RASTI Measurements in St. Paul’s Cathedral, London

by John Anderson, The City University, London, and Torben Jacobsen, Brüel & Kjær

Introduction

St. Paul’s Cathedral in London is one of the largest buildings in Great Britain. Its internal space, of volume 152000 m”, is dominated by a dome at the crossing of internal height 66m. The dome is seen in Fig.1, which shows the external elevation as viewed from the South. The Cathedral is also well known for its whispering gallery. The reverberation time in the Cathe- dral is large, as much as 12s at low frequencies, and not surprisingly the intelligibility of speech is generally poor without the use of a speech rein- Fig. Elevation of the Cathedral from the South forcement system.

Traditionally the method used to assess speech intelligibility, and car- ried out previously in the Cathedral (Ref.[l]), is for different speakers to read out a series of phonetically bal- anced words in which each word is buried in a carrier sentence so that it cannot be recognised from the con- 60 text. Listeners in different locations write down what they hear the words to be. The method is described in de- tail by Beranek (Ref.[2]). Normally if a 40 word is misunderstood it is because the consonants are not heard properly. The number of words understood out =N+BP of the total is a fraction less than unity = peakcl. = AGCiRev which, when expressed as a percent- = 56% age, is almost the same as the phoneti- cally balanced (PB) word score. The 0, I I 0 60 ’ 80 100 PB word score correlates well with an Bad Fair J_Good,/_ Excellent 4 index of speech intelligibility, the I- -I- speech transmission index (STI) Sti (%) c%?am

(Ref.[3]), as shown in Fig.2. Fig. 2. Relation between the objective STI and PB-word score for 167 different transmission channels. The disturbances were combinations of bandpass limiting, noise, peak The measurement of the PB word clipping, automatic gain control and reverberation. From reference 5 score may be described as a subjective method as it depends upon the clarity of the speakers and the personal as- quires 98 measurements and conse- form measurements of the rapid sessments of the listeners. These tests quently a rapid speech transmission speech transmission index and com- also require a great deal of time and index (RASTI) has been devised pare them with earlier subjectively ob- organisation. An objective method, us- which requires only 9 measurements. tained results for speech intelligibility ing a loudspeaker and electronic Apparatus for the measurement of in the Cathedral. Furthermore, it is equipment, has been developed by RASTI is now commercially available now possible to take quickly a large Houtgast and Steeneken and their col- from Briiel & Kjær. number of measurements at different leagues at the Institute for Perception positions so that iso-RASTI contours TN0 in the Netherlands (Ref. [3, 4, The availability of this new instru- may be constructed. These tests are 5 However, even this method ~ ment presented an opportunity to per- described in this Application Note.

BO0116-11 3 The RASTI-Method As already mentioned, the RASTI- is placed at the speaker’s position and The final RASTI-value is based method is based on the Speech Trans- simulates the real speaker. upon these nine modulation indices. mission Index (STI). The RASTI- The RASTI-value ranges from 0 (= no method is very quick; it is possible to The receiver is placed at the listen- intelligibility) to 1 (= complete intelli- measure the intelligibility at one lis- er’s position. It measures how the test gibility). tening position in less than 10 sec- signal has been modified. The change onds. in the signal (reduction in the modula- The RASTI-method can be used in tion) at the listener’s position is quan- many different applications, among The RASTI-method is being stan- tified in terms of the modulation them being: dardized by the IEC (Ref.[7]). The transfer function for nine different method relies on the use of a special modulation frequencies. The reduc- evaluation of speech intelligibility transmitter and receiver. The trans- tion of the modulation is interpreted in rooms mitter sends out an acoustic test sig- in terms of an apparent signal-to- evaluation of reinforcement - and nal (“an artificial voice”) which con- noise ratio, irrespective of the cause of PA-systems tains information about the frequency the reduction, which can be reverbera- intelligibility in industrial facilities range covered by speech, and the fluc- tion, echoes or background noise. assessment of sound masking sys- tuations in the voice. The transmitter tems

The B & K RASTI-Instruments

Brüel & Kjær produces RASTI-in- The internal loudspeaker mounted human being, and fulfils the IEC draft struments (system 3361) which fulfil in the transmitter has a directivity in- standard with regard to directivity. the requirements of the IEC draft dex similar to that of the voice of a standard (Ref.[7]), and which give fur- ther useful information. A short de- scription is given below of each of the two instruments which make up the system.

