A Study on establishing benchmarks for acoustical ... 2013 Toronto, Canada International Symposium...

ISRA2013

Toronto, Canada International Symposium on Room Acoustics

2013 June 9-11

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A Study on establishing benchmarks for acoustical parameters derived from impulse responses

Kazuma Hoshi ([email protected]) Atsushi Marui ([email protected]) Dept. of Architecture and Living Design, Musical Creativity and the Environment Course, Junior College, Nihon University Faculty of Music, Tokyo University of the Arts 7-24-1 Narashinodai, Funabashi, 1-25-1 Senju, Adachi-ku, Chiba 274-8501, Japan Tokyo 120-0034 Japan Hiroyuki Okubo ([email protected]) Hideo Miyazaki ([email protected]) NHK Sci. and Tech. Res. Lab. YAMAHA Corporation 1-10-11 Kinuta, Setagaya-ku, 10-1, Nakazawacho, Hamamatsu, Tokyo 157-8510, Japan Shizuoka 430-8650, Japan Jun Kanda ([email protected]) Motoki Yairi ([email protected]) NHK Integrated Technology Inc. Kajima Technical Research Institute 1-4-1 Jinnan, Shibuya-ku, 19-1, Tobitakyu 2-chome, Chofu, Tokyo 150-0041, Japan Tokyo 182-0036, Japan Takumi Asakura ([email protected]) Yosuke Yasuda ([email protected]) Institute of Technology, Dept. of Architecture, Faculty of Engineering, SHIMIZU Corporation Kanagawa University 3-14-17, Etchujima, Koto-ku, 3-27-1, Rokkakubashi, Kanagawa-ku, Tokyo 135-8530, Japan Yokohama, Kanagawa 221-8686, Japan

ABSTRACT

Acoustical parameters of a room are measured to obtain its quantitative acoustic characteristics. The techniques for calculating the acoustical parameters have been previously reported in ISO3382-1:2009. However, the values of the acoustical parameters often include some errors, which are due to a variety of measurement conditions, and loosely defined calculation procedures. A Working Group on Analysis of Acoustical Parameters by the Architectural Institute of Japan (AIJ) has been organized to set benchmarks for calculating and evaluating acoustic parameters derived from room impulse responses, with the ultimate aim of reducing the errors in calculation. First, for calculating parameters such as T or EDT used for evaluating the slope of decay, we discuss the signal-to-noise (S/N) ratio of the impulse response and the length of the impulse response. Second, for calculating the parameters used for evaluating the energy ratio, such as C80 or D50, we discuss detecting the starting point of the impulse response, and the order of time-windowing and octave band filtering. Appropriate impulse responses as benchmarks are designed on the basis of these discussions. Finally, in this paper, the framework of benchmarks is described and the details are to be published online.

1 INTRODUCTION

The properties of a linear system are characterized by its impulse response [1]. Therefore, in such a system, most acoustic parameters such as reverberation time can be calculated from the impulse response. The techniques for calculating acoustical parameters are described in the

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ISO3382-1 standard [2]. However, values of the acoustical parameters often include some errors owing to a variety of measurement conditions and loosely defined calculation procedures. According to our previous work [3], these errors are caused by the following three factors:

1. Variation due to the type of sound sources, such as loudspeaker and microphone.

2. Variation due to the computing method, i.e., method for detecting direct sound, type of filter, or sequential order of filtering*. (*filtering should be done prior to dividing the signal into early and late part of impulse responses.)

3. Variation due to the measurement conditions in the sound field, such as position of sound source and microphones, or time variance of sound field.

We conducted a round robin test on the measurement and analysis of acoustic parameters, comparing some computing methods, and directivity of dodecahedral omnidirectional loudspeakers. We found that the computing method and the directivity of loudspeaker significantly affect the acoustic parameters [3]. However, it is needed to further investigate the variation in the calculated parameter values, as well as the effect of S/N of impulse responses or the method to find direct sound The Working Group on Analysis of Acoustical Parameters, by the Architectural Institute of Japan (AIJ), is organized to investigate these subjects and to set benchmark problems for calculating and evaluating the value of acoustic parameters derived from room impulse responses, with the aim of reducing the errors in calculation.

Before making benchmark problems, procedures and methods to calculate the value of parameters were discussed with the WG (Working Group) members. We identified two factors affecting the calculation error. First concern is about the integrated impulse response method, especially the correspondence between the S/N of impulse response and the calculated reverberation time. The second one is about the computing method to calculate the ratio of early part and part of the latter period of impulse responses, such as effect of type of filter, filtering order and method of finding direct sound. We defined artificial impulse response as a reference material to investigate the tendency of the calculation error. We made various calculation programs by a wrong method regarded as a right method and gathered benchmark data by comparing the results. The details are discussed in Chapter 3 and 4 of the reference placed on the World Wide Web (see below for the link).

