Acoustical investigation of ceiling performance in an open office

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Acoustical investigation of ceiling performance in an open office

ACOUSTICAL INVESTIGATION O F CEILING

PERFORMANCE

IN

A N

OPEN

OFFICE

by

A. C.

C.

Warnock

50640

+ C + - CANADA

SAUE DES

MACK!NES

NsaE

rnoTs

a

a-

I

0

D A T E ~ ~ ~ N I I I . ~ L E S

fiJ

.

-

j - 1

ill

-/,*

ng Researc

I Ser THl s92 no. 88 c . 2 BrnE - -D l Y l S l O N O F BUILDING R E S E A R C H

-

M A T l O M A P R E S E A R C H C O U N C I L O T f A W A C A N A D A

NO

B f i

ACOUSTICAL

INVESTIGATION

O F CEILING

PERFORMANCE IN

AN O P E N OFFICE

by

A . C . C . Warnock

ABSTRACT

This Note d e s c r i b e s an acoustical investigation of flat ceiling

properties in an open office. The experiments w e r e concerned p a r - t i c u l a r l y with the e f f e c t s of different ceiling materials, baffle c a n -

figurations and screen a r r a n g e m e n t s , The r e s u l t s obtained are

analyzed in terms of articulation index giving a single figure rating t h a t facilitates comparisons. A short theoretical outline of screen-

ceiling interaction emphasizes the i m p o r t a n t c e i l i n g and screen para

-

meters and r e f e r s to areas where new t e s t procedures a r e needed.

INTRODUCTION

The acoustical privacy of an open office i s extremely dependent

on the reflection of sound f r m the ceSling assembly. The experiments

d e s c r i b e d in this Note w e r e designed t o compare the performance of three

t y p e s of ceiling materials used with and without ceiling baffles. T o determine the influence of screen height on the acoustical privacy b e -

hind a screen, two screen heights were used, 4 f t 8 in. and 6 ft. The experiments w e r e designed and carried out by the Building Physics

Section of the Division of Building Research, N . R . C . , in a sixth f l o o r

office of the Kenson Building,Metcalfe Street, Ottawa.

1 E X P E R M E N T A L TECHNIQUE

The privacy conditions in an open office are closely related

to sound propagation in the office space, and can be examined by m a k i n g

the appropriate acoustic measurements.

In

the open air, in the absence

of obstruction and atmospheric disturbances, the sound pressure level

from a continuous s o u r c e w i l l decrease by 6 d B each t i m e the distance

v e r y absorbent r o o m such as an anechoic c h a m b e r , At the

other extreme, in a reverberant space bounded by highly reflective surfaces, one quickly e n t e r s the r e v e r b e r a n t field where the sound

pressure l e v e l does n o t decrease appreciably with distance from

the source.

The objective of an open office i s to approach, as closely

as i s practicable, an open air case, hence the specification of a b s o r -

bent surfaces and furnishings, The propagation experiments described here were intended to measure the effects of ceiling materials, ceiling b a f f l e s m d s c r e e n configurations on the propagation of sound in an

office.

The office used f o r these studies was fairly narrow and, in

an attempt to reduce possible reflections from a nearby wall, the wall

was covered with about 2 in. of glass fibre. The ceiling boards in the office measured 4 f t by 2 f t and w e r e supparted on T - b a r s . Details

of the t h r e e types of ceiling boards u s e d are given in Appendix A. The baffles w e r e simply constructed f r o m piece s of ~ / 4 - i n , plywood measuring

4 ft by 1 f t and covered a n one face with 518-in. g l a s s f i b r e which had

a perforated vinyl facing. W h e n used these baffles w e r e suspended on

hooks from the T-bars with the glass fibre side facing the sound source.

The screens used w e r e cornmercialy manufactured and measured

4 f t 4 in. high, 5 f t wide and 3 in. thick. The bottom edges of the screens

w e r e 4 in. from the floor raising the tops t o a height of 4 f t 8 in. One of t h e screens was modified by clamping to it a piece of 1/8-in. hardboard

which was covered on both s i d e s with 1 1 1 2 in. of g l a s s f i b r e thereby i n c r e a s i n g the aver-all height to

6

f t ,

Figure 1 outlines the experimental procedure u s e d in making the continuous sound propagation measurements. The long axis of the

column laudspeaker was placed horizontally in a f u r t h e r attempt t o

reduce the effects of the walls and windows. The recordings w e r e

analyzed in the l a b o r a t o r y an a r e a l time analyzes and the third-octave band levels obtained w e r e t h e n reduced to octave band readings, The results a r e presented and discussed i n the next section.

