小型無響室校準用之標準音源研製

Development of the Reference Sound Source for Calibrating Miniature Anechoic Rooms

李建鋒(Chien-Feng Lee)

指導教授 ： 陳永樹

元智大學/工程學院/機械工程學系/碩士(2009年)

摘要

噪音(noise)問題經常存於現代之生活環境中，為了解決擾人的噪音問題，許多電子產品與機械零件，諸如電腦散熱風扇、冷氣壓縮機等旋轉機械等，在大量生產前需於無響室(anechic rooms)或半無響室(hemi-anechoic rooms)中進行噪音測試。然而這無響室或半無響室是否合乎標準，會影響噪音量測結果。因此，都需以標準音源(reference sound source)進行測試，可以確保所測試產品不會嚴重影響到人類生活品質，才能生產。然而，無響室有一定之空間大小規範，即使量測極小之諸如電腦風扇等，用諾大空間則顯得浪費也不切實際。如何對應使用小型無響室，且有類似大型無響室之標準音源加以鑑定，在應用上極具實用價值。大型無響室用之標準音源運轉時會產生相對而言太大之風量，因此並不適用於小規模之量測環境。因此本研究據此需求，研製小型無響室專用之標準音源。
標準音源係利用高轉速下之橫流風扇(Cross Flow Fan)，產生平穩之氣動噪音，而葉片的相關設計：如氣流進入葉片時入口與出口角度…等，決定風扇產生之噪音與風量。所以本研究首先針對變更葉片之相關設計，對噪音之影響進行探究。
由於橫流扇的主要特性由風扇的葉型、內外葉輪直徑的比率、葉片的間距與葉片的數目來決定，產聲的音壓值(sound pressure level)亦與這些參數息息相關。本研究中變化這些參數，利用FLUENT軟體，對不同葉片角度進行流場噪音數值分析，以期找出何種葉片角度能穩定產生噪音，最後設計出能產生小風量之橫流扇葉片，期望產生風量的範圍能控制在1 m以內。
由分析結果得知，內周角度於95°時會產生最小風速；由實驗中得知葉片數愈多，愈能有效提升中高頻音。依據上述製做出小型標準音源後，再經由風速量測比對，距離小型標準音源0.2m時，其產生之風速為0.06m/s。小型標準音源於中低頻之指向性較中高頻平穩，由其在4kHz之後的高頻音，其指向性較不穩定。

關鍵字

無響室 ； 半無響室 ； 標準音源 ； 橫流風扇 ； 音壓值 ； 流場數值分析

並列摘要

In present day, noise is a common problem in our living surrounding. For finding the solvent, many of electronic and mechanical parts, like computer cooling fan, air compressor and so on, needed to proceeding noise test in anechoic or hemi-anechoic rooms before manufacture. But it would affect the result of the measurement of noise that depend on the rooms are correspond to standard or not. Thus, we need to use reference sound source to processing test, it could ensure all the products would not affect our life. However, it have standard of space for the rooms and it’s unrealistic for using a huge space to measure a tiny fan of computer. How to using the reference sound source in miniature anechoic rooms to simulate large-scale anechoic rooms is very practical. The reference sound source for large-scale anechoic rooms would cause more air flow in operation, and it not applies to miniature measurement environment. And the main major for this research is developing specific reference sound source for miniature anechoic rooms.
The theory of reference sound source is using high-speed cross flow fan produce stable areoacoustic noise. And the design for fin would change noise and air flow of the fan, such as the angle of the fin when aero passes through and so on. This research is discussion on the effect of noise for changes fin’s design.
The characteristics of fan including style of fan, ratio of impeller inner and outer diameter, gap between each fin and number of fin, and the sound pressure level also depend on all of these parameter. In this research, changing these parameter and proceed flow field noise numerical analyses for different angle by FLUENT. Here’s hoping to find out which angle can make the stable noise. Finally, design the cross flow fan that can produce less wind flow, and control wind flow ambit within 1 m. From the analysis results, the smallest wind speed would occur at 95° of angle of circumference, the fan with more fins could occur widespread audio range. With these conditions, make a improvement reference sound source could apply to miniature anechoic rooms.

並列關鍵字

anechic rooms ； hemi-anechoic rooms ； reference sound source ； Cross Flow Fan ； sound pressure level ； Computational Fluid Dynamics

參考文獻

[1]ISO 3745, "Acoustics-Determination of sound power levels of noise sources using sound pressure-Precision methods for anechoic and hemi-anechoic rooms," Dec, 2003.

[3]S.P. Dragana, M. Mijic and H. Kurtovic, "A simple impulse sound source for measurements in room acoustics, "Applied Acoustics, Vol.69, pp.378-383, 2008.

[8]Young J. Moon and Yong Cho, "Discrete Noise Prediction of Variable Pitch Cross-Flow Fans by Unsteady Navier-Stokes Computions, "ASEM, Vol.125, pp.543-550, 2003.

[12]Junichiro Fukutomi and Rei Nakamura, "Performance and Internal Flow of Cross-Flow Fan withInlet Guide Vane, "The Japan Society of Mechanical Engineers, Vol.48, No.4, 2005.

[13]Go-Long TSAI, Tsung-Hsien TU, Tung-Chen LI and Kuang-Hsieng WANG, "Flow Style Investigation and Noise Reduction of a Cross-Flow Fan with Varied Rotor-Skew-Angle Rotor," The Japan Society of Mechanical Engineers, Vol.49, No.3, pp.695-704, 2006.