Sound propagation measurement using swept-sine signal Fumiaki Satoha, Jin Hiranob, Shinichi Sakamotoc, Hideki Tachibanad a,d
Chiba Institute of Technology, 2-17-1 Tsudanuma, Narashino-city, Chiba, 271-0016, Japan Insitute of Industrial Science, the University of Tokyo, 4-6-1 Komaba, Meguro-ku Tokyo, 153-8505, Japan
b,c
a
[email protected]
Abstract In the measurement of sound propagation outdoors, the synchronous averaging techniques using MLS signal or swept-sine signal can not be applied to reduce the influence of the background noise because of the time-variance of the sound field influenced by the meteorological conditions. Therefore, we have been investigating the method of measuring impulse response using swept-sine signals with a long duration time (up to 600 sec. per a octave band) to avoid the influence by wind and atmospheric turbulence and to get a high signal-to-noise ratio. In order to examine the applicability of this method to outdoor sound propagation measurement, numerical study, scale model experiment using a 1/40 wind tunnel and field measurement of sound reduction index of a building facade have been conducted. As a result, it has been found that the results obtained by the method using a long swept-sine signal and those obtained by the ordinary method using a stationary-random noise are in high correspondence. As another application, the measurement of sound propagation characteristics from a semiunderground road will be presented. 1. INTRODUCTION To obtain sufficient S/N ratio in various kinds of acoustic measurements, the methods based on the correlation technique such as synchronous averaging, MLS method and Swept-sine method are being widely used [1]-[10]. At the same time, some researches on the influence of time-variance and harmonic distortion of loudspeakers on the measurement results have been conducted up to date [11]-[13]. Even in an ordinary room, the assumption of time-invariance has to be carefully considered. For example, reverberation time can be measured shorter at high frequency range when applying the synchronous averaging technique. When the MLS method is used, the S/N ratio in the impulse response measurement tends to be remarkably poor in high frequencies by the influence of time-variance. In addition, harmonic distortion of the loudspeaker makes the S/N ratio of the impulse response poor. As a result of our experimental studies, the superiority of the Swept-sine method in room acoustic
measurements has been derived. In this paper, the applicability of this method to outdoor sound propagation measurement is presented. 2. FINDINGS FROM THE STUDIES FOR INDOOR MEASURMENTS 2.1 Influence of the time-variance In actual measurement condition, the air in a room is continuously moving and the temperature is changing more or less in general. Figure 1 shows the excess decay of reverberation time caused by eight times averaging under time-varying conditions [12].The values obtained from the impulse response measured by eight times averaging were compared with those without averaging. In this study, the duration time of the Swept-sine signal was 1.35 s and it takes about 50 s for eight times radiation. The impulse responses were obtained by linear convolution using the inverse Swept-sine signal. The extent of the time-variance in the medium was estimated by the method using short-term running cross correlation function [12]. The value of the error remarkably increased in high frequencies when the temperature change exceeded 0.05 degrees. Thus, careful consideration is needed when using the synchronous averaging even in room acoustic measurement. As another method to improve the S/N ratio in impulse response measurement without synchronous averaging, the application of a source signal with long duration time would be effective. Figure 2 shows the effect of signal duration in the case of Swept-sine method. By using a signal with eight times duration length instead of the eight times averaging, the same S/N ratio improvement, approximately +9 dB, was obtained without excess decay. The effect of the signal duration is ought to be seen in theory if the MLS method is used. Regarding this point, the MLS method and the Swept-sine method were compared under the same condition. The result is shown in Fig.3, an excess delay is seen in the results using the MLS signal with a long duration time without averaging even under the very slight time-varying condition (estimated temperature change was 0.103 degrees per 174.8 seconds). In addition, the S/N ratio was remarkably lower compared to the result obtained by the Swept-sine method. In the case of MLS method, the source signal consists of a lot of impulses. The responses to each of the pseudo-random impulses are accumulated in some point by taking correlation with the source signal. On the other hand, in the case of Swept-sine method, the signal can be considered to consist of pure-tones and an impulse response is obtained as an accumulation of the pure-tone responses. In the case of MLS method, time averaging processing is inherent and therefore this method is weak against the time-variance. 2.2 Influence of the non-linearity of the measurement system Careful consideration is needed not only to the time-variance but also to the non-linearity in the measurement system. Focusing on this point, the MLS method and the Swept-sine method were compared by performing an experiment in a reverberation room. In the experiment, the same equipment (which were D/A-A/D converters, amplifier, loudspeaker, microphone and host PC for the D/A-A/D) was used. To make the measurement condition even between the two methods, the duration time of the source signals and their energy were adjusted equal in the PC when their orders (N) were equal. The D/A and A/D converters were driven with the same master clock. To obtain the impulse response, the fast Hadamard transform was applied in the MLS method and the circular convolution was applied in the
Swept-sine method. To realize the time-invariance condition in the reverberation room, a series of measurements were made after more than 24 hours after closing the door of the room.
