NBSIR 75-964

An Evaluation and Assessment of

Existing Data and Procedures for

the Measurement of Noise from

Truck Transport Refrigeration

Units

Applied Acoustics Section

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

December 1975

Final Report

Prepared for Office of Noise Abatement and Control

U. S. Environmental Protection Agency

Washington, D. C. 20460

NBSIR 75-964

AN EVALUATION AND ASSESSMENT OF

EXISTING DATA AND PROCEDURES FOR

THE MEASUREMENT OF NOISE FROM

TRUCK TRANSPORT REFRIGERATION

UNITS

Applied Acoustics Section

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

December 1975

Final Report

Prepared for Office of Noise Abatement and Control

U. S. Environmental Protection Agency

Washington, D. C. 20460

U.S. DEPARTMENT OF COMMERCE, Rogers C.B. Morton, Secretary

James A. Baker, III, Under Secretary

Dr. Betsy Ancker-Johnson, Assistant Secretary for Science arid Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler. Acting Director

Table of Contents

Page

Executive Summary ii

Acknowledgements lv

1. Introduction and Scope 1

2. Description and Design Characteristics of Truck Transport

Refrigeration Units 2

2.1. Definition 2 2.2. Design Characteristics/Normal Operation 2

3. Effects of Noise and Parties Affected 4

4. Existing Measurement Procedures and Data Base 4

5. Overview of Truck Transport Refrigeration Unit Noise

Measurement Difficulties 5

5.1. Characteristics of the Radiated Sound 5 5.2. Choice of Appropriate Metric 8

5.2.1. Relationship of Appropriate Metric to the Subjective

Data Base 8

5.2.2. Quantity to be Measured 8

5.3. Noise Measurement Considerations 10

5.3.1. Facilities and Measurement Procedures 10

5.3.2. Special Site and Environmental Considerations ... 12

5.3.3. Provision of a Physical Data Base 13

5.4. Summary 14

6. References 14

i
Executive Summary

EPA is charged with taking strong, comprehensive action to protect

public health and welfare from increasing noise. Accordingly, the Agency

initially identified medium and heavy trucks and portable air compressors

as major noise sources and promulgated noise emission regulations covering

these products. Additional products have recently been identified as major noise sources, including truck transport refrigeration units (truck

mounted auxilliary equipment) . Although EPA recognizes that the noise

impact from truck transport refrigeration units alone is not as great as some other noise sources, control of this source is required to avoid

reducing the effectiveness of actions already taken to control noise

emissions from medium and heavy trucks.

This report reviews existing noise measurement procedures with regard

to their usefulness in the regulation of truck transport refrigeration

noise as well as the availability, extent and applicability of existing

data. On the basis of this review, the following probable or potential

measurement difficulties have been identified that could hinder the

promulgation and/or enforcement of future EPA regulations to control the

noise emission from truck transport refrigeration units.

# NEED TO ESTABLISH A SUBJECTIVE DATA BASE . There exists a need to

conduct a psychoacoustic study to determine what characteristics of

the truck transport refrigeration unit noise spectrum annoy people.

A more complex measure than the A-weighted sound level may be

necessary. This subjective data base is necessary before a suitable

physical measure can be selected which correlates with human response

and before an appropriate measurement procedure can be established.

# NEED TO DEFINE APPROPRIATE METRIC . There exists a need to define

the appropriate acoustic quantity (ies) to be measured. Since the

frequency spectrum typical of a diesel-powered truck transport

refrigeration unit is dominated by the low frequency (less than 100

Hz) fundamental associated with the engine firing frequency, the use

of the A-weighted sound level as a metric must seriously be

questioned. The use of a simple metric such as the A-weighted sound

level may result in a situation in which adequate incentives for

reduction of low frequency components of the noise are not provided.

# NEED TO DEVELOP AN APPROPRIATE MEASUREMENT PROCEDURE . No one standardized procedure presently exists which can be directly

utilized for measurement of truck transport refrigeration unit noise.

