🌊 Key Characteristics | 主要特徵

• Longitudinal Wave: The particles of the medium vibrate parallel to the direction of wave travel, forming compressions (high pressure) and rarefactions (low pressure). (縱波：介質粒子的振動方向與波的傳播方向平行，形成密部和疏部。)
• Requires a Medium: Sound cannot travel through a vacuum. It needs a solid, liquid, or gas to propagate. (需要介質：聲音不能在真空中傳播，必須依賴固體、液體或氣體。)
• Speed of Sound: Sound travels fastest in solids, slower in liquids, and slowest in gases. (e.g., \( v_{solid} > v_{liquid} > v_{gas} \)). In air at room temperature, it is about \( 340 \text{ m s}^{-1} \). (聲速：在固體中最快，液體次之，氣體中最慢。在室溫空氣中約為 340 m/s。)

🎵 Three Characteristics | 三大特徵

• Loudness vs Amplitude: The loudness of a sound depends on its amplitude. A larger amplitude produces a louder sound. (響度與振幅：聲音的響度取決於其振幅。振幅越大，聲音越響。)
• Pitch vs Frequency: The pitch of a sound depends on its frequency. A higher frequency produces a higher pitch (e.g., a squeaky voice). (音調與頻率：聲音的音調取決於其頻率。頻率越高，音調越高。)
• Sound Quality (Timbre) vs Waveform: The quality of a sound depends on its waveform. This allows us to distinguish between different instruments (like a piano and a violin) playing the same note at the same loudness. (音色與波形：聲音的音色取決於其波形。這使我們能夠區分以相同響度彈奏相同音符的不同樂器。)

🦇 What is Ultrasound? | 什麼是超聲波？

Ultrasound refers to sound waves with a frequency higher than 20,000 Hz. Because the frequency is too high, it is inaudible to humans, though some animals like bats and dolphins can hear and use it. 超聲波是指頻率高於 20,000 Hz 的聲波。由於頻率太高，人類無法聽見，但蝙蝠和海豚等動物可以聽見並使用它。

🚢 1. SONAR (Echolocation) | 聲納 (回聲定位)

Ships use ultrasound to measure the depth of the ocean or detect submarines/fish. A pulse is sent down, and the time taken for the echo to return is measured. 船隻利用超聲波測量海洋深度或探測潛艇/魚群。發射脈衝後，測量回聲返回所需的時間。

(Where \( d \) is depth, \( v \) is speed of sound in water, and \( t \) is the total round-trip time. 除以2是因為時間包含了去程和回程。)

🏥 2. Medical Imaging | 醫學影像

• Prenatal Scans: Ultrasound is used to examine fetuses in the womb. It is much safer than X-rays because it does not use ionizing radiation. (產前檢查：超聲波用於檢查子宮內的胎兒。它比X射線安全得多，因為它不使用電離輻射。)
• Detecting Flaws: Similar principles are used in industry to detect cracks inside metal pipes or airplane parts without breaking them. (無損檢測：工業上利用類似原理檢測金屬管或飛機零件內部的裂紋。)

✨ 3. Ultrasonic Cleaning | 超聲波清洗

High-frequency vibrations in a liquid create tiny bubbles that rapidly expand and collapse (cavitation). This process effectively knocks dirt off small or intricate items like jewelry, watches, and spectacles. 液體中的高頻振動會產生微小氣泡，這些氣泡迅速膨脹並破裂（空化效應）。這個過程能有效清除珠寶、手錶和眼鏡等微小或複雜物品上的污垢。