- Email: [email protected]
Development of sound absorption measuring system with acoustic chamber
437
Technical Notes / JSAE Review 20 (1999) 421}438
Technical Notes
Development of sound absorption measuring system with acoustic chamber Masakiyo Takahira , Mikio Noba , Hisanaga Matsuoka Toyota Motor Corporation, Toyota-cho 1, Toyota-shi Aichi 471-0826, Japan Nippon Soken Incorporation, 14, Iwaya, Shimohasumi-cho, Nishio-shi Aichi 445-0012, Japan Received 27 November 1998; received in revised form 10 February 1999
1. Introduction Sound absorbing material is one of the most important countermeasures against vehicle exterior noise. It is understood that noise reduction by sound absorbing material, obtained from vehicle pass-by noise tests, corresponds to that measured by the reverberation room method. In order to easily obtain sound absorbing capability, a simple measuring system was developed by using an acoustic chamber. The correlation between sound absorbing capabilities obtained from measuring by the system and those obtained by the reverberation room method were investigated.
2. Test equipment The following development targets were set, and the shape of equipment and measuring method were examined: (1) Since the sound absorbing e!ects on vehicle pass-by noise test appear to be in the 500 Hz to 5 kHz band, the order of the sound absorbing capability should correspond to that of the reverberation room method in this frequency band. (2) Dispersion of the measured value should be equal to or below that of the reverberation room method. (3) The device should be compact (2 m wide ;2 m deep ;2 m high, or smaller). (4) Measuring time should be 5 min or less per sample. The acoustic chamber was shaped into an irregular septilateral shape in which no faces are directly opposite each other, so as to provide a di!used sound "eld in the frequency range of 500 Hz and above inside the chamber. The measuring method was devised to obtain the sound absorption coe$cient from the di!erence in the pressure levels by constantly generating sounds at
random, for miniaturization of the acoustic room subsequently shortens the reverberating time [1,2]. Moreover, to eliminate the process of averaging the results of multiple point measurement inside the acoustic chamber, the provision of an opening in the chamber and the placing of the sound receiving point outside of it were examined.
3. Test results 3.1. Evaluation of the equipmenmt The sound pressure level of arbitrary points inside the acoust chamber were measured to verify the sound "eld di!usion, (Fig. 1). The range of dispersion of the sound pressure level at six points 0.5 m or more away from the speaker was smaller than that of dispersion at 15 sound pressure level points inside the chamber. Although it increased slightly in the range for 500 Hz to 1 kHz, the dispersion was similar to that of a large reverberation room having a dispersion range of approximately 5 dB maximum over the 500 Hz to 5 kHz range. In other words, it was veri"ed that a su$cient di!used sound "eld was obtained at a position 0.5 m or more away from the speaker. It was also veri"ed that the sample should ideally be placed at this position. Moreover, the relationship between the sound "eld in the acoustic chamber and that in the vicinity outside of the opening was veri"ed in a test. The test was conducted by measuring the sound pressure level at six points 0.5 m or more away from the speaker placed inside the acoustic chamber and the same level at a position 50 mm outside the center of the opening (Fig. 2). Fig. 3 shows the result of the test. It shows that both the characteristics matched comparatively well. In
0389-4304/99/$20.00 1999 Society of Automotive Engineers of Japan, Inc. and Elsevier Science B.V. All rights reserved. PII: S 0 3 8 9 - 4 3 0 4 ( 9 9 ) 0 0 0 3 3 - 8
JSAE9931413
438
Technical Notes / JSAE Review 20 (1999) 421}438
Fig. 3. Comparison of shape of sound spectrum. Fig. 1. Measuring points in chamber (N"15).
Fig. 2. Measuring points (inner N"6, outside of opening N"1).
other words, it was veri"ed that multi-point measurement of the sound pressure level inside the chamber was expressed by the sound pressure level measured at a single point 50 mm outside the center of the opening. 3.2. Comparison of reverberation room method and the current equipment To maximize the di!erence of the sound pressure level by sound absorbing material, the length of one side was set at 400 mm for measurement. Fig. 4 shows the response to variation of the mean sound absorption coe$cient by the reverberation room method in the 500 Hz to 5 kHz band. The percentage of variation of the mean sound absorption coe$cient using this equipment and that of the reverberation room method, that is, the inclination, remained almost constant. From the above, the sound absorption coe$cient of the current equipment corresponded su$ciently with
Fig. 4. Correlation of 500}5000 Hz averaged sound absorption coe$cient.
that of the reverberation room method. The size of the sample was also veri"ed to be appropriate. 4. Conclusion (1) It is now possible to evaluate, with ease, the relative positioning of the capability of sound absorbing materials during vehicle pass-by noise tests using current measuring equipment. (2) Measuring the e!ect of sound absorbing material using this equipment produced sound absorption coe$cients that corresponded su$ciently with that obtained by the reverberation room method. References [1] Japan Society of the Manufacturers of Acoustic Materials (Nihon Onkyo Zairyo Kyokai), The Handbook on Noise and Vibration Measures (1982). [2] Kiyoshi Okada and Musha, H. The Handbook on Concrete Engineering (1981).
