Ultrasound — Cambridge IGCSE Co-ordinated Sciences 0654 - Physics Revision Notes & Topical Questions

Section 1 of 10What ultrasound is

Hearing range

• Hearing range humans hear about 20 Hz to 20 000 Hz and the range decreases with age

Ultrasound

• Ultrasound sound with a frequency higher than 20 kHz

Transducers

• Transducers emit and receive ultrasound, and the received signal is displayed on an oscilloscope

Behaviour

• Behaviour partially reflected at boundaries between media, with the rest transmitted

Section 2 of 10Measuring depth

Method

• Method use the time between the ultrasound being emitted and received and its speed in the material

Equation

• Equation depth = ½ × speed × time
• Time is the total time from emission to receipt
• Speed is the speed of the ultrasound in the medium

Why halve

• Why halve the total distance is twice the depth

Section 3 of 10Sonar

Echo sounding

• Echo sounding a pulse is sent down from the surface and reflected off the sea bed

Depth

• Depth found from the time to return, and the distance travelled is twice the depth

Other uses

• Other uses detect objects underwater such as shoals, wrecks and submarines

Section 4 of 10Material testing

Use

• Use check for cracks inside metal objects and generate images beneath surfaces

Cracks

• Cracks waves reflect back from a crack earlier than waves that continue through the object

Oscilloscope

• Oscilloscope reflected waves show as pulses, and a bolt with no crack gives only two pulses at its start and end

Section 5 of 10Medical uses

Imaging

• Imaging foetus in the womb and 2D images of organs, as long as they are not surrounded by bone

Method

• Method waves are reflected at boundaries between tissues such as fluid and soft tissue or tissue and bone
• Sweeping across an area builds up an image from the time measurements

Treatment

• Treatment removing kidney stones

Safety

• Safety non-invasive and believed to be harmless, unlike many other imaging methods

Section 6 of 10Why ultrasound is useful

Short wavelength

• Short wavelength detects small features and forms narrow beams with little diffraction

Non-ionising

• Non-ionising safe for medical use at low intensity

Gel

• Gel applied to the skin so there is no air gap, since air reflects almost all ultrasound

Section 7 of 10Cleaning

Cleaning tanks

• Cleaning tanks ultrasound vibrations shake dirt loose from jewellery, instruments and electronics

Reach

• Reach cleans crevices a brush cannot

Section 8 of 10More from the specification

1Longitudinal Sound Waves

• Sound propagates as a longitudinal mechanical wave through solid, liquid,
• or gas media, consisting of alternating regions of high pressure (compressions) and low pressure (rarefactions) parallel to wave travel.

2Measuring the Speed of Sound

• In a practical acoustics experiment, the time taken for sound waves to travel a measured distance between a noise source and a detector or listener is recorded and averaged to calculate speed (v = d / t).

3Pitch, Loudness, and Wave Parameters

• The pitch of a sound is directly determined by the frequency of the acoustic wave,
  • (higher frequency corresponds to higher pitch)
• whereas loudness is governed by the wave amplitude.

4Properties and Uses of Ultrasound

• Ultrasound consists of acoustic sound waves with frequencies higher than the upper limit of human hearing, utilized in non-destructive testing, sonar ranging,
  • (greater than 20 kHz)
• and medical imaging.

5Control variable

• A quantity kept the same to make the test fair

6Average speed

• Total distance divided by total time.

7Compression

• Decrease in length under a squashing force.

8Rarefaction

• Region of a longitudinal wave where particles are spread apart

9Loudness

• How loud a sound is, set by its amplitude

10Pressure

• Force per unit area, P = F/A.
  What the symbols mean: p = pressure (Pa = N m⁻²) · F = force acting at right angles to the surface (N) · A = area (m²)

11Microphone

• Device that converts sound into an electrical signal

12Longitudinal wave

• Wave whose vibrations are parallel to the direction of travel

13Independent variable

• The variable deliberately changed, here time

14Dependent variable

• The variable measured, here temperature

Section 9 of 10Key terms

Term	Meaning
Ultrasound	Sound with a frequency above 20 kHz
Transducer	Device that emits and receives ultrasound
Sonar	Use of reflected sound to measure depth or find objects underwater
Non-invasive	Not requiring entry into the body
Boundary	Surface between two media at which waves are partially reflected

Section 10 of 10Exam technique

1

Forgetting the factor of one half — depth is half the total distance travelled.

2

Saying ultrasound is harmful like X-rays — it is non-ionising and believed harmless.

3

Using the speed of sound in air for tissue or steel — use the speed in that medium.

4

Saying ultrasound is a transverse wave — it is longitudinal like all sound.