PERCENTAGE ARTICULATION LOSS OF CONSONANTS IN THE ELEMENTARY SCHOOL CLASSROOMS

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1 The 21 st International Congress on Sound and Vibration July, 2014, Beijing/China PERCENTAGE ARTICULATION LOSS OF CONSONANTS IN THE ELEMENTARY SCHOOL CLASSROOMS Dan Wang, Nanjie Yan and Jianxin Peng* Department of Physics, School of Science, South China University of Technology, Guangzhou, 5640, China Speech intelligibility was investigated by using directional source with its directivity similar to human condition for grades 2, 4 and 6 children in 27 elementary classrooms from 9 different elementary schools in Guangzhou. Objective acoustical measurements were also performed in these classrooms. The percentage articulation loss of consonants (%Alcons) was calculated from reverberation time (T 30 ) and signal to noise ratio (SNR) at each listening position by using the revised Peutz algorithm. The subjective speech intelligibility scores were also obtained at each listening position. Relationships between %Alcons and subjective Chinese speech intelligibility scores were analyzed for grades 2, 4, and 6 children respectively. To obtain good Chinese speech intelligibility, the desired T 30 and SNR values in classrooms were discussed. 1. Introduction For children aged from 6 to 13, most of their time is spent in the primary school classroom. A good classroom acoustic environment is beneficial to the teachers' teaching and students' learning. The high speech intelligibility is the first demand of children learning in the classroom. However, the school classrooms lack the necessary economic investment and the acoustical environment control measures during the architectural design, which causes that all sorts of interference noises are excessive and reverberation time is too long in the classroom. These factors not only reduce the indoor speech intelligibility, but also directly affect children's classroom learning. Bradley and Sato 1,2 indicated that the optimal range of reverberation time is from 0.4s to 0.6s and the corresponding suitable indoor noise level is from 30dBA to 40dBA for small elementary school classrooms that total volume is less than 283m 3. The favorable signal-to-noise ratio (SNR) generally is between 12dB and 25dB. In recent years, some researchers have proposed that the sound amplification system should be adopted to improve SNR in the classroom. Larsen, Vege and Ribera 3 pointed out that the cost to build or modify school classrooms to meet the ANSI standard has been viewed as prohibitive by some educators and industry groups. They also demonstrated that classroom amplification could be beneficial for college-age listeners in a classroom with reverberation time that exceed recommended values. Jonsdottir, Laukkanen and Siikki 4 pointed out that the sound amplification system could reduce the teacher's speech sound pressure level, then reduce the teacher's vocal stress and fatigue. The study of Dockrell and Shield 5 indicated that the sound-field systems improved performance on children s understanding of spoken language. But, ICSV21, Beijing, China, July

2 they also pointed that classroom acoustics were a significant factor influencing the efficacy of sound-field systems. There was no additional benefit in listening comprehension for children in classrooms with better acoustics. Moreover, they did not further discuss how poor acoustical condition where the sound amplification system should be installed in classroom. In general, the sound amplification system reproduces the acoustical signal using the loudspeaker system. For the space of the employed loudspeaker system, the percentage articulation loss of consonants (%Alcons) developed by Peutz 6,7 is used to evaluate the speech intelligibility in room. The algorithm for predicting %Alcons is further revised by Bistafa and Bradley 8. In this paper, the subjective Chinese speech intelligibility evaluation and objective acoustic parameters measurement in 27 real elementary school classrooms were performed by children in grades 2, 4 and 6 (7 to 12 years old) from 9 different elementary schools in Guangzhou. The %Alcons was calculated from the reverberation time (T 30 ) and the signal to noise ratio (SNR). To obtain good Chinese speech intelligibility, the desired T 30 and SNR values in classrooms were discussed for grades 2, 4 and Methods 2.1 Objective acoustical measurement in classrooms In this study, 27 classrooms from 9 different elementary schools in Guangzhou with volume between 150m 3 and 280m 3 were investigated. They were 24 conventional rectangular shaped classrooms, and 3 hexagon shaped classrooms. All classrooms had no special acoustical treatment. The sound source was located in the teachers position at the platform and the four listening positions located in the seat areas outside of reverberation radius during the testing. A sine sweep signal was used to obtain the room impulse response at the four listening position. The reverberation time (T 30 ), early decay time (EDT) and early-late sound energy ratio (C 50 ) from the four listening position were calculated using the Dirac 4.0 software. 2.2 Subjective Chinese speech intelligibility test The subjective speech intelligibility evaluation was conducted using a Mandarin Chinese test word lists as specified by GB The test uses ten word lists in which contains 25 five-word rows of similar-sounding Chinese monosyllabic words and is similar to modified Rhyme Test of English. The five words in each row are randomly arranged and differ only in the initial consonant sound (for example, hao, sao, gao, zao, kao). The test word in carrier phrase is The row reads -, where the was row number and the - was read word. The Chinese speech intelligibility test signals were recorded in an anechoic chamber by a male and a female speaker respectively, then were edited using Cooledit Pro, and the mute signal was added between the stimuli of carrier sentence so that listeners could select the object word that they heard. Speech-shaped noise with a frequency spectrum equivalent to the long-term speech spectrum of test signals was obtained by filtering from pink noise. The noise and Chinese speech intelligibility test signals were mixed according to the certain SNR and reproduced by a JBL- LSR6325P loudspeaker with its directivity similar to human speaking. The speech level at 1m directly in front of the loudspeaker was set at 65dBA by adjusting the volume of the loudspeaker. To obtain the SNR at the listening position, the speech signals and noise at the four listening positions were recorded by using microphones. A total of 432 children in grades 2, 4 and 6 from 9 different elementary schools participated in the tests. All subjects were no known hearing problems, who were able to speak and hear standard Mandarin. The necessary training prior to the test was performed. 16 subjects averagely sit around the four listening positions in each classroom during the testing. For each testing condition, two test word lists (one is man speaker, the other woman speaker) were used. The subjects were asked to circle the key words which they heard. The percentage rate of each test word lists which were ICSV21, Beijing, China, July