Adequate data do not exist to allow one to decide whether the noise

emission from these units should be regulated on the basis of the

sound pressure level at a particular location, or set of locations,

or on the sound power output of the device. It is not possible to

select either the appropriate acoustic quantity to be measured or the

appropriate measurement procedure and test environment without

adequate subjective and physical data bases.

ii
# NEED TO ACCOUNT FOR ENVIRONMENTAL/ SITE EFFECTS . If an outdoor test

environment (e.g., free field over a reflecting plane) is deemed

appropriate, there is a need for the various environmental and test

site effects on noise generation, radiation and/or propagation 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. If an indoor test environment (e.g., indoor

semi-anechoic or reverberant) is chosen, then it is necessary to

establish the relationship between ,! real world" noise radiation

characteristics such as directionality or possible secondary source radiation due to vibration of the truck body and the characteristics

observed in the indoor environment.

# NEED TO DEFINE PROCEDURE FOR OBTAINING MAXIMUM UNIT LOADING . The

maximum cooling capacity, and therefore maximum unit loading, for

truck transport refrigeration units is typically specified for an outside air temperature of 100°F (37.8°C). If an outdoor test

procedure is eventually selected some alternative environmental

loading procedure will have to be specified. If, an indoor test

utilizing a well-characterized acoustic environment is selected, the

chamber will have to be capable of supplying the required thermal load

to the refrigeration unit and be capable of handling the exhaust

effluent from the engine.

# NEED TO ESTABLISH A PHYSICAL DATA BASE . Almost no objective data

exist in the public domain on the noise levels associated with truck

transport refrigeration unit operation.

In summary, existing data — both objective and subjective — are not

sufficient to allow EPA to (1) select a suitable physical metric that

correlates with human response, (2) establish an appropriate measurement

procedure, and (3) perform a comprehensive analysis of the economic costs

and technical feasibility associated with a given regulation. Thus, it

appears that EPA should generate an extensive physical and psychoacoustic

data base and investigate alternative measurement procedures prior to

formulation of the Notice of Proposed Rule Making.
Acknowledgements

The National Bureau of Standards (NBS) gratefully acknowledges the

assistance of the individuals and organizations whose contributions have

helped make it possible to produce this report.

Of great assistance to the NBS was the Air Conditioning and

Refrigeration Institute's (ARI) Sub-Committee on Transport Refrigeration

Sound which provided technical expertise and back-up data — both acoustic

and product. The sub-committee members included: Mr. Charles E. Ebbing

and Mr. L. Thomas Lane (Carrier Corporation), Mr. Donald D. King

(Thermo-King Corporation), Mr. Daniel L. Saxton (Therm-Air Mfg. Co.,

Inc.), Mr. Robert G. Harold and Mr. Richard Vaughn (The Trane Company) and

Mr. Robert Stabley (York). We would also like to thank Mr. Robert J.

Evans (ARI) who assisted in assembling this sub-committee.

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

iv
An Evaluation and Assessment of Existing Data and Procedures for the

Measurement of Noise from Truck Transport Refrigeration Units

This report reviews existing noise measurement procedures with regard

to their usefulness in the regulation of truck transport refrigeration

noise as well as the availability, extent and applicability of existing

data. On the basis of this review, probable or potential measurement

difficulties are identified that could hinder the promulgation and/or

enforcement of future EPA regulations to control the noise emission from

truck transport refrigeration units.

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

standard; noise measurement; refrigeration; truck transport

refrigeration unit.

1. Introduction and Scope

EPA has identified^ truck transport refrigeration units as a major

noise source. Even though EPA recognizes that the noise impact from such

special purpose equipment alone is not as great as some other noise

sources, control of this source is required to avoid reducing the

effectiveness of actions already taken to control noise emissions from

medium and heavy trucks.

The National Bureau of Standards (NBS), under the sponsorship of the

Office of Noise Abatement and Control (ONAC) , U. S. Environmental

Protection Agency (EPA), has attempted to identify probable or potential

measurement difficulties that could hinder the promulgation and/or

enforcement of future noise regulations to control the noise emissions

from truck transport refrigeration units. A search of the open literature

in conjunction with numerous industrial and private sector contacts

established the basis for discussion of:

1. The basic characteristics of truck transport refrigeration

systems and their design.

