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Measurements of Acoustic Impedance and Their Data Application to Calculation and Audible Simulation of Sound Propagation

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Measurements of Acoustic Impedance and Their Data Application to Calculation and Audible Simulation of Sound Propagation Acoust. Sci. & Tech. 29, 1 (2008) #2008 The Acoustical Society of Japan PAPER Measurements of acoustic impedance and their data application to calculation and audible simulation of sound propagation Teruo Iwase1, Yu Murotuka2, Kenichi Ishikawa3 and Koichi Yoshihisa4 1Faculty of Engineering, Niigata University, 8050 Igarashi 2-no-cho Nishi-ku, Niigata, 950–2181 Japan 2Graduate School, Niigata University, 8050 Igarashi 2-no-cho Nishi-ku, Niigata, 950–2181 Japan 3Oriental Consultants Co. Ltd., Shibuya Bldg. 16–28 Shibuya, Shibuya-ku, Tokyo 150–0036 Japan 4Faculty of Science and Technology, Meijo University, 1–501 Shiogamaguchi, Tempaku-ku, Nagoya, 468–8502 Japan ( Received 20 April 2007, Accepted for publication 17 August 2007 ) Abstract: Acoustic impedance determines the boundary condition of each sound field, but collections of actual values to evaluate sound fields are insufficient. Therefore, measurements of acoustic impedance using a particle velocity sensor were taken on different fields. Such measurement results were used for sound propagation calculations. Frequency characteristics of sound propagation on grass, snow-covered, and porous drainage pavement surfaces showed fair correspondence with field measurement results. Subsequently, fine calculations in the frequency domain were converted to impulse responses for each sound field model. Convolution operations based on the impulse response and on voice, music, and other noise sources readily produced an ideal sound field for the audible sound file. Furthermore, simulations of noise from a car running through a paved drainage area, with noise reduction effects, were attempted as advanced applications. Keywords: Acoustic impedance, Sound propagation, Road traffic noise, Snow field, Drainage pavement, Impulse response, Convolution PACS number: 43.28.En, 43.58.Bh [doi:10.1250/ast.29.21] example, the prediction method for traffic noise has 1. INTRODUCTION become a widely known numerical evaluation method Acoustic impedance is a very important parameter for [2]. There is a boundary that has not yet been exceeded determining boundary conditions of sound fields. There- between a numerical evaluation method and an audible one fore, collections and collection projects of the acoustic and there are still some problems to be solved before impedance data for acoustical calculations are desired by widespread adoption of the audible evaluation method. many acoustical researchers and engineers who are To realize audible simulation by acoustical calculation, interested in evaluating and simulating the sound propaga- many acoustic impedance data have been required to have tion in various sound fields. Then, the establishment of an fine and wide continuous frequency values without disorder audible confirmation technique for easy evaluation of the from the point of view of hearing tests for the calculated calculated result using familiar sound signals such as results. Although there has been considerable research and human voice, music and noise signals produced by vehicles developing work on the measurement technique for the or machines is strongly expected by many researchers and impedance of actual materials or actual surface conditions engineers who have researched noise control and architec- of boundaries around us, only the sound tube method or its tural acoustics [1]. However, collections of actual values improved version for computer-aided measurement [3] can are insufficient for calculations and simulations of sound be established as a practical method that produces highly propagation and for evaluation of many sound fields. As for reliable values available to audible applications such as the method of evaluating the acoustical effects, the audible those mentioned above. Many development studies based simulation technique is not yet generalized though; for on the sound tube method using a pair of microphones 21 Acoust. Sci. & Tech. 29, 1 (2008) applied to an actual field have been conducted but effects, we tried many natural methods of correcting the significant success has not been achieved. Such methods measured impedance data. still have the common problem in which unexpected At the last stage of this study, simulations of noise disorder appears in measured results on a highly reflective radiation including noise reduction effects from a car test object or those in some frequency area where high- running through a drainage pavement area were carried out reflectivity characteristics are exhibited by accompany the as advanced applications. test object. For example, Takahashi et al. showed detailed In this paper, we describe a series of studies that starts acoustic impedance data of only fibrous materials and only from the development of methods of measuring acoustic for middle-range and high frequencies higher than several impedance, continues to the execution of measurements hundred Hz, and the method proposed by one of us was and the application of data to sound