3 correctly identified was counted. The subjective Chinese speech intelligibility score at each listening position was the average score of eight word lists of the four listeners in the position. 2.3 Calculation of %Alcons The revised Peutz formula for calculating %Alcons at the listening position is given by equation (1) 8. 2 d r d r % 0 i i i i Al 1.5 (1) cons where i d and i r are as follows: LnLr 1 id 0.32log 2 LnLr 1 Dc D LnLr ir 0.32log 0.5 log T 12 L L n r and the critical distance D c is given Dc 0.21 VQ T, where L n is the ambient noise level, L r is the reflected speech level, D is the distance between the source and the receiving position, T is the reverberation time, Q is the speaker directivity factor ( Q 3 in this study), V is the classroom volume. 3. Results and discussions The subjective Chinese speech intelligibility scores obtained from grade 2, grade 4 and grade 6 respectively are plotted against the corresponding %Alcons in the different test conditions and 2 listening positions in Figure 1. The determination coefficient R are 0.68, 0.79, 0.69 respectively. The Figure 1 shows that there are high correlations between subjective Chinese speech intelligibility scores and %Alcons for grades 2, 4, and 6 respectively. It also can be seen that the Chinese speech intelligibility scores among different age groups are decreasing with the increase of %Alcons. The Chinese speech intelligibility scores increase with the age increasing under the same %Alcons conditions. To obtain the 95% Chinese speech intelligibility scores, the %Alcons required by the grades 2, 4, and 6 are 2.5%, 4.4%, 5.8% respectively. 0 Chinese speech intelligibility scores /% Grade 2 Grade 4 Grade % ALcons /% Figure 1. The relationship between the Chinese speech intelligibility score and %Alcons ICSV21, Beijing, China, July

