（b）A parallel ultrasound beam is incident on the boundary between two media，as illustrated in Fig．10．1． The media have specific acoustic impedances $$\(Z_{1}\)$$ and $$\(Z_{2}\)$$ ． At the boundary，a fraction $$\(\alpha\)$$ of the incident intensity of the ultrasound beam is reflected．The remainder is transmitted． （i）State what is meant by specific acoustic impedance． . . ［2］ （ii）Describe how $$\(\alpha\)$$ depends on the relative values of $$\(Z_{1}\)$$ and $$\(Z_{2}\)$$ ． . . . ［2］

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