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Figure 4 shows the results for the condition that the radiation level of the source signals was considerably low. The power level was about 59 dB in 1k Hz octave band, in spite of using a large loudspeaker system with 15 inch woofer. If it is realized such a linear condition with time-variance, almost same effect of averaging and signal duration upon background noise suppression can be obtained in both methods. Figure 5 shows the results for the condition that the sound power level was increased by 17 dB compared to the former case. In the result obtained by the Swept-sine method (N=17), the influence of harmonic distortion of the loudspeaker is seen at the latter part of the decay curve. Although such an influence is not seen in Fig. 5(b) in case of N=20, it can be seen much latter part outside the figure. Anyway, in case of the Swept-sine method, the influence of the harmonic distortion of the loudspeaker is seen in local. The influenced part is not suppressed by averaging but another noise part is suppressed by averaging. On the other hand, in case of the MLS method the influence extended over the whole part of the impulse response and it was not suppressed by synchronous averaging (see Fig. 5(c)). Figure 6 shows the results under the condition that the radiation level was further increases by 14 dB. This condition is considered within a linear range in ordinary measurements, but the decay curves are extremely influenced by the harmonic distortion. In the results by the MLS method, the S/N ratio deteriorated compared to the former conditions, although the power level of the source signal was increased. In the results by the Swept-sine method, the influence extends widely but the tendency was same as Fig.5(c) mentioned above.
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Figure 1: Underestimate of reverberation time caused by 8 times synchronous averaging -10 -20 -30
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Figure 6: Comparisons of reverberation decay (squared envelope) in 8k Hz octave band Power level (MLS) is about 90 dB in 1k Hz octave band
3. APPLICABILITY TO VARIOUS MEASUREMENTS IN ROOMS AND BUILDING ACOUSTICS In sound insulation measurement, the influence of background noise is often serious problem. Even in case of laboratory measurement, the internal noises in the measurement system can become a problem when the test specimen has a high sound insulation performance. For example, when measuring sound insulation between a large hall and a small hall included in a complex building often become difficult in high frequency ranges due to the influence of the background noise. In such a case, the impulse response measurement method by the MLS or Swept-sine method can be applied, in which the sound pressure exposure level (LpE) is obtained by integrating the square of the impulse response. In this kind of measurement, it is desirable that the sound source radiates a big sound power. In this respect, the Swept-sine method has an advantage from the point mentioned above. In the case of filed measurement of sound insulation of building facades, the measurement is disturbed not only the effect of background noise but also that of the time-variance. In such kind of field measurement, the impulse response measurement method by the Swept-sine method can be applied. Although the phase characteristics is mainly distorted by the effect of time-variance, the amplitude characteristics is not so influenced. When applying the Sweptsine method, it is advantageous to use a source signal with a long duration time not only to get sufficient S/N ratio but also to average the long term wind fluctuation. 3.1 Numerical study on the relationship between S/N ratio and signal duration Figure 7 shows the relationship between S/N ratio and signal duration, where S/Nsound field indicates the S/N ratio under the field measurement condition, S/NLpE,ti indicates that in LpE obtained by integrating the impulse response for ti seconds integration and T indicates the duration time of the source signal. That is, when ti = T, S/NLpE,ti = S/Nsound field. By limiting the integration time (ti) in the calculation of LpE, the influence of the background noise can be suppressed. In Eq. 1, under the condition that S/Nsound field = -20 dB, T = 600 s and ti = 1 s, S/NLpE,ti =7.8 dB, even though which include 0.7 dB errors. To confirm the relationship between the S/N ratio and the signal duration time mentioned above, a numerical study was performed under the assumption of time-invariant condition. Band-limited swept-sine signals with a duration time of 10 min. were used, which were directly generated in time domain using a PC without FFT calculation [10]. As the background noise, three kinds of noises (stationary-random noise, road traffic noise and construction noise) were used. The Swept-sine signal and respective background noise signals with the same duration time and frequency range as those of the source signal were adjusted so that their RMS values were equal (S/Nsound field = 0 dB). S/Nsound field was changed in five steps from 0 dB to -40 dB. The integration time to calculate LpE was 1 seconds (ti = 1). To suppress the influence of the background noise, furthermore, the value of the error including LpE,1 was estimated from the noise floor of the impulse response and subtracted on power base. By performing such an operation, the contribution of the background noise can be cancelled furthermore (see Fig. 8 and Eq. 2). Figure 9 shows the errors in LpE,1 caused by the effect of background noise, where the reference value is that of LpE,1 under the condition of S/Nsound field = ∞. If the correction is not performed, the values of error are within 1 dB
under the S/Nsound field condition of -20 dB as mentioned above. Concerning the variation of the background noise, the results are almost the same In the case of the stationary-random noise, the effect of suppression of the background noise can be seen up to -30 dB S/Nsound field. In order to confirm the results of the numerical study mentioned above, an experimental study was performed in the set of a reverberation room and a hemi-anechoic room. In the reverberation room, a loudspeaker was set and the source signal was radiated. The adjacent hemi-anechoic room was used as the transmission room, in which an additional loudspeaker was set and a broad-band noise was radiated to simulate the background noise. Under such condition, the impulse response measurement was performed at a point in each room. The experiment was performed in five step S/Nsound field conditions from 0 dB to -40 dB for 125 Hz, 500 Hz and 2k Hz octave bands. Besides, the sound pressure level difference between the two points was measured by the ordinary method using a broad-band noise without the background noise and the result was used as the reference value. Figure 10 shows the errors in the sound pressure level difference. A similar tendency to the numerical study can be seen. The error is within 1 dB under a very adverse S/Nsound field condition up to -30 dB by the correction. When the stationary-random of the background noise is guaranteed, the additional correction method is effective. T
