Reverberation room qualification studies at the National Research Council of Canada

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Reverberation room qualification studies at the National Research

Council of Canada

(2)

BUILDING

RESEARCh

NOTE

REVEFWiRATION ROOEi

QUALIFICATION

STUDIGS AT

TEE

HATIOIAL lU3SEARCH COUNCIL

OF

CANADA '

by

W.T. Chu

D i v i s i o n of Building

Research,

National Research

Council

Canada

Ottawa, May 1983

I

L I B R A R Y

4

- - -

03-

05- 1

B I S L ~ Q T H Z Q U E

Rech.

Bairn.

C N R C - I C l P T

(3)

NATIONAL RESEARCB COUNCIL CANADA DIVISION

OF BUILDING RESWCH

REYP,RBERATION ROOM

QUALIFICATION STUDIES

AT

THE

NATIONAL RESEARCH COUNCIL

OF CANADA

(4)

REVERBERATION ROOM QUALIFICATION STUDIES AT THE

NATIONAL RESEARCH COUNCIL OF CANADA

by

W.T. Chu

INTRODUCTION

This report p r e s e n t s the results of a series o f measurements undertaken in t h e DBR-NRC reverberation room t o determine whether t h e room m e e t s both the broad-band and the discrete-frequency qualification

requireraents of the American National Standard for sound power

measurements.

Our results give additional support to other researchers' findings

that loudspeakers w i t h a d i a m e t e r larger than 200 mm are not suitable

Eor discrete-frequency t e s t s at high frequencies and t h a t averaging over

source posttfons is not very e f f e c t i v e at low frequencies,

For the broad-band requirements, it f s p o s s i b l e to qualify the DBR

room with its present configuration of fixed diffusers and a rotating vane. This is a l s o true for t h e discrete-frequency requirement for

p r a c t i c a l l y any single source p o s i t i o n e x c e p t for the t w o lowest

frequency bands (100

Hz

and 125 Hz). To qualify the room f o r these two bands, four resonant type l o w frequency absorbers are required and it is necessary t o average over at l e a s t two s p e c i f i c a l l y chosen source

locat ions.

Since the DBR room configuration is still under continuous

improvement f o r other purposes, we have not optimized the amount of

absorption to qualify the room for any single source position. The main

purpose of this report is to summarize the steps i n v o l v e d in the

qualificatfon procedure and to p o i n t our b e n e f i c i a l modificatione that might be useful f o r future q u a l f f i c a t i o n t e s t s .

FACILITY DESCRIPTION

The

_{reverberation}_{room to}_be_{qualified for sound power measurement}

is t h e larger of the two rooms comprising t h e transdssion loss s u i t e in

t h e Division of Building Research. It 1s rectangular, w i t h a nominal volume of 255 m3 and dimensions of 8.0 x 6.5 x 4 . 9 m- In i t s general use, the room is equipped with fixed diffusers and a rotating vane. The fixed dfffusers are made of 12.7 mm thick sheet plywood, 1.21 x 1.21 m.

Nine of these are hung at random fa space. An additional panel, 0-9 x 2-2 m, i s placed along one of the lower edges of the room to

improve diffusion in the lower portion of the room. The rotating vane

consists of four 1.21 x 2.42 _m_{sheets of}_12.7 _mm_{t h i c k}_plywood_arranged

(5)

middle of

the

upper portion of the room and rotates at a rate o f

12 r p .

The room has automaEic environmental controls and is generally

maintained at 21°C

and

55% relative humidity.

The existing instrumentation system in the Noise and Vibration Secttan for commefcial testing is quite readily' adapted for these qualification t e s t s . Generarim of the sequence of test tones, and

acquisition and numerfcal processiug of the data can be perforfed

automatically under carnputer control.

A black diagram

of t h i s system is

s h m in Pig, 1.

An array of nine GR Type 1961-9602 25.4

mm electret laicrophones

is

placed in the reverberation room according to the standard's

requirement. These microphones are mounted an GR Type 1560-P42

preaotplifiers, which are fa turn connected t o the GB Hultfchannel Amplifier and Multiplexer. Under computer control, any one of the microphones

can

be sc1l;cted

and

I t s w t p u t

fed into

the

GR

Real-Time

Analyzer. The latter consists of a Type 1925 Multifilrer and a Type

1926 Multichannel

RHS

Detector. The

W S

Detector computes the root-

mean-square voltage levels, referenced to 1 mV, from the digitized

samples obtained for each one-third octave band over the integration

p e r i o d . These data are transmitted t o the computer, which controls the i n t e g r a t i o n tine and s t a r t s the data transfer process.