It is g e n e r a l l y true that t h e r e are several acoustical paths between two points in an office, Continuous measurements d o not separate these paths and give only the total effect produced by all of them.

h

_{an effort to s o r t out these paths, measurements were made}

using pulse s of sound, It w a s found that if the duration of the pulse

w e r e short enough it was then possible, under appropriate conditions,

t o identify and separate the different components

.

The apparatus used to carry out the pulsed study is shown in

Other horizontal reflecting paths could be present but it was hoped

that they would not be a problem in this case as precautions were

taken. The acoustic pulses were picked up by microphones and the electrical signals w e r e displayed on an oscilloscope screen which was photographed. The interpretation

of

the results i s discussed in the

next section.

U

RESULTS

(i] Continuous sound source: (a) dB(A) analysis

Table 1 lists t h e A-weighted sound p r e s s u r e levels measured

a s a function of distance under the different ceilings. Data f r o m this

table are plotted in Figures 3 to 5 to demonstrate the effects of the

ceiling baffles. F o r material A , the b a f f l e s have an easily observable

effect in that they generally reduce the received signal. T o obtain a

significant reduction however, the baffles should be used on 2-ft centres.

The performance of the combination is then about the sarne a s material

C

without baffles. This was perhaps a predictable result since the total

absorption of ceiling A plus baffles is about the s m e as that provided

by ceiling

C.

By contrast it was observed that the addition of baffles to ceiling C made a negligible improvement.

Figure 6 facilitates the comparison of ceiling materials, A

and

C.

It can be seen that the u s e of the more absorbent m a t e r i a l results in a lower signal at a given distance behind the screen. The difference

is generally about 3 dB(A).

Ebl Octave band analysis

Tables 2 to 8 show the octave band analysis of the signals and

are included for completeness. The octave band analysis of the propaga-

tion m e a s u r e m e n t s leads to essentially the s m e conclusions as the

d B ( A ] results. F i g u r e 7 is included to illustrate what is obtaixled f o r materials A and

C.

In the next section it is shown that an

analysis of these octave band results, in terms; of the articulation index,

Beads to a single figure rating for a given screen-ceiling combination.

Articulation Index

In the open-plan office the acoustician is generally concerned

with those factors which have an e f f e c t on speech privacy, It is convenient, therefore, to express the results in t e r m s of a quantity that measures the

i n t e l l i g i b i l i t y of speech. The most common unit i s the articulation index

(AI).

The articulation index is calculated by e s t b a t i n g speech s i g n a l - t o -noise

the ratios by weighting factors and summing the products [ _{l ) , The}

number resulting from this i s the

A1

and lies between 0 and I . _The dependence of sentence intelligibility on

A1

i s shown in Figure 8 where

it i s c l e a r that intelligibility increases rapidly

as

A1 increases from

z e r o to 0.40. For acceptable privacy, free f r m distraction, the A1 in a n open office should be l e s s than 0.15. F o r AICO. 3 a decrease in A1 of 0 - 0 5 is very important and produces a decrease i n sentence

intelligibility of about 1570.

The

data tabulated i n T a b l e s 2 to 8 can be used to calculate

the changes in A1 which occur as a result of changes in the ceiling or

screen configuration. These calculations w e r e made for three distances

of interest, viz. 9, 1 2 and 15 f t , and the results a r e given in Tables

9-12. The results are expressed as the improvement over a reference

condition consisting of material A with no intervening screen or baffles. Considering the effects of ceiling baffles on the performance of material A , it can be seen that on 4-Et c e n t r e s the baffles produce an

insignificant decrease in AI. W h e n the baffles are 2 f t apart the decrease

in A1 is quite marked, consistently about 0.1.

A S

has already been pointed out, such changes are very isnportant.

The use of baffles with material.

C

a l s o decreases the A1 i n a given situation. In this case the decrease is smaller and the differences

b e t w e e n the 4-ft o. c. and the 2-ft o. c . case are even l e s s and a r e same

-

what random in nature. The fact that material

C

is ahmost acoustically

transparent could account f o r the inconsistency of its behaviour; also, plenum reflections could introduce complications.