2. The effects of noise emission from such units and the parties

affected.

3. The normal or typical mode of operation characteristic of

various types of refrigeration systems.

4. The usefulness of existing measurement procedures for use in

regulation of the noise from truck transport refrigeration units

considering the viewpoint of EPA, manufacturers and enforcement

personnel.

5. The availability, extent and applicability of existing data that

could be utilized by EPA in their efforts to promulgate noise

emission regulations for truck transport refrigeration units.

—See Federal Register, Vol. 40, No. 103 - Wednesday, May 28, 1975.
This report is limited to those factors related to the measurement of

truck transport refrigeration unit noise. EPA/ONAC will independently

investigate the technical feasibility and economic implications of truck

transport refrigeration unit noise regulation.

2. Description and Design Characteristics of Truck

Transport Refrigeration Units

2.1. Definition

A working definition for truck auxiliary equipment was established at

the Williamsburg Conference on Noise Research[l]— , which was sponsored by

the Vehicle Research Institute, Society of Automotive Engineers (SAE) —

"Auxiliary (ancillary) equipment shall consist of any device capable of

generating noise which is mounted on (as opposed to readily detachable

from) a motor truck, trailer, or truck-trailer combination and which is

utilized to perform some task, subsidiary to normal truck propulsion

functions." The important concept is the fact that only the noise

generated by the truck transport refrigeration unit itself is of interest;

truck passby noise is already covered under previous EPA actions.

2.2. Design Characteristics/Normal Operation

Truck transport refrigeration units provide the cooling or heating

necessary to maintain various types of perishable (temperature dependent)

commodities. The majority of these units are front mounted on trailers or containers; however, under-mounted, side-mounted and roof -mounted

configurations are also available.

Most refrigeration units have automatic thermostatic temperature

control which maintains a pre-selected temperature through cooling or heating as necessary. The units contain both an evaporator and a condenser section. The evaporator section, which generally extends into

the trailer or container, typically consists of an evaporator coil, fan,

heat exchanger and expansion valve while the condenser section typically

contains the engine, compressor, condenser coil, fan, radiator, generator

or electric standby motor and unit controls. The engine, compressor and

standby motor, when used, are installed on a vibration isolated base.

This resilient-mounted system serves to reduce noise caused by the

refrigeration unit's engine vibration.

In general, these units have a maximum of two possible operational

modes — high speed operation (e.g., 2200 rpm) and low speed operation

(e.g., 1400 rpm) — for either heating or cooling. Single speed models

also exist. The net cooling capacity (air to coil) and maximum engine

speed for these units is typically specified for an ambient air

temperature of 100°F (37.8°C).

Truck transport refrigeration systems are either self-contained

— Numbers in brackets indicate the literature references at the end of

this report.

2
(i.e., run off their own power-plant) or depend upon the vehicle engine

for operation.

Self-contained units normally employ one of the following engine/ com- pressor configurations

• Diesel engine driven compressor unit.

• Diesel engine driven compressor unit with standby electric motor.

• Gasoline or propane engine driven compressor unit.

• Gasoline or propane engine driven compressor unit with standby

electric motor.

• All electric-driven compressor unit.

Most of the refrigeration units that depend on the vehicle power-plant are typically restricted to single-chassis vehicles rather than truck-trailer

combination vehicles. These type units can be categorized as follows:

• Truck engine (gasoline or diesel) driven compressor unit.

• Truck engine (gasoline or diesel) driven compressor unit with

electric standby motor.

•Hydraulic driven compressor unit from truck engine.

•Power take off driven compressor from truck transmission.

All units tend to be quietest when running in the electric-powered mode.

The engine exhaust for self-contained units is directed from the

engine manifold, through a muffler, which is generally located within the

refrigeration unit housing, and out the top or side of the unit depending

on the particular mounting configuration. An air intake is usually

located on the side of the condenser section.

The maximum brake horsepower for truck transport refrigeration units

ranges from 8 to 32 hp. The diesel engine is capable of generating the

maximum horsepower, while the other engines generally fall in the range of

8-15 hp.

