Echoes and Ultrasound

Echoes

An echo is the reflection of sound that reaches the listener after a delay from the direct sound.

For example, when a person speaks, the sound waves will travel to the wall in front of them. The wall will absorb some of the sound waves and some will be reflected back. So, the person will be able to hear the echo.

Sound waves bounce off smooth surfaces similarly to how a bouncy ball does. Empty rooms, as well as spaces where there are many smooth and hard surfaces, can increase the effect of echoes. This is why echoes can be heard in caves or new unfurnished houses.

Soft furnishings can reduce the effect of echoes, as they absorb more sound waves, so fewer waves will be reflected back. Some examples are carpets, cushions and curtains.

We can use echoes to measure the speed of sound, using the equation:

Speed is equal to distance divided over time.

  • Speed is measured in metres per second (m/s)
  • Distance is measured in metres (m)
  • Time is measured in seconds (s)

Example

This example involves timing how long it takes to hear an echo.

Two people are standing next to each other:

  • Person 1 claps or produces a sharp, distinct sound
  • Person 2 uses a stopwatch or electronic timer to record the time interval between the clap and the moment the echo is heard.

When person 1 claps, this sound will produce a sound wave that will travel to the wall. The wall will absorb some of the sound waves and the rest will be reflected as an echo. Then, person 2 will record how long it takes to hear the echo.

As we are calculating the speed, there are two pieces of information we need:

  • The distance between person 1 and the wall
  • How long it takes for the echo to be heard after person 1 claps

Using a tape measure or other accurate measuring device, determine the distance between the persons and the wall. Let’s assume this distance is 686 metres (m).

We have to multiply 686 m by 2 because the sound waves travel from person 1 to the wall and back to person 2. So, 686 × 2 = 1372 m.

Person 2 records the time it took to hear the echo as 4 seconds (s).

So now, we use the equation: Speed = Distance ÷ Time

  • The distance is 1372 m
  • The time is 4 s

Then, we put these values into the equation:

Speed = 1372 ÷ 4 = 343 m/s.

The speed of sound in dry air at 20°C is around 343 m/s, so this is the speed you should aim to get in this experiment.

Considerations

When carrying out this experiment, take into account the following:

  • Environmental conditions such as temperature, humidity and altitude can affect the speed of sound.
  • The wall’s material can affect the intensity of the echo due to absorption, but won’t affect the speed of the echo.
  • Aim for a clear path between the sound source, the reflecting wall, and the receiver to minimise interference or multiple echoes.
  • It is beneficial to repeat the experiment multiple times, calculating an average for a more accurate result.
  • When comparing to standard values, make sure the conditions match. The speed of sound in dry air at 20°C is typically 343 m/s, but this varies with conditions.

Linking Ultrasound and Echoes

The range of human hearing is between 20 hertz and 20,000 hertz. Ultrasound waves have a frequency above 20,000 hertz (Hz). So although we can produce ultrasound waves, we can’t actually hear them.

Various animals use ultrasound to communicate, for example, bats.

Echolocation in bats

Bats use echoes and ultrasound to navigate and detect their prey. They emit ultrasound calls, then listen for the echoes and this process is called echolocation.

  • How bats detect prey – The sound waves emitted by the bats reflect off their prey, then return to the bat’s ears.
  • How bats navigate – The sound waves emitted by the bats reflect off the buildings or trees, then return to the bat’s ears.

The bat’s brain calculates the time it took for the echo to return to them, which allows them to calculate how far away their prey is. This means that their brain is essentially using the speed = distance ÷ time equation.

Ultrasound in pregnancy

Doctors also use echoes and ultrasound to produce an image of an unborn baby.

Ultrasound imaging involves using a probe to transmit sound waves and record the waves that echo back.

1. The ultrasound waves are transmitted through the mother’s belly.

2. The sound waves travel until they detect a boundary between tissues.

3. At these boundaries, some of the sound waves are reflected.

4. The computer uses the speed of the sound waves and the time taken to generate a two-dimensional image of the baby on a screen.

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