The effects of screen s i z e and position can be _{found b y reading} v e r t i c a l l y down a column in Tables

9-LZ.

One finds, predictably, that

a 6-ft screen provides more isolation than a 4-ft 8-in. screen and that

moving the s c r e e n closer to the source is also an _improvement. The relative efficacy of the three ceiling materials can be

determined by concentrating on the results obtained without ceiling

b a f f l e s . W i t h one exception, material C performs best, The exception

is again a possible manifestation of the acoustical transparency of

material

C ,

C e i l i n g materials with N o i s e Reduction Coefficients ( N R G )

approaching 1.0 a r e now readily available and should be an improvement

o v e r the b e s t s y s t e m dealt with here.

It

was not possible t o t e s t such a

material during the t h e available for this study.

The Information found in the tables can be summarized as follows: the larger the screen, the Larger the insertion l o s s ; the more absorbent

the ceiling, the better a given screen will perform. The limit is reached

when there are no flanking reflections. In practice, one should use a

baffles can be used to improve a reflective ceiling but, when used, they need be spaced s o closely that they become economically unfeasable.

T h e y can also slightly improve a m o r e absorbent ceiling; again, however, benefits cannot be justified economically since highly

absorbing materials are easily available and are probably m o r e effective.

j i i ) Pulse M e a s u r e m e n t s

F i g u r e 9 illustrates the electrical signals produced by the r e c e i v i n g microphone shown i n F i g u r e 2. The figure caption identifies

the e x p e r b e n t a l conditions, It i s clear that there a r e several sources

of reflection which complicate the trace,

From a knowledge of the path l e n g t h s in F i g u r e 2 the time

d e l a y between two corresponding points on the diffracted pulse and the pulse reflected from the ceiling can be evaluated. G i v e n the value of the time delay, i t i s then p o s s i b l e t o delineate the expected

ceiling pulse. In a f e w c a s e s the stmilarity between the pulse shapes was close enough to allow identification of the ceiling pulse by d i r e c t

oh servation. T h i s permitted the verification of the calculated time

delays. With this knowledge in mind, it b e c m e s , in theory, a

s t r a i g h t f o r w a r d task t o m e a s u r e the amplitude of the diffracted signal

and the ceiling reflection signal. Unfortunately, i n m a n y c a s e s , the ceiling reflection signal was extremely distorted a s a result of i n t e r

-

ference e f f e c t s . This resuLted in a large spread of values obtained

f o r the ceiling reflectionmeasurements. The values o b t a i n e d f a r the d i r e c t and diffracted pulse s behaved in a more predictable fashion

and the results obtained f o r these pulses are displayed in F i g u r e 10.

These results a r e the average of the values obtained under all

ceiling conditions. A s expected, the d i r e c t pulse decreased 6 dB f o r

each doubling of distance from the source, The small screen has an

i n s e r t i o n l o s s about 1 0 d B less than that of the l a r g e s c r e e n . Theory,

discussed l a t e r , predicts a larger value but this discrepancy is probably due to the relatively l a r g e s i z e af the source.

T h e results for the reflected ceiling pulses a r e shown in

F i g u r e 11. The scatter in these measurements was v e r y large and i s

n o t shown here. Upon exarmination same inconsistencies w e r e v e r y quickly discovered, Figure 1 l ( a ) s e e m s t o show, in contradiction t o F i g u r e s l l h - d , that baffles on 4-ft centres are about as effective as

those an 2 - f t centres. Comparison of the levels of the pulse diffracted

around the small screen with the levels of the ceiling bounces leads

t o t h e conclusion that the ceiling material and baffle configuration are

not important. T h i s i s c o n t r a r y to the continuous sound r e s u l t s and t o theoretical considerations. Because sf these and o t h e r discrepancies

ink r e sting, a r e not reliable for quantitative information,

UI

THEORETICAL CONSLDERATIONS

Sound, at the i m p o r t a n t speech frequencies, reaches the other side of a screen in various ways:

i) by direct transmission through the screen, ii) by diffraction a r o u n d the screen,

iii) by reflection from the ceiling,

iv) by reflection from vertical flanking surfaces.

