Room response measurements in a reverberation chamber containing a rotating diffuser

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Room response measurements in a reverberation chamber containing

a rotating diffuser

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N21d

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National Research

Conseil national

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ROOM RESPONSE MEASUREMENTS I N A REVERBERATION CHAMBER CONTAINING A ROTATING DIFFUSER

by W.T. Chu

A N A L Y Z E D

Reprinted from

L I a R A R Y

Journal of the Acoustical Society of America i

Vol. 77, No. 3, March 1985 _II

p. 1252

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1256

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06-

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DBR Paper No. 1278

Division of Building Research

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Ltapplication de sequences periodiques pseudo-aldatoires 21 la

mesure du comportement dtune piece a Qt6 Qtendue aux pieces

dotQes dtun diffuseur rotatif.

Les rhultats obtenus avec

cette nouvelle technique correspondent assez bien 2 ceux

obtenus par la methode de balayage

3 fr6quence unique.

Pourtant, la nouvelle methode ntexige qutune fraction du temps

requis par ltancienne

pour rkaliser le mgme travail.

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Room response measurements in a reverberation chamber

containing a rotating diffuser

W . T. Chu

National Research Council of Canada, Divison of Building Research, Ottawa, Ontario, Canada KIA OR6

(Received 27 July 1984; accepted for publication 26 November 1984)

The application of periodic pseudorandom sequences for room response measurements has been extended to chambers containing one rotating diffuser. Results obtained by the new technique compare very well with those measured by the single frequency sweep method. Yet it takes only a

-

small fraction of the time required by the latter method to accomplish the same task.

PACS numbers: 43.55.Br, 43.85.Fm, 43.55.Nd

INTRODUCTION pure-tone or narrow-band sound power measurements.'

Conventional single frequency sweep method is very time Accurate determinations of room responses in rever- consuming and new techniques should be developed. beration chambers are required for the qualification test of In an earlier paper,2 it was shown how a periodic pseu- 1252 J. Acoust. Soc. Am. 77(3), March 1985;OOOl-4966/85/031252-05$00.80; @ 1985 Acoust. Soc. Am.; Letters to the Editor 1252

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dora ndom sec Iso name

_..

..

.

lgth se-

quence (a d a maximum-1e1 quence) can be used eftectively as a mulitone source in a chamber with fixed geometry. The new technique can shor- ten the testing time by more than one order of magnitude.

In this paper, the extension of this technique to rooms ~ p e d with one rotating diffuser is discusssed. To ap- imate such a room as a time invariant system, it is neces-

.

to match the period of the sequence with that of the rotating diffuser and perform all analyses over exactly one period. The different frequency components of a room re-

I sponse can be determined by the fast Fourier transform

(FFT) technique. However, measurements showed that the I

room response varied with the phase relationship between the periodic motion of the rotating diffuser and the period of the sequence. Thus, some ensemble averaging is necessary. Very good agreement has been obtained between this new technique and the single frequency sweep method. Yet the new technique takes a much shorter time to achieve the same results.

I. SINGLE FREQUENCY SWEEP METHOD

Measurements were taken for one fixed microphone and source position in a rectangular model reverberation chamber with dimensions 3.2 X 2.6 X 1.96 m. The room is equipped with both fixed diffusers and a rotating diffuser. The latter consists of a 0.76

x

0.9 1 m wooden panel mounted at about 45" inclination on a rotating shaft in the middle of the room; it rotates at about 15 rpm.

The room was excited with pure tones generated by a stable frequency synthesizer (HP 3325A) fed through a power amplifier to a speaker placed at a fixed position on the floor of the room. Two hundred frequencies were used start- ing from 400 Hz with an increment of 1 Hz. At each frequency, the sound pressure at one chosen location in the reverber- ant sound field about half a meter from the rotating diffuser

1

R U R U N N o . N N 2 O

1

F R E Q U E N C Y , H z

FIG. 1. Repeated measurements of room responses in the 400-600 Hz range

by the single frequency sweep method.

FIG. 2. Initial portion of two separate time traces measurea rrom n

reference position of the rotating diffuser under periodic pseudoranc quence excitation.

le same ~ ----

dom se- ,

was measured with a B&K 1/2-in. microphone whos put was fed into an Analog Devices 4425 true rms-to-d

verter with a time constant of 0.03 s. The output of t t - _ . _ _

detector was sampled every 0.1 s. A total of 40 samples were used to compute the averaged mean-square pressure over one period of the rotating diffuser. Results from two repeated runs are plotted in Fig. 1 as sound pressure levels versus frequency. The excellent agreement indicates that the procedure of averaging over one period of the rotating diffuser is correct.

