Ultrasonic Waves Overview

Ultrasonic Waves

• Sound waves with frequencies above the human audible range (greater than 20,000 Hz)
• Often referred to as high-frequency waves
• Used by bats for echolocation
• Employed by many animals, including whales and dolphins.
• Rhinoceroses use infrasonic frequencies for communication

Sound Waves

• A form of energy that travels through a medium
• Longitudinal waves – the particle motion is parallel to the direction of propagation
• Travels from the source in all directions
• Wavelength (λ): Distance between successive compressions or rarefactions
• Frequency determines type with 3 Categories
• Infrasound (0-20 Hz)
• Audible sound (20–20,000 Hz)
• Ultrasound (>20,000 to 5MHz)

Properties of Ultrasonic Waves

• High energy content
• Reflect, refract, and absorb like ordinary sound waves
• Negligible diffraction due to short wavelength, allowing transmission over long distances with less energy loss
• Intense ultrasonic waves can create bubbles in liquids
• Can form stationary waves that act like diffraction gratings (acoustic gratings) in liquids

Applications of Ultrasonic Waves

• Medical imaging (sonography): Visualizes internal organs; fetal ultrasound for prenatal care
• Industrial applications: Detecting flaws in metals (Non-Destructive Testing - NDT)
• Cleaning: Cleaning hard-to-reach parts on machines, or in various parts of electronic assemblies (watches, armatures)
• Drilling: Used to make holes in hard materials like glass
• Soldering: Soldering metals that are typically hard to solder on their own (aluminum), by melting solder on the metal and removing metal oxide layers
• SONAR (Sound Navigation and Ranging): Detecting and locating underwater objects/targets.

Methods of Ultrasonic Production

• Magnetostriction Method:

  • Applying a magnetic field to a ferromagnetic rod causes it to elongate or contract, producing ultrasonic waves.
  • The frequency of the vibrations is based on the length, Young's modulus, and density of the material.
  • Generates low frequency ultrasonic waves
  • Advantages: Simple design, low cost, high power output at low frequencies
  • Disadvantages: Low upper frequency limit, affected by temperature, energy loss due to hysteresis and eddy currents

• Piezoelectric Method:

  • Applying mechanical pressure to certain crystals (like quartz) generates electrical charges across other faces.
  • Applying an electric field causes a change in crystal dimensions.
  • Advantages: Can produce high-frequency (up to 500 MHz) ultrasonic waves; unaffected by temperature and humidity
  • Disadvantages: Expensive crystal materials; complex cutting and shaping of quartz crystals;

Detection Methods

• Piezoelectric Detector: Ultrasonic waves generate voltage differences across the piezoelectric materials, which can then be amplified.

• Kundt's Tube Method: A tube with powder sprinkled in it, where the powder collects at the nodes and is blown away at the antinodes based on the waves produced.

• Sensitive Flame Method: A narrow flame reacts to pressure changes in the medium, allowing the location of nodes and antinodes to be determined.

• Thermal Detector: Observing the changes in resistance of a fine platinum wire within the medium from alternating compression and rarefaction during the passing of ultrasonic waves.