Considering first the d i r e c t transmission, it i s shown i n

standard texts ( 2 ) that the transmission loss far sound waves normally

incident on a solid wall is

T L (dB)

=

- 2 8 . 6 -C 20 log Mf w h e r e M = rnass/unit area in lb/ft2

f

=

frequency in Hz,

The reduction of a

signal

caused by diffraction around a semi-infinite

screen is given by ( 3 ) ( 4 ) ,

IIL /dB)

-

20 log N r - , 0 2

The distances A , B , and d are shown i n Figure 2. The distance d is the

difference between the path taken by the diffracted wave and that taken

by the direct wave.

In Figure 12 the i n s e r t i o n l o s s due to diffraction for two semi- infinite s c r e e n s is compared with the attenuation of signals propagating through the screen.

It

i s quite clear from this figure that the loss due

to transmission through the screen i s easily made negligible f o r typical screen heights by using an acoustically opaque material with a surface d e n s i t y of 1/4 ib/ft2 or greater as a membrane t o separate the layers

of absorbing material,

Because the diffraction loss is g r e a t e r f o r higher screens that a x e occasionally used, the mass per unit area of the separating membrane

m u s t be i n c r e a s e d .

If

_{one adopts as a c r i t e r i o n the requirement that the.} transmission loss should be about

6

d B

greater than the diffraction 10s s at 1 0 0 0 Hz, then the surface density of the screen membrane should

satisfy

W i t h t h i s condition met the screen will perform close to the limit

A theoretical consideration of specular reflection from a flat,

absorbing ceiling is straightforward. Referring again to F i g u r e 2, the

reflected signal arriving at the microphone i s smaller because of

absorption and because of the longer path length travelled; in fact, the attenuation relative t o the direct signal is

20 log Z G / ~

-

1 0 logp

where p i s the reflection coefficient of the ceiling at the particular £re

-

quency and angle of incidence considered,

En o r d e r to demonstrate the effects af ceiling absarptioiz on

screen performance, Figure 1 3 has been constructed, This figure

shows the variation of articulation index as a function of ceiling re-

flectivity calculated at the l i s t e n e r ' s position when a screen is inter-

posed halfway between listener and talker. F o r the purpose of t h i s figure a background noise of 48 dB(A) has been assumed. Table 1 3

shows the voice levels, noise levels and diffraction Ioase s used i n

the calculation of the articulation index, The talker -1i stene r separa

-

t i o n assumed was 12 E t and the ceiling height was taken as

9

ft. The absorption coefficient was assumed t o be equal En all bands.

Specular reflection from vertical surface s can a l s o create problems. Vertical s u r f a c e s are usually treated w i t h an absorbing

Sayer. A short study of the effectiveness of different treatments is

described in another Note ( 5 ) .

IV

CONCLUSIONS

The following conclusions can be d r a w n f r o m this study:

1 . Assuming a screen constructed in accordance with the r e c o m -

mendation made in the previous section, the larger a screen, the greater i t s insertion loss.

2. The ceiling system in an open office should have a reflection

coefficient low enough t o permit the screens to perform close to their

rnax'lmurn, diff raction-limited capahilitie s

.

The simple st way of

achieving this condition for a flat ceiling i s to use highly absorbent

ceiling boards. Ceiling baffles are not recommended except possibly

as a way of improving the performance of a poor ceiling.

V

F U T U R E

W O W

The t h e o r e t i c a l discussion of screen performance under an absor -

bent ceiling emphasizes the need for further work in the field. At this

writing, flat and caffe red ceiling systems are tested in reverberation

chambers where the absorption coefficient averaged over -all angles is measured. It would be m a r e appropriate to have a t e s t procedure

where the reflection coefficient is measured as a function of frequency and angle.

If

such a t e s t were available it would also be possible to deal with those ceiling systems which are composed entirely of

ab sorbent baffles suspended f r a m exposed ductwork.

At present, screens are typically rated in terms of their surface absorptiorr {averaged at the N o i s e Reduction Coefficient).

It

might a l s o be relevant to introduce a simple t e s t to measure the insertion

loss of screens to ensure that as well as being absorbent they a l s o

have a high enough transmission loss.

REFERENCES

1

.

American National Standard Methods f o r the Calculation of the

Articulation Index. ANSI S3.5, 1969.

