Refraction of Ultrasound Waves--The Secret behind XY-TEK's Clamp-on Ultrasonic Flow Sensors

Many ultrasonic devices, such as our clamp-on ultrasonic flow sensors, operate based on the refraction of ultrasound waves. But what is refraction? The article here offers you a glimpse of refraction.

  Refraction of Ultrasound Waves

Refraction, or ray bending, occurs when ultrasound waves travel through an interface between media of different sound speeds at an oblique angle. This is the same behavior that can be observed with light.   

The Refracted Wave Propagation Angles

The refracted wave propagation angles can be calculated using Snell’s Law. Snell’s Law describes the relationship between the wave velocities (c₁, c₂) and propagation angles (θ₁, θ₂), as shown in the diagram and equation below.

One Calculation of a Refracted Wave Propagation Angle

For example, when ultrasound waves cross through the interface between an Acrylic plastic block and water, refraction angle can be calculated using Snell’s Law. The sound speed of Acrylic plastic is 2670 m/s, and the sound speed of water is 1496 m/s.  When the incident angle θ₁ is 45 degrees, the refraction angle θ₂ is calculated to be 23.3 degrees.  

Refraction, the Principle behind some Ultrasonic Devices

A lot of ultrasonic devices make use of the sound beam refraction for measurement and process control, such as angle beam transducers for inspection of weld defects/cracks,  clamp-on flow sensors for non-invasive flow measurements, etc.  

2D FEA Simulation of the Ultrasound Refraction

Below is a 2D FEA simulation illustrating the ultrasound refraction.

The Setup

An ultrasonic transducer is mounted on an Acrylic wedge with a 45 degree angle, and the Acrylic wedge is pressed on an Acrylic block with water under it.  

The Process:

When the ultrasonic transducer is excited by short voltage bursts at its resonant frequency, it vibrates and generates ultrasonic waves.  The ultrasonic waves propagate first in the Acrylic block without changing its direction, but refraction can be clearly observed at the interface between Acrylic and water. The ultrasound wavelength also becomes smaller after entering from Acrylic into water, since water has lower sound speed.

In case you would like to view the whole process clearly. Below is the video of the 2D FEA Simulation of the Ultrasound Refraction:

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