What are the key learning points about echoes and sonar?
Recall that sound waves are A wave that moves in the same direction (parallel) as the direction in which the particles are vibrating. .
Use the equation for speed with sound waves to calculate distances.
Distinguish between sound and Sound with a frequency greater than 20,000 Hz (20 kHz)..
Use the echo principle to work out depths or distances.
State some practical applications of ultrasound in industry or medicine.
What type of waves are sound waves?
Sound waves are A wave that moves in the same direction (parallel) as the direction in which the particles are vibrating. .
They cause particles to vibrate parallel to the direction of the wave.
Image caption, How does sound travel?
When there is no sound, the air particles are still.
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The vibrations can travel through solids, liquids or gases.
The speed of sound depends on the A material through which a wave can be transmitted (propagate). through which it is travelling.
When travelling through air, the speed of sound is about 340 m/s.
Sound cannot travel through a A volume of space that contains no matter. because there are no particles to carry the vibrations.
What is the range of human hearing?
What is the range of human hearing?
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).
Any sound above 20,000 Hz is called ultrasound.
How do sound waves reflect?
Sound waves can reflect off surfaces.
We hear reflected sound waves as echoes.
Image caption, 1. The speaker emits a sound wave.
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Hard, smooth surfaces are particularly good at reflecting sound.
This is why empty rooms produce lots of echoes.
Soft, rough surfaces are good at absorbing sound.
This is why rooms with carpets and curtains do not usually produce lots of echoes.
Sound travels at a constant speed in air or water.
If we know the speed of sound and the time it takes for the echo to be detected, we can use the equation:
Speed = \(\frac{\text{distance}}{\text{time}}\) to work out distances.
Or
Distance = speed x time.
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).
Example
A fishing boat sounds its foghorn.
The echo from a nearby cliff is heard after 5 s.
If the speed of sound is 340 m/s calculate the distance between the fishing boat and the cliff.
Answer
Distance = speed x time
Speed = 340 m/s
Time for sound to travel to the cliff and back again = 5 s
Time for sound to travel to the cliff = \(5\div 2 = 2.5 s\)
Distance to the cliff = 340 m/s x 2.5 s
Distance to the cliff = 850 m
The distance between the fishing boat and the cliff is 850 m.
How to measure the speed of sound in air
Although sound travels quite fast, it is still possible to measure its speed in air.
To do this, you need to measure the time it takes a sound to travel a measured distance.
To reduce errors, particularly timing errors, you should either:
use a large distance (preferably over 100 metres), or
use an electronic timer or data logger to record the time taken.
What is the clap-echo method?
This method involves measuring the time taken for you to hear an echo from a sharp clap, either made by hands or by banging two wooden blocks together.
You stand a long distance from a wall, clap, and listen for the echo.
The distance travelled is twice the distance from you to the wall (because the sound has to travel to the wall and back).
Measure the distance to the wall using a 20 m tape measure or a trundle wheel.
Record the distance in a suitable table.
Stand beside the person who is going to clap.
Start a stop clock when the clap is made and stop it when you hear the echo.
Repeat ten times and calculate the average time to hear the echo.
Results
Distance to wall = Total distance travelled = 2 x distance to wall
Time to hear echo:
| Clap | Time in s |
|---|---|
| 1. | |
| 2. | |
| 3. | |
| 4. | |
| 5. | |
| 6. | |
| 7. | |
| 8. | |
| 9. | |
| 10. | |
| Average time |
Speed of sound = \(\frac{\text{distance}}{\text{time}}\).
Speed of sound = m/s.
The main source of error in this experiment is reaction time when starting and stopping the stop clock.
The error is reduced by repeating the timing ten times and calculating the average time.
Example
A girl stands 102 m away from a wall and claps two wooden blocks together.
She hears an echo after 0.6 s.
Calculate the speed of sound in air.
Answer
Speed = \(\frac{\text{distance}}{\text{time}}\).
The sound travels 102 m to the wall and 102 m back to the girl, so the total distance travelled by the sound = 102 + 102 = 204 m.
Time = 0.6 s.
Speed = \(\frac{204}{0.6}\)
Speed = 340 m/s.
The speed of sound in air is 340 m/s.
What is the flash-bang method? (Higher tier only)
A person fires a starting pistol.
A distant observer stood 200 metres away (measured using a 20 m tape measure or trundle wheel) records the time between seeing the flash of light from the starting pistol and hearing the sound.
The sound takes more time to cover the same distance as the light because sound travels much more slowly than light.
The speed of sound can be calculated using the equation:
Speed = \(\frac{\text{distance}}{\text{time}}\)
The main source of error in this experiment is reaction time when starting and stopping the stop clock.
The error is reduced by repeating the timing ten times and calculating the average time.
The starting pistol could be replaced by two blocks of wood as shown below.
How to carry out the flash-bang method for calculating the speed of sound
Image caption, WHAT YOU NEED: Two or three people, two blocks of wood, a stopwatch, a notepad and pen, and a large, open space.
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Example (Higher tier only)
An observer 400 m away records a 1.2 s time difference between seeing the flash of a starting pistol and hearing the bang.
Calculate the speed of sound.
Answer
Speed = \(\frac{distance}{time}\)
Distance = 400 m.
Time = 1.2 s.
Speed = \(\frac{400}{1.2}\)
Speed = 333 m/s.
The accepted value for the speed of sound in air is 340 m/s.
