Section 1 of 5What Ultrasound Is
Ultrasound is a sound wave whose frequency is too high for humans to hear.
Definition
- Definition Ultrasound is sound with a frequency above 20 000 Hz, which is higher than the upper limit of human hearing.
Mechanical wave
- Mechanical wave Ultrasound is a longitudinal wave that needs a medium to travel through, so it cannot pass through a vacuum.
Production
- Production A piezoelectric transducer produces ultrasound when an alternating voltage makes its crystal change shape rapidly.
Detection
- Detection The same kind of transducer detects returning ultrasound, converting the waves into electrical signals.
Speed
- Speed Ultrasound travels at the same speed as sound of the same frequency in the same medium, and that speed depends on the material.
Section 2 of 5Behaviour at Boundaries
When ultrasound reaches a boundary between two materials, part of the wave is reflected.
1
A pulse of ultrasound is sent into a material by a transducer.
2
The wave is partially reflected at each boundary between two materials with different densities.
3
The reflected pulse returns to the transducer and is converted into an electrical signal.
4
The time between sending the pulse and receiving the echo shows how far away the boundary is.
5
This principle is applied in echo sounding and in medical and industrial imaging.
| Medium | Approximate speed of ultrasound (m/s) |
|---|---|
| Air | 330 |
| Water | 1500 |
| Soft body tissue | 1500 |
Section 3 of 5Uses of Ultrasound
Ultrasound is useful because it can pass into materials and reflect from hidden boundaries.
Uses of Ultrasound
Cleaning
- Cleaning High-frequency vibrations in a liquid loosen dirt from delicate objects such as jewellery and surgical instruments.
Flaw detection
- Flaw detection Ultrasound finds cracks inside metal parts, because waves are reflected at the edge of the flaw.
Medical imaging
- Medical imaging Ultrasound scans create images of soft tissue, including a developing baby in the womb, without using ionising radiation.
Safety
- Safety Ultrasound is non-ionising, but very high intensities can heat tissue, so exposure is controlled in practice.
Section 4 of 5Worked Example
Question
- Ultrasound of frequency 50 000 Hz travels through water at a speed of 1500 m/s.
- Calculate its wavelength.
- Wave equationspeed = frequency × wavelength, so wavelength λ = v ÷ f.
- λ = 1500 m/s ÷ 50 000 Hz = 0.030 m.
- 0.030 m = 3.0 cm.
Answer The wavelength of the ultrasound is 0.030 m, or 3.0 cm.
Section 5 of 5Exam Checklist & Tips
6.1.11 – Key Points for the Exam
- Ultrasound Sound waves with a frequency above 20 000 Hz.
- Transducer A device that converts electrical signals into waves, and waves back into electrical signals.
- Piezoelectric effect The change in shape of certain crystals when an alternating voltage is applied across them.
- Longitudinal wave A wave in which the vibrations are parallel to the direction of energy transfer.
- Reflection The bouncing back of a wave at a boundary between two materials.
Common Mistakes
- Describing ultrasound as a type of electromagnetic wave; it is a mechanical sound wave.
- Confusing the audible range with the ultrasound range; ultrasound is above about 20 000 Hz, not 20 Hz.
- Forgetting to convert centimetres to metres before using the wave equation with SI units.
- Believing ultrasound can travel through a vacuum, which is impossible for a mechanical wave.