Speech Transmission Meter, Transmitter 4225 The Transmitter 4225 produces the special “RASTI-sound” and sends it out by means of a built-in loudspeak- er. In Fig.3 the front and the back of the instrument are shown.

The power On/Off switch (battery powered) is located at the front. The output level is the reference level as given in the IEC-standard (a speech level of Leq,A = 60 dB at 1 m distance). A further + 10 dB compared to the standard level may also be selected. The latter output level can be used to evaluate the influence of background noise in the room.

Three different output possibilities exist: the internal loudspeaker, output to an external loudspeaker or an elec- trical output. For each of these out- puts it is possible to calibrate the out- put level.

The internal loudspeaker is located at the back along with connectors for External Power and Remote Control, and the battery box. Provision is also made for mounting the microphone Fig. 3. Front and rear panels of the Speech Transmission Meter, Transmitter 4225. Note during calibration. the loudspeaker on the left of the rear panel

4 Output all the values may be output through the serial interface ( RS 232). This can be done automatically after each measurement or by, pressing Dig- ital Output. There are different out- put formats, one is shown in Fig.5. It is also possible to control the instru- ment through the interface.

In the measurement in St. Paul’s a portable Epson computer was used to print out the result. The RASTI in- strument checks for errors during the measurement and if these occur, error codes are generated.

Measurement Procedure

The Brüel & Kjær Type 4225 was Fig. 4. Front panel of the Speech Transmission Meter, Receiver 4419 used as the sound source and the Type 4419 was connected to a microphone placed at the receiver positions. At each receiver position at least one measurement of 32 seconds duration Speech Transmission Meter, Re- whether an intelligibility problem was performed. ceiver 44 19 comes from background noise or from The Speech Transmission Meter, reverberation. Also the RASTI-value Source positions: in order to obtain Receiver 4419 receives the test signal for both octave bands may be read a direct comparison with the earlier at the listener position by means of a out. The level in each octave band tests (Ref.[l]) the sound source (Type prepolarized microphone. In the 4419 may be displayed. 4225) was placed at either the pulpit the signal passes through an auto- or the lectern, both of which are indi- range stage and through an anti-alias- RASTI-values less than one can be cated in the plan shown in Fig.6. The ing filter. The rest of the treatment is caused by background noise or by re- sound source was placed on its tripod in digital form. Each of the modula- verberation in the room. It is not pos- at a position where the head of the tion frequencies is found by means of sible by means of the RASTI-method clergyman or preacher would normally a Discrete Fourier Transform. After to evaluate which of these parameters be. At both the pulpit and the lectern that, the different values are calculat- is more important. But the instrument the axis of the source was pointed ed. The instrument is shown in Fig.4. can calculate a S/N ratio if it is as- down the nave, as shown by the dotted sumed that the reverberation time is lines in Fig.6. The level of the source The 4419 has two inputs, a pream- zero; and, conversely, if it is assumed was + 10 dB compared with the IEC plifier input and a direct input. In the that the S/N ratio is 3 15 dB the re- RASTI standard level. measurement in St. Paul’s a 4129 Mi- verberation time can be calculated. crophone and 2642 Preamplifier were These two values are called S/N connected via a 30m extension cable Equivalent and EDT Equivalent. to the preamplifier input. Pressing “S/N Equiv. dB”, the RASTI-value is translated to the S/N Before a measurement is started a in the room, assuming that the rever- RASTI 0.56 calibration is performed using a B&K beration time is zero in the room. Usu- SPECIllL 21 4230 calibrator. After this calibration ally it is possible to evaluate if back- ERROR 6 the measurement time is chosen (8, 16 ground noise or reverberation time is ocT*YP HZ 500 * FREQ fro I 0.85 0.;: or 32 seconds) and the measurement is the most important parameter, and H FAEP w 2 0.72 0.15 0.41 0.58 started by pressing Single or Run. In the instrument can give the value. 0.14 0.38 Run the instrument will start a new 0.34 ST1 (OCT) 0.52 0.63 measurement again after the previous In connection with the RASTI- LEVEL DB 60.6 47.5 one is finished. A longer measurement method, the reverberation time con- FLOOR Da a.0 SH EUJIV oa ::t 3.1 time will produce a more accurate sidered is Early Decay Time (EDT), EDT EQJIV 5 7.29 0.92 RASTI-value. When the measurement because the measurement corresponds is finished the RASTI-value is dis- to the first 5 dB of the decay curve. OCT a 0 .2 .4 .6 .a I played. The RASTI-value is based on Furthermore, it is the reverberation as 2X 1 X measured between the speaker and 500 , 0 the 9 modulation reduction indices, i I and it is possible to read out each of the listener positions, so it can differ 5:: 2 0 2K X these values. Four of these modulation from the reverberation of the room as 505 : 0 X reduction indices are obtained in the a whole. 520;: : 3 500 Hz octave band and five in the 2K 5 X 2000 Hz octave band. By means of All the above mentioned values can these values it is possible to evaluate be displayed. By use of the Digital Fig. 5. An example of printout from 4419

