可變容積消音器消音能力之模擬分析

Abstract

由前人製造之可變容積消音器與對其做的實際測量，吾人可知共鳴腔大小與其共鳴頻率有關係，本文研究目的是使用傳輸損失與噪音減量的方式，雙管齊下驗證可變容積消音器的消音能力。本文主體可分為兩部份，一部份是使用平面波理論假設，推導同心共鳴管所得之傳輸損失方程式，再使用Matlab軟體將之撰寫為一程式，使吾人只要輸入不同的共鳴腔長度時就可得到不同的傳輸損失值。另一部分則是使用兩個電容式麥克風實際測量對可變容積消音器進行噪音減量的實驗，這個部份會先進行簡單直通式共鳴消音器之噪音減量實驗，證明其具有消音能力後，再進行可變容積消音器的噪音減量實驗。在理論方面吾人發現當共鳴腔長度越長時，傳輸損失的頻帶會向較低頻的方向移動，這跟劉之研究結果相符：而實驗方面則當共鳴腔長度增長時，其波形並不一定朝向低頻的方向移動，但消音器整體的消音能力的確會有所變化，這可能是模擬的設定與實驗的誤差所致。

A variable-volume resonant muffler was made by Liu in 2005.The relationship between the size of resonant chamber and the resonant frequency can be obtained by his study.The purpose of this research is to use two indices：transmission loss and noise reduction to find out the performance of this type of muffler.The work can be divided into two parts.The first part is to use the plan wave theory to get a transmission loss equation of the variable-volume resonant muffler.A software called Matlab is used.If the length of the resonant chamber can be changed,different transmission losses can be obtained.The second part is to use two microphones to measure a variable-volume resonant muffler and the noise reduction can then be obtained.A simple straight-through muffler will be tested at first in order to prove whether the procedure of measurement is correct.After that,the experiments on a variable-volume resonant muffler will be done.In theory,we can find that the wave tendancy of transmission loss moves toward to lower frequency region when the length of the resonant chamber becomes longer.This conclusion is corresponding to Liu’s result.The result of noise reduction experiment doesn’t match this phenomenon,however. It may result from some experimental errors.