Unit 1 Physics Detailed Study 3.6 Chapter 15.1: Ultrasound and how it is made. Chapter 15.2: Ultrasound Interactions

Section 15.1 Ultrasound and how it is made Sound is an example of energy travelling in the form of a wave Humans can hear sound waves in the frequency range 20-20 000 Hz. Tone generator Demo! NCH Tone Generator 2

Section 15.1 Ultrasound and how it is made Infrasound frequencies of sound below our hearing range (<20 Hz). Ultrasound frequencies beyond our hearing range (> 20 000 Hz)

Section 15.1 Ultrasound and how it is made Ultrasonography is the use of ultrasound in medical and industrial applications. In medical applications ultrasound is used in two different modes: diagnostic aid - the production of images of internal parts of the body. therapeutic - used to treat an ailment such as kidney stones or to carry out delicate brain surgery.

Section 15.1 Ultrasound and how it is made Diagnostic medical Ultrasound Relies on the reflection of sound waves. When sound enters a medium (substance) some of the energy will be: Reflected Transmitted and Absorbed Different types of tissue will reflect the waves, a computer can analyse the time taken to construct an image.

Section 15.1 Ultrasound and how it is made Sound is a longitudinal mechanical wave Mechanical waves involve the transfer of energy without the net transfer of matter. The particles vibrate within the medium allowing the energy to travel.

Section 15.1 Ultrasound and how it is made Sound is a longitudinal mechanical wave

Section 15.1 Ultrasound and how it is made Sound is a longitudinal mechanical wave

Section 15.1 Ultrasound and how it is made Below is a similar animation of a transverse wave

Section 15.1 Ultrasound and how it is made Waves (mechanical) Waves can be transverse Particles travel perpendicular to the direction of travel of the wave In this section we do not care about this! 10

Section 15.1 Ultrasound and how it is made Waves (mechanical) Waves can be longitudinal Particles travel parallel to the direction of travel of the wave Sound waves are longitudinal waves 11

Section 15.1 Ultrasound and how it is made Sound is a longitudinal mechanical wave All sound waves must have a vibrating item at their source. Examples: Feel your throat (larynx) when you speak Stereo speakers (doof doof) Ultrasound technician holds a transducer (device) containing vibrating crystals

Section 15.1 Ultrasound and how it is made Properties of (sound) waves Frequency, f is the frequency of the vibrating source (measured in hertz) Period, T is the time taken for one cycle to be completed f = 1/T T = 1/f

Section 15.1 Ultrasound and how it is made Sound is a longitudinal mechanical wave As a sound wave travels regions of high pressure (compressions) and areas of low pressure (rarefactions) are created.

Section 15.1 Ultrasound and how it is made Properties of (sound) waves Wavelength, λ Distance between two points with same displacement Amplitude, A Relates to the amount of energy the wave has.

Section 15.1 Ultrasound and how it is made Producing Ultrasound In a diagnostic ultrasound the sound waves are both generated and detected by a small handheld probe. In response to sound waves the transducer produces an alternating electrical signal due the piezoelectric effect.

Section 15.1 Ultrasound and how it is made Producing Ultrasound

Section 15.2 Ultrasound interactions Intensity Sound waves carry energy through a medium and so the amount of energy passing through a square metre area each second is called the intensity of the sound wave. Intensity is therefore the power (in watts) passing through a square metre of area. The unit of intensity is watts per square metre (W m-2).

Section 15.2 Ultrasound interactions Attenuation As a sound wave is transmitted through a medium some of its energy will be absorbed by the particles of the medium. Much of this absorbed energy contributes to an increase in temperature. The gradual reduction in the intensity of a sound wave as it travels away from its source is called the attenuation The amount of attenuation as a sound wave travels through a given medium depends largely on the physical properties of the medium.

Section 15.2 Ultrasound interactions Impedence Since the passage of a sound wave through a medium involves the passing on of vibrations, it can be said that all media will hinder the progress of the wave to some extent. The level of efficiency with which sound waves can pass through a given medium is called the acoustic impedence Acoustic impedance is strongly related to the speed at which sound can travel in the medium. The density of the medium is also a key factor.