A-LevelPhysicsMedical physics9702/41 · May/June 2020 · Q47 Marks

By reference to an ultrasound wave, explain what is meant by specific acoustic impedance.

(a)(i)By reference to an ultrasound wave, explain what is meant by specific acoustic impedance.[2]
(a)(ii)An ultrasound wave is incident normally on the boundary between two media. The media have specific acoustic impedances Z₁ and Z₂. State how the ratio intensity of ultrasound reflected from boundary intensity of ultrasound incident on boundary depends on the relative magnitudes of Z₁ and Z₂.[2]
(b)(i)State what is meant by the attenuation of an ultrasound wave.[1]
(b)(ii)A parallel beam of ultrasound is passing through a medium. The incident intensity I₀ is reduced to 0.35 I₀ on passing through a thickness of 0.046 m of the medium. Calculate the linear attenuation coefficient μ of the ultrasound beam in the medium.[2]

Mark scheme

Marking points from the official mark scheme (May/June 2020). The mark or mark type is shown in brackets.

[M1] product of density and speed
[A1] speed of ultrasound in medium
[B1] the greater the difference between Z₁ and Z₂, the closer the ratio is to 1 / if difference between Z₁ and Z₂ large, ratio is close to 1
[B1] the closer together Z₁ and Z₂, the closer the ratio is to 0 / if difference between Z₁ and Z₂ small, ratio close to 0
[B1] loss of intensity/amplitude/power (of the wave)
[C1] I = I₀ e⁻ᵤₓ
[A1] 0.35 = e⁻⁰.⁰⁴⁶ᵤ AND μ = 23 m⁻¹

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