These benchmark problems were gathered, and are shown on the World Wide Web page in English and Japanese (http://news-sv.aij.or.jp/kankyo/s20/benchmark/). The framework and contents are introduced in Chapter 2.

It is expected that the benchmarks could be used for reducing the error of a value calculated using one's own program, or as a reference to compare data calculated by a different program.

2 FRAMEWORK OF BENCHMARK PLATFORM

2.1 Classification

The web site consists of four parts, i.e. Introduction, Motivation, Benchmark Problems, and Column. The main languages of the site are English and Japanese, we can change the language interactively. Benchmark problems are categorized into the following three parts:

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ü Basic Problems (B): The impulse responses in these problems consist of very amenable impulse responses. We recommend that you start from these problems for checking own programs.

ü Problems for checking algorithms (C): These problems are useful to check algorithms in your programs and attitude to the acoustical parameters. Please challenge these problems and check the reliability of own programs.

ü Problems for the measured impulse responses in actual rooms (A): The impulse responses in these problems are the measured responses in actual rooms. These responses are well measured. Therefore, calculating the value of parameters from them is not very difficult.

Table 1 shows the latest version of classification of the benchmark problems, which is one of the main parts in the web site.

Table 1: Classification of Benchmark Problems

Category Index Type Target parameters Notes

Basic

B-11

Reverberation Time (T), EDT (Early Decay Time), C80, Ts

Reverberation time: 0.5 (s)

B-12 Reverberation time: 1.5 (s)

B-13 Reverberation time: 3.0 (s)

Check

C-11 Evaluation of Decay Curve

Reverberation Time (T), EDT

Back ground noise

C-12 Large direct sound

C-13 Short data

C-21

Energy Ratio C80, Ts

Detection of direct sound

C-22 Order of time-windowing and octave band filter

C-23 Back ground noise

Actual room A-11

Reverberation Time (T), EDT, C80, Ts

Concert hall of about 500 people

A-12 Echoic chamber

2.2 Regulation

Regulations consist of two items, and are as follows:

• Find values of 6 bands from 125 Hz to 4 kHz, 1/1 octave band center frequency.

• The two kinds of format (Floating Binary format and RIFF wave audio format) of impulse responses are ready in each problem. You can use either of the data format that is easier to deal with.

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2.3 Contents

Any one of the benchmark problems typically consists of three sections, namely, Problem Statement, Note, and Calculation Example. The information of a benchmark problem, like the characteristics of impulse response, or significant points is written in the “Note” section, following problem statement. The calculation results by several institutes and the WG are given to compare with your own results. Figure 1 shows an example page of a typical benchmark problem.

Figure 1: An example page of a benchmark problem

Additionally, the explications about how to find the procedure and approach to calculate the parameters are summed up in other page "Column.” Column page is the other part of main contents in the benchmark platform. These columns can assist you to get the value with the right procedure and attitude.

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3 PARAMETRICAL INVESTIGATION FOR ESTABLISHING BENCHMARK PROBLEMS

3.1 Influence of the Background Noise on the Estimation of Reverberation Time

In this section, the influence of the background noise upon the accuracy of the estimated reverberation time is investigated.

3.1.1 Procedure

In this study, two kinds of impulse responses were used. One was a response made by the decaying white noise in time domain, assuming an envelope of exponentially-decayed curve. The decay curve was determined so that the reverberation time was estimated as 1.5 sec. The other was a response actually measured in a concert hall. This response has reverberation time of approximately 2.0 sec.

In order to simulate the background noise, white noise was artificially superimposed on the two responses described above. The amplitude of the superimposed white noise was set so that the S/N of the response was at the desired level. The S/N was set to 12 levels from 10 dB to 120 dB in 10 dB steps. In addition, the condition in which the noise was not superimposed was named as ∞ dB. In this section, acoustical indices of T30 and EDT were calculated based on the integrated impulse response method.

In this section, two kinds of S/N definition, S/NAverage and S/NPeak, were used. Influence of the background noise was discussed through the S/N conditions based on the two definitions. The definitions of the S/NAverage and S/NPeak are as follows. Note that the S/NPeak was calculated based on the procedure reported in ISO3382-1:2009 [2].

i. S/NAverage is defined as the ratio of the average energy for the time domain of 5 ms length that includes the direct sound to the average energy for the time domain of 100 ms length that includes the background noise.

ii. S/NPeak is defined as the ratio of the energy of the maximum value corresponding to the peak amplitude of the direct sound in many cases to the average energy for the time domain of 100 ms length that includes the background noise.