The qualiEicatlon t e s t tone8 ate generated by a Rocklaad Frequency

Synthesizer. 'L'he signal frequency and amplitude from this synthesizer

are also controlled by the computer.

ACOUSTIC

SOURCES

For the broad-band qualification test, an ILG Reference Sound

Source was used as the noise generator. Two loudspeakers of d i f f e r e n t

dimensions were used for the pure-tone qualification t e s t . One was

a

120 um KEP B l l O loudspeaker mounted

in

a closed cubic box 23

cm

on each

side. The other was a 250 naa

YBL

E l l o loudspeaker

mounted

in a closed

rectangular box aeasuring 30 x 35 x 25 em. The loudspeaker responses

were measured in an anechoic

room

over a reflecting plane with the

speaker cone facing upward in accordance with the standard.

T n performing these ~seasurements, another GR microphone of the same

t y p e w a s used. It was placed about 10

mm above

the plane of the speaker

b a f f l e on the speaker axis w i t h the microphone diaphragm perpendicular to the speaker haEEle so chat sound f r o m t h e speaker etruck it at

grazing i n c i d e n c e . According to the manufacturer, t h e frequency response of t h i s type

of

ndcrophune is practically

the

same at both

(6)

microphones measuring the near-field response have the same response a s

those measuring the reverberant f i e l d . As s h m in

Tables

I

and 11, the

near-ffeld responses of both loudspeakers s a t i s f i e d the standardls

requirement that smnd pressure l e v e l s at adjacent fxequencles do not

d i f f e r by more than 1 dB.

ROOM QUALIFICATLON

The qualffication t e a t s were performed according to the procedure described in the t w o h r i c a n National Standards, mS1 S1.31-1980 and

ANSI S1.32-1988. g 2 Broad-Band T e s t

For the broad-band case, the procedure is f a i r l y sfmple. Sound

pressure l e v e h at the nine microphone p o s i t i o n s w e r e measured for eight

source locations using the ILG as the nofse source. The r e l a t i v e source

locatfans are sham in Fig. 2

Since spatial standard deviation I s required for t h i s test, all t h e

microphones have to be c a l i b r a t e d before the experiment. The

calibration was performed with a I3 &

K

Type 4230 Swnd Level Calibrator

at 1 W e .

Ir

has

an

accuraq of f

0.3 dB, as required

by the standard.

For each frequency band

far

which the test ram is to be qualified,

t h e s p a t i a l standard deviation ( S s ) in d e c i b e l s , was computed u s i n g the

formula:

where: (L,Ij = sound pressure level averaged over a11 microphone

positions when the ILG is in the jth source location,

(a);

= arithmetic mean of (L ) values, averaged over a l l source locations Jnd,

Ng = number of reference sound source positions ( e l ght were

used).

The test room will. quallfy for the measurement of broad-band noise

sources i f the computed standard deviation does not exceed the l i d t s

specified in t h e standard. Results presented in Table 111 show that the existing DBR room,

with

f i x e d d i f f u s e r s and rotating vane, qualified for

a l l the frequency bands from 100

Hz to

10 ZtHz.

Pure-Tone Test

From a practical point of v i e w , it 5 s important to know t h e effects of various room configurations and meawtrearent pararmeters on the results of the qualification tests. The main objective is to find a room

(7)

configuration

such that

the roam can be

qualified for

a single swrce

p o s i t i o n placed anywhere w i t h i n a certain area

fn

the roam. With this

i n mind, we bave carried out t e s t s

i n

the Large DBR-NRC reverberation chamber f o r three different configuratiws. The alternative procedure

for the measurement of discrete-frequency components as s e t

f ~ r t h

in Section 5 of ANSI ~1.32-1980~ was f o l l m e d .

For

each ode-third octave band, the sound pressure l e v e l e

were

measured at nine independent microphone positions at the prescrfbed test

frequencies l i s

t s d in

Table

V

of ANSI S1.32-1980, using one of the loudspeakers as the source. The near-ffeld characteristic o f the

laudspeaker was then corrected for at each frequency before the arithmetic mean, elp>, end the standard deviation, Sf, were colpufed using the following equation:

where: (Lp)* = average sound pressure level (corrected for loudspeaker

response) produced in the test room by the loudspeaker

source when excited at

the

kth test: frequency, average

over

a l l microphone positions (and if appropriate, over

all

loudspeaker source locattons) (dB);

(Ld

-

arithmetic mean of

(I

) values, averaged over all n

p k

t e s t frequencies (dB),

n

= number of t e s t frequencies fn a gfvea one-third octave

band.