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Figure 7: Principle of the Swept-sine method S/NLpE,ti = S/Nsound field + 10 log10( T / ti ) [dB] 0
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p(t) Figure 8: Additional operation to suppress the influence of the background noise ⎡ t2 t 2 − t1 L pE ,correct = 10 log10 ⎢ ∫ p 2 (t )dt − t 3 − (t 2 − t1) ⎣ t1
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Figure 10: Errors in the sound pressure level difference between a reverberation room and a hemi-anechoic room (experimental study)
3.2 Scale model experiment using a wind tunnel In order to examine the applicability of the Swept-sine method to measurements under timevarying condition, scale model experiment using a boundary layer wind tunnel (testing area: W:2.2m, H:1.8m, L:16.5m) was made. As a confirmation study, furthermore, field measurement of sound reduction index of a building facade was done on a very windy day, which was the next day of a typhoon. Turbulent flow behind the building Turbulent flow occurs behind building. The influence upon the Swept-sine method was examined. A wind at the mean velocity of 8 meters per second was blown in a 1/40 model wind tunnel. A loudspeaker was set in the tunnel. The loudspeaker was regarded as a building and a receiving position where the wind velocities fluctuate violently was selected on the leeward of the loudspeaker. The LpE,1 measurements by the Swept-sine method were performed. As the source signal, four kinds of signal durations which were 0.17 s, 1.37 s, 10.92 s and 87.38 s (linear Swept-sine, 0 to 48000 Hz) were used. The measurements are repeated five times in each signal. Figure 11 shows the errors from the reference value, which is the value obtained by the ordinary method using white noise (10 s averaging) without wind. The variation in five times measuring was not so big even in the case of short signal. As a confirmation study, a field measurement of sound reduction index of a building facade was done on a very windy day, which was the next day of a typhoon. As the source signal, the band-limited swept-sine signal with a duration time of 10 min. was used. The conditions of the S/Nsound field were controlled roughly by adjusting the sound radiation level of source signals. Figure 12 shows the errors from the reference value, which is the value obtained by the ordinary method using band noise under the +20 dB S/Nsound field condition. Almost equal
values with ordinary method can be obtained by the long Swept-sine method under a very adverse S/Nsound field condition up to -20 dB. Sound propagation measurement in the field with wind gradient The outdoor propagating sound is refracted by wind gradient. Of course, this is time-varying phenomenon. The applicability of the Swept-sine method under such a condition was examined. In the wind tunnel, the vertical wind gradient was simulated so as to approximately correspond to 1/40 scale of the real one. The experimental geometries are shown in Fig. 13. The LpE measurements by the Swept-sine method were performed five times in each signal duration length which were 0.17 s, 1.37 s, 10.92 s and 87.38 s (linear Swept-sine, 0 to 48000 Hz), under three conditions of wind (no wind, following wind of 6 m/s and adverse wind of 6 m/s). The propagation characteristics were compared with those by the ordinary method using white noise (10 s averaging). The results in 20k Hz octave band are shown in Fig. 14. The reinforcement caused by following wind and the excess attenuation caused by adverse wind can be observed. In such experimental conditions, the results obtained by the Swept-sine method were equal to those obtained by the ordinary method. Figure 15 shows the variation in 5 times measurement by the Swept-sine method under the condition at adverse wind of 6 m/s in 40k Hz octave band. This condition was highly timevarying, as the fluctuation of the sound pressure level at the measuring point of 3 m is shown in Fig.16. Sufficient averaging time is required even in the ordinary method. Although the physical meaning of the averaging is different between the signal duration of the Swept-sine and the averaging time by the ordinary method, it can be seen in Fig. 15 that the variation in five times measurement by the Swept-sine method is decreasing as the signal duration increases. 1.0
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Figure 16: Fluctuation of the sound pressure level at the measuring point of 3 m under the condition of adverse wind of 6 m/s in 40k Hz octave band (time constant: 3 ms)
4. APPLICATIONES Measurement of sound radiation characteristics from semi-underground road As an application of the Swept-sine method, sound radiation characteristics from a semiunderground road was measured. The measurement environment was not so good, in which there were various kinds of extraneous noises such as construction noises, road traffic noises, and so on. Even then, the measurements were performed by selection of appropriate signal duration corresponding to the extraneous noise level. Figure 17 shows the measured wave forms at the receiving position of 100 m from the sound source position. From the noise floor level, it can be judged that correct measurements were done. Details of the measurement are presented in reference [15]. Measurement of house filter The authors have been investigating “house-filter”, which is a concept to discuss living environments and a tool to perform subjective experiments for sound amenity in laboratory using a sound field simulation system. As a trial, the impulse response from a sound source located in a roadside to a receiving position located in a living room was measured. As a source signal, a pink Swept-sine signal with a duration time of 2 min. was used. High S/N ratio can be obtained as the measured wave forms are shown in Fig. 18. Details of this topic are under.
