應用珈瑪啁啾濾波器組模擬聽障者之耳蝸聽覺機制

Abstract

對於聽障人士來說，因為無法正確聽到完整的語音訊號，往往就會造成學習與溝通上的困難，故正常人士會對聽障人士產生誤解。若聽力正常人士能透過某種工具感受到聽障人士所聽到的聲音及語音，必能更加體會聽障人士對於聽力損失所造成生活與學習上的不便，更能針對其感受協助改善由於聽障所造成的問題。 耳蝸(cochlea)對聲音訊號的分析佔聽覺系統相當重要的部分，因為耳蝸內的基底膜(basilar membrane)對聲音訊號的振幅與頻率有不同的響應，其功能就像一個帶通濾波器組(band pass filter bank)。聲音的高頻成份對耳蝸底部(base)有較大的響應；相反地，聲音的低頻成份對頂部(apex)有較大的響應。本文的研究目標以耳蝸基底膜上的每一點有其特性頻率(characteristic frequency)，對於聲波的大小頻率響應圖曲線呈現帶通濾波器組的特性。依數位濾波器組的原理，設計並聯濾波器組，其濾波器組分別以三分之一音程(one-third-octave)與臨界頻帶(critical band)作為濾波器頻帶劃分的方式，模擬耳蝸對於頻率的選擇性。另一方面利用符合心理與生理聲學耳蝸量測實驗所提出的珈瑪調(gammatone)與珈瑪啁啾(gammachirp)濾波器組，以模擬內耳蝸聽覺機制。最後再串聯外耳、中耳與珈瑪調或珈瑪啁啾濾波器組，以模擬由外耳、中耳及內耳整體組成之周邊聽覺系統。 依本文所設計的四種濾波器組，測試語音訊號，來驗證這些濾波器組是否符合耳蝸之頻率選擇上的特性。再依據不同程度的聽力損失，設計不同的聽力損失濾波器組，將語音訊號通過其濾波器組，進而模擬聽障人士所聽到之語音訊號。將此語音讓聽力正常人士身歷其境感受，透過這樣的體驗，對於聽障人士所接受到的語音訊號有所瞭解，進而協助他們，改善其因為聽力所造成的不便。

As far as hearing-impaired people is considered, they might encounter the difficulties in the process of learning and communication due to not able to perceive the complete signal of sounds in the correct way, which leads normal people have misunderstanding and wrong perception towards hearing-impaired people. If normal people without hearing problems can sense or really hear the voices or sounds which hearing-impaired people actually receive by means of certain instrument or tool, we can truly understand and realize such inconveniences of life and learning process for hearing-impaired people; moreover we can assist to improve their lives by solving the problems caused by hearing impairment. The cochlea plays a very important and critical role of analysis of signal of sounds in entire hearing system. Since the basilar membrane inside cochlea reacts different responses on amplitudes and frequency of the signal of sounds, its function works as one band pass filter bank. The high frequency of sound causes bigger response to the base of cochlea; on the contrary, the low frequency of sound results in bigger response to the apex of cochlea. The research objective is to explore that each point of the basilar membrane inside cochlea has its property of the band pass filter bank while shows in the frequency responses to voice wave, since each point has its characteristic frequency. In accordance of the principle of digital band pass filter bank, we specifically design the parallel connection of band pass filter bank, and the band pass filter bank respectively utilize one-third-octave and critical band as the method to divide the band, which is to simulate the selectivity of the cochlea towards the frequency intensity. In another hand, the filter bank of gammatone and of gammachirp, published and designed by the cochlea measurement experiment consistent of physical and psychological auditory mechanism, will be used to simulate the auditory cochlea mechanism of the inner cochlea. Then, the outer ear, middle ear will be connected with the filter bank of gammatone and of gammachirp by the way of cascade connection to simulate the peripheral auditory system and mechanism of outer ear, middle ear and inner ear altogether. In this research, four different types of filter banks will be designed to test the signal of sounds, which can verify and prove whether these filter banks can really function and be consistent of the properties of frequency selectivity of the cochlea. Accordingly we can design different hearing loss filter banks in accordance with the different level of hearing loss. When the signal of sounds passes through filter banks, we can simulate and imitate the signal of sounds which hearing impaired people actually receive. Through such a devise, the normal people can actually hear this type of sounds; they can really understand when they put themselves in people’s shoes. We hope that we can really understand and realize their difficulties and inconveniences which they have to face everyday by this experience; then we can further assist them and help them to improve their lives and make their lives better and easier.