) NATIONAL BUREAU OF STANDARDS REPORT

6387

A STUDY OF

STANDARD TEST PROCEDURES FOR SOILING ACOUSTIC TILE

by

Selden D. Cole

and

Paul R. Achenbach

Report to

Engineering Division

General Services Administration

Washington, D. C.

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

Functions pnd Activities

The functions of the National Bureau of Standards are set forth in the Act of Congress, March

3, 1901, as amended by Congress in Public Law 619, 1950. These include the development and

maintenance of the national standards of measurement and the provision of means and methods

for making measurements consistent with these standards; the determination of physical constants and properties of materials; the development of methods and instruments for testing materials,

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Inquiries regarding the Bureau’s reports should be addressed to the Office of Technical Information, National Bureau of Standards. Washington 25, D. C.
NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT

1003-20-4836

NBS REPORT

April 23, 1959 6387

A STUDY OF

STANDARD TEST PROCEDURES FOR SOILING ACOUSTIC TILE

by

Selden D. Cole and Paul R. Achenbach

Air Conditioning, Heating, and Refrigeration Section

Building Technology Division

to

Engineering Division

General Services Administration

Washington, D. C.

IMPORTANT NOTICE

NATIONAL BUREAU OF E intended for use within th to additional evaluation am

listing of this Report, eithe the Office of the Director, however, by the Governmei

to reproduce additional co[

U. $. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 201 5.

progress accounting documents

:ormally published it is subjected

g, reproduction, or open-literature

ission is obtained in writing from Such permission is not needed,

y prepared if that agency wishes

A STUDY OF

STANDARD TEST PROCEDURES FOR SOILING ACOUSTIC TILE

by

Selden D. Cole and Paul R. Achenbach

1

.

Introduction

After several discussions between members of the Design

and Construction Division, Public Buildings Service, and members of the staff of the National Bureau of Standards, the

study of the mechanisms by which acoustic tile become soiled

with air-borne particulates was reactivated for the purpose

of developing a standard test procedure for comparison of the

susceptibility of various commercial products to soiling.

The following tasks were envisioned as necessary steps

in developing such a standard test procedure:

1. Modify the existing soiling enclosure to provide an

octagon shape of the interior space and insulate the walls of

the test structure.

2. Rebuild the dust injector and blower system to permit standardization of air flow rate and dust loading of the

air.

3. Select or develop a standard air contaminant.

4. Select or determine standard air delivery rates and

velocities, and length of exposure.

5. Determine reproducibility of the results during

repetitive tests, and the resolution of the methods used.

6. Evaluate the usefulness of the methods proposed by

measuring the soiling characteristics of white plaster and

six categories of commercial acoustic tile when exposed to

the mechanisms of impingement, filtration, and thermal precipitation.

A study of the effect of repetitive painting of acoustic

tile on its sound absorption coefficient was included in the

program even though this effect is only indirectly related

to the remainder of the project.

- 2 -

Earlier work on the apparatus required to produce controlled soiling of such surfaces indicated that difficulties

might be encountered in obtaining satisfactory dispersal of

the test dust in air and in producing uniform soiling for the

eight mounting stations during any given test and reproducible

results during successive tests under identical test procedures .

2 . Apparatus Development

The development of a standard test procedure for measuring the susceptibility of acoustic tile to dust soiling

involved modification of existing test equipment, and the design and construction of some new components for the apparatus.

Modification and development of apparatus was carried out primarily on the test enclosure, tile mounting, dust dispersal

mechanism, the equipment for discoloration measurement, and

the components for aerosol impingement on the test specimens.

2 . 1 Test Enclosure

The test apparatus that had been used for previous studies

of soiling of acoustic tile was modified in two respects. Whereas

the plywood enclosure for soiling tile specimens had been square

in plan view, it was changed to an octagon shape so the geometrical relationship between each of the eight tile specimens

and the adjacent vertical walls would be identical. The walls

of the enclosure were insulated with two inches of glass fiber

blanket to provide an approach to isothermal conditions inside

and to minimize convection currents at the surrounding walls.

The enclosure was carefully sealed against air leakage to eliminate stray air currents during the soiling procedures.

The enclosure. Fig. 1, was 5’ x 5’ x 6* in size with the

top one foot sealed off as a plenum chamber from the bottom

five feet. The top deck of the plenum served as a base for

a centrifugal exhaust blower used to create a negative pressure within the plenum relative to the space in the lower

part of the box and/or the outside atmosphere. An inclined

manometer Indicated the differential pressure through pressure

taps. An access door, as seen in Fig. 1, in one side of the box
;

: ; .