4 Monosyllabic words were used for the subjective Chinese articulation test. The initial of each monosyllabic testing word is different which is similar to the consonants in English. Knudsen, by quantifying the nature of errors in the syllable articulation test, found that the number of errors due to incorrectly perceived vowels was smaller than for consonants. This happens mainly because the energy and the duration of vowel sounds are greater than the consonant sounds. Nevertheless, most of the speech information is carried by the sound of consonants. The greater duration and energy of vowel sounds increases the masking of the consonant sounds by vowels as the reverberation time increases and SNR decreases 11. Peutz 6 also found that the articulation loss of vowels was much smaller than that for consonants. He then defined the articulation loss of consonants Alcons. Children s phoneme identification abilities may not mature until the late teenage years 12. The ability to identify the consonants requires listeners to be sensitive to the rapid spectrum changes over time intervals of ~30ms. The younger children s age is, the harder they are able to identify consonants under the long reverberation and the high noise level condition 13. Masking by noise and reverberation reduces the amount of acoustical information available to listeners, so the distorted probability of receiving the information especially for the consonant sounds of the speech signals is considerable for the younger children. On the other hand, children are less able than adults to focus attention on task-relevant information and resist interference from irrelevant sounds, which leads to a great difference for the consonants articulation of different age groups under the same %Alcons condition When T 30 in the classroom is longer or SNR is lower, the sound amplification system is often installed. The speech sound pressure level at the seat areas can be increased to improve the SNR and reduce the %Alcons at the listening position. Eventually, the speech intelligibility is improved in classroom. However, the official position of the Acoustical Society of America is that sound amplification should not be routinely employed in typical small mainstream classrooms. All new or renovated small mainstream classrooms should be designed to conform with ANSI S12.60 to ensure satisfactory speech communication for learning 17. It is very important that the sound amplification system should be used under what conditions, namely that how long the reverberation time is and how low the SNR is. It is assumed that 95% Chinese speech intelligibility score in classrooms is good for children. Therefore, the %Alcons value can be determined for 95% Chinese speech intelligibility score according to the relationship between subjective Chinese speech intelligibility scores and %Alcons in Figure 1. From equation (1), it can be seen that %Alcons relates to SNR, T 30, and D. The %Alcons values decrease with the increase of D. The experiment data indicates that the ratio of Dc and maximum D isd 2 c D 0.6. Table 1 shows the %Alcons values under different T 30 conditions with the SNR db, 15dB, 20dB and 25dB when Chinese speech intelligibility score reaches 95%. Table 2 shows the %Alcons values under different SNR conditions with the T s, 0.8s, 1.2s and 1.5s when Chinese speech intelligibility score reaches 95%. Table 1. The %Alcons under the different T 30 conditions SNR /db T 30 /s As mentioned above, to reach the 95% subjective Chinese speech intelligibility scores, the critical %Alcons for the grades 2, 4 and 6 are 2.5%, 4.4%, 5.8% respectively. It can be seen from the Table 1 and Table 2 that the %Alcons in most conditions for the grades 2, 4 and 6 children are higher than their corresponding critical %Alcons values when the SNR is lower than 15 db and T 30 ICSV21, Beijing, China, July

5 ranges from 0.4s to 2.0s in classrooms. Table 1 shows that the %Alcons for the grade 6 children are lower than their corresponding critical values when the SNR is 20dB and the T 30 is 0.4s in the classroom, and the %Alcons for the grades 6 children are also lower than their corresponding critical %Alcons values when the SNR is 25dB and the T 30 is 0.4s and 0.6s. Moreover, the %Alcons for the grades 4 children are lower than their corresponding critical %Alcons values when the SNR is 25dB and the T 30 is between 0.4s. However, the %Alcons for grade 2 children is higher than their corresponding critical %Alcons values in all conditions. Similarly, Table 2 also shows that the %Alcons for the grades 2, 4 and 6 children are higher than their corresponding critical %Alcons values when the T 30 is higher than 0.8s and the SNR is less than 25dB in the classrooms. The %Alcons for grades 4 and 6 children are lower than their corresponding critical %Alcons values when the SNR is 25dB and the T 30 is 0.5s. It can be seen from Table 2 that the %Alcons for grade 2 children are always higher than their corresponding critical %Alcons values. Table 2. The %Alcons under the different SNR conditions T 30 /s SNR /db Combined with Table 1 and Table 2, it can be guessed that the %Alcons for the grades 4 and 6 children are lower than their corresponding critical %Alcons values in some cases when the SNR is from 20dB to 25dB and the T 30 is between 0.4s and 0.6s in the classroom. At this time, the subjective Chinese speech intelligibility scores for the grades 4 and 6 children can reach above 95% speech intelligibility scores in the classroom. However, for grade 2 children, the %Alcons values in all conditions in Table 1 and 2 can not meet the requirements of 95% speech intelligibility scores. As also can be seen, when the SNR is in the range of 20 ~ 25dB and the T 30 is between 0.4s and 0.6s in classrooms, Chinese speech intelligibility scores can reach 95%, the sound amplification system might be unnecessary for grades 4 and 6 children. However, due to grade 2 children are 7~8 years old, their auditory function may not yet mature and the ability of speech identification is lower than that of the grades 4 and 6 children. Chinese speech intelligibility scores for grade 2 children are lower than those grades 4 and 6 children and can not reach the 95% in the classrooms with the SNR 25dB and the T s. The sound amplification system may increase SNR for grade 2 children and improve the speech intelligibility in the classroom. 4. Conclusions The subjective Chinese speech intelligibility evaluation and objective acoustic parameters measurement were performed for grades 2, 4 and 6 in 27 elementary classrooms from 9 different elementary schools in Guangzhou. The subjective Chinese speech intelligibility scores were obtained and the %Alcons was calculated from T 30 and SNR at each listening position by using the revised Peutz equation. Relationships between subjective Chinese speech intelligibility scores and %Alcons were investigated for grades 2, 4 and 6 children respectively. The relationship of the %Alcons with the SNR and T 30 were also analyzed. The results were as follows: 1) There are high correlations between the subjective Chinese speech intelligibility scores and the %Alcons for grades 2, 4 and 6 respectively. The Chinese speech intelligibility scores decrease with the increase of %Alcons and increase with the increase of age under the same %Alcons condition. To obtain the 95% Chinese speech intelligibility scores, the corresponding %Alcons for grades 2, 4 and 6 are 2.5%, 4.4%, and 5.8%, respectively. ICSV21, Beijing, China, July