2. K i n s l e r , L.

E.

and A. R. Frey. Fundamentals of A c o u s t i c s .

J.

W i l s y and Sons,

1962.

pp 36-140.

3 . Maekawa, 2. Noise Reduction by Screens. Applied Acoustics

1 (1968). p 157.

-

4. Noise andVibratianCsntro1. L.L, B e r m e k , Editor, M c G r a w -

Hi11,1971.

p

175.

5. Warnock, A.

C.

C . Laboratory h v e stigation

of

L o s s of Privacy due to Flanking Transmission by a Column. National Research Council of Canada, Division of Building Research, Technical

Q) "3 C vl .d Lfl m Ln tn 1 . In m m . I . $ - ! : $ m m m c ' m c m m r - m .--I d A 4 p l d d & + . - I

TABLE 2 : O C T A V E BAND S L G R A L

ATTENUATION

IN

D B RELATIVE

TC

LEVEL

3-FT IN F 8 3 N T '

OF

LOUDSPEAKER, C E I L I N G llATERIAL

A USED WITH 3 0 : E I L f N G BAFFLES

O C r a V E B A N D CENTRE FREQUENCY

(HZ)

DISTAffCE

('FT)

325

250

5 0 0 1000 2000 40 00

N O

INTXBY

Z NIWG SCREEN

6 - F T HIS3 SCREEN

AT

3-FT

6-FT A l S d SCREEN AT 6 - F T

TABLE 3 : O C T A V E B A N 3

S I G N A L

ATTEWUATIOM I N Dl3 R E L A T I V E

TC

LEVEL 3-FT

IN

P R D M T

OF

LOUDSPEAKER. C E I L I N G AATERLAL A

USED RTTM

CEELIWG BAFFLES ON 4-FT C E N T R E S

ocrrvE

B A N D CENTRE F R E G U E N C Y

(HZ)

D I S T A N C E (FT) 1 2 5 2 5 0 500 10 00 2 0 0 0 40

00

NO I N T E R V E N I N G S C R E E N

6-PT

RIG3

SCREEN AT 3-FT

TABLE

4 : O C T R V E

B A N D

SIGNAL ATTENUATIOH

IN DB

RELATIVE

T C

LEVEL 3-FT IN P 2 3 N T OF LOUDSPEAKER. C E I L I N G HATERIAL

A USED WITH C E I L I N G BAFFLES O N

2FT

CENTRES

O C T h V E BAND CENTRE FRECDENCY {HZ)

DISTANCE {FT) 125

250

5 0 0 1000 2000 40 06

HO I N T E B J B N ING SCREEN

6-FT H I S 3 S C R E E N AT 3-FT

6-FT

HISii SCREEN AT 6-FT

TABLE

5 : OCTAVE B A H D S I G N A L ATTENUATTDN

IN D B

R E L A T I V E

TQ

LEVEL 3-PT IN F R 5 N T OF LOUDSPEAKER. C E I L I N G flATERZAL

B USED WITH N O Z E I L I N G BAFFLES

O E r h Y E

B A N D

CENTRE

FREQUENCY

(NZ)

D I S T A N C E

(FT)

125

25fl

5 6 0 1 0 0 0 21300 40 00

NO

I N T E R V E N I B G SCREEN

6-PT H I S 3 SCREEN

AT

3-FT

TABLE 6 : OCTAVE

B A N D

S I G N A L ATTENUATION IN DB RELATIVE

TG

LEVEE 3-FT IN F 3 O N T OF LOUDSPEAKER, C E I L I N G YATERIAL

C USED

WITH

NO : E E t I N G

BAFFLES

O C E n V E

B A N D

CENTRE FREQUENCY

(HZ)

DISTANCE

(FT)

125 2 5 0 5 0 0 10 00 2000

4000

H O INTEBPENING S C R E E N 6-FT HI23 SCREEN AT 3-FT

6-PT

HLZd

SCREEN

AT 6-FT 4-PT 8 - I H H I G H SCREEN

AT

3-FT

TABLE 7 : OCTAVE BANI) 3 f G N A L ATTENIIATIOW

IN

318

RELATIVE

T C

LEVEL 3 - F T

IN

F B O B T OF LOUDSPEAKER,

CEILING

RATERIAL

C U S E D

WITH

C E L L F N G BAFPXES OR 4-FT CENTRES

ioCP4VE BaND CENTRE FREQUENCY [HZ]

