Lab Analysis : Acoustic Mismatch

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1. * Ultrasound techniques are used to detect structure inside the body. A gel is used when a 1MHz transducer is placed on the skin to avoid acoustic mismatch at the skin-transducer interface. (a) Define acoustic mismatch (in this particular case). Acoustic mismatch, by definition, is the discrepancy between the acoustic impedances of two or more mediums (MacLennan, 2006). This occurs when a propagated soundwave, passing through one medium, travels into another medium of unequal impedance. In the context of ultrasounds, the soundwaves emitted by the transducer are affected by changes in acoustic impedance, this is acoustic mismatch. Much like any other forms of mechanical waves, a reflection of these waves occurs at interfaces and boundaries between mediums (most likely skin/muscle or organ/fluid in this case). In cases of high acoustic mismatch, reflections are so severe that a readable ultrasound is improbable. (b) Describe what would happen if air was between the transducer and the skin. Acoustic impedance is the resistance a substance’s molecules have to a change in their state of mechanical vibration. A material with a high acoustic impedance, such as bone, will be much more resistant to a change in vibration than a material with a low acoustic impedance, such as air or water. Air, having an acoustic impedance much lower than an (c) If the density of skin was 1010 kg/m3 and the velocity of sound through the skin was 1540 m/s, calculate the optimum acoustic impedance

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