Research On Calibration Method And Calibration Device Of Anechoic Chamber

With the development of science and technology and social progress, the requirement of acoustic noise quality has been constantly improved. As the principal place of acoustic measurement environment, the measurement results of acoustic noise are seriously influenced by the acoustic characteristic of anechoic chamber. The existing calibration method of anechoic chamber has some difficulties, such as unable to automatically realize the multi-point continuous measurement, the large deviation of theoretical sound pressure level (SPL) calculation and the simplicity of evaluating method. The dissertation work mainly includes:1. The propagation principle of free sound field and metrological parameters of acoustic characteristic are analyzed, and the calibration method and procedure of anechoic chamber are introduced, which includes the measurement methods of background acoustic noise and free sound field, the uncertainty evaluation method of anechoic chamber. Moreover, the attenuation principle of SPL with distance and the calculation method of theoretical SPL are discussed.2. The acoustic influence factors on anechoic chamber calibration accuracy are analyzed, which includes the background acoustic noise, air absorption, ground reflection and the directivity of sound source. A modified model is established for the SPL measurement in free sound field, a novel method of calculating SPL is proposed for realizing fast calibration and accurate calibration, and a SPL curve evaluation method based on moment estimation is further proposed to evaluate quality of the tested anechoic chamber.3. A new automatic calibration device is designed for the anechoic chamber, which can realize the acoustic data collection by employing the Pulse system, sound devices, microphone and its automatic traction devices. Furthermore, the SPL data processing system based on LabVIEW is also developed, which can obtain the theoretical SPL, free sound field characteristic of anechoic chamber, frequency range and space range calibration.The theoretical simulation and experimental results show that the high frequency acoustic noise is influenced by the air absorption greatly, in which the SPL attenuation coefficients are 0.02dB/m and 0.3dB/m when the frequencies are 1kHz and 10kHz respectively. With the SPL deviation smaller between the background and sound source acoustic noise, the SPL experimental result error was greater. The result further shows that the small deviation between ground noise and sound source noise results in the larger error of real SPL measurement. For example, the error is 0.4dB when the deviation is lOdB. Moreover, the SPL measurement is also influenced by the ground reflection and sound source directivity seriously, and thus results in larger SPL measurement error. As a result, the proposed theoretical SPL calculation method can overcome several problems in the national standard, where the SPL deviation is larger when the measurement distance from sound source is closer, and the SPL is smaller when the measurement distance is further. Moreover, it also decrease the SPL calculation accuracy due to the single point calculation based on ZHONG’s method and not considering the air absorption based on PU’s method. The SPL curve measurement automatically at each point, the theoretical SPL calculation and the free sound field characteristic evaluation can be realized by employing the proposed automatically calibration device of anechoic chamber and the SPL comprehensive analysis system, which can improve the efficiency and accuracy of anechoic chamber calibration and provide the theoretical basis and practical guidance to the future calibration standard revision of anechoic chamber and the acoustic noise measurement.