.

• . .. . :

•

'

:

• -

‘

•
ACOUSTIC TILE TEST

FIG. l

3-

opened into the airtight chamber. The ceiling of the chamber

contained eight openings, each eleven inches square, over which

the acoustic tile specimens were placed for test. A small

round opening at the center of the ceiling served as a pressure

relief or air exhaust opening.

2 . 2 Tile Mounting

Initially, the tile specimens were arranged in the pattern

shown in Fig. 2 . The eight specimens were placed in a symmetrical pattern with the outer edge of the tile located about

1 1/2 inches from the adjacent inner wall surface of the enclosure .

It was Impossible to obtainv ;a uniform discoloration of the

specimens in a radial direction with this tile arrangement. The

discoloration pattern was the same on each tile but the discoloration increased with distance from the wall of the enclosure. It was thought that the close proximity of the specimens

to the enclosure walls interfered with the pattern of air motion

and affected the dust deposit at the outer edge of the tile.

Subsequently, the entire ceiling of the enclosure was fitted with acoustic tile to determine the area In which the discoloration would be uniform. A number of such tests indicated that eight specimens arranged in the pattern shown in Fig. 3

would be uniformly soiled, so the original ceiling of the enclosure was replaced with another incorporating this pattern of

openings

.

2 . 3 Dust Dispersal

The test dust was introduced into the enclosure through

a tube penetrating the sidewall about 20 inches above the floor.

The dust was picked up from a circular groove in a turntable

by means of an aspirator using compressed air. The amount of

dust deposited in the groove could be varied and the turntable

was rotated by a variable speed drive. The air pressure applied

to the aspirator was regulated with a valve.

A non-oily carbon black made from acetylene was used as

the test dust. Some experiments were conducted with bone char

as the test dust, but it did not adhere well to the tile surfaces.
.
CEILING AT PLENUM

60

FIG. 2.

CEILING AT PLENUM

GO"

- 4-

Consequently, an undesirably long operating time was required

to obtain an appreciable discoloration with bone char.

2 . 4 Discoloration Measurement

The degree of discoloration of the acoustic tiles was

measured with a** photoelectric reflectance meter commercially

known as ’’Photovolt . M The meter had a two-inch scanning spot

diameter and was used with a green tristimulus filter. Measurements with the green tristimulus filter served for determining

’’luminous apparent reflectance" or "whiteness." The reflectance of a surface as measured by this meter is its reflectance

expressed in percent of the reflectance of magnesium oxide. The

instrument is adjusted so that the 100 mark on the scale corresponds to the reflectance of magnesium oxide. Therefore., all

readings obtained with the reflection meter are a percent reading of the reflectance of magnesium oxide.

A jig was constructed for holding the tile specimens while

measurements were made of discoloration with the photovoltmeter. The jig located the tile specimen precisely with respect

to a plastic cover plate containing three holes through which

the viewing aperture of the photovoltmeter was inserted for

reflectance measurements. The three holes were located along

a diagonal through the center line of the specimen at an angle

of about 30 degrees with the base. One hole was at the center

of the tile and the other two were on opposite sides of the

center hole along the diagonal line. The holes were nonoverlapping and the field of reflectance measurement in the

three positions did not overlap. Fig. 4 shows the jig and photovoltmeter with the viewing element located in one of the three

holes in the cover of the jig.

2 . 5 Dust Impingement Apparatus

Fig. 5 and 6 show the rotating mechanism and distribution

box designed for the impingement method of soiling the acoustic

tiles. Fig. 5 shows the motor, the reduction gear box, and belt

drive of the rotating arm apparatus. The upright member of the

rotating arm was fitted with a pair of slip rings to conduct the

electric current to the blower motor in the distribution box.

This rotating arm was mounted at the center of the test enclosure.



* w R07rAi IM J A P hA H A for )

’ s \

t a D 1 b t /: a

I / Vi ! h V

!

Fir,, * V..-' * Vfy .

IMPINGEMENT DlSTRiBl T!Of ; V. V

WIDTH REDUCE P TO S

"

h

_L

f'h.

