Unit 1 PhysicsDetailed Study 3.6

Chapter 15.1: Ultrasound and how it is made.

Chapter 15.2: Ultrasound Interactions

Sound∗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!

Section 15.1Ultrasound and how it is made

Sound∗Infrasound

∗frequencies of sound below our hearing range (<20 Hz).

∗Ultrasound∗frequencies beyond our hearing range (> 20

000 Hz)

Section 15.1Ultrasound and how it is made

Sound∗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 15.1Ultrasound 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 15.1Ultrasound 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 15.1Ultrasound and how it is made

Sound is a longitudinal mechanical wave

Section 15.1Ultrasound and how it is made

Sound is a longitudinal mechanical wave

Section 15.1Ultrasound and how it is made

Below is a similar animationof a transverse wave

Section 15.1Ultrasound and how it is made

Waves (mechanical)∗Waves can be transverse

∗Particles travel perpendicular to the direction of travel of the wave

Section 15.1Ultrasound and how it is made

Waves (mechanical)∗Waves can be longitudinal

∗Particles travel parallel to the direction of travel of the wave

Section 15.1Ultrasound 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 15.1Ultrasound 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 15.1Ultrasound 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 15.1Ultrasound 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 15.1Ultrasound 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 15.1Ultrasound and how it is made

Producing Ultrasound

Section 15.1Ultrasound and how it is made

IntensitySound 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 15.2Ultrasound 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 15.2Ultrasound interactions

ImpedenceSince 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.

Section 15.2Ultrasound interactions