A-LevelPhysicsMedical physics9702/42 · May/June 2019 · Q47 Marks

State what is meant by the specific acoustic impedance of a medium.

(a)State what is meant by the specific acoustic impedance of a medium.[2]
(b)A parallel beam of ultrasound of intensity I₀ is incident on the boundary between two media A and B, as illustrated in Fig. 4.1. The two media A and B have specific acoustic impedances Z_A and Z_B respectively. The intensity of the beam transmitted through the boundary is I_T. State how the ratio intensity I_T of transmitted beam -------------------------------------------------------------- intensity I₀ of incident beam depends on the relative magnitudes of Z_A and Z_B.[2]
(c)The linear absorption (attenuation) coefficient μ of medium B is 23 m⁻¹. Calculate the thickness of medium B required to reduce the intensity of the ultrasound beam to 34% of its initial intensity in medium B. thickness = ................................................................ m[3]

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Mark scheme

Marking points from the official mark scheme (May/June 2019). The mark or mark type is shown in brackets.

4(a) product of density and speed M1 speed of sound in medium A1
4(b) Any two from: • if ZA >> ZB then ratio is (nearly) zero or if ZB >> ZA then ratio is (nearly) zero or if ZB and ZA are very different then ratio is (nearly) zero or the greater the difference the lower the ratio • if ZA ≈ ZB then ratio is (nearly) 1 or if ZA = ZB then ratio is 1 or the smaller the difference the closer the ratio to 1 (not 'large') • IT / Io = 1 - [(ZA – ZB)2 / (ZA + ZB)²] B2
4(c) I = Ioe-ux C1 0.34 = exp(-23 × x) C1 x = 0.047 m A1

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