Bringing together two sets of constraints

Focusing on the learners:

Distinguishing–eliciting–connecting. How to:

• focus on the physical aspect of the transmission of sound
• build a source–medium–detector model
• put the source–medium–detector model to use
• make connections with travelling electromagnetic vibrations
• connect vibrating objects to travelling vibrations
• connect hearing to the source–medium–detector model

Teacher Tip: These are all related to findings about children's ideas from research. The teaching activities will provide some suggestions. So will colleagues, near and far.

Focusing on the physics:

Representing–noticing–recording. How to:

• draw the spread of the vibrations
• describe and make real the to and fro vibrations
• prepare the ground for frequency and amplitude being the fundamental characteristics
• draw vibrating objects
• unify a wide range of experiences with a single model

Teacher Tip: Connecting what is experienced with what is written and drawn is essential to making sense of the connections between the theoretical world of physics and the lived-in world of the children. Don't forget to exemplify this action.

Pupils' views on how we hear

Our experience tells us that there is not a big variation in the views that children of this age bring to their science lessons. Nevertheless, it is a good idea to probe the understanding of your pupils at the start of any lesson sequence or module.

Here is a sheet that might be used to probe pupils' thinking on sound, allowing them to show what they understand. Included are the responses of six pupils to the sheet.

Support sheet

See if you can spot any common patterns, things to avoid and things you'd want to reinforce in your teaching.

Vibration

Wrong Track: If you hit a cymbal it vibrates to make the sound. If you drop a spoon on the kitchen table it just makes a noise.

Right Lines: All sounds are produced by vibrations. If you drop a spoon on the table, the sound you hear comes from the vibrations that are set up in the table and spoon.

Teaching and learning about vibrations

Thinking about the learning

The challenge here is for pupils to come to appreciate that all sounds are generated by the vibration of a source.

In some situations the vibrating object is obvious; in others it is less so. Where the vibration is less obvious, pupils tend to revert to ad-hoc explanations for the generation of the sound, often focusing on human action. For example, The hammer makes the bang because you hit the wood hard with it. The learning challenge for pupils is to develop the general idea that all sounds are produced by vibrations.

Thinking about the teaching

In your teaching it is worth identifying where the sound is coming from each time you consider a new sound-making context. Take the time to chase down what it is that is vibrating to act as the source of the sound.

A teaching colleague entertains her classes each year with the invitation:

Teacher: Let's play spot the vibration!

She uses a range of examples in class:

• In a reed instrument (clarinet, saxophone, oboe): the reed vibrates, setting all of the air in the instrument tube vibrating.
• In a stringed instrument: the strings are first set into vibration, by either bowing or plucking, then air is set in motion in the box behind the strings.
• In a flute or recorder: the mouthpiece is shaped so that the air striking one edge of it is set into vibration. This small vibration sets up a larger vibration of the air in the instrument tube.
• When two stones are clinked together: it is the stones that vibrate, changing shape as they do so, much like the surface of a drum.

The physical nature of sounds

Wrong Track: Sounds travel to your television through the cable from the TV company.

Right Lines: Electrical signals pass down the cable to your television. Here they are decoded and the loudspeaker in the television produces the desired sounds.

Codes are not sounds

Wrong Track: You buy CD sounds at a record shop.

Right Lines: The sound does not just sit on the CD. The code on the CD is read by the CD player, which drives the speakers in the headphones and they produce the sounds (music to your ears!).

The nature of sound in teaching and learning

Thinking about the learning

This learning challenge involves being clear about the physical nature of sounds… what they really are.

The challenge here is for pupils to come to recognise and to understand the scientific view of what sound is: the disturbance created by the source, which travels out through the surrounding medium. The disturbance itself consists of successive regions of high

Thinking about the teaching

It is worth emphasising the message:

Teacher: If it does not involve the to-and-fro movement of a medium, is is not a sound.

So sound is not stored on a CD; it is not transmitted to a radio set; it is not delivered down a cable to a television.

Sounds can be produced by decoding what is stored on these artefacts. You need to take care to refer to sound only when you want to speak of the to and fro movements of the medium. Sound only exists in each of these cases after the loudspeaker.

Vibrating sources

A fundamental property of sound is that it is produced by a source that vibrates. This to and fro motion applies to all sounds, whatever the medium.