II. PERIODIC PSEUDORANDOM SEQUENCE TECHNIQUE

e out- c con-

Le rms

For comparison purposes, the room response at the same microphone location was measured under identical conditions. The instrumentation setup is similar to that used in Ref. 2. Since it has been established that the spectrum of the source signal is flat,2 it is only necessary to compute the microphone output spectrum rather than the transfer function.

Matching the period of the sequence with that of the rotating diffuser is important. To measure the period of the rotating diffuser accurately, a small rod, attached to the shaft of the rotating diffuser, interrupted the beam of a light sensor once every revolution. The output from the light sensor was used to trigger a frequency counter. The same signal was also used as a trigger for the A to D converter-so that data could be obtained from the same reference position of the rotating diffuser.

In order to meet the requirements imposed by the matching of periods and the frequency range of interest, the length of the sequence L was chosen to be (2'' - 1) and a clock frequency of 32 998 Hz was used. The sampling frequency was 1/16 of the clock frequency as used in the pre- vious paper. For this case, the total number of data points

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400 500 600

r n E Q U E N C Y , H Z

FIG. 3. Comparison of room responses computed from the two time traces shown in Fig. 2 in the 400-600 Hz range.

used became 8 192. The Nova 4 minicomputer can handle the 8 192 point FFT producing 4096 frequency components of approximately 0.25-Hz spacing. However, only every fourth point (giving approximately 1-Hz resolution) will be used for better graphic presentation of all results reported here. For smaller computers that cannot handle large FFTs, either the method proposed by Chu3 or the one by Aoshima4 can be used.

First it will be shown that when the periods of the sequence and the rotating diffuser are matched, the sound field

TABL

aged sl

figural

,E I. Standard deviations comvuted from the different ensemble aver-, x n 400 and

is also periodic and deterministic. This was accomplished by analyzing two separate digitized time records of 8 192 points, each taken not necessarily consecutively, but from the same reference position of the rotating diffuser while a fixed phase relationship between the rotating diffuser and the sequence was maintained. Indeed, the two time traces show good re- peatability. An initial portion of each of them is shown in Fig. 2. Figure 3 depicts the spectra computed from these time records between 400 and 600 Hz. The two curves are nearly identical.

It is evident from Figs. 1 and 3 that the new technique has not produced a room response equivalent to that obtained by the single frequency sweep method. Further studies showed that the measured room response for the fixed microphone and source position varied with the phase relationship between the periodic motion of the rotating diffuser and the periodic pseudorandom sequence. Figure 4 shows significant differences between two spectra obtained after the sequence had been turned off and on again to alter this phase relationship. Thus ensemble averaging for different phase relationships is necessary.

-

R U N No. 2

F R E Q U E N C Y . H z 400

FIG. 4. Comparison of room responses obtained under different phase rela- F R E Q U E N C Y . H z

tionships between the periodic motion of the rotating diffuser and the peri- FIG. 5. Repeated measurements of the averaged room response in the 400- odic pseudorandom sequence in the 400-600 Hz range. 600 Hz range obtained under periodic pseudorandom sequence excitation.

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t

'O - - t tKtYUtNLY b W t t Y - - - MAXIMUM-LENGTH SEQUENCE

i

I I , A.

-

SINGLE FREQUENCY

-..-,.

- - - _{MAXIMUM-LENGTH SEQUENCE} 1 1 I ,

I

400 500 600 500 5 80 FREQUENCY, Hz FREQUENCY. H :

FIG. 6 . Comparison of room responses in the 400600 Hz range obtained by FIG. 7. Comparison of room responses in the

two different methods. two different methods in a full scale reverbel

The number of ensemble averages required was determined by comparing the standard deviations computed from the different averaged responses between 400 and 600 Hz. Table I indicates that the standard deviations varied from 5.86 dB for a single observation to a final value of 3.60 dB after six or more ensemble averages. At least for the present room condition and the frequency range of interest, it seems that six to eight averages are adequate, but eight were used for the results presented here. Figure 5 shows that the averaged room response is quite repeatable. This also compares well with the response obtained by the single frequency sweep method, as depicted in Fig. 6. The standard deviation of the single frequency sweep curve is 3.30 dB.

Ill. DISCUSSION

A single measurement of the room response using the periodic pseudorandom sequence showed no effect of the rotating diffuser. The measured standard deviation was 5.86 dB, which seems to correspond to the value of 5.57 dB for no rotating diffuser. A possible explanation can be presented based on the modulation theory5p6 of a rotating diffuser.