Since refrigeration units are designed for mounting on various types

of trucks, a convenient categorization is according to intended use: — Single-chassis truck units — Trailer units — Container units

3
Single-chassis (or "straight") truck mounted units are utilized on light,

medium and heavy van type vehicles typically involved in short haul operations. For long haul applications, units are typically mounted in the

trailer of a truck-trailer combination vehicle. Containers refer to a special class of storage container which may be transported by means of a flat-bed truck, railroad car or by ship. Many of these identical units

can also be utilized on refrigerated railway cars and on prefabricated

warehouses

.

3. Effects of Noise and Parties Affected

It is important to identify the parties affected by truck transport

refrigeration unit noise and the nature of the effects. Tn general, com- plaints have come from areas where commodity distribution or storage

facilities, such as warehouses or dairies, are located in close proximity

to residential areas. At such facilities, large numbers of trucks and/or

trailers equipped with refrigerator units are required to operate in order

to keep perishable commodities from spoiling until they can be unloaded

from their storage containers or for truck pre-cooling prior to loading.

Although hearing loss is not a potential problem for persons living near

such facilities, task interference and/or annoyance can result from the

noise from truck transport refrigeration unit operation. These units are considered especially annoying to the neighboring residents during the

late evening due to otherwise low ambient conditions.

A critical deficiency at the present time, is that the subjective

data do not exist to determine conclusively if the complaints are primarily due to the characteristic low frequency spectral peak

(associated with the engine firing frequency), to the change in noise

level as the u^^y is thermostatically switched from low speed to high

speed operation— and vice-versa, or to some other complex spectral

characteristic

.

Without such information it is not possible to select the appropriate

acoustic quantity to be measured or an appropriate measurement procedure,

since the objective of the measurement data is to permit a reliable

prediction of acceptable noise emission characteristics.

4. Existing Measurement Procedures and Data Base

There are no standardized measurement procedures directly applicable

to the measurement of the noise generated by truck transport refrigeration

uniLS anu, as a consequence, there is practically a non-existent data base

available in the public domain on the noise levels associated with these

units. This is due, in part, to the fact that in the past the noise

levels of the auxilliary equipment tended to be lower than the noise level

— To a person located in a house with the windows closed, the automatic

change from low speed operation to high speed operation may be perceived as simply an off-on operation of the unit.
of the vehicle on which they were mounted and little effort has been made

to isolate auxilliary equipment as a potential major noise source and to

determine the environmental impact associated with the operation of the

auxilliary equipment as a separate source.

Discussions with manufacturers indicates that some proprietory data do

exist; however, these data are not sufficiently extensive to provide EPA

with even a fraction of the information needed for a comprehensive analysis

of the economic costs and technical feasibility associated with a given

regulation. Furthermore, it is evident that there are essentially no data

available upon which to base the establishment of an appropriate

measurement procedure for truck transport refrigeration units.

Thus, it appears that EPA should investigate alternative measurement

procedures and should generate an extensive physical data base prior to

formulation of the Notice of Proposed Rule Making.

5. Overview of Truck Transport Refrigeration Unit

Noise Measurement Difficulties

5.1. Characteristics of the Radiated Sound

There is little question that subjective response to noise is due to some measurable characteristics of the radiated sound. While there is

little reliable technical data at the present time, it is prudent to con- sider the characteristics of the sound which are important both to the

subjective response (perhaps resulting in task interference or annoyance)

and to the related measurement difficulties.

There are two principal characteristics which are important

the time variability of the sound, and

the frequency spectrum.

Refrigeration units typically cycle from high to low speed operation

in response to varying thermal demands. This cyclic variation in engine

speed results in a corresponding cyclic variation in the sound. While the

details of the changes in sound levels are subject to unit-to-unit

variability and to the metric selected, at a 50 foot (15.2 m) distance from

a typical truck mounted refrigeration unit, the A-weighted sound levels

have been shown to vary from 65.5 dB to 72 dB — a difference in level of

6.5 dB. Considering the "linear" levels, the difference in levels amounts

to 8.5 dB . These differences in level are substantial enough that the

subjective loudness differential may be nearly a factor of two to one.

