Physics 9702 · AS & A Level · Production and use of ultrasound

Production and use of ultrasound — practice question

(a)[2]

By using ultrasound waves, state what the specific acoustic impedance of a medium means.

(b(i))[2]

The intensity reflection coefficient $\alpha$ for two media with specific acoustic impedances $Z_1$ and $Z_2$ is given by $\alpha = \dfrac{(Z_1 - Z_2)^2}{(Z_1 + Z_2)^2}$. Calculate the fraction of the ultrasound intensity that is reflected at the muscle-bone boundary.

(b(ii))[3]

Calculate the fraction of the ultrasound intensity that passes through $3.4\,\text{cm}$ of muscle.

(b(iii))[2]

Use your answers in (i) and (ii) to determine the ratio $\dfrac{I_R}{I}$.

(c(i))[2]

The intensity reflection coefficient $\alpha$ for two media with specific acoustic impedances $Z_1$ and $Z_2$ is given by $\alpha = \dfrac{(Z_1 - Z_2)^2}{(Z_1 + Z_2)^2}$. Calculate how much of the ultrasound intensity is reflected at the muscle-bone boundary.

(c(ii))[3]

Calculate the fraction of the ultrasound intensity that passes through a thickness of $3.4\,\text{cm}$ of muscle.

(c(iii))[2]

Use your answers in (i) and (ii) to determine the ratio $\dfrac{I_R}{I}$.

Worked solution & mark scheme

This 16-mark question has a full step-by-step worked solution and mark scheme. One marking point: Multiplication of the medium’s density by the wave speed in the medium

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