Activity 1.2 - Do muscle, fat and bone sound the same?

• To explain when an echo occurs
• To define the concept of reflection interface
• To be able to interpret the recorded sound graphs
• To identify on what the strength of reflection depends
• To apply the concept of reflection interface for ultrasound and human body tissues

Hand out the activity 1 worksheet. Let students work in small groups of 2 or 3. They have to first describe when an echo occurs. Summarize their answers in a class discussion.

In activity 2 students are introduced to the concept of reflection interface and are asked to answer the research question: ‘Does the strength of the reflection, in other words strength of echo, depend on the material which reflects sound?’ Let the students formulate their hypotheses.
To test their hypotheses let them perform the experiment in which two different reflection media are compared: hard (e.g. plastic) and soft (e.g. foam) material. They should find out that hard material reflects sound better than soft material. The evidence for this is that the first echo pulse is stronger for hard material than for soft material (in each trial the initial sound should be the same and have similar intensity).

In activity 3 students have to answer the next research question: ‘Does sound travel with the same speed in different media?’
To answer this question the students perform investigation in which they compare the speed of mechanical waves in water and in vegetable oil. Each student group is provided with the following materials: two plastic droppers, a metric ruler, a stopwatch that measures tenths of seconds, water and vegetable oil in plastic bottles, two ripple tanks (e.g. aluminium foil lasagne pans with mirrors at the bottom).

The students have to design their experiment. Check their plans before they begin their experiments. The ripple tanks should be filled one with water, one with vegetable oil, to the same depth of 1-2 cm. In the most likely setup a student will release a drop of medium into one end of a ripple tank and start a stopwatch at the same time. In each trial, the drop should be released from the same height and distance from the edge of the tank. When the first wave produced by the drop reaches the opposite side of the tank, the second student will call out to stop the watch. The students should record this time in their data table. They should also measure and record the distance from the point where the drop hit the liquid to the opposite side of the ripple tank. The speed of wave can be calculated by dividing the distance by the time measured on the stopwatch. Students will find that, under similar conditions, waves travel faster in water than in oil.

In activities 4 and 5 students apply the concepts and realise how ultrasound pulses reflect from human body tissues.

• Why echo occurs?
• In what kind of place would you expect to hear echoes?
• Does the strength of the sound reflection depend on the material which reflects sound?
• Give examples of good sound reflectors
• Does sound travel with the same speed in different media? Give examples.
• How do ultrasound waves reflect from human body tissues?