Because of the wide range of this time variation in sound level, and

because of the cyclic nature of the variations, it is probable that

subjective annoyance to the sound may be higher than if the levels were

constant. In effect, the time variation draws listeners attention to the

noise source at each transition between operating conditions.

5
An adequate psychoacoustic data base for the prediction of annoyance

from a sound source with this type of time variability does not exist, and

should be obtained prior to selection of the appropriate metric. These

psychoacoustic considerations should dictate whether or not special

consideration ought to be made of the magnitude and temporal duration of

the cyclic variations in sound emissions.

The frequency spectrum of the sound emitted by truck mounted diesel

engine powered refrigeration units appear (on the basis of the limited data

base presently available) to be characterized by strong discrete frequency

or pure tone components at the fundamental firing frequency of the diesel

engine as shown in Figure 1.

This line component may be as much as 20 dB higher than any other com- ponent in the spectrum, which gives rise to severe measurement problems due

to the possibility of interference effects in the measurement of sound

pressures at discrete locations in a free field over a reflecting plane

measurement environment, or due to the requirement for extremely large

facilities and stringent performance criteria for the measurement of sound

power using reverberation rooms.

Furthermore, the frequency of the line component is correlated with

operating speed, and when operating speed is lowered for the low load con- dition, the frequency is reduced and the detailed nature of the

interference effects change.

In addition to these complications, if the A-weighting network is

employed for measurement of the sound pressure, the low frequency roll-off

characteristic of this weighting curve is such that the resultant sound

pressure levels are not principally determined by the large amplitude

discrete frequency components. Thus, even if the spectrum is radically

changed by the incorporation of sufficient muffling capacity to

substantially reduce the strength of the line component, the A-weighted

sound pressure levels may not be changed.

The simple use of the A-weighting characteristic may not provide much

incentive on the part of industry for the reduction of the low frequency

line component through better muffling of engine exhaust.

Psychoacoustic studies to determine whether the presence of such a prominent low frequency tone may lead to a high degree of annoyance, and

for which specific pure-tone correction procedures ought to be employed as part of the appropriate metric, are essential elements prior to selection

of the metric to be employed by EPA in its regulatory actions. Further

subjective response considerations must account for the greater annoyance

which is probable when these units are operated during evening hours,

although these operational considerations do not present difficult physical

measurement problems.

6
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

FREQUENCY, kHz

Figure 1. Refrigeration unit noise at maximum load at 50 feet (15.2 m) from

truck mounted unit, directly in line with truck body[2]. (Bandwidth of the analysis used to obtain these data was not specified.)

7
5.2. Choice of Appropriate Metric

5.2.1. Relationship of Appropriate Metric

to the Subjective Data Base

The physical characteristics of the radiated sound have been shown to

include an unusually strong, discrete, low-frequency component and a cyclic

time variation in the level and spectral characteristics. These

considerations point to the possible inadequacy of simple measures such as A-weighted sound pressure level or equivalent sound level as reliable

predictors of subjective response. Since the choice of an appropriate

metric must be one that provides good correlation between the identified

acceptable emission level and generally acceptable subjective reaction, it

is imperative to obtain a relevant psychoacoustic and/or subjective data

base upon which the choice of an appropriate metric can be based.

5.2.2. Quantity to be Measured

Measurements of sound pressure must be conducted in order to

quantitatively characterize the noise emissions; however, the selected

metric can be expressed in terms of either sound pressure level or sound

power level.

The principal distinction between metrics based upon sound power or sound pressure level lies in the fact that sound power levels represent a spatially averaged or integrated measure of sound pressure emissions;

whereas metrics based upon sound pressure levels are inherently sensitive

to the details of the directionality of sound radiation. Therefore, when

it is apparent that a source may be highly directional, it is important to

clearly specify the directionality characteristics together with any

(spatially averaged) sound pressure data. For moderately directional

stationary sound sources, there is little preference between the two

metrics in terms of ability to predict levels at remote locations.

