# 02Quantifying and using sound

So02TL of the Sound topic
• ## 01 Things you'll need to decide on as you planSo02TLnugget01 Decisions

### Bringing together two sets of constraints

Focusing on the learners:

Distinguishing–eliciting–connecting. How to:

• keep amplitude and frequency separate, each with their own vocabulary
• separate the to and fro movement of the particles that forms the vibration from the propagation of the vibration, which is also a movement
• explore the range of hearing, along both the amplitude and frequency axes
• connect human hearing to what other species can hear
• ensure that the need for particles as a medium is always there
• link each sound heard back to the source, via the medium

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:

• reinforce the source–medium–detector model
• represent the vibrations of the source
• represent the vibrations in the medium
• represent the vibrations of the detector
• account for reductions in intensity with distance from the source
• link delays in hearing sounds to the trip time of propagation from the source
• show sounds travelling through solids and liquids, as well as gases
• measure the frequency of a sound
• measure the amplitude of a sound

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.

• ## 02 Separating loud/soft (loudness) from high/low (pitch)So02TLnugget02 Challenge

### Frequency and pitch

Wrong Track: There are more vibrations, so it will be louder.

Right Lines: There are more vibrations each second, so the frequency will be higher, giving a higher pitched sound.

### Amplitude and loudness

Wrong Track: With bigger vibrations it will be a higher sound.

Right Lines: If the amplitude of the vibration of the source is bigger, the sound produced will be louder.

### Describing sound

The central learning challenge for pupils is for them to develop a clear understanding of the difference between describing sounds in terms of loudness (amplitude) and pitch (frequency).

There are two separate ways of describing sounds here, and the aim must be to help pupils differentiate between them.

Using terms such as more vibrations or it vibrates more is likely to lead to confusion among your pupils.

Be clear in specifying either:

• more vibrations in a second (pitch or frequency)
or
• each vibration involves a greater to-and-fro movement of the source, or of the air (loudness or amplitude)
• ## 03 Teaching stories about frequencySo02TLnugget03 Teaching tip

### Two stories, in different styles

Teacher Tip: It is perhaps one of the the injustices of life that by the time you can afford the ultimate hi-fi music system you are unlikely to be able to hear the benefits! That's because one of the features that you pay for is the facility to reproduce a wide range of sounds, from very low to very high pitched, and our ability to detect such a range decays with age.

Teacher Tip: I knew an old man who was losing his ability to hear higher-pitched sounds. He could hear my voice fine, but he had trouble hearing his wife, because her voice was of a higher pitch. She often became angry with him because she thought he was just ignoring her… especially when she was asking him to do the washing up. In truth he never seemed to miss being called for a meal. She called it selective hearing.

• ## 04 Speed of sound: speed of lightSo02TLnugget04 Teaching tip

### Different speeds of sound and light

Pupils need to differentiate between the speed of sound in air (about 340 metre / second) and the speed of light in air (about 3 × 108 metre / second).

Teacher Tip: Introduce everyday examples to emphasise that sound and light travel at different speeds.

Many pupils are familiar with the situations that can be referred to in drawing attention to the difference between the speeds of sound and light. For example, they may know that you can see a flash of lightening before you hear the peal of thunder. They may not, however, be clear about how this observation can be accounted for.

We see the flash virtually instantaneously because light travels so fast. Sound, on the other hand, travels more slowly (about 300 m / s). If the storm is one mile away – about 1500 m – the sound will take about 5 s to arrive.

Other situations that you might talk about with pupils include:

Sprinters start on the flash of a gun, rather than the bang. On school sports day, the starter actually stands half way down the 100 m track with the starting pistol. How long will it take for the bang to arrive? (About 0.3 s).

• ## 05 A teaching storySo02TLnugget05 Teaching tip

### Different speeds of sound and light

You probably have a story like this one:

It's a Saturday afternoon and Anita, a keen football fan, is revising for her exams. She has the radio on quietly in the background to follow the progress of the match. In the last seconds of the match, the home team scores. Anita jumps up to the window, throws it open and puts her head outside to see if she can hear the roar of the crowd from the ground.

Nothing!

She is just about to close the window when there it is: the roar arrives from the football ground!

The football ground is about 3 km away. Sound travels at about 300 metre /second, giving about 10 second for the sound to travel from the ground to the house.

The radio wave signal to Anita's radio set travels at the speed of light, arriving instantaneously while the sound takes rather longer to get there.

• ## 06 Sounds through solids and liquids as well as gasesSo02TLnugget06 Teaching tip

### Sounds through solids and liquids

Pupils need to appreciate that sounds can travel not only through gases but also through solids and liquids.

Teacher Tip: Use examples where sound travels through media other than air.

You can have an interesting discussion with pupils about sounds travelling through solids and liquids. Most of them will be able to tell you that they can still hear muffled sounds while swimming underwater in a swimming pool. They may have seen TV or films in which submarine crews are able to hear the sound of the engine on a passing ship. Some may have played at passing messages down iron railings by tapping one end and getting a friend to put their ear next to the railing at the other end.

• ## 07 Showing sounds on an oscilloscopeSo02TLnugget07 Teaching tip

### Sounds on an oscilloscope

It's great fun to connect a microphone to an oscilloscope and to use the scope to monitor different sounds. Can you sing a perfect note? However, this exercise can be misleading on more than one account.

First, if the time-base of the oscilloscope is turned on, the trace on the screen will look exactly like the crests and troughs of a transverse wave, but the sound wave is a longitudinal wave.

Second, it is very tempting to point out the trace on the screen and to identify wavelength (as the distance between the adjacent crests). This would be wrong because the horizontal axis of the oscilloscope provides a measure of time.

The best way around these problems is to switch the time-base off and see the sound represented as a vertical line on the screen.

Teacher Tip: Using an oscilloscope thoughtfully is essential to not misleading pupils. There is a case for using a computer program to represent the sound more naturally – that is representing amplitude and frequency in as unambiguous a way as possible.

• ## 08 Thinking about actions to takeSo02TLnugget08 Suggestions

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

Try these

• showing large-scale, slow vibrations producing a sound
• emphasising that all sounds have a source
• tracing the chain from source to detector, via medium, often
• showing changing frequency without changing amplitude
• showing changing amplitude without changing frequency
• measuring frequency directly, linking this to counting the vibrations
• relating delays in hearing sounds to trip times due to the speed of propagation

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

• showing waveforms on an oscilloscope
• introducing wavelength prematurely
• using a strobe without explaining how you see what you see

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.

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