Teacher Tip: Regions of high pressure and low pressure happen as a result of to and fro motion in air. (The to and fro causes increases and decreases in the density of the air, thus changing the pressure). Changes in pressure can only happen in fluids; sounds can travel in solids as well. So keep it simple and general: to and fro is both accurate and sufficient.

Sound as a disturbance in the air

Wrong Track: The air just in front of the loudspeaker is pushed forward by the cone and it then travels to your ear so that you can hear the sound.

Right Lines: Vibrations travel as the disturbance from the speaker cone spreads out through the air. First of all the air next to the cone is made to move to and fro. Since neighbouring particles affect one another this pattern is passed on to groups of particles all around. So the pattern of vibration travels from one block of particles to the next, and the sound travels without the particles needing to make the trip themselves.

Vibrations travelling

Thinking about the learning

Some children have the idea that sound is an entity that itself travels from speaker to ear. This view needs to be countered with the scientific view that sound is not an object but a process – the travelling disturbance of the medium.

Thinking about the learning

The fact that sound travels through solids, where the particles are restricted to vibrating about fixed positions, provides a good counter-argument to those who think that the particles need to move from source to detector in order to carry the sound.

Sound travels

Wrong Track: I had my radio on very loud and the music just filled the house.

Right Lines: Sound travels from the radio to all parts of the house.

Filling or travelling

Thinking about the learning

Some pupils talk about sound as if it is something that can fill a space. Sound can only fill a space to the extent that it travels through that space.

Thinking about the teaching

Each time you talk about sound, try to emphasise the source–medium–detector model, making it the most natural thing in the world to look for paths along which the sound travels.

Sounds dissipating

Wrong Track: Sounds run out as they get farther away from the source, and eventually stop.

Right Lines: The movement (or the energy) from the vibrating source is spread across more and more particles.

Sound energy

Thinking about the learning

Typically pupils are quite happy to say that sounds get quieter as they travel farther away from the source and suggest they die away. Pupils are not concerned with accounting for where the initial motion (or energy) of the vibrating cone and medium ends up.

Thinking about the teaching

One of the ways in which the sound get quieter is that the energy of the vibrations is transferred to more and more particles as the vibrations spread out over a larger and larger sphere.

Imagine throwing a pebble into the middle of a pond, creating waves that spread out over the surface (just like the pattern of high and low density travelling through the air) in an ever-expanding circle. As the circle gets bigger the energy available for each centimetre of the circumference gets less.

Since a sound travels out over a three-dimensional space (rather than a circle), the energy from the source is spread out much more for each centimetre moved away from the source.

In addition, some of the sound may be absorbed by the stuff through which it travels. In this case not all of the vibration of one block of particles is passed onto the next – some of the energy gets spread around, resulting in a disordered jiggling, rather than in the organised vibration of the sound. Materials and structures that do this particularly well are good sound insulators – good at insulating source from detector.

Don't use the idea of rays in discussing sound

Teacher Tip: Use rays are used to model light, but not sound.

Here is why.

In explaining the properties of light we suggest constructing diagrams based on light rays, which are imaginary. Rays can be imagined to represent infinitely thin beams of light.

In optics, light rays are used a lot because real light beams act in much the same way as the model rays. In other words, light really does travel along well defined paths, which do not diverge too much with distance.

Sound does not travel in sharply defined beams. Instead it spreads much more as it goes – beams are very hard to maintain.

There's a good chance you could improve your teaching if you were to:

Try these

• make explicit use of the source–medium–detector model
• model the vibrations as changes in density
• link the movements of the sources to changes in density
• keep the language simple: 'to and fro' may be more effective than 'longitudinal'
• relating what's pictured as a part of the model to human experiences
• draw simple sketches, rather than hanging too much on words

Teacher Tip: Work through the Physics Narrative to find these lines of thinking worked out and then look in the Teaching Approaches for some examples of activities.

Avoid these

• using specious energy descriptions
• drawing or showing transverse waveforms
• asserting that sound is a wave without clarifying explanation of the idea of a wave
• conflating models with the lived-in world

Teacher Tip: These difficulties are distilled from: the research findings; the practice of well-connected teachers with expertise; issues intrinsic to representing the physics well.