FIG 6

- 5

-

Fig. 6 is a dust distribution box designed to deliver a controlled jet of dusty air toward the test specimens. It was

built to deliver a stream of well-mixed air and carbon black

through a short duct of high width to depth ratio such that the

dimensions,, direction, and velocity of the stream could be controlled. The aerosol could be picked up directly from the test

enclosure through the side inlet openings of the blower, or the

test dust could be injected directly into the distribution box

through the rotating arm. The distribution box could be supported on the rotating arm at any height, the angle of impingement of the aerosol stream was adjustable, and the width of the

impingement duct or slot could be changed to form any desired

width of dust pattern on the specimens.

The distribution box shown in Fig. 6 was originally built

in a cubical shape. However, it was found that carbon black

was deposited in the lower corner of the box opposite the blower

discharge. This deposition was greater during the first test

made after cleaning the box than during successive tests under

the same conditions. Thus, when one test was made after another

under the same conditions, the discoloration produced on the

tile became progressively greater when clean tiles were used

for each test. It was necessary, therefore, to provide a curved

side for the distribution box opposite the discharge of the blows

to prevent this dust deposit. The revised shape of the distribution box is shown in Fig. 6.

3 . Study of Test Procedures

The nature of this problem indicated trial and error experiment and the use of judgment in developing test procedures

that might serve as a standard for soiling tests of acoustic tile.

This process led to a reasonably good procedure for attaining

repetitive results under impingement conditions, but a much lower

degree of success for the filtration process. The project had to

be terminated for lack of funds before much experimentation could

be carried out on the thermal precipitation process. The studies

made of test procedures are described in the following sections

of the report.

- 6 -

3 . 1 Dust Impingement

Under impingement conditions the principal problem was to

obtain repetitive results of equal and uniform discoloration of

the test specimens. The following elements of the test procedure were studied to arrive at a satisfactory method for

measuring comparative discoloration by impingement: (l) the

amount of carbon black needed to produce enough discoloration

to minimize the percentage error in successive tests, (2) the

best positioning of the tile specimens on the ceiling of the

enclosure, (3) the optimum distance between the outlet of the

rotating impingement box and the tile, (4) the angle and velocity of impingement of the aerosol, ( 5 ) the place of injection of the carbon black into the enclosure and the duration of the injection period, and (6) the design of the impingement distribution box that would inhibit deposition of

carbon black inside the box.

These problems were resolved by experiment to attain the

desired control of one variable at a time. Different weights

of carbon black were used to determine the amount necessary to

give a reduction in reflectance of 10 to 25 percent. The ceiling of the enclosure was completely covered with tile to determine the best location of the test specimens. The termination

of the aerosol injection tube within the enclosure was varied

until the greatest uniformity of discoloration was attained.

Several angles and velocities of impingement of the aerosol on

the specimens were used. The shape of the distribution box

was redesigned one or more times to eliminate deposition of

carbon black inside the box during the test. The modifications

to the apparatus are described in the preceding sections of

this report.

A procedure of introducing 40 grams of carbon black in two

batches of 20 grams each, using 1 1/2 hours injection time for

each batch and a one-hour interval between injection periods,

provided a satisfactory soiling rate. The nozzle of the rotating box was positioned about 17 in. from the tile surface,

the impingement angle was almost 90 degrees, and the aerosol

velocity at the nozzle outlet was about 1000 ft/min. The eight

tile specimens were arranged as shown in Fig. 3. When all of

these detailed procedures were followed the dust distribution

on each tile was essentially the same and the reduction of

reflectance of eight specimens of the same tile was equal within

- 7

-

about one percent reflectance. In successive tests using the

same procedure the reflectance was reduced more in a given

test than in the preceding one by about 1.7 percent reflectance

in a total of about 15 percent change.

3 . 2 Dust Filtration

Many methods were explored for providing a dusty atmosphere in the enclosure without significant air currents at

the ceiling level where the tile specimens were located so

deposition could occur under the sole influence of a slight

pressure difference across the tile. In all of these tests

the carbon black was injected into the enclosure with an air

jet. Small propeller fans of two different diameters were

used near the floor of the enclosure forcing the air upward

and downward in different tests to attain a uniformly dusty

atmosphere In the enclosure with low air motion at the ceiling

level. Several designs of housing were placed around the fans

in an effort to attain a uniform deposit on all eight specimens. During these exploratory tests it was not difficult to

attain substantially equal discoloration on eight identical

specimens, but the discoloration was shaded from lighter to

darker toward the inner edge of the tile. Screens and sheet

metal baffles were used in attempts to attain uniform radial

distribution of the carbon black deposit.

