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RASTI Measurements in St. Paul's Cathedral, London (Bo0116)

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RASTI Measurements in St. Paul's Cathedral, London (Bo0116) 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.
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