Echoes and sonar - CCEA

• The display of different sound waves as traces on a cathode ray oscilloscope (CRO)

• Amplitude (loudness) of a sound.

• Frequency (pitch) of a sound.

• The range and limit of human hearing.

• The uses of ultrasound.

• Echo calculation in industry and medicine.

Sound waves are longitudinal waves.

They cause particles to vibrate parallel to the direction of wave travel.

The vibrations can travel through solids, liquids or gases.

The speed of sound depends on the medium through which it is travelling.

When travelling through air, the speed of sound is about 340 m/s.

Sound cannot travel through a vacuum because there are no particles to carry the vibrations.

Oscilloscope traces are a visual way to display sound waves.

The amplitude of a sound wave is related to the volume of the sound:

• Small amplitude sound waves are quiet.

• Large amplitude sound waves are loud.

The frequency of a sound wave is related to the pitch that is heard:

• High frequency sound waves are high pitched.

• Low frequency sound waves are low pitched.

Humans can only hear certain frequencies.

The range of normal human hearing is 20 Hz to 20,000 Hz (or 20 Hz to 20 kHz).

Sound waves can reflect off surfaces.

We hear reflected sound waves as echoes.

Key fact

Remember in echo, the sound has travelled to the object and back again. To calculate the distance to the object use half of the time (or calculate half the total distance there and back).

Ultrasound waves are sound waves which have a frequencyhigher than the upper limit for human hearing - above 20,000 Hz.

Ultrasound waves are longitudinal because they are simply high frequency sound waves i.e. above 20 kHz.

Different species of animal have different hearing ranges.

This explains why a dog can hear the ultrasound produced by a dog whistle, but humans cannot.

Ultrasound imaging creates a picture of something that cannot be seen directly, such as an unborn baby in the womb (a foetus), or faults and defects inside metals.

These uses rely on what happens when ultrasound waves meet the boundary between two different materials.

What are the medical uses of ultrasound?

The best known example of the use of ultrasound is medical imaging.

An ultrasound scanner is simply run over the skin to obtain an image of what's inside.

The scanner probe has a built-in transducer that directs ultrasound pulses down into the body.

As the waves travel through the different bones and tissues, some are reflected off a boundary but some travel further to be reflected back up again off another boundary, as some parts have different densities.

• Some of the ultrasound waves are reflected at a boundary.

• The time taken for the waves to leave a source and return to the detector is measured.

• The depth of the boundary can be determined using distance = speed of sound in the material x the time taken.

The probe receives the reflected waves and a computer connected to the scanner uses them to draw an image on a screen.

Scans of foetuses (unborn babies developing in the womb) are made this way and are used, for example, to measure the diameter of the head of a foetus so that growth can be monitored.

Ultrasound imaging also helps to diagnose problems with the:

• heart;

• kidneys;

• blood vessels;

• bladder.

What are the advantages of using ultrasound in medicine?

• Ultrasound waves are non-ionising, they pass through tissue without causing harm, unlike x-rays which can damage DNA inside cells.

• Ultrasound equipment is relatively cheap, portable and easy to use.

• Images of internal organs can be seen without having to operate on patients.

Detecting defects in metals

Ultrasound can be used in industry to detect defects in metals.

Materials can be tested for internal faults and cracks that could lead to the failure of a structure under certain conditions.

Ultrasound imaging provides a quick method of detection and perhaps prevents serious accidents.

Cleaning jewellery

Ultrasound can be used to clean jewellery.

The vibrations caused by the ultrasound shake apart the dirt, breaking it up.

The principle is the same as the opera singer's trick, where a glass may shatter if the singer makes a high-pitched sound near to the glass which causes it to vibrate with large amplitude.

High frequency sound waves can be used to detect objects in deep water and to measure water depth.

The time between a pulse of sound being transmitted and detected and the speed of sound in water can be used to calculate the distance of the reflecting surface or object.

The process is very similar to ultrasound imaging.

For deep water, 50 kHz is the preferred frequency of the ultrasound.

This is because water absorbs sound waves at a slower rate than for lower frequencies and so the signal can travel farther before becoming too weak to use.

This technique is applied in sonar systems used to measure the depth of the seabed and to find shipwrecks, submarines and shoals of fish.

SONAR stands for SOund Navigation And Ranging.

Bats and dolphins use a similar method, called echolocation, to detect their surroundings and to find food.

Example

A sonar system on a boat sends an ultrasound pulse towards the seabed.

The pulse is reflected, and it is detected 0.1 s later by the system.

Calculate the depth of water if the speed of sound in water is 1,480 m/s.

Answer

Distance = speed × time

Speed = 1,480 m/s

Time for ultrasound to travel to seabed and back again = 0.1 s

Time for ultrasound to travel to seabed = 0.1 s ÷ 2 = 0.05 s

Distance to seabed = 1,480 × 0.05 = 74 m

The depth of water is 74 m.

Questions

Ultrasound can be used to measure the diameter of the head of a baby in the womb.

When ultrasound reaches the baby’s head at A, some ultrasound is reflected back to the detector and produces pulse A on the CRO.

Some ultrasound passes through the head to point B, and is reflected back to the detector.