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

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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

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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

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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).

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