In the single frequency sweep method, each frequency point of the measured response function is effectively an average over several frequency components, due to the modulation effect of the rotating diffuser and the way the mean- square values are obtained by including the contributions from all the sidebands. No such summation or averaging exists, however, in the case of the pseudorandom sequence excitation, where the response function was obtained by the FFT procedure. In addition, the redistribution of energy of any frequency into sidebands might be different because the sideband frequencies coincide with the existing driving frequencies of the pseudorandom noise source.

500-660 H ration cham

-

' ZWFFP !range obtai ber. side-

----.

Since the number and relative ies oithe bands cannot be predicted for each fleyue~lc;y, it is not 1

ble to perform any frequency averaging of the measurc sponse function. Other averaging techniques must be uc reveal the effect of the rotating diffuser. The ensemble aging chosen for the present investigation works well rotating diffuser with a relatively low figure of merit (i

1.5 in the 400-600 Hz range). For a more effective rot,,..-, diffuser, the number of ensemble averages required sl be increased.

Further experiments carried out in a full scale revc ation chamber of 255 m3 equipped with a larger roi diffuser (see Ref. 7 for detailed description) confirme validity of the present technique. As shown in Fig. '

averaged response curve compared reasonably well witn r n n r

obtained by the single frequency sweep method. Fo case, 40 ensemble averages were required to bring the dard deviation to its final value of 1.35 dB. The fig^,,

,.

merit of this diffuser is about 4.1 in the 500-600 Hz range. The corresponding value for the single frequency sweep curve is 1.24 dB. Juaal- :d re- sed to aver- - . -- for a xbout - * t i n o tating :d the 7, the I .l

.

--

-

r this stan- 1rP nf IV. CONCLUSION

It has been demonstrated that the periodic pseudorandom sequence can also be used as a multitone source for rooms equipped with a rotating diffuser. Ensemble averaging is required, however, to eliminate variations caused by the phase relationship between the periodic motion of the rotating diffuser and the sequence. The new technique signif- icantly reduces the testing time compared to the single frequency sweep method. Coupled with the nearfield and farfield transfer function technique as suggested by Chu," future qualification tests for sound power measurements can be performed more efficiently.

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:KNOWLEDGMEh I a

This paper is a contribution of the Division of Building :search National Research: Council of Canada, and is pub- hed with the approval of the Director of the Di-li~inn

ational Stan rer Levels o

unerican N, dard Precision Methods for the D, n

Sound Poa f Discrete-Frequency and Narrow e

urces in Reverberation Rooms," ANSI S1.32-1980.

. T. Chu, "Architectural Acoustic Measurements Using Periodic Pseu-

-Random Sc d FFT," J. Acoust. Soc. Am. 76,475478 (1984).

I . Lnu, A I ecnnlque for Zoom Transform and Long-TUG olgruu

Analysis," Can. Acoust. 11 (3) (1983).

4N. Aoshima, "New Method of Measuring Reverberation Time by Fourier Transforms," J. Acoust. Soc. Am. 67, 1816-1817 (1980).

'D. Lubman, "Spatial Averaging in Sound Power Measurements," J. Sound

Vib. 16,4348 (1971).

6J. Tichy and P. K. Baade, "Effect of Rotating Diffusers and

Techniques on Sound-Pressure Averaging in Reverberation C h Acoust. Soc. Am. 56, 137-143 (1974).

'W. T. Chu, "Reverberation Room Qualification Studies at the National

Research Council of Canada," National Research Council Canada, Divi- sion of Building Research, BR Note 203, Ottawa (1983).

'W. T. Chu, "Near and Farfield Transfer Function Technique for Rever-

beration Room Response Studies," J. Acoust. Soc. Am. Suppl. 1 72, S18 (1982).

l Sampling mbers." J.

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T h i s paper, w h i l e b e i n g d i s t r i b u t e d i n r e p r i n t form by t h e D i v i s i o n of B u i l d i n g R e s e a r c h , remains t h e c o p y r i g h t of t h e o r i g i n a l p u b l i s h e r . It s h o u l d n o t be r e p r o d u c e d i n whole o r i n p a r t w i t h o u t t h e p e r m i s s i o n of t h e p u b l i s h e r . A l i s t of a l l p u b l i c a t i o n s a v a i l a b l e from t h e D i v i s i o n may be o b t a i n e d by w r i t i n g t o t h e P u b l i c a t i o n s S e c t i o n , D i v i s i o n o f B u i l d i n g R e s e a r c h , N a t i o n a l R e s e a r c h C o u n c i l of C a n a d a , O t t a w a , O n t a r i o ,

K1A

0R6.