Âm thanh = theo chiều dọc, các hạt sóng cơ học di chuyển song song với hướng của âm thanh du lịch âm thanh kHz Audible 20 không có thể đi du lịch thông qua Four chân không Mật độ biến âm thanh (g / l) áp (kPa) Nhiệt độ (K) Hạt chuyển động (m) nén: | 1 Physics of ultrasound Basic principles Nature of ultrasound Sound longitudinal mechanical wave particles move parallel to direction of travel Audible sound 20 kHz Ultrasound 20 kHz Sound cannot travel through a vacuum Four acoustic variables Density g l Pressure kPa Temperature K Particle motion m Compressions high density pressure temperature motion Rarefactions low density pressure temperature motion Fig. Transthoracic echo TTE 2-5 MHz Transoesophageal echo TOE MHz Sound is described by Propagation speed m s Frequency Hz Wavelength m Physics of ultrasound 3 Table Speed of sound in different media Tissue Speed of sound m s Air 331 Lung 500 Fat 1450 Brain 1541 Liver 1549 Muscle 1585 Bone 3000 Frequency f f number of cycles per second Units Hz U S 20 kHz Determined by sound source Affects penetration and axial resolution Period T T length of time to complete one cycle Units s U S ps Determined by sound source Reciprocal of frequency T 1 f Wavelength k k distance occupied by a single cycle Units m U S mm Determined by sound source and medium k influences axial resolution Velocity v frequency f and wavelength k associated by the equation v .