6 2) When the SNR is in the range of 20 ~ 25dB and the T 30 is between 0.4s and 0.6s, the sound amplification system might be unnecessary for grades 4 and 6 children but necessary for grade 2 children to improve speech intelligibility in the classrooms. Acknowledgements This work is supported by National Natural Science Foundation of China (Grant No , ), Science and Technology Planning Project of Guangdong Province, China (Grant No. 2011B ), and Science and Technology Planning Project of Guangzhou City, China (Grant No. 2012KP021). REFERENCES 1 Bradley, J. S., and Sato, H. The intelligibility of speech in elementary school classrooms. The Journal of the Acoustical Society of America, 123(4), , (2008). 2 Sato, H., and Bradley, J. S. Evaluation of acoustical conditions for speech communication in working elementary school classrooms. The Journal of the Acoustical Society of America, 123(4), , (2008). 3 Larsen, J. B., Vega, A., and Ribera, J. E. The effect of room acoustics and sound-field amplification on word recognition performance in young adult listeners in suboptimal listening conditions. American Journal of Audiology, 17(1), 50-59, (2008). 4 Jónsdóttir, V., Laukkanen, A. M., and Siikki, I. Changes in teachers voice quality during a working day with and without electric sound amplification. Folia phoniatrica etlogopaedica, 55(5), , (2003). 5 Dockrell, J. E., and Shield, B. The impact of sound-field systems on learning and attention in elementary school classrooms. Journal of Speech, Language, and Hearing Research, 55(4), , (2012). 6 Peutz, V. M. A. Articulation loss of consonants as a criterion for speech transmission in a room. Journal of the Audio Engineering Society, 19(11), , (1971). 7 Davis, D., and Patronis, E. Sound system engineering. Taylor & Francis US, (2006). 8 Bistafa, S. R., and Bradley, J. S. Revisiting algorithms for predicting the articulation loss of consonants Alcons. Journal of the Audio Engineering Society, 48(6), , (2000). 9 GB , Methods of measurement for the characteristics of sound reinforcement in auditoria(in Chinese), Standard of P. R. China, (1985). Knudsen, V. O. Architectural Acoustics ~Wiley, New York, 373, , (1932). 11 Bistafa, S. R., and Bradley, J. S. Reverberation time and maximum background-noise level for classrooms from a comparative study of speech intelligibility metrics. The Journal of the Acoustical Society of America, 7(2), , (2000). 12 Johnson, C. E. Children's phoneme identification in reverberation and noise. Journal of Speech, Language and Hearing Research, 43(1), 144, (2000). 13 Stevens, K. N. Acoustic correlates of some phonetic categories. The Journal of the Acoustical Society of America, 68(3), , (1980). 14 Klatte, M., Lachmann, T., and Meis, M. Effects of noise and reverberation on speech perception and listening comprehension of children and adults in a classroom-like setting. Noise and Health, 12(49), 270, (20). 15 Dempster, F. N. Resistance to interference: Developmental changes in a basic processing mechanism. In Emerging themes in cognitive development, Springer New York, 3-27, (1993). 16 Elliott, E. M. The irrelevant-speech effect and children: Theoretical implications of developmental change. Memory & Cognition, 30(3), , (2002). ICSV21, Beijing, China, July

7 17 Acoustical Society of America Position on the Use of Sound Amplification in the Classroom, Acoustical Society of America. [Online.] available: ICSV21, Beijing, China, July