The room d l 1 qualify for measuremente of narrPotj-band or discrete-

frequency noise sources if the c o q u t e d standard deviations do not

exceed the limits s p e c i f i e d in the standard. Results and discussions of

these t e s t s will be presented in the following sections,

Room with fixed dilfusers

-

For this case, the rotat.ing vane was not In

operation but was f i x e d at a particular ortentation f b r the complete s e t of experiments. Six source positions have been tested, usiw the JBL

loudspeaker. Locations of theae source poaitione are the same .as those

shown

in

P i g . 2.

Results presented in Table

IV

ahow that the room in t h i s

configuration d i d not q u a l i f y for

any

me of the swrce locations used.

Even the averaged r e s u l t s over the six source positions d i d not qualify

the roam at the lower frequency bands, The r e a m

for

the

ineffectiveness

ia source

position averaging has been given by Ebbing and &ling. They suggest that a t the

lower

frequencies, because t h e

modal overlap is not sufficieatly great, the sound power output of the source a t a particular frequency may 'be high (or lm) regardless of the posttion of the source in the room.

Thus, p l o t s

of spacedaveraged sound pressure l e v e l s

in

the room versus frequency for several source

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S i d L a r results were obtained by Lang and ~ e n n i e . ~ Out present results

shm the behaviour at

rhe

lwer frequencies depicted in Fig. 3. The relatively poor performance at the high frequency bands, as

indicated in Table

IV, was

f w n d t o be

a

consequence of the large

JBL

foudspeairer being used, Further discussion

on thi6

will be given in t h e

next sect ion.

Room with f i x e d d3ffusers and a rotating vane - Significant improvement

was obtained in general when the rotating vane was in operation, as

indicated by results tabulated in Table

V.

The room in t h i s

configuration qualified for a l m e t any source position tested, e x e p t at

the two lowest frequency bands. The larger standard d e d a t i o n s measured

at t h e high frequency bands were due to the use of the large JBL

loudspeaker. More acceptable values were obtained when the JBL loudspeaker w a s replaced by the smaller REF loudspeaker, as s h m i n Table V. The anomaly might be cauaad by the directionality of the

loudspeaker. Fig. 4 _{compared the}_{dftectivities}_of_the_{t w o}_loudspeakers measured in an anechoic chamber. The JBL loud~peaker shows a more

severe direct1 onal characteristic than the KEF loudspeaker

in

the

2.5

kHz

band. Unfortunately, the latter d w s not have enough acoustic output at law frequencies to be used for the f u l l frequency range of the

teats,

When averaging over source position was applied t o the data, tt was

Ewnd

that the room in t h i s configuration s t i l l f a i l e d t o qualify at the

100

Hz one-third

octave band even when seven source positions were u6ed.

In fact, plots of the space-averaged sound pressure l e v e l in the room

versus frequency show rhat the ratating vane d i d not produce s i g n i f i c a n t changes in the shape of the reepoase curves from those of t h e f i x e d diffusers at t h i s lowest frequency band. T y p i c a l curves for two

d i f f e r e n t source lacations are

shown

in Figs. 5 and 6.

It

fs not clear at t h i s point whether i t is the size or the paddle design of the

rotating vane that is the source af the problem. Another p o s s i b l e

explanation is that the ratio of room dimension to wavelength has

approached a limit that makes it difficult t o change the modal structure

of the

room.

For the 100 Flz band, the modal overlap of the room is

about 0.4. The next logical s t e p is to add abeorption t o broaden the

modal bandwidth

and

smooth out the response curves. Derailed

discussion will be given in the next section.

Room with fixed diffusers,

rotatina

vane and low-frequency absorbers -

In order to provide absorption only at the b e s t frequency bands, some form of resonant type absorbers has t o be used. The one chosen

coneisted of a rectangular wooden box 61 x 122 x 15 cm. Ordinary

pegboard was used for rhs t o p face with alternate rows and columns of

the holes taped. T h i s left a spaetng of about 7.6 cm between holes to

provide a resonance frequency at about 120

Hz.

The box was filled with

l a w d e n s i t y fiberglass. Four of these w e r e used, two hung on the eide

walls and two placed along the edgee of the room. These absorbers

provide f a i r l y good absorptians at

low

frequencfea, as i n d i c a t e d by the

(9)

on

the

mom responses is also evident from the p l o t s shown

tn

Figs, 5

and 6 .

