] Warnix, J. L. and Sharp, B. H., Cost Effectiveness Study of Major Sources of Noise, Volume 1, Medium and Heavy Trucks.

[34] Staadt, R. L. , Truck Noise Control, SAE Report SP-386 (Society of Automotive Engineers, New York, New York, 1974)

[35] Close, W. H. and Atkinson, T., Technical Basis for Motor Carrier and Railroad Noise Regulations,

Sound and Vibration, October 1973.

29
NBS-n4A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUHIJC ATION OR RIvPORT NO. NBSIR-74-5172. Ciov't Accession

No.

4. TITLF AND SUBTITI.I' Methodology and Supporting Documentation for the Measurement

of Noise from Medium and Heavy Trucks

3. Recipient's Accession No.

5. Publication Date June 1974

6. Performing Organization Code

7. AUTHOR(S)

W. A. Leasure, Jr. and T. T.. Qninrlry

8. Performing Organ. Report No. NBSIR-74-517

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 2130491

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Office of Noise Abatement and Control

U.S. Environmental Protection Agency

Washington, D.C. 20460

13. Type of Report & Period Covered

FINAL

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

This report contains a measurement methodology which defines the procedures by

which the noise from mediiam and heavy trucks, excluding tire noise, may be

measured to ascertain whether the noise emitted is in compliance with noise emission standards to be promulgated by the U.S. Environmental Protection Agency

pursuant to Section 6 of the Noise Control Act of 1972 (P.L. 92-574). Associated

supporting technical documentation for the methodology is included.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Acoustics (sound); measurement methodology; noise emission standard; noise measurement, truck.

18. AVAILABILITY Unlimited

I X! For Official Distribution. Do Not Release to NTIS

I I Order From Sup. of Doc, U.S. Government Printing Office Washington, D.C. 20402, SD Cat. No. CH

I ! Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151

19. SECURITY CLASS

(THIS REPORT)

UNCL ASSIFIED

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

21. NO. OF PAGES

22. Pr

USCOMM-DC 29042-P74