熱巧克力效應之迷思
“熱巧克力效應”是當你將巧克力粉加入裝有熱水之馬克杯後用湯匙輕敲杯壁,可以聽出攪拌前後敲擊所得聲音頻率明顯不同,攪拌後巧克力粉溶解頻率會較高。一般論文的解答是「當粉末溶解的時候,藏在粉末裡的空氣就會跑出來,在空氣與水混合的環境裡,音速比在水裡低。當水裡不斷有空氣與水混進去時,這個容器的共振頻率和它裡面的音速有關,所以也會降低。因此你會聽到較低之音調,直到空氣全跑光。」但根據我的實驗我覺得這樣的解釋並不對。我們提出新的模型來解釋“熱巧克力效應”。When we put an ounce of dry chocolate powder into a mug filled with hot water, then tapping the side with your spoon will generate a sound. The pitch of this sound will rise after stirring. This is called “the hot chocolate effect”. According to a published paper, the explanation is “The air trapped in the powder is released as the powder dissolves. Since the speed of sound is lower in air than that in water, the speed of sound in the air-water mixture is lower than that in pure water. During that period while the air escapes the container, the resonant frequencies of the water, which depend directly on the speed of sound, will also be lower. Hence, you hear a lower tone until the air escapes”. However, our experiments clearly demonstrated that the explanation is plainly wrong. A new theoretical model is also proposed for the observed phenomenon.
調幅超聲波解調高指向可聽音之研究
可聽聲有向四周擴散繞射特性,而超聲波具有指向性,改以超聲波載送可聽音訊號後,其載波與旁頻帶均在超聲波範圍,實驗中人耳卻可聽到高度指向性聲音,且調幅解調後的可聽聲衰減率比純超聲波來的低。那為什麼超聲波會解調可聽音?我們以非線性的數學轉換概念,成功以數學推導解釋實驗中所聽到的可聽聲,是由旁頻經由非線性轉換而來的。為了證實空氣中的超聲波有非線性現象,以發射40KHz單頻訊號,除了接收到40KHz訊號外還可接收80KHz訊號,而80KHz訊號振幅,會隨著發射強度而遞增,也會隨著傳輸距離增加至穩定狀態,這所我們從文獻中的非線性理論所吻合。接下來進行調幅超聲波實驗,我們經理論計算旁頻帶強度為頻率響應與調變率乘積的一半,而解調可聽聲的強度為調變率、頻率響應與非線性係數三者乘積,我們也由實驗數據證實理論計算結果,在實驗中,換能器在40KHz有最佳的頻率響應,其非線性係數與所載送可聽聲頻率高低約略成正相關,並且與換能器距離遞增而越遠而增加。此外在提高高指向可聽音輸出功率方面,除製作專屬的放大器、運用方波取代正弦波來載波、配合陣列換能器輸出;在改善音質方面,利用等化器調整訊源頻譜分佈,降低低頻振幅,增強高頻振幅,讓各頻率的原始訊號都能有適當的調變,達到最佳音質。The audible sound has the characteristics of spreading and diffracting. And ultrasonic is directive. We modulate sound into ultrasonic signal. The carrier and sideband are ultrasonic frequency bands. But in the experiment, human can hear highly directive sound. In terms of attenuation rate, AM demodulation sound is lower than pure ultrasonic wave. Why can human hear the directive sound? By using the nonlinear mathematical transform, we managed to explain the audible sound which is transformed from sideband with nonlinear effect in the experiment. In order to confirm that nonlinear phenomena in the air ultrasonic, we launch 40KHz single tone ultrasonic signal. Besides the 40KHz signal, we also received 80KHz signal. The amplitude of 80KHz signal will increase with the emission intensity, and also with the transmission distance to increase its stability. These are consistent with nonlinear theory in the literature. Next we began AM ultrasonic experiment. We calculated the sideband intensity that is the product of frequency response and modulation index. The demodulation sound intensity is the product of modulation index, frequency response, and nonlinear coefficient. We also proved the calculated consequence through the experiment. In the experiment, the ultrasonic transducer has a best frequency response in 40KHz. The nonlinear coefficient has positive correlation with the modulation frequency, and increases transmission distance. To boost the power of directive audible sound, we made an amplifier, using square wave to replace sine wave of carrier, and in conjunction with array transducer output. To improve the sound quality, We use the spectrum-Equalizer to adjust the frequency distribution of the origin signal. The EQ reduces the low-frequency amplitude, and boost high-frequency amplitude, which enables every frequency of the original signal to be properly modulated, achieving the best sound quality.