However, this experimental method is flawed because of human reaction time when using the stop clock.
This explains why the answer of 333 m/s is slightly below the accepted value for the speed of sound in air.
How to use microphones and a data logger (Higher tier only)
A data logger can measure and record the time taken for sound to reach two microphones.
Unlike the clap-echo method, these can be quite close together.
For example, two microphones are 3.4 m apart.
The data logger switches on when the sound from the bell reaches the first microphone, and off again when the sound reaches the second microphone.
The data logger recorded a time of 0.01 s for the sound to travel between the microphones.
Speed = \(\frac{distance}{time}\)
Speed = \(\frac{3.4}{0.01}\)
Speed of sound = 340 m/s.
This method works because there is no human reaction time involved starting and stopping the clock and because the microphones are more sensitive than human ears.
How does sound travel through different materials?
Sound travels faster through liquids and solids than it does through air and other gases.
The table gives some examples.
| Substance | Speed of sound |
|---|---|
| Air | 343 m/s |
| Water | 1493 m/s |
| Steel | 5130 m/s |
This is because the particles of gases are further apart than liquids and solids.
Sound waves move more slowly when particles are further apart.
What are ultrasound waves?
Ultrasound waves are sound waves which have a The number of waves produced each second. The unit of frequency is hertz (Hz). higher than the upper limit for human hearing - above 20,000 Hz.
Ultrasound waves are A wave that moves in the same direction (parallel) as the direction in which the particles are vibrating. because they are simply high frequency sound waves ie 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.
Question
A builder uses an ultrasonic device to measure the length of a room.
The device shows that the distance from one wall to the opposite wall is 8.25 m.
If the speed of ultrasound in air is 330 m/s, how long does it take for the ultrasound to travel to the far wall and back again?
Answer
Speed = \(\frac{\text{distance}}{\text{time}}\)
Time = \(\frac{\text{distance}}{\text{speed}}\)
Distance to opposite wall = 8.25 m
Speed of sound = 330 m/s
Time taken from device to opposite wall = \(\frac{\text{8.25 m}}{\text{330 m/s}}\)
Time taken from device to opposite wall = 0.025 s
Time taken from device to opposite wall and back again
= 0.025 s x 2 = 0.050 s
The time taken for the ultrasound to travel to the far wall and back again is 0.050 s.
How do car parking sensors work?
Many car parking sensors send out ultrasound pulses that reflect off objects such as walls, kerbs and nearby cars.
A receiver detects the reflected ultrasound, and the time taken for the waves to leave a sensor and return to the detector is measured.
Using this time, the distance from the car to the object is calculated electronically, using distance = speed of ultrasound in air multiplied by the time taken.
This is an echo method so it is important that only the outward time from the sensor to the object is used in the calculation.
If the car is too close to an object, warning bleeps are sounded, and the dashboard displays warning lights.
Ultrasound is used rather than sound waves so that pedestrians and other road users don’t hear the pulses of waves sent out from the parking sensors.
Problems:
The sensors can be affected by snow, rain and wind which can reduce accuracy or cause the system to fail.
Individual sensors are mounted on the car’s bumper – dirt and mud can cause the system to fail.
How does ultrasound imaging work?
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, to see inside a body.
An ultrasound scanner is simply run over the skin to obtain an image of what's inside.
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.
What are the advantages of using ultrasound in medicine?
Ultrasound waves 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.
What are the industrial uses of ultrasound?
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.
How is ultrasound used to clean 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 The amplitude of a wave is its maximum displacement from its undisturbed position..
How are sound waves used in detection?
High The number of waves produced each second. The unit of frequency is hertz (Hz). 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 using:
Distance = speed of ultrasound in water x time taken.
Remember this is an echo method and so the time between a pulse of ultrasound being sent out and detected must be divided by two to work out the time to the seabed.
The process is very similar to ultrasound imaging.
For deep water, 50 kHz ultrasound is used.
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.
WATCH: How do sonar and radar work?
Give me 90 seconds to explain how sonar and radar work.
Both sonar and radar use echoes to detect, locate and measure the distance to objects, a bit like how bats fly through a cave.
They emit a sound and the sound waves reflect off the walls of the cave.
The quicker the sound comes back the closer they are to a wall, allowing them to know when they need to turn.
Sonar, which stands for Sound Navigation and Ranging, uses sound waves to detect, locate and measure the distance to objects underwater.
Sound waves are used underwater because they travel much further than radio waves in water.
Submarines moving in the pitch‑black deep ocean use Sonar to guide them safely through the water.
They measure the time taken for a pulse to be sent out and returned to the submarine to find out how close they are to colliding with an object.
This is calculated using the formula distance equals speed times time over two.
We divide by two because the time measured is to the nearest object and back again, but the distance required is just the distance to the object.
Radar, which stands for Radio Detection and Ranging, uses radio waves to detect the presence, direction, distance and speed of objects.
Radio waves are used because they travel much faster than sound in air.
Radar is used in aviation so pilots can detect weather conditions ahead of them, allowing them to attempt to avoid thunderstorms and turbulence.
It also allows pilots to guide the aircraft onto the runway in difficult conditions where visibility may be reduced.
So to recap, sonar and radar work by sending out waves that reflect off objects and return to the source.
Sonar sends out sound waves underwater, while radar sends out radio waves in air.
The equation used for measuring distance using sonar is distance equals speed times time over two.
How much do you know about echoes and sonar?
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