Technical Notes / JSAE Review 20 (1999) 421}438
Technical Notes
Development of sound absorption measuring system with acoustic chamber Masakiyo Takahira , Mikio Noba , Hisanaga Matsuoka Toyota Motor Corporation, Toyota-cho 1, Toyota-shi Aichi 471-0826, Japan Nippon Soken Incorporation, 14, Iwaya, Shimohasumi-cho, Nishio-shi Aichi 445-0012, Japan Received 27 November 1998; received in revised form 10 February 1999
1. Introduction Sound absorbing material is one of the most important countermeasures against vehicle exterior noise. It is understood that noise reduction by sound absorbing material, obtained from vehicle pass-by noise tests, corresponds to that measured by the reverberation room method. In order to easily obtain sound absorbing capability, a simple measuring system was developed by using an acoustic chamber. The correlation between sound absorbing capabilities obtained from measuring by the system and those obtained by the reverberation room method were investigated.
2. Test equipment The following development targets were set, and the shape of equipment and measuring method were examined: (1) Since the sound absorbing e!ects on vehicle pass-by noise test appear to be in the 500 Hz to 5 kHz band, the order of the sound absorbing capability should correspond to that of the reverberation room method in this frequency band. (2) Dispersion of the measured value should be equal to or below that of the reverberation room method. (3) The device should be compact (2 m wide ;2 m deep ;2 m high, or smaller). (4) Measuring time should be 5 min or less per sample. The acoustic chamber was shaped into an irregular septilateral shape in which no faces are directly opposite each other, so as to provide a di!used sound "eld in the frequency range of 500 Hz and above inside the chamber. The measuring method was devised to obtain the sound absorption coe$cient from the di!erence in the pressure levels by constantly generating sounds at
random, for miniaturization of the acoustic room subsequently shortens the reverberating time [1,2]. Moreover, to eliminate the process of averaging the results of multiple point measurement inside the acoustic chamber, the provision of an opening in the chamber and the placing of the sound receiving point outside of it were examined.
3. Test results 3.1. Evaluation of the equipmenmt The sound pressure level of arbitrary points inside the acoust chamber were measured to verify the sound "eld di!usion, (Fig. 1). The range of dispersion of the sound pressure level at six points 0.5 m or more away from the speaker was smaller than that of dispersion at 15 sound pressure level points inside the chamber. Although it increased slightly in the range for 500 Hz to 1 kHz, the dispersion was similar to that of a large reverberation room having a dispersion range of approximately 5 dB maximum over the 500 Hz to 5 kHz range. In other words, it was veri"ed that a su$cient di!used sound "eld was obtained at a position 0.5 m or more away from the speaker. It was also veri"ed that the sample should ideally be placed at this position. Moreover, the relationship between the sound "eld in the acoustic chamber and that in the vicinity outside of the opening was veri"ed in a test. The test was conducted by measuring the sound pressure level at six points 0.5 m or more away from the speaker placed inside the acoustic chamber and the same level at a position 50 mm outside the center of the opening (Fig. 2). Fig. 3 shows the result of the test. It shows that both the characteristics matched comparatively well. In
0389-4304/99/$20.00 1999 Society of Automotive Engineers of Japan, Inc. and Elsevier Science B.V. All rights reserved. PII: S 0 3 8 9 - 4 3 0 4 ( 9 9 ) 0 0 0 3 3 - 8
JSAE9931413
438
Technical Notes / JSAE Review 20 (1999) 421}438
Fig. 3. Comparison of shape of sound spectrum. Fig. 1. Measuring points in chamber (N"15).
Fig. 2. Measuring points (inner N"6, outside of opening N"1).
other words, it was veri"ed that multi-point measurement of the sound pressure level inside the chamber was expressed by the sound pressure level measured at a single point 50 mm outside the center of the opening. 3.2. Comparison of reverberation room method and the current equipment To maximize the di!erence of the sound pressure level by sound absorbing material, the length of one side was set at 400 mm for measurement. Fig. 4 shows the response to variation of the mean sound absorption coe$cient by the reverberation room method in the 500 Hz to 5 kHz band. The percentage of variation of the mean sound absorption coe$cient using this equipment and that of the reverberation room method, that is, the inclination, remained almost constant. From the above, the sound absorption coe$cient of the current equipment corresponded su$ciently with
Fig. 4. Correlation of 500}5000 Hz averaged sound absorption coe$cient.
that of the reverberation room method. The size of the sample was also veri"ed to be appropriate. 4. Conclusion (1) It is now possible to evaluate, with ease, the relative positioning of the capability of sound absorbing materials during vehicle pass-by noise tests using current measuring equipment. (2) Measuring the e!ect of sound absorbing material using this equipment produced sound absorption coe$cients that corresponded su$ciently with that obtained by the reverberation room method. References [1] Japan Society of the Manufacturers of Acoustic Materials (Nihon Onkyo Zairyo Kyokai), The Handbook on Noise and Vibration Measures (1982). [2] Kiyoshi Okada and Musha, H. The Handbook on Concrete Engineering (1981).