Science and Technology K–6 · Year 2

Tuned water bottles: how the amount of water changes the pitch

Physical World (NSW 2017); Science understanding, Physical sciences (ACARA v9)

Practical, model not builtLow risk

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The idea

Blowing across a bottle sets the air above the water vibrating, so more water shortens that air and raises the note, while tapping the same bottle sets the vessel and the water vibrating together, so more water lowers the note.

What you need

5 identical straight-sided plastic measuring cylinders of 250 mL, 200 mm tall and 40 mm across, or 5 identical straight-sided plastic bottles with the labels removed
A 250 mL jug of water and a small funnel
A millimetre ruler taped up the outside of each cylinder, or the cylinder's own printed scale
A free tuner app on a tablet that displays the frequency of a note in hertz
A metal teaspoon for tapping
A thermometer reading in degrees Celsius, for the room
One short drinking straw per child, to blow across the rim without touching it, and wipes for the rims
Recording table: air column height (mm), water poured (mL), blown reading (Hz), and whether the tapped note is higher or lower than the last cylinder

How to do it

Leave one cylinder empty. Fill the other four so the air above the water measures 150 mm, 100 mm, 75 mm and 50 mm, measuring the air with the ruler rather than the water.
Record the water each one took. For a cylinder 40 mm across, 50 mm of water is about 63 mL, 100 mm about 126 mL, 125 mm about 157 mL and 150 mm about 188 mL.
Blow steadily across the rim of each cylinder in turn, holding the straw just above the rim so nothing touches the mouth. Read the tuner and record the hertz. Take three readings of each cylinder and keep the middle one.
Predict the reading for a cylinder holding half the air of one already measured, write the prediction down, then set that cylinder up and test it.
Tap each cylinder gently on the side with the teaspoon, in the same order, and record whether each note is higher or lower than the one before.
Read the room temperature and write it beside the table.
Line the cylinders up in order of pitch when blown, then in order of pitch when tapped, and compare the two orders.

What you should see

Blown, the note rises as the air column shortens. Taking the speed of sound in air as 344 m/s at 20 degrees Celsius from the HyperPhysics table, an air column of length L in a cylinder of radius 20 mm sounds at v divided by 4 times (L plus 0.6 r): 406 Hz for the empty 200 mm cylinder, 531 Hz at 150 mm, 768 Hz at 100 mm, 989 Hz at 75 mm and 1387 Hz at 50 mm. Halving the air from 150 mm to 75 mm therefore multiplies the frequency by 1.86 rather than 2, because the 12 mm end correction is added to both lengths: 10.8 semitones instead of a full 12-semitone octave. Classroom readings within about 5 per cent of these figures are a good match, since a cylinder rim is not a perfect open end. Tapped, the order reverses: the fuller the cylinder, the lower the note, because the wall and the water swing together and more water makes a heavier, slower swing. Room temperature is worth recording, because the same 150 mm column moves from 521 Hz at 10 degrees Celsius to 541 Hz at 30 degrees Celsius, 3.7 per cent or 0.6 of a semitone, using the HyperPhysics values of 331.5 m/s at 0 degrees Celsius and 344 m/s at 20 degrees Celsius. The learner knows it worked when the blown order and the tapped order come out opposite ways round and the blown readings climb as the air shortens.

What changes

What you change: the height of the air column above the water (mm)
What you measure: pitch: the tuner reading in hertz when blown, and higher or lower by ear when tapped
What you keep the same: the same five cylinders, the same rim and the same steady blow
the air column measured rather than the water
the same room temperature, recorded beside the readings
the middle of three readings taken each time

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

More water always makes a higher note, whatever is done to the bottle.
Blowing harder raises the pitch rather than the loudness.
The water makes the sound when you blow across the top.
Halving the air must double the frequency exactly.

Safety card

Low riskLearners carry it out

Hazards

Sharing a rim passes on illness
A glass cylinder breaks if knocked over
Loud sustained notes close to an ear
Spilt water on a hard floor

Controls

Plastic cylinders or plastic bottles, never glass
One short straw per child, held just above the rim, and every rim wiped between users
Moderate blowing, and no cylinder held against an ear
A tray under the cylinders and a cloth kept on the bench

Note

No chemicals and no heat. Record the activity on Primary School RiskAssess (riskassess.com.au).

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Science and Technology K-6 Syllabus (2017), NESA. Current syllabus; NESA's timeline is 2026 plan and prepare, 2027 start teaching the 2024 syllabus. Code read from the official syllabus document on 2026-09-22.ST1-8PW-S ST1-1WS-S
Science and Technology K-6 Syllabus (2024), NESA. Implementation from 2027, so this code describes the future syllabus. Code read from the outcomes page on 2026-09-22.ST1-SCI-01 ST1-PQU-01 ST1-DAT-01
Australian Curriculum v9AC9S2U02AC9S2I01AC9S2I02AC9S2I03AC9S2I04AC9S2I05

Sources

The pages the author read to write this activity.

All Concept Studio activities