There is some evidence to indicate that truck transport refrigeration

units exhibit moderate directional radiation, although detailed data,

characterizing an adequate sample of these units in many frequency bands,

are not yet available. Figure 2 illustrates the nature of the

directionality data for one unit for both A-weighted, linear and several

octave bands.

It will be important, in the event that the chosen metric specifies

sound pressure levels, that the associated measurement procedure

appropriately identifies the presence of high sound pressure level lobes

and that either appropriate spatial averaging or specification of

measurement at the direction of maximum sound pressure levels be required.

It is important to note, however, that sound power inherently accounts for

the possible presence of source directionality since it is a measure of

spatially integrated source emission.

8
210° 200° 190° 180° 170° 160° 150°

330° 340° 350° 0° 10° 20° 30°

Figure 2. Directional radiation patterns for truck-mounted transport

refrigeration units (A-weighted, flat response, and 63 Hz, 250

Hz, 1 kHz, and 4 kHz octave band data). Unit mounted on truck

facing toward 180°; measurements taken at 50 feet (15.2 m) from

unit. Note that the original data shows flat response 0.5 dB

less than the sound pressure level in the 63 Hz octave band at

180°[2]

.
5.3. Noise Measurement Considerations

5.3.1. Facilities and Measurement Procedures

Facilities for the measurement o refrigeration units can be classified

differing in their environmental and

(a) Outdoors, in situ , or free

(b) Indoors, free field over a

f noise emissions from truck transport

into three basic categories,

acoustical properties:

field over a reflection plane.

reflecting plane.

(c) Indoor, reverberation room.

Specification of one or more of these test facilities for the

measurement is complicated by the existence of mutually exclusive or contradictory preferential factors. For example, thermal load control and

control of external environmental factors are important to accurate

specification of test conditions. These factors favor the use of one of

the indoor test facilities. However, because of the presence of engine

exhaust, indoor semi-anechoic facilities could potentially suffer wedge

soiling with attendant acoustical performance degradation. Indoor

reverberation room facilities also would experience wall soiling.

Furthermore, special considerations are required in the design, operation,

and maintenance of indoor facilities due to the presence of the exhaust

fumes from many of these units. These considerations favor the use of

outdoor test facilities.

Two physical factors indicate that indoor facilities, if selected,

must be large. The presence of discrete frequency sources in reverberation

rooms imposes special performance limitations upon the facility. Unusually

large reverberation room facilities may be required to obtain truly

reverberant conditions in the 40-100 Hz range. To some extent the source directionality characteristics as well as the absolute levels are affected

by the presence of the truck or trailer body. These effects are due not

only to the acoustic baffle effects provided by the large truck or trailer

body, but also may include secondary radiation due to structural resonances excited by vibrations induced by unit operation. Unless it can be shown

that directionality data are unimportant, that effects due to baffling by

the truck body are predictable, and that, for typical units, secondary

radiation from the truck or trailer body is negligible, the tests should be

conducted with the units mounted on trucks or trailers. Indoor facilities

large enough to accomodate such large units are not common. No unusual

requirements exist for outdoor facilities, except in terms of

characterization of the acoustical properties of the reflecting surface.

In the presence of discrete frequency sources, interference effects

between the directly propagating sound energy and the sound energy

reflected off the ground, test pad surface, or reflecting plane in general

can lead to appreciable errors in the measurement of sound pressure levels.

10
Careful characterization of the acoustical properties of the reflecting

surface are essential, though difficult to specify and obtain for the

typical out-of-doors ground surface. Qualification procedures for the

acoustic environment such as those contained in several recent draft

international and national standards [3-5] provide an important means to ensure that the measurement uncertainty due to reflecting plane effects will

not be excessive.

Errors due to reflecting plane interference effects can be minimized by

conducting measurements "close in" to the source (in the acoustic "near

field") in order to estimate the acoustic power. In this case it may not be

critical whether the surrounding environment is precisely specified and well

controlled, which introduces a relaxation in the stringency of test site

specification and may result in reduced facility cost. However, it has not

been conclusively established at the present time that near field

measurements allow an adequately accurate extrapolation of the data to yield

the far field pressure levels, which are usually of principal interest.