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Figure 17: Measurement of sound radiation characteristics 5 from semi-underground road by band limited Swept-sine (receiving position: 100 m from the sound source position) 5. CONCLUSIONS
As described above, the measuring results of LpE,1 obtained by the method using a long swept-sine signal are highly corresponding with the results obtained by the ordinary method using a stationary-random noise even under the time-varying conditions. Measuring the LpE,1 is possible under a very adverse S/Nsound field condition up to -20 dB by using a signal of 10 min. duration. If the stationary-random of the background noise is guaranteed, it is possible under the S/Nsound field condition of -30 dB. 6. REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
M.R. Schroeder: Integrated impulse method measuring sound decay without using impulses. J. Acoust. Soc. Am., 66, 497-500 (1979). H. Alrutz and M.R. Schroeder: A fast Hadamard transform method for the evaluation of measurements using pseudrandom test signals. Proc. ICA 1983, Vol. 6, pp.235-238 (1983). D.D. Rife and J. Vanderkooy: Transfer-function measurement with maximum-length sequences. J. Audio Eng. Soc., 37, 419-444 (1989). A.J. Berkhout, D. de Vries and M.M. Boone: A new method to acquire impulse responses in concert halls. J. Acoust. Soc. Am., 68, 179-183 (1980). N. Aoshima: Computer-generated pulse signal applied for sound measurement. J. Acoust. Soc. Am., 69, 1484-1488 (1981). Y. Suzuki, F. Asano, H. Kim and T. Sone: An optimum computer-generated pulse signal suitable for the measurement of very long impulse responses. J. Acoust. Soc. Am., 97, 1119-1123 (1995). H. Tachibana, H. Yano and F. Satoh: Sound insulation measurement by various kinds of digital signal processing techniques. Proc. Inter-Noise 2001, Vol.3, pp.1137-1142 (2001). M. Vorlander: Categorization of modern measurement techniques in building acoustics. Proc. InterNoise 2001, Vol.4, pp.2145-2154 (2001). S. Yum, K. Kawasaki, F. Satoh, S. Shinichi and H. Tachibana: Application of the Long-TSP method to sound propagation measurements. Proc. Inter-Noise 2003, N560 (2003). F. Satoh, J. Hirano, S. Sakamoto and H. Tachibana: Sound insulation measurement using a long sweptsine signal. Proc. ICA 2004, Vol.5, pp.3385-3388 (2004). F. Satoh, Y. Hidaka and H. Tachibana: Influence of time variance on room impulse response measurement. Acustica united with Acta Acustica, Vol. 85, p.441 (1999.Jan./Feb.). F. Satoh, M. Nagayama and H. Tachibana: Influence of time-variance in auditorium on impulse response measurement. Proc. Forum Acusticum Sevilla 2002, PHA-Gen-025 (2002). F. Satoh, S. Sakamoto and H. Tachibana: Comparison between the MLS and TSP methods for room impulse response measurement under time-varying condition. Proc. RADS 2004 (A satellite symposium of ICA 2004), B05 (2004). H. Tachibana and K. Ishii: Scale model experiment of the effect of wind on sound propagation. Proc. Inter-Noise 1975, pp.627-630 (1975). S. Sakamoto, J. Hirano, F. Satoh, H. Tachibana, O. Funakoshi and T. Mori: Experiment and calculation of sound radiation characteristics from semi-underground road. Proc. Inter-Noise 2005
