CAIE A-Level Physics 24.1.6 Ultrasound Attenuation

Practise using I = I₀e^(−μx) to calculate ultrasound attenuation or μ and explain reduced transmitted intensity through tissue and boundaries.

Exam points

use I = I₀e⁻μx to calculate ultrasound attenuation or the attenuation coefficient
explain reduced transmitted intensity using attenuation and repeated boundary reflections

[Maximum number: 4]

The linear attenuation (absorption) coefficient for a parallel beam of ultrasound waves in air is $1.2 \mathrm{~cm}^{-1}$ .

The parallel beam passes through a layer of air of thickness 3.5 cm .
Calculate the ratio, in dB,
intensity of beam after passing through the layer of air intensity of beam entering the layer of air
ratio = dB

$I=I_{0} \mathrm{e}^{-\mu x}$

C1

$\begin{aligned} I / I_{0} =\mathrm{e}^{-1.2 \times 3.5} =0.015 \end{aligned}$

C1

ratio $/ \mathrm{dB}=-10 \lg (1 / 0.015)$ or $10 \lg (0.015)$

C1

$=-18 \mathrm{~dB}$

A1

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24.1.6—I = I0e–µx for the attenuation of ultrasound in matter question 1