Once an appropriate metric is selected and a facility has been

specified, a decision must be reached as to whether or not the metric is to

be expressed in terms of sound pressure level at a particular,

well-specified location or set of locations or in terms of the sound power

level. While it is realized that direct measurements of sound power

emissions are not possible, estimation of power emission is possible using

several measurement methodologies, and specification of sound power level

enables ready estimation of levels at large distances, without the

requirement for specification of directionality and/or source/receiver

orientation information.

Several measurement procedures can be cited as potentially relevant.

American National Standard Methods for the Measurement of Sound

Pressure Levels, S1.13-1971[6]

.

American National Standard Method for the Physical Measurement of

Sound, S1.2-1962[7]

.

Draft International Standard Acoustics - Determination of Sound

Power Levels of Noise Sources - Engineering Methods for Free Field

Conditions over a Reflecting Plane, ISO/DIS 3744[3].

American National Standard Methods for the Determination of Sound

Power Levels of Small Sources in Reverberation Rooms,

S1.21-1972[5]

.

Draft International Standard Acoustics - Determination of Sound

Power Levels of Noise Sources - Precision Methods for

Discrete-Frequency and Narrow Band Sources in Reverberation Rooms,

ISO/DIS 3742 [4].

11
However, no one of these measurement procedures is specifically directed

to this type of source. Thus, an adaptation of these measurement

procedures to the specific chosen metric and facility, appropriate to the

special constraints imposed by the nature of these sources, will have to

be performed prior to regulation of the noise emission.

5.3.2. Special Site and Environmental Considerations

It is generally impossible to control all of the important

environmental and site parameters affecting data at outdoor test sites.

Therefore, correction factors are often desirable so that data can be

"corrected" or converted to indicate the values which would have been

obtained under a single, standard, set of test conditions. Parameters

which are known to affect the generation and radiation of sound include

atmospheric parameters (which affect the operation of internal combustion

engines) and mechanical properties of the mounting surfaces (which may

involve the vibration response of the truck or trailer body) . A parameter

important to the propagation of sound is the characteristics of the

"reflective plane". In particular, if the surface is paved, it may

degrade mechanically due to wear and weathering effects. If the surface

is grass covered, the condition of the grass may vary considerably. In

both cases, the acoustical properties may be essentially uncontrolled. A

further environmental parameter which is difficult or impossible to

control is the detailed nature of temperature and wind gradients above the

reflecting plane. These gradients may vary appreciably throughout the course of a day or even during a brief series of tests.

The development of suitable correction factors to account for these

environmental and site variables is an area where research is necessary.

If the use of correction factors is not feasible, then there is need for a site calibration procedure or a highly specific (and possibly severely

restrictive) limiting set of test conditions.

Indoor test environments are not representative of the environment in

which truck mounted refrigeration units are typically operated. Thus, if

a" indoor test site is selected, it is necessary to establish the

relationship between the characteristics observed in the indoor test

environment and the "real world". In particular, it would be necessary to

determine whether source directionality effects and possible secondary

sources of radiation would be altered due to the absence of the truck

body

.

It is desirable to ensure that noise emission data are obtained under

conditions of maximum thermal loading. These conditions are typically

obtained at temperatures on the order of 100°F (37.8°C), which are not

frequently encountered in large portions of the United States. Thus if

the chosen metric requires a measurement methodology predicated upon use

of an outdoor test site, it will be essential to specify some alternative

environmental loading condition. If, on the other hand, an indoor test

methodology is selected, the facility must have the capability of

12
supplying the required thermal load to the refrigeration unit in addition

to handling the exhaust effluent from the engine.

5.3.3. Provision of a Physical Data Base

A physical data base is an essential element in the reduction of noise

radiated by truck transport refrigeration units. This data base is

essential for at least two reasons:

(a) An adequate data base is required for the identification of

acceptable emission levels, using the appropriate metric and

measurement methodology. The present data base is so limited

that adequate accounting for such factors as unit-to-unit

variability, details of mounting and operational characteristics,

directionality, etc. — all of which will be vital to effective

implementation and enforcement of noise emission regulations —

is not possible.