3.1.2 Results of the artificially-made impulse responses

The calculated results of the T30, and EDT of the artificially made responses in case of S/NAverage are shown in Fig. 2. The T30 is calculated accurately for the condition of S/NAverage over 50 dB. In addition, EDT is calculated accurately for the condition of over 30 dB. On the other hand, for the conditions of S/NAverage lower than the thresholds described above, the reverberation time calculation gives higher values than the accurate ones. The calculated results for the case of S/NAverage are shown in Fig. 3. The T30 and EDT are calculated as accurate values for the conditions of over 60 dB and over 40 dB, respectively. As described above, a level difference of 10 dB arose between the thresholds for the conditions of S/NAverage and S/NPeak, which are required for the calculation of an accurate reverberation time.

Reverberation decay curves for the conditions of S/N at 40 dB, 50 dB, and 60 dB are shown for both conditions of S/NAverage and S/NPeak in Fig. 4. As can be seen in these figures, it can be said that the reason for the errors occurring for the lower S/N conditions is that the time at which the

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63 125 250 500 1k 2k 4k 8k1

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s]

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Figure 2: Results of the calculated reverberation time of the artificially made impulse responses for the case of S/NAverage.

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Figure 3: Results of the calculated reverberation time of the artificially made impulse responses for the case of S/NPeak.

0 500 1000 1500 2000 2500 3000-60

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(a) S/NAverage (b) S/NPeak

Figure 4: Energy decay curve of the 500 Hz band of the artificially made impulse responses. Left one is the result for S/NAverage and the right one is that for S/NPeak.

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relative energy level results in -35 dB is not accurately identified because of the influence of the background noise.

3.1.3 Results of the measured impulse responses

The calculated results of the T30 and EDT of the measured responses for the case of S/NPeak are shown in Fig. 5. Note that the impulse responses and the superimposed noise used in this section have frequency components only in the 500 Hz band. As can be seen in the figure, T30, and EDT are calculated as accurate values for the conditions of S/NPeak over 60 dB and over 40 dB, respectively. The results indicate similar tendency compared to the results of the artificially made impulse responses, and it can be said that the tendency of the thresholds for calculation of the reverberation time for the conditions of S/NAverage and S/NPeak are almost equivalent between the artificially made and measured conditions.

10 20 30 40 50 60 70 80 90 100 110 120 1301

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n tim

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S/N ratio [dB]


T30

T 30, EDT

[s]

EDT

Figure 5: Results of the calculated reverberation time for the case of the measured impulse responses.

3.2 Influence of Background Noise on the Calculation of C80

In this section, the C80 that is utilized to evaluate the clarity is investigated as an acoustic index calculated from impulse responses. There are two issues to be considered when calculating C80; one is whether the starting points of impulse responses are determined correctly under the condition of a certain background noise, and the other is whether the time-window and filtering to impulse responses are correctly undertaken. Here the study regarding the former issue is introduced.

3.2.1 Procedure

In the Annex A of the article ISO3382-1-1:2009 [2], it is mentioned that “the start of the impulse response for the equations given in A.2 should be determined from the broadband impulse response, where the signal first rises significantly above the background but is more

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than 20 dB below the maximum.” However, under the condition of a background noise, the starting points may not be determined correctly depending on the threshold value and may cause some errors in the calculation of the C80. In this section, the influences of the background noise are investigated while changing the threshold value for determining starting points.

Using the same impulse responses introduced in the section 2, the C80 values of each response were calculated while changing the threshold value from 0 dB to -60 dB below the maximum value of the signals. Figure 6 shows the echo time pattern of the S/NPEAK 30 dB, as an example, together with the threshold lines. The point where the signal first rises above each threshold line is determined as a starting point.

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5 10 15 20Tim e [m s]

Max値に対する比率 [%]

Max Point

-5dB(56%)

-10dB(32%) -15dB(18%) -20dB(10%)

-30dB(3%)

Rat

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Max

val

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Figure 6: Echo time pattern of S/NPEAK 30 dB with the threshold line

3.2.2 Results of the artificially-made impulse responses

Figure 7 shows the detected starting point of each S/NPEAK S/N and the corresponding threshold. Since the S/NPEAK is defined as the ratio of the maximum of the impulse responses to the background noise, the starting point is determined within the background noise when setting the threshold lower than S/NPEAK (for example -35 dB as a threshold with S/NPEAK 30 dB). Furthermore, since the maximum point of the signals does not match to the correct starting point, the detected starting point differs widely from the correct starting point when setting the threshold to 0 dB.