Although further improvement t o the measured standard deviation has been abtalned, it was s t i l l not s u f f i c i e n t to -qualify the

room at any

single source position, as indicated

by

results sbwn in Table

VI.

We

are, however, approaching the possiblli~y of qualifying the room .with averaging results over any p a i r of source positions, as suggested by results shown in Table V I I .

With the l o w frequency absorber8 added, the average Sabin

absarptlon coefffcient of the room st 100 He was about 0 . 0 5 , quite a b i t helow the value of 0.16

allowed by

the standard. Thus, w e are

o p t i m l s t l c that the room can q u a l i f y even for any single source p o s i t i o n

if more low frequency absorption is added. These additional

investigations

will

be p e r f o e d when we have perfected an alternate

that is Easter and

more

accurate.

No data above 1 kFIs far this room configuration has been presented

because

we are

confident the roam

&I1

qualify for these frequencies If

the smaller KEF loudspeaker is used.

CONCLUSIONS

The basic conclusions d e r i v e d from t h i s study are as follows: (1) loudspeakers w f t h diameter larger than 200 mm are not suitable for the discrete-frequency qualtfication t e s t at high frequencies; ( 2 ) at

low frequencies, the space-averaged room response shows significant correlation over d i f f e r e n t source l o c a t i o n s , thus rendering averaging

o v e r source p o s t t i o n s r e l a t i v e l y ineffective; ( 3 ) to qualify the

existing DBR-NRC reverberation room for discrete-f requency measurement

over the whole frequency range, it is necessary t o add low frequency a b s o r p t i o n and use t w o source p o s i t i o n s ; (4) qualification of the room f o r broadrband measureent is possible without the addition of l o w

frequency absorptions.

1. American National Standard Precision Methods for the Determination of Sound Power Levels of Broad-Band Noise Sources in Reverberation Booms, ANSI S1.31-1980. American Institute of Physics, New York.

2. A m e r i c a n National Standard Precision Methods for the Determination of Sound Power L e v e l s of Discrete-Frequency and Narrow-Band Noise

Sources in Reverberation Rooms,

ANSI

S1.32-1980. American Institute

of Physics, New Y o t k ,

3 . Ebbing, C.E. and Haling, G.C. J r . , Reverberation Rooa Qualtfication

f o r Determination of Somnd Power o f Sources of Discrete-Frequency Sound, .T. Acoust. Soc. Am.

54,

935-949 (1973).

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4 . _tang, _M.A.

_.and

_Rennie,

_J.,

_{ExamLnation of}_{the Effect of Source}

Location on Sound P o w e r Measurements at L& F'requermcies, J. A c w s t . Sac. Am, 6 5 S9(A) (19811.

5 , Chu, W.T., Near and Far F i e l d Transfer-Function Technfque for

Reverberation RoomResponse S t u d i e s , J. Aeoust. Soc, Am,

-*

72 S18(A) (1982).

(11)

6 0 ~ ~ ~ ~ w 0 d 4 ~ d ~ ~ 0 m ~ 0 ~ ~ ~ 0 O m 0 0 m ~ ~ ~ * ~ ? ~ ? ~C .~. ~ ~ ~

.

~ * ~ ~ " o " ~ h " ! v ) ~ O o I m m " m ~ ~ ~ h h ~ m m ~ w m ~ m ~ m a D 0 0 a S m m c r ) O D m m m m m m m m m m a m m m m m m m m m m m m ~ m m ~ ~ m O ~ u 0 ~ ~ 0 a 0 ~ m ~ ~ o ~ ~ O O O O O O m

~ ? ~ ~ " ~ ? ~ ~ y " ~ " " " o o ? O O ~ ~ O ~ O b

I . . . . 9

. .

~ h h h h m O w m m m m a m ~ m m m m m Q I Q I b \ O I b r a ~ m m m m ~ m m m m m m m m m m m m m m ~ ~ m m ~ m

(12)

TA3LE 11

NEAR-FIELD

RESPONSE OF THE KEF LOUDSPEAKER

Center Frequency of One-Third Octave Bands

(Hz)

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BRQAD-BANI QUALIFICATION

DATA

FOR ROW WITH

FIXED DIFFUSERS

AND

A

ROTATING VANE

h e - T h i r d Octave

Band Maximum Allomble Measured

Center Frequency Standard Deviations Standard Deviations

(14)

T m E IV

UISCReTE-FREQWCI C@ALIFICATION DATA FOR ROOM WTIi FIXED DIFE'USERS

Measured Standard Deviations

One Third At Dffference Source P o s i t i o n s Averaged Octave Band Maximum Allmable