DISTANCE (FTF 1 2 5 250 5 0 0 1a00 2000 40 00

BO

IWTEBIE AING

SCREEN

6-PT HI33 SCREEN AT 3-PT

SIX

FT

81 3 8 SCREEN

AT

S I X PT

TABLE 8 : O C T A V E B A N D S I G N A L A T T E N U A T I O N IN D B RELBTIVE T C LEVEL 3-FT IN F R D M T OF L O U D S P E A K E R . C E I L I N G MATERIAL

C

USED

WITH

C E Z L f N G BAFFLES

ON

2FT CENTRES

O C r I L V E BAWD CENTRE FREQUENCY [ H Z )

DISTANCE (FT) 125 250 5 0 0 10 00 2000 40 00

N O I N T E B J E N I N G SCREEN

F - F T HIZ3 SCREEN AT 3-FT

6-PT HXSil SCREEN AT 6-FT

T A B L E 9

CALCULATED RELATIVE DECREASE I N ARTICULATION INDEX

( W I T H R E S P E C T

T O

CONDITIONS UNDER MATERLAL A W I T H NO I N T E R V E N I N G SCREEN A N D NO CEILING BAFFLES) F O R

A DISTANCE

OF

9

FT

_FROM

THE

_LOUDSPEAKER

Material A NRC 0 . 5 5 2'0.

c .

Reference N o S c r e e n 0

-

. 0 4 . 0 1 4-ft 8-in. S c r e e n at

3

f t . 2 0 a 19 . 2 8 6-ft Screen at 6 f t .30 31 .36 6-ft s c r e e n at 3 f t . 2 9 3 2 . 4 3 M a t e r i a l B NRC

=

0.65 N o B a f f l e s 0.0

,24

, 33

.46

Material

G

NRC

=

0.85

No

B afflc s

-

-

-

-

--

.

Q 1 . 2 9 -_ . _ p p p . . 3 4 . 3 9 4'0. c . 0 1 . 3 6 - 4 1

.46

2Io. c.

.

o r

. 3 0 . 4 3

46

T A B L E 10

CALCULATED RELATIVE DECREASE I N ARTICULATION INDEX

(WITH R E S P E C T T O CONDITIONS UNDER MATERIAL A WITH

NO I N T E R V E N N C SCREEN AND N O CEILING B A F F L E S ) F O R

A

_DISTANCE

_OF

_{1 2}

_{F T}

_{F R O M}

_{THE LOUDSPEAKER}

..

Material A M a t e r i a l

B

M a t e r i a l

C

NRC = 0 - 5 5 NRC

=

0 . 6 5 NRC

=

0,85

r

No

No

No

C onditions B a f f l e s 4'0. c . 210. c . Baffles Baffle s 410. c, 2'0, c .

TABLE 1 1

C A L C U L A T E D RELATIVE DECREASE I N ARTICULATION I N D E X ( W I T H

RESPEGT TO CONDITIONS UNDER

MATERIAL A WITH N O I N T E R V E N I N G

S C R E E N AND NO CEILING BAFFLES) F O R A DISTANCE: O F 15 F T

TABLE 1 3

SOUND

LEVELS

IN

dB USED I N T H E CALCULATION

O F ARTICULATION INDEX BEHIND A SCREEN

/ / / / / / / / / / / / / J / J / / J / / / J / J / / / / / / / / / J / / / / / / / / '

A

I T R U E C E I L I N G

A

S U S P E N D E D C E I L I N G F I G U R E 1 M E T H O D U S E D FOR C O N T l N U O U S P I N K

N O I S E

P R O P A G A T I O N M E A S U R E M E N T S . S C R E E N S W E R E I N S E R T E D A T D I S T A N C E S O F 3 F t O R 6 F T E . 4 5 0 5 B

-

I

I S U S P E N D E D C E I L I N G / / \ \ COLUMN L O U D S P E A K E R

,'

A

,f

+

P O W E R AMP ,

I

T O

O S C I L L O S G A T E D S I N E W A V E \ M E A S U R I N G O S C I L L A T O R MI CROPHONE M I C R O P H O N E

r

TO O S C I L L O S C O P E F I G U R E 2 C O P E A P P A R A T U S U S E D

TO

M E A S U R E P R O P A G A T I O N O F S O U N D P U L S E S A N D T H E P A T H S I M P O R T A N T I N D E T E R M I N I N G THE S C R E E N P E R F O R M A N C E

