Attack on the eardrums
The aim is for the pupils to understand the function and importance of the ear, so that they will turn their MP3 players down to prevent damage to their auditory systems. On a journey from a sound source to the inner ear, sound production and the anatomy of the ear are explained.
Auditory walking tour
Get the pupils to take a walk through the grounds of the school or kindergarten, once with and once without earplugs, to experience the loss of environmental impressions when they partially ‘switch off’ their hearing. They will also learn about the dangers that (partially) deaf people are exposed to.
Do we really hear everything?
The human ear can perceive sounds with 20 to 20 000 oscillations per second. The number of oscillations per second is called the frequency. As we get older, we lose the ability to hear very high frequencies. Dogs can hear sounds with up to 35 000 oscillations per second (35 kHz), bats even higher-frequency sounds. Use a normal whistle and a dog whistle for the children to compare. Typically, a dog whistle is within the range of 16-22 kHz, with only the frequencies below 20 kHz audible to the human ear (and depending on the individual state of your hearing, you may not even hear these).
Adjust the volume of a signal generator with amplifier and loudspeaker to a medium level at an audible frequency. Then turn up the frequency to 50 kHz, and slowly tune it down from there. Ask the first child who can hear something to describe the sound (a high-pitched whistle).
Do our ears have favourite sounds? An individual auditory diagram
By testing our own hearing range, we can estimate the state of our hearing. Connect a signal generator with an oscilloscope and loudspeaker as indicated (see image).
Click to enlarge image
Image courtesy of Nicola Graf
Using the signal generator, generate sounds between 250 and 16 000 Hz according to Table 1. To make the sounds comparable, make sure that they all generate a ‘wave line’ of equal ‘height’ on the oscilloscope.
Ask each child to complete the table (which can also be downloaded as a worksheet from the Science in School websitew4), recording whether they experience each sound as being very loud, loud, medium loud, quiet or very quiet. This can be a little tricky.
To make it easier, start with 16 000 Hz and do pairwise comparisons between neighbouring frequencies to be measured, i.e. ‘How do you perceive the sound at 16 000 Hz? Now listen to the sound at 8000 Hz – how do you perceive it in comparison?’ And so on. Typically, the human ear is most sensitive to the frequencies at which we usually speak (about 200–3500 Hz).
With the help of a secondary-school mentor (or teacher), each child should plot the perceived volume (e.g. loud = 8) against the frequency of the sound.
How do you perceive the sound? |
Sound 1 |
Sound 2 |
Sound 3 |
Sound 4 |
Sound 5 |
Sound 6 |
Sound 7 |
Table 1: individual auditory diagram
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16000 Hz |
8000 Hz |
4000 Hz |
2000 Hz |
1000 Hz |
500 Hz |
250 Hz |
Very loud (10) |
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Loud (8) |
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Medium loud (5) |
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Quiet (3) |
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Very quiet (1) |
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Auditory diagram of an
eight-year-old girl. Click
to enlarge image
It is useful to make the same measurements with adults (e.g. teachers or parents) as well, because as we age, we hear high-pitched sounds less well. You may notice this when the TV is on – young children may hear a high-pitched whistling noise whereas adults do not.
How does sound reach the ear? The swinging candle
Because sounds are transported by variations in air pressure, sound moves air particles. The movement of a candle flame is used to illustrate this. Sound with a low frequency can even blow out a candle flame.
Materials
- A CD player with a bass loudspeaker playing techno music
- A candle and matches
- A paper funnel
- A (bass) drum with a hole in the back
- A signal generator with amplifier
- A loudspeaker suitable for low frequencies (at least as low as 100 Hz)
- Cables
Images courtesy of Werner
Stetzenbach
Procedure
- Place a burning candle in front of a CD player with a bass loudspeaker playing techno music. The flame will flicker in time with the music. If the effect is not very visible, use a paper funnel between the loudspeaker and the candle to enhance it.
- Place the burning candle in front of the hole at the back of a drum. Beat the drum on the other side and watch the flame move or be blown out.
- Using the cables, connect the loudspeaker to the signal generator and turn it to a low frequency (100 Hz). The candle will be blown out. To enhance the effect, you can use a paper funnel between the loudspeaker and the candle.
What happens in the ear?
Use a plastic or paper model (which may be homemade) of the ear to illustrate the different parts of the ear, which will be explained in the following experiments.
The outer ear: the auricle and eardrum
The auricle collects sound like a funnel. Use a paper funnel as an ear trumpet to improve hearing: whisper into it and see how it magnifies the sound.
Image courtesy of Werner
Stetzenbach
The outer ear acts like an organ pipe that is closed at one end, so that the air in it can vibrate. This vibration is passed on to the eardrum, a membrane that behaves like a drum, and then through mechanical linkage to the three ossicles (small bones).
Materials
- A bass loudspeaker
- A signal generator with amplifier or a CD player with amplifier
- Cables
- Jelly babies
Procedure
Image courtesy of Werner
Stetzenbach
Using the cables, connect the bass loudspeaker to the signal generator or CD player. Place some jelly babies on the speaker. Watch them ‘dancing’ with the vibrations of the loudspeaker membrane, which represents the eardrum.
The middle ear: the ossicles
Materials
- Two tambourines
- A drumstick
- A table tennis or styrofoam ball tied to a string
- A baking tray
- A wooden mallet or similar wooden instrument
- A bowl
- Aluminium foil or cling film
- Rice grains or sugar
Click to enlarge image
Procedure
- Hang the table tennis or styrofoam ball (representing the ossicles) in front of one of the tambourines, touching its surface (T2 in the image below). Beat the other tambourine (T1, the sound source) with the drumstick and watch the ball move, as the sound waves reach T2.
- Cover the bowl with foil or film pulled taut, and place rice grains or sugar on top. Hold the baking tray close, bang it with a mallet and watch the rice or sugar (representing the ossicles) jump.
The inner ear: the cochlea
Click to enlarge image
There are auditory nerves in the hair cells of the cochlea. Sound (changes in air pressure) makes them move, which triggers information to be transmitted to the brain. The louder the sound, the more the hairs move. Very loud noises can even damage the hair cells.
A glass tube is used as a model for an uncoiled cochlea. Cork dust or talcum powder inside the tube represents the hair cells.
Click to enlarge image
Image courtesy of Werner
Stetzenbach
Materials
- A glass tube
- Cork dust or talcum powder
- Two stands (see image)
- A signal generator
- A loudspeaker
- Cables
Procedure
- Fill the bottom of the glass tube with cork dust or talcum powder and mount it horizontally on the stands.
- Using the cables, connect the loudspeaker to the signal generator and place it in front of one opening of the glass tube.
- Adjust the frequency of the sound (depending on the tube’s length) until the tube resonates (for any given length, there are multiple frequencies at which it will resonate), making the dust vibrate visibly. This represents the movement of the hair cells.