5 It is important to note that the pul- pit was elevated above the floor of the cathedral, whereas the lectern rested on the floor. Above the pulpit was a canopy, but the lectern was not cov- ered.

Receiver position: in the first series of tests the receiver was placed at the positions marked A to F in Fig.6. These positions had been used in the earlier tests. In the second test series the receiver was moved around to dif- ferent positions on a pre-arranged grid that covered the floor space of the area below the dome, the nave and transepts. In all cases the receiver height was 1.2 m.

The first series of tests, with receiv- er positions A to F, was carried out with the source both at the lectern and in the pulpit, and with the speech re- inforcement on and off, i.e. four sets of measurements.

In the second series in which mea- surements at grid points were ob- tained for the iso-RASTI contours the source was placed in the pulpit only, and measurements were made when the speech reinforcement system was both on and off. Fig. 6. Plan of the Cathedral showing the measuring postitions All of these measurements took less than four hours and took place during two evenings (11 and 12 of September, 1984) when the Cathedral was empty apart from the experimenters. The in Figs.7 and 8. The results will be The fact that the speakers read with empty Cathedral without any speech discussed below. above average clarity might account reinforcement is referred to as the for the higher score in the subjective normal state. From the second series of tests the tests. In addition it it known that iso-RASTI contours of Fig.9 were speaking in the Cathedral is difficult, drawn from the measurements at the so speakers do try harder. The earlier Results grid positions. For these contours the subjective tests were based on mean- Cathedral was in the normal state and ingful English words buried out of The RASTI values may be convert- the source was placed in the pulpit. context in a carrier sentence. The con- ed into PB word scores using Fig.2. A The RASTI value is an average of re- version of STI- and RASTI-indices in RASTI value above 0,75 indicates that sults in the 500 Hz and 2000 Hz octave Fig. 2 is based on nonsense words speech intelligibility is excellent. From bands. It is possible to construct con- which are not so easily understood, 0,6 to 0.74 it is good; from 0,45 to 0,59 tours based on the results in the and this is an additional reason why fair; 0,3 to 0,44 poor; and below 0,3 500 Hz and 2000 Hz bands and these the subjective method gives greater bad. are shown in Fig.10. values. It is interesting to note that in a similar comparative test in the Fig.7a shows a comparison of the Grundtvigs Kirk in Copenhagen in phonetically balanced (PB) word Discussion which the speaker and RASTI source scores for the source in the pulpit of were at the altar, the subjective articu- the Cathedral in its normal state In Fig.7a the comparison between lation test also resulted in PB word (speech reinforcement system off) for the subjective tests (Ref.[l]) and the scores slightly greater than those ob- the earlier subjective tests (Ref.[l]) RASTI tests is quite good. The PB tained with the RASTI equipment. and the present RASTI tests. Fig.7b word score for the subjective test is in shows a similar comparison, but with general slightly greater. With the ear- As mentioned previously, the pulpit the speech reinforcement system lier tests the two speakers who read in St. Paul’s is covered by a canopy. switched on. Figures 8a and 8b show out the sentences with the PB words This canopy reflects sound radiated similar results, but with the source on were both clear speakers who project- upwards from the source or speaker the lectern. The receiver positions A ed their voices well. One of the speak- and prevents the first reflection of to F are shown in relation to the dome ers was a clergyman at the Cathedral. sound from occurring in the dome.