I

4 F T 8 l N . S C R E E N A T 3 F T O N 0 B A F F L E S a 4 F E E T O . C . A 2 O . C . ~ ~ ~ ~

M A T E R E A L

A

1

. 5

3

6

1 2

24

D l

S T A N C E FROM L O U D S P E A K E R , FEET

4 F T 8 I N . S C R E E N A T 3 F T " N O B A F F L E S n 4 F T 0 - C .

D l

S T A N C E F R O M

L O U D S P E A K E R ,

FEET

F I G U R E

3

C O N T I N U O U S P I N K

N O !

S E

P R O P A G A T I O N ,

d B

( A )

A N A L Y S I S

I

NO S C R E E N

6 F T S C R E E N A T 6 F T a N O B A F F L E S U 4 F T 0 . C . Q: A 2 F T O . C .

I

M A T E R I A L

A

D l

S T A N C E FROM

L O U

D S P E A K E R .

FEET

6 F T S C R E E N A T 6 F T o N O B A F F L E S

3

6

1 2

24

4 8

D l S T A N C E

FROM

L O U D S P E A K E R ,

FEET

F I G U R E

4

C O N T I N U O U S P I N K

N O !

S E

P R O P A G A T I O N ,

d B

( A )

A N A L Y S I S

M A T E R I A L

A

t

3

1 2

24

-

4 8

- d

0

a

B I D

-

& Q J a

8

- d 6 F T 5 C R E E N A T 3 F T Q N O B A F F L E S

-

U 4 F T O . C .

-

A 2 F T O . C .

D l

S T A N C E

F R O M

L O U D S P E A K E R , FEET

I 6 F T S C R E E N A T 3 F T O N 0 B A F F L E S n 4 F T 0 . C - A 2 F T 0 . c :

M A T E R I A L

C

D l

S T A N C E

F R O M

L O U D S P E A K E R ,

FEET

F I G U R E 5

C O N T I N U O U S

P I N K

N O I S E P R O P A G A T I O N ,

dB

( A )

A N A L Y S I S

4 F T 8 l N . S C R E E N A T 3 F T - O M A T E R I A L A " M A T E R I A L C, ld0 B A F F L E S

D l

S T A N C E FROM

L O U D S P E A K E R ,

F E E T

I

N O

S C R E E N

D l S T A N C E FROM

L O U D S P E A K E R , FEET

LY

*

2 3 0 -

F I G U R E 6

6 F T S C R E E N A T 3 F T O M A T E R I A L A

-

D M A T E R I A L C , NO B A F F L E S

C O N T I N U O U S

P I N K N O 1

SE

P R O P A G A T I O N .

d B

( A 1

A N A L Y S I S

40

-

I

I

I

3

6

1 2

24

48

w l M A T E R I A L A

i

W 1 0 1

'

g

1 d, 0 A W ~ F S C R E E N T A T 3 F T o N O B A F F L E S M U 4 F T 0 . C . v A 2 F T O . C .

-

2:

o

40 I B 4 F T 8 1 N . S C R E E N A T 3FT o N O B A F F L E S n 4 F T O . C . A 2 F T O . C . M A T E R I A L

C

6 F T S C R E E N A T 3 F T 0 NO B A F F L E S 0 4 F T O . C . A 2 F T O . C .

I

1

I

M A T E R I A L

C

- - g -

#

- LI 4 F T 81N. SCREEN A T 3 F T & O N 0 B A F F L E S - - a 4 F T O . C . * ~ F T O . C .

