Echoes and Ultrasound

Echoes

An echo is the reflection of sound that reaches the listener after a delay, following 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 this person will be able to hear the echo.

Sound waves are able to bounce off smooth surfaces similar 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. Some examples include carpets, cushions and curtains, as they absorb more sound waves, meaning fewer waves will be reflected back.

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 typically around 343 m/s, although this can vary with different conditions.

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. Sounds above 20,000 hertz (Hz) are called ultrasound. So although we can produce ultrasound waves, we can’t actually hear them.

There are various animals that use ultrasound to communicate, for example, bats.

Echolocation in bats

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

  • How bats detect prey – The ultrasound calls reflect on their prey
  • How bats navigate – The ultrasound calls reflect on trees or buildings

Their 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 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.

1. An ultrasound scanner has a microphone that gives off sound waves (ultrasound). These ultrasound waves are then transmitted through the mother’s abdomen.

2. The sound waves bounce off the organs in the body, and the microphone picks them up, therefore detecting the baby.

3. These echoes are used to form an image of the baby.

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