The test procedure most often used for these exploratory

tests consisted of introducing 20 grams of soiling material

into the chamber over a period of one hour followed by a onehour settling period before the door of the enclosure was

opened for removing the tile. This amount of carbon black produced a reduction In reflectance ranging from about five percent

under one set of conditions to about 55 percent under different

conditions

.

During all of these tests the discoloration of the specimens was greater toward the inner edge of the tile. It was

concluded that the specimens were mounted too near the vertical

sidewalls of the enclosure, the distance from the outer edge of

the tile to the wall being only 1 1/2 inches for the mounting

pattern shown in Fig. 2. The project was terminated before tests

could be made under filtration conditions with the revised mounting pattern illustrated in Fig. 3-

- 8 -

3 . 3 Thermal Precipitation

Pour test runs were made during which the air in the test

enclosure was heated and the impingement apparatus was not operated. Low-wattage, “lead-sheathed heating cable was distributed

on the floor of the enclosure to provide a reasonably uniform

heating of the air from a low level. A different amount of

heat was used for each test. However, the entire space and the

insulated wall and ceiling surface were at nearly the same

temperature so little discoloration was observed. Based on

previous studies, it is believed that a temperature difference

between the air and tile surface would have to exist before significant thermal precipitation would bd observed.

3 . 4 Effect of Painting on Sound Absorption

The horn coupler, under development by the Sound Section of

the National Bureau of Standards for measurement of the sound

absorption of acoustic tile and for studying the effect of repeated painting on absorptivity, did not produce the flat wave

front required for such tests and was abandoned. The alternative method was the use of a 10 ft by 10 ft panel in the sound

chamber. However, this method appeared to be too expensive for

this study, so a new approach to this objective is needed.

4 . Soiling Tests of Commercial Tile

A series of tests were made with the impingement apparatus

developed during this study to determine the comparative soiling characteristics of 19 different types of acoustic tile and

a specimen of white plaster prepared at this Bureau. The 19

types of acoustic tile were stock materials supplied by 7 manufacturers . The test specimens were classified in six broad categories

by representatives of the Acoustic Materials Association, as

follows

:

Type I, Regular Perforated Cellulose Fiber Tile. Specimens 1-4 inc.

Type II, Textured or Fissured Cellulose Tile or Board. Specimens

5 & 6.

Type III, Perforated Mineral Tile. Specimens 7-9 inc.

Type IV, Fissured Mineral Fiber Tile. Specimens 10-12 inc.

Type V, Textured or Smooth Mineral Fiber Tile or Board. Specimens

13-16 inc.

Type VI, Perforated Metal Pans with Mineral Fiber Pads.

17-19 inc.

Specimens

-9-

A twentieth specimen consisted of white plaster over brown

scratch coat on metal lath, 3/4 in. thick.

Eight specimens were attached to the ceiling of the test

enclosure at one time and subjected to the test procedure described earlier in this report for the impingement conditions.

One of the eight specimens in each case was a control, taken

from a group of identical specimens specially selected for

uniformity. The control specimens were used in each group as

a basis for establishing whether or not substantially equal

soiling conditions had existed in successive tests so all the

test results could be compared.

The reflectance of three areas on each specimen, as determined by the construction of the measuring jig, was measured

with the photovoltmeter initially and at the end of the soiling

test to determine the decrease in reflectance.

4. 1 Test Results

Table 1 summarizes the initial and final reflectance values

at the center of each specimen, and the average of the initial

and final values at the three stations of observation along a diagonal line on each tile. The table also shows the change in

reflectance during the tests expressed as a difference between

initial and final values and as a ratio of final to initial

reflectances

.

It will be noted in Table 1 that the initial reflectance

of the 19 specimens ranged from 66.5 to 79.5 with only two

having higher reflectance values than the white plaster specimen. Initially, the average of the reflectance at the three

stations of measurement differed from that at the center by a

few tenths of a percent in all cases except for specimens 11

and 15. The random fissures in some of the specimens might

have caused a variation in the observed reflectance in different areas depending on the number and depth of the fissures

covered by the viewing head.

At the conclusion of the impingement tests, the average

of the reflectances at the three stations of measurement differed

from that at the center by less than one percent in all cases

except for specimens 12, 13 and 18 .