Figure 8 shows the zoomed result of the Figure 7. The same point (528 samples) is determined as a starting point at the threshold of –20 dB or lower, and higher than each S/N. However, for the threshold higher than -20 dB, the starting point is detected forward with a few samples.

In order to examine how much these errors influence the results of the C80 calculation, the relation between the threshold and C80 value was investigated. The results at the frequency 1 kHz is shown in Figure 9, as an example. The C80 values are same when the threshold is set higher than S/NPEAK under the condition of S/NPEAK higher than 40 dB. However, when the

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threshold is set lower than S/NPEAK, the C80 values differs from the correct value by 1 dB or more. On the other hand, when setting the threshold higher than –20 dB, the C80 value is almost correct even though the starting point is not detected correctly. This is because the density of the reflections is same throughout the signals, since each signal is created from the white noise. Furthermore, under the S/NPEAK of 30 dB or lower, the C80 is lower than the correct value even with the starting point detected correctly by setting the threshold as –20 dB. This is because of the integration of the background noise energy.

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Figure 7: Results of the detected Figure 8: Results of the detected starting starting points points (around 528 samples)

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C80 [dB

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Figure 9: The influence of S/NPEAK and threshold on the calculation of C80

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10dB 20dB 30dB 40dB

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RT=0.5s

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3.2.3 Study of a Benchmark for the C80 calculation

Based on the studies mentioned above, the proper benchmark is examined in this section. The S/NPEAK should be set 40 dB or more in order to prevent the influence of the integration of the background noise. However, this issue is excluded from the benchmarks since this is related to the environment of the measurement site. Here, the benchmark to find whether the threshold is set correctly is studied.

Assuming the error to detect the point is delayed from the correct starting point by the threshold of –20 dB or higher, the impulse response in which the artificial pulse noise is added just after 80 ms from when the starting point is created, is shown in Figure 10. When the starting point is detected incorrectly, the point of the added pulse is judged within 80 ms after the starting point, which should be judged after 80 ms, and the C80 value is expected to be calculated higher than the correct value.

The figure11 shows the results of the C80 calculation without and with the pulse noise. The results without pulses are almost same except for the threshold of –45 dB or lower which detects the background noise as the starting point and for the threshold of 0 dB, which detects the maximum point as the starting point. This result makes it difficult to evaluate whether the calculation is undertaken correctly. On the other hand, there are some differences in the results with the pulse, when setting the threshold below –15 dB. With these responses, the C80 is calculated correctly only when setting the threshold between –20 dB and –40 dB. Consequently, this response can be a benchmark on how to judge the correct procedure.

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0 100 200Tim e [m s]

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-10dB(32%) -15dB(18%)

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Figure 10: Impulse response with the artificial reflection

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Figure 11: Results of calculated C80 (L: without reflection, R: with reflection)

4 CONCLUSIONS

The framework of benchmarks, using the index calculation method described in the article ISO3382-1:2009, is introduced in this paper. Furthermore, a practical methodology for establishing benchmark problems is also described. A correspondence between the S/N of impulse response and calculated reverberation time is investigated. The correspondence between computing method for calculating ratio of early part and part of the latter period of impulse responses and the calculated C80 is also studied. We are planning to examine further sound indexes such as STI and the results of future work are to be published in the WWW (World Wide Web) pages of the benchmark problems referenced earlier. It is expected that the benchmarks could be used for reducing the error of a value of calculations by one's own program, or as a reference to compare data calculated by a different program. It could improve the precision of the sound field design, and enrich the designed sound.

ACKNOWLEDGMENTS

This project was investigated by the Working Group on Analysis of Acoustical Parameters by the Architectural Institute of Japan (AIJ).

REFERENCES

1 H. Kuttruff, “Room Acoustics Fourth Edition,” Spon Press (2000).

2 International Standard ISO3382-1:2009 Acoustics – Measurement of room acoustic parameters – Part 1: Performance spaces (2009).

3 T. Koga, H. Okubo, “Round Robin Tests on Measurement and Analysis of Acoustic Indexes,” Proc. of the RADS 2004 (2004).

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Threshold [dB]

C80 [dB]

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50dB 60dB 70dB 80dB

90dB 100dB 110dB ∞

RT=0.5s

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