Consider the following statements: Statement I: When an ultrasound travels from one medium to the other medium, its wavelength, frequency and speed change. Statement II: The frequency of an ultrasound wave remains almost unchanged while travelling from one medium to the other medium. Statement III: The frequency and speed of an ultrasound wave change when travelling from one medium to the other medium. Statement IV: ‘Echocardiography’- a technique used in medical diagnostics, employs ultrasound. Which of the following combination of above statements are true?

Sound waves

• Longitudinal waves are sound waves. They cause waves to travel in the same direction as the vibration of particles.
• Vibrations can travel through gas, liquid, or solid materials.
• The medium through which sound travels determines its speed.
• The speed at which sound travels through air is approximately 330 meters per second (m/s).
• There are no particles in a vacuum to carry the vibrations, so sound cannot travel through it.

Ultrasound

• Ultrasound is sound with frequencies greater than 20 kilohertz.
• This frequency is the approximate upper audible limit of human hearing in healthy young adults.
• Ultrasonic devices operate with frequencies from 20 kHz up to several gigahertz

Refraction

• The velocity (or speed), frequency, and wavelength of sound waves will change as they move from one medium to another. Refraction, also known as this change in velocity, can also cause the sound wave to change direction.
• For example, refraction occurs when sound travels from warm air into cold air. When this happens:
  • the wavelength of the sound wave decreases.
  • the frequency of the sound wave stays the same.
  • the velocity of the sound wave decreases (since wave speed = frequency × wavelength).
• When a sound wave goes from one medium to another, the frequency remains unchanged. The relation between wavelength, frequency, and speed v is ⇒ v = f x λ. The velocity of sound depends upon the elasticity and inertia of the medium. So, when a particle moves from one medium to another its amplitude increases or decreases, depending upon the medium. So, when a sound wave travelling from one medium enters another medium, then the quantities that changes are velocity and wavelength. Frequency does not change. Hence statement I and III is incorrect and II is correct.

Echocardiography

• Echocardiography is a diagnostic test that uses ultrasound waves to create an image of the heart muscle. Hence statement IV is correct.
• Ultrasound waves that rebound or echo off the heart can show the size, shape, and movement of the heart's valves and chambers as well as the flow of blood through the heart.
• Echocardiography may show such abnormalities as poorly functioning heart valves or damage to the heart tissue from a past heart attack.