口琴簧片振動與氣流的影響
本研究主題在測量口琴簧片受到各種氣流因子影響後,所產生音色、音頻等變化之探討。在過去我們認為,一片簧片不論如何吹奏,其發出的頻率皆相同。但是事實上,演奏家控制氣流的強弱、方向、渦流等,便可吹奏出多樣的音頻。探討形狀因子對簧片頻率的影響,如:長度、寬度、厚度對頻率所造成的影響。自製口琴,利用變壓器控制送風機風速。探討氣流因子對簧片主頻之影響,利用各種不同的自製吹嘴,改變風速、角度、渦流…等,找出可能使簧片改變頻率的氣流因素。實驗結果發現改變風速會影響簧片主頻的變化,風速越大,頻率越大,為一條平滑線。但並非一直都會上升,當簧片頻率上升至某一極限,便無法再利用風速使頻率上升。例如實驗四吸音標準狀態下,風速大於8 Kt 後,頻率一直停在429Hz。在外加障礙物時(模擬吹奏舌頭時隆起)和標準狀態(正常零度入射)下頻率比較吹音和吸音有明顯的差異。吹音時,同風速下,其頻率比標準狀態高,發生音升;吸音時,同風速下,其頻率比標準狀態低,發生音降,具應用性。我們發現在頻率改變時,簧片的振動型態會有所不同,所以利用高畫素像機拍攝和電腦相位差算出簧片之曲折點至尾端的距離,發現頻率和簧片之曲折點至尾端的距離成正向關係。如實驗五中頻率從414 至419Hz,簧片的曲折點到振動端距離也明顯變大。我們發現吹嘴和口琴只要稍有一點空隙(大約在0.2cm 左右),便會和完全吻合時有顯著的頻率差距(吻合後大約比有空隙低20Hz 左右),此實驗頻率變化現象和現實壓音頻率變化極為相近。實驗過程中發現,改變簧片吹嘴的吻合程度,吹入口琴的風速相近,但頻率變化卻也有壓音的音頻變化。在實驗三加入各種氣流因子發現入射角度和標準情形差異不明顯,因此推論壓音的頻率變化和風力強度、入射角度關係不大,壓音主要為渦流所造成的現象。簧片振動模式改變,導致簧片振動頻率發生變化,且簧片的自然頻率不變。當壓音產生時,氣流在振動面造成妨礙簧片振動的抗力,但琴格內部同時也給簧片的風壓,使簧片產生一種非自然振動的頻率。The theme of the research is to explore the changes on its timbre and frequency after the harmonica reed is influenced by each kind of air current factor .In the past ,most people think no matter how to play the reed ,the frequency it produced was supposed to be the same. But in fact the frequency will be changeable under different direction, turbulent flow and air intension by the perform. First to explore the basic feature of harmonica reed, for example: The length, the width, thickness cause the influence on the frequency. To make the self-made harmonica, using the transformer control air feeder wind speed. To discussion the influenced caused by air current factors,and use each kind of different self-restraint to boast, change the wind speed, angle, turbulent flow ,in order to discover possible factors the reed causes to change the frequency of the air current factor. The experimental result discovered the change of wind speed can affect the change of basic frequency , the stronger speed cause the bigger frequency, It will be a curve. But it will not be rising continuously, when the reed frequency rise to some limit, it is unable to cause the frequency rise again by using the wind speed. For example experiment four sound absorption standard conditions, after the wind speed is higher than 8 Kt, the frequency continuously stops in 429Hz. To compare obstacle (simulation plays when tongue sticks out) and the standard condition (normal zero degree incidence) , comparison blows the sound agreement sound absorption to have the obvious difference. When blows the sound, under the same wind speed, its frequency is higher than the standard condition, has the sound to rise; When sound absorption, under the same wind speed, its frequency is lower than the standard condition, has the sound to fall. The harmonica terminology for presses the sound, extremely has the application. We discovered when frequency change, the reed vibration condition have differently, therefore use the camera photography and the computer phase different figures out the reed winding point to the end distance, discovered the frequency and the reed winding point relate to end distance is being connected. If tests five medium frequencies from 414 to 419Hz, the reed winding point is away from to the vibration end also obviously changes . The different reed vibration condition cause the frequency to change. Natural frequency is constant. When cause “bending” (the frequency is lower than the standard condition), the airflow make a force keep from reed vibration. But the chamber air pressure still drive reed. therefore cause the reed to give off not natural frequency sound