Table 1

Reflectance of Acoustic Tile Before and After Impingement Soiling

Average of Diff. Between Ratio of

Type Specimen Center Station 3 Stations Initial & Final Final to Initial

No. No. Initial Final Initial Final Center 3 Stations Center 3 Stations

I 1 71 .5 46.5 71.6 46.3 25.0 25.3 65 .O 64.7

I 2 68.5 49.0 68.8 49.1 19.5 19.7 71.5 71.4

I 3 68.5 43.5 68.5 43.6 25.0 24.9 63.6 63.7

I 4 67.0 48.5 67.3 48.3 18.

5

19.0 72.4 71.8

n 5 74.0 52.5 74.3 53.5 21.5 20.8 71.0 72.0

n 6 68.0 42.5 67.6 42.5 25.5 25 .I 62.5 62.9

hi 7 71.0 51.0 71.1 51.3 20.0 19.8 71.8 72.2

hi 8 73.5 47.5 73.6 47.5 26.0 26.1 64.7 64.6

hi 9 77.0 64.5 76.8 63,5 12.5 13.3 83.8 82.7

IV 10 72.5 46.5 73.0 46.8 26.0 26.2 64.1 64.1

IV 11 71.5- 46.0 73.6 48.5 25.5 25.1 64.4 65.9

IV 12 76.5 47.0 77.0 46.0 29.5 31.0 61.5 59.8

V •13 66.5 46.0 65.8 43.8 20.5 22.0 69.2 66.6

V 14 79 .5 69.5 79 .5 68.6 10.0 10.9 87.4 86.3

V 15 76.5 63 .O 75.5 61.3 13.5 14.2 82.4 81.2

V 16 66.5 51.5 65.8 50.0 15.0 15.8 77 .5 76.0

VI 17 72.0 59.0 72.0 58.6 13.0 13.4 82.0 81.4

VI 18 69.5 57.5 68.8 58.8 12.0 10.0 82.7 85.5

VI 19 72.0 59.0 72.0 59.5 13.0 12.5 82.0 82.7

Plaster 20 76.5 58.0 76.5 58.0 I8.5 18.

5

75.8 75.8

Control I

Control II

78.5

80.5

79.5

68.0

69.0

69.0

79.1

80.0

79.6

67.6

68.5

68.8

10.5

11.5

11.5

11.5

86.7 85.5

87.3 85.7

Control III 10.5 10.8 86.7 86.5
.

.
- 10 -

The change In reflectance of the three specimens used for

controls in the three successive tests differed by a maximum

of one percent in a total change of about 11 percent. This

agreement indicates that all of the observed changes in reflectance on the 19 commercial tiles and the plaster specimen

are comparative within a tolerance of about one percent. This

agreement in the results on the three control specimens also

indicates the ability of the test method to produce identical

results during repetitive impingement tests.

Table 1 shows that tile specimen 14 had the lowest change

in reflectance of all the specimens and also the highest ratio

of final to Initial values. Seven of the specimens exhibited

a change in reflectance ranging from 2 1/2 to 3 times that of

specimen 14. The three specimens of Type VI had the lowest

average decrease in reflectance of any of the types taken as

a group. However, the decreases in reflectance of specimens 9 , 14, and 15 were of the same order of magnitude as those for

Type VI tile. In all types of tile other than Type VI there

were rather large variations in discoloration of the several

kinds of tile within the group.

The final values of reflectance of six of the specimens

were higher than the final value for white plaster.

5 . Conclusions

The apparatus and procedure described in this report appear

to provide a satisfactory method for comparing the rate of discoloration of acoustic tilp by the impingement of an aerosol

containing carbon black. Further experimentation would be

required to develop equally good procedures for comparing the

effects of filtration and thermal precipitation on the soiling

of acoustic tile. Enough study was made of the processes of

impingement, filtration, and thermal precipitation during this

and previous work to establish that all three processes are important in some applications. Further study would also be required to evaluate the effect of different aerosols on the comparative rates of discoloration of different tile specimens.

USCOMM-NBS-DC

tl. S. DEPARTMENT OK COMMERCE

Ltiwln L. Straits*, Srrrrlrtfy

NATIONAL lUJIttiAU OK STANIMIHIH

A. V, Antiit, iJIrrcinr Till? NATIONAL BUREAU OI<" STANWAR.WM

The scope of activities of the National Bureau of Standards at its headquarters in Washington,

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