| Summary: | 碩士 === 元智大學 === 機械工程學系 === 98 === In the calibration of anechoic or semi-anechoic chambers, it is required to have a reference sound source (RSS) to generate a standard noise for testing whether they can meet the ISO international standards. In general, the dimensions of the standard anechoic chamber are relatively too large for testing the small products such as those 3C products. It is better to have smaller chambers for saving the construction cost and reducing occupied space. In the mean time, it also needs a small RSS for calibrating the chambers.
This study developed a new type of reference sound source by focusing on the two critical factors of its sound intensity and directionality as specified in the ISO standards. Based on the RSS for the large standard chambers, an initial design of a small reference sound source (RSS-0.25) which was about the quarter size of RSS was developed and tested. Analysis with the Fluent software and experiment of wind speeds were conducted and compared with each other for the verification of the analysis model. By checking the wind speeds of the existing RSS and RSS-0.25, a new type of small reference sound source (RSS-0.5) was designed and simulated for its wind speeds. The results showed that when at a distance of 0.5 m, the wind speed analyzed is 1.46 m/s. According to the relationship between wind speed and noise measured on the RSS and RSS-0.25, the wind speed will not affect the accuracy of microphone measurement when it is less than 1.5 m/s. The model of RSS-0.5 with the corresponding analyzed geometric dimensions is then used for manufacturing the new RSS-0.5. The wind speed and noise intensity and directionality are investigated further.
The experimental results of RSS-0.5 showed that the wind speed was 1.44 m / s when measured at a distance of 0.5 m. Also, the maximum sound intensity generated was 71.4 dB though its intensity will vary as the motor speed changed. For the two designs with and without an opening on the cover plate of the fan top, it was found that the former still had directionality problem at the 90∘location and the latter had totally solved this problem no matter in the low or medium-high frequency ranges. Besides, the sound pressure differences are all within 6 dB between 100 Hz ~ 10 kHz following the ISO standards except that of 2.5 kHz which is 6.9 dB. This slight deviation at the solely high frequency is believed not affecting much on the newly developed reference sound source RSS-0.5 for the calibration of a small anechoic chamber.
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