(b) Only a limited amount of data is available to the manufacturers

for guidance in implementation of noise control programs. These

data are to a large extent, proprietary, and are not available to

many of the manufacturers. While the technology exists for a modest reduction of noise emissions, the cost effectiveness of

existing noise control technology has not been adequately

determined, and a substantial reduction of unit noise will

require extensive redesigns in most cases.

Provision of an adequate physical data base should incorporate studies

of the following factors:

(1) Noise versus load

(a) Engine noise alone

(b) Fan and compressor alone

(c) Total unit

(2) Unit directivity

(a) Octave band or one-third octave band; with trailer

(b) Octave band or one-third octave band; without trailer

(c) A-weighted and/or "linear"; with trailer

(d) A-weighted and/or "linear"; without trailer

(3) Sound power output

(a) With trailer

(b) Without trailer

(4) Correlation of indoor and outdoor tests

»

13
(5) Study of environmental factors in outdoor tests

(6) Effects of mounting rigidity upon noise emission

(7) Measurement uncertainties associated with

(a) Outdoors

(b) Semi-anechoic

(c) Reverberant measurement environments

5.4. Summary

In summary, at the present time existing data bases — both

subjective and objective — are not sufficient to allow EPA to (1) select

a suitable physical metric that correlates with human response, (2)

establish an appropriate measurement procedure, and (3) perform a comprehensive analysis of the economic factors and technical feasibility

associated with a given proposed regulation. Thus it appears that EPA

must generate an extensive psychoacoustic data base as well as obtain an adequate physical data base prior to formulation of the Notice of Proposed

Rule Making.

6. References

[1] The Williamsburg Conference on Noise Research, Vehicle Research

Institute, Society of Automotive Engineers, Inc., New York, New York

(1973)

.

[2] Diesel-powered Heavy-Duty Refrigeration Unit Noise, Donaldson

Company, Inc., Minneapolis, Minnesota (1975).

[3] Draft International Standard Acoustics - Determination of Sound Power

Levels of Noise Sources - Engineering Methods for Free Field

Conditions over a Reflecting Plane, ISO/DIS • 3744

.

[4] Draft International Standard Acoustics - Determination of Sound Power

Levels of Noise Sources - Precision Methods for Discrete-Frequency

and Narrow Band Sources in Reverberation Rooms, ISO/DIS 3742.

[5] American National Standard Methods for the Determination of Sound

Power Levels of Small Sources in Reverberation Rooms, SI. 21-1972,

American National Standards Institute, New York, New York (1972).

[6] American National Standard Methods for the Measurement of Sound

Pressure Levels, SI. 13-1971, American National Standards Institute,

New York, New York (1971).

[7] American National Standard Method for the Physical Measurement of

Sound, SI. 2-1965, American National Standards Institute, New York,

New York (1962)

.

USCOMM-NBS DC

14
N BS-1 1 4 A (REV. 7-7J)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBSIR 75-964

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

AN EVALUATION AND ASSESSMENT OF EXISTING DATA AND

PROCEDURES FOR THE MEASUREMENT OF NOISE FROM TRUCK

TRANSPORT REFRIGERATION UNITS

5. Publication Date December 1975

6. Performing Organization Code

7. AUTHOR(S)

B. R. Fuller, W. A. Leasure, Jr., and D. S. Pallett

8. Performing Organ. Report No. NBSIR 75-964

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 2000451

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Srreef, 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 Report

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 reviews existing noise measurement procedures with regard

to their usefulness in the regulation of truck transport refrigeration noise

as well as the availability, extent and applicability of existing data. On

the basis of this review, probable or potential measurement difficulties are identified that could hinder the promulgation and/or enforcement of future

EPA regulations to control the noise emission from truck transport refrigeration

units

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; refrigeration; truck transport refrigeration unit.

18. AVAILABILITY ("^ Unlimited

[XJ '' or Official Distribution. Do Not Release to NTIS

Order From Sup. of Doc, U.S. Government Printing Office Washington, D.C. 20-102, SD Cat. No. CH

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

20

22. Price

U5COMM-DC 29042-P74