6 With the speaker at the lectern, the Normal - Pulpit situation is quite different, and from this position a great deal of sound is radiated directly into the dome. The lack of a canopy over the lectern may be one reason why there is a poor com- parison between the subjective and RASTI results for the source at the lectern, as shown in Fig. 8a. Of course, sound is radiated into the dome for both the human speaker and for the RASTI source, but once again the hu- man speaker may automatically com- pensate for the known disadvantage by increasing his volume. In St. Paul’s he is also able to hear the decay of the 100 X----X Subyxtive reverberant sound, particularly at the

PB 60 M RASTI lectern, and as a result tends to reduce his speaking rate. Furthermore we Word 60 have to take into account the standard score 40 deviation in the RASTI results and especially in the subjective results. (%) 20 01 I I I I I I A B C D. E F The comparisons with the speech 0 10 20 30 m reinforcement system in Figs.7b and Listening position Scale a%?,! 8b, are not believed to be of much significance because some adjust- Fig. 7a. Comparison of results for the Transmitter placed at the pulpit, without speech reinforcement ments to the system had been made in the period between the two tests.

It is proposed that the RASTI method should be restricted to audito- ria where the reverberation time is not strongly dependent on frequency (Ref.[6]). In a huge space like St. Paul’s Cathedral, air absorption be- comes important at high frequencies, and causes the reverberation time to decrease. This decrease in reverbera- tion time with frequency is shown in Speech reinforcement - Pulpit Fig.11 for the source in the pulpit and the receiver at the positions which are indicated as 3 and 4 on the Cathedral plan in Fig.6. These measurements were obtained in the empty Cathedral using conventional apparatus (a ran- dom noise generator Type 1024, third octave filters Type 2112 and level re- corder Type 2305) in third octave bands. At the octave frequencies of 500 Hz and 2000 Hz used for the RASTI method the reverberation time was measured to be about 11 s and 7 s, respectively. The full Speech Trans- mission Index method makes use of loo- x----x Subjective octave bands from 125 Hz to 8 kHz, 0---o RASTI PB 60- ,X\. and for the rapid (RASTI) method to be successful the speech intelligibility Word at 500 Hz and 2 kHz should be repre- Score 40- sentative of the entire octave range of speech. The full STI method might be (%) 20- expected to give rise to a higher PB 0 I I I I I I A B C D E F word score than the RASTI method 0 10 20 30 m because the full analysis would take Listening position Scale a5 into account results at 4 kHz and 8 kHz where the reverberation time is Fig 7b. Comparison of results for the Transmitter placed at the pulpit, with speech lower, and hence the speech intelligi- reinforcement bility better. On the other hand, the

7 average speech level is lower for the higher frequencies so these frequen- Normal - Lectern cies contribute less to the intelligibil- ity.

The most noticeable feature about the iso-RASTI contours is that rela- tively high values are obtained, with the Cathedral in its normal state and the sound source in the pulpit, in an area within the dome almost diametri- cally opposite from the pulpit. The improved intelligibility occurs partic- ularly in the octave band at 2000 Hz, as shown in Fig.lOb. This area is just ? in front of one of the great stone pil- lars which support the dome. Such a 100 X----X Subjective pillar is shown in Fig. 12. Saucer domes form the arches between the piers flanking the transepts. i $ x&,x,,,-;’

In the area of better speech intelligi- bility listeners receive direct sound from the elevated pulpit and also 0 I I I I I I A B c D E F - - sound reflected behind from the stone 0 10 20 30 m Listening position pillars. As the transmission paths are Scale asI not much different in length there is Fig 8a Comparison of results for the Transmitter placed at the lectern, without speech no noticeable echo and both transmis- reinforcement sion paths contribute to the under- standing of speech. The beneficial ef- fect of the reflection is more pro- nounced at the higher frequency of 2000 Hz because at that frequency the sound radiates like rays and is reflect- ed in a similar manner to light, where-

as at the lower frequency of 500 Hz Speech reinforcement - Lectern the sound is in part scattered in all directions. Furthermore the reverber- ation time is longer for 500 Hz than for 2000 Hz.