propagation calcu- limited to the measurement of only sound absorption lations, and finishes with an audible simulation. From our coefficients and was not applicable to the measurement of results, it will be shown that the proposed measuring complex impedance data [4,5]. We found that the main method is effective for the collection of acoustic impe- reason for the lack of collected acoustic impedance data dance data in actual various fields and that not only a was the absence of a practical measurement method and we numerical method but also an audible simulation method were able to conclude that not only collections of acoustic under the condition of a sound source moving in both impedance values but also the establishment of practical methods based on the measured impedance values is measuring methods for field measurement of the acoustic effective as a new way to acoustically evaluate many sound impedance is still greatly needed. fields and provide a virtual experience. At the first stage of our approach towards the goal of 2. METHOD OF MEASURING ACOUSTIC realizing evaluation that includes audible simulation, we IMPEDANCE IN FIELD measured the acoustical surface impedances of various fields. In these measurements, we tested the ordinal A method that employs two-channel microphones is method using a pair of microphones to confirm its often used for measuring acoustic impedance and the sound features and to check the acoustical conditions that could absorption coefficient [4,5]. Figure 1(A) shows that when free the obtained acoustic impedance data from disorder. a pair of microphones with equal sensitivity and a small In addition, we have recently tested a new measuring interval are arranged near a test target surface under the method that uses a hot-wire-type particle velocity sensor assumption of a plane wave sound field, both the sound and a microphone. pressure components of the incident and reflected waves At the second stage of this study, to evaluate measured can be separated from the two microphone outputs pa and ÀikÁr impedance data on the basis of the measure of quality pb using the phase delay operation e , corresponding to applicable to numerical calculations including advanced a small interval of Ár. Thus, the complex sound reflection audible simulations, results from these measurements were ratio r can be obtained from both components. The acoustic applied to some sound propagation calculations focused on impedance ratio z and sound absorption coefficient can the ground with the surface condition varied from being be obtained continuously using the following equations: acoustically very soft to hard. Typical frequency character- P eikÁr À p r ¼ a b ; ð1Þ istics of sound propagation along surfaces of snow, turf, ÀikÁr pb À pae long grass, and a porous asphalt pavement field were 1 þ r obtained from both measurements and calculations. The z ¼ ; ð2Þ 1 À r calculated characteristics showed good agreement with 2 results obtained from inspecting field measurements on the ¼ 1 jrj ; ð3Þ sound propagation. Then, fine calculations in the frequency where i is the imaginary number unit and k is the wave domain were converted to the impulse response for each number. sound field model. Convolution operations based on the impulse response and on voice, music and some noise sources readily produced an ideal sound field for the audible sound file. Loudspeaker Loudspeaker These processes are based on the principle of digital signal processing and very simple, but the obtained impulse response had to be evaluated from other points of view of (A) by a Pair (B) by a Particle Velocity hearing tests on the calculated results. It was confirmed that of Microphones Sensor and a Microphone a little irregularity in the impulse response signal brought a strange and unrelenting tone. To avoid such unexpected Fig. 1 Two measurement models for sound reflection. 22 T. IWASE et al.: MEASUREMENTS AND APPLICATIONS OF ACOUSTIC IMPEDANCE (A) for Acoustic Impedance and Sound Absorption Coefficient Loudspeaker: for White Noise Amplifire DAT Pair microphones B&K 3519 1.0m Power Supply DAT 50mm, 12mm ∆r Amplifire Sony TCD-D5 Snow Layer Fig. 2 Block diagram for field measurements on acoustic impedance of snow surface and sound propagation on the snow field. If acoustic particle velocity can be measured by any istics above the same field were also carried out to confirm method [6] with the sound pressure measured using a the simulation calculation of sound propagation character- microphone, we can obtain the acoustic impedance in istics [8]. For this study, we updated the already measured accordance with the definition. parts of data by reanalyzing the original recorded data and The following new method, which uses a set of sensors by recalculation. In these measurements, as shown in consisting of a particle velocity sensor and a microphone, Fig. 2(A), the previously described methods based on a is used in our measurements of acoustical impedance closed pair of microphones were used to obtain sound for many sound fields. Figure 1(B) shows that a particle absorption characteristics for snowy surfaces. The reason velocity sensor and a microphone are arranged close to for the use of the ordinal method, the only method for each other.
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