I

I 1 I 1 O C T A V E B A N D C E N T R E F R E Q U E N C Y F I G U R E

7

O C T A V E B A N D A T T E N U A T I O N M E A S U R E D A T 1 2 FEET FROM L O U D S P E A K E R

A R T I C U L A T I O N I N D E X

F I G U R E

8

R E L A T I O N B E T W E E N

A R T I C U L A T I O N

I N D E X

A N D

S E N T E N C E

E N T E L L I G

I & I I I T Y

-

4

I!-

( 0 1

T Y P I C A L R E F E R E N C E S I G N A L AT 6 I N a l O . S v / d i u N O B A F F L E S , 0.01 v / d i v N O B A F F L E S , 0.01 v l d i v I ( d ) M A T E R I A L C

,

B A F F L E S 2 F T O.C., 0.01 v f d i v I 1

I

I

1 I I S I G N A L S R E C E I V E D AT 12 F T

FOR

( b )

,

(c), I d )

F I G U R E

9

E L E C T R I C A L S I G N A L S F R O M

M I C R O P H O N E S , 5 m s / c m

E R 5 6 5 8 - 9

0 6 F T S C R E E N A T 3 FT a4 F T 81N. SCREEN AT 3FT A A 6 F T S C R E E N A T 6 FJ

D l S T A N C E F R O M L O U D S P E A K E R , F E E T

- o 6 FT SCREEN AT 3 FT Q 4 f T 81N. SCREEN AT 3 F T AbFT SCREEN AT 6 F T

D l S T A N C E

FROM

L O U D S P E A K E R . FEET

F I G U R E

10

D l

R E C T

P U L S E

MEASUREMENTS

I N O F F I C E

0 N O B A F F L E S a 2 FT O . C .

I

1

I

M A T E R I A L A

-

- - -

-

o N O B A F F L E S

\P

- 0 2 FT O . C . D * ~ F T O . C .

I

1

0 N O B A F F L E S 2 F T O . C . 4 FT O . C .

1

M A T E R I A L

C

2000 H z

-

- o N O B A F F L E S 0 2 F T O . C . 4 F T 0 . C .

I

6 1 2 24 6 1 2 24 D l S T A N C E F R O M L O U D S P E A K E R , FEET F I G U R E 11

B O U N C E

S I G N A L S

FROM C E I L I N G S W I T H AND W I T H O E I T B A F F L E S . THE C O N T I N U O U S L I N E I S F R E E - S P R E A D I N G L E V E L

F I G U R E

1 2

C O M P A R I S O N

O F T H E O R E T I C A L D I R E C T

A N D

D I F F R A C T E D

S I G N A L S

F O R

S C R E E N S . S O U R C E

R E C E I V E R S E P A R A T I O N

0

0 . 1

0.2

0.

3

0.

4

A R T I C U L A T I O N I N D E X

F I G U R E

1 3

A R T I C U L A T I O N

I N D E X

B E H I N D S C R E E N S

A S F U N C T I O N

OF

C E I L I N G R E F L E C T I O N

C O E F F I C

l

ENT

Sound absorption t e s t s conducted an lay-in type ceiling panels from the Kenson Building Landscaped Office in Ottawa, Sample tested on

Mounting N u m b e r 7 .

Sarnple A: M i n e r a l fibre painted and perf orated on one face (installed in

ceiling), Panel size

-

23 3 / 4 in. _{wide by 47 3/4 in. long by}

_9/16

_in.

thick. Panel W e i g h t

-

9.5 lb. Approximate number of perforations per

sq 5t

-

11,

400.

Size

and n u m b e r of perforations

-

600

-

1/16 in. _{d i m . by 318 in. deep}

800

-

1/32 in. d i m . by 3 / 8 in. deep

Sample B: Mineral f i b r e panel painted and perforated on one face (new

sample). Panel s i z e

-

2 3 3/4 in. wide by 47 3 / 4 in. long by 518 in.

thick. Panel weight

-

7.5 lb. Approximate number of perforations

per s q f t

-

1,400. Size and number a£ perforations

-

600

-

1 / 1 6 in. _{d i m . by 1/2 in. deep} 800

-

1/32 in. d i m . by 1/2 in. deep

Sample Cz Glass fibre panel with 0.0 10 in, thick painted porous face.

Panel Size

-

2 3 3/4 in. w i d e by 47 3/4 in. long by 11/16 in. thick.

Panel Weight

-

5 . 2 Ib.

Sound Absorption Coefficients

N o i s e Reduction Coefficient 0 , 5 5 0.65 - . 0.85 Sample A B C Fresuencv in H z 1 2 5 250 500 1000 2000 4000 0.29 .28 ,52 .70 -67 .44 0 . 2 9 . 3 8 . 6 3 - 7 9 .71 .46 0.45 -

. 6 9

. 7 5 - 9 4 1 . 0 4 .99