The saucer domes may also play a part in reflecting the sound into the area of improved speech intelligibility.

Because the reinforcement system had been changed since the earlier subjective tests were made, these re- sults are not described in detail here. But it should be emphasized that the RASTI-method is an excellent meth- od for evaluation and adjustment of *c‘\ reinforcement - and PA systems. By PB 60 means of this method it is possible to Word 60 examine the effect of such systems ac- Score 40 curately and quickly. X----X Subjective wj 20 w RASTI 0 I I I I I I Conclusion I’ A B C D E F The RASTI method has been ap- plied to a large, reverberant space, St. Fig 8b Comparison of results for the Transmitter placed at the lectern, with speech Paul’s Cathedral in London. Results reinforcement have been compared with previously obtained speech articulation tests us- ing speakers and listeners. With the source in an elevated pulpit below a phonetically balanced word score tive tests. With the source at the lec- canopy, the RASTI method gave a slightly lower than that of the subjec- tern the RASTI results were in some

8 Fig. 9. The iso-RASTI contours for the source in the pulpit without reinforcement (maximum value = 45) cases considerably lower. The RASTI method is believed to have given dif- ferent values in this application be- cause: a: 500 Hz Octave Band a) speakers make more effort to pro- ject their voice and speak more slowly in a large, reverberant space which is known to have poor acous- tics, b) the subjective method was based on meaningful words, whereas the conversion of the RASTI results to a PB-word score is based on non- sense words, c) the RASTI method does not take into account octave frequencies at 4 kHz and 8 kHz where the rever- beration time is much less and b: 2000 Hz Octave Band speech intelligibility better.

a) would indicate that the RASTI values are more correct than the sub- jective, whereas c) indicates that the RASTI method is underestimating speech intelligibility. However, the er- rors from c) are not likely to be as great as those in the subjective meth- od described in a).

St. Paul’s Cathedral is a particularly severe test of the RASTI method and the agreement with the subjective method shown in Fig.7a is encourag- Fig. 10. The iso-RASTI contours for the source in the pulpit for each octave band (maxi- ing. mum value for 2000 Hz octave = 60)

9 Acknowledgements

The authors wish to acknowledge the help of Martin Armstrong and Tim Lewers in performing these tests. They are grateful to the Cathedral au- thorities for permission to carry out the tests.

References

VI T.H. LEWERS and J.S. ANDER- SON, “Some Acoustical Properties of St. Paul’s Cathedral, London”, Journal of Sound and Vibration 92(2), 2855297, 1984 PI L.L. BERANEK, “Acoustic Mea- surements”,John Wiley, 1949 t31 T. HOUTGAST, H.J.M. STEEN- EKEN and R. PLOMP, “Predict- ing Speech Intelligibility in Rooms from the Modulation Transfer Function, I. General Room Acous- Il~~ll~l~ll~~l~llIII~~I~~I~ I tics”, Acustica 46, 60-72, 1980 ’ 31,5 63 125 250 500 lk 2k 4k 8k [41 T. HOUTGAST and H.J.M. Frequency (Hz) STEENEKEN, “The Modulation Reverberation time in the empty Cathedral BJWZI Transfer Function in Room Acoustics as a Predictor of Speech Fig 11. Reverberation time in the empty Cathedral Intelligibility”,” Acustica 28, 66-73, 1973 151 H.J.M. STEENEKEN and T. HOUTGAST, “A Physical Method for Measuring Speech Transmis- [6] P.V. BRÜEL, “RASTI. Rapid [71 IEC draft publication 268: Sound sion Quality”, Journal of Acousti- Speech Transmission Index”, System Equipment Part 16: “Re- cal Society of America 67, Brüel & Kjær DK 2850, Nærum, port on the RASTI-method for the 318326, 1980 Denmark objective rating of speech intelligi- bility in auditoria”

10

. Fig. 12. Stone pillars which support the dome

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