Physics 11–12 · Year 11

Sound intensity against distance: the inverse square law with a phone sound meter

Module 3: Waves and Thermodynamics (Sound Waves)

Practical, model not builtLow risk

This site has no interactive model of its own. Where a step or a material names a Concept Studio model, simulation or tool, it has not been built; an external simulation a step names (for example PhET) is not part of this site.

The idea

Energy from a small source spreads over a sphere whose area grows with the square of the distance, so intensity falls as one over distance squared.

What you need

Small loudspeaker playing a steady 1 kHz tone, or a mechanical buzzer
Sound level meter or phone sound meter app, 10 m tape measure
An open outdoor area or large hall to reduce reflections

How to do it

Place the source at 1 m above the ground and read the level at 1.0, 1.4, 2.0, 2.8, 4.0 and 5.7 m (each step multiplies the distance by the square root of two).
Take three readings at each distance and average.
Convert levels to relative intensity with I/I_ref = 10^(L/10) and plot intensity against 1/r^2.
Check the slope: every doubling of distance should drop the level by 6 dB.

What you should see

Levels fall about 6 dB per doubling of distance: a source reading 70 dB at 1.0 m reads about 64 dB at 2.0 m and 58 dB at 4.0 m in open air. Intensity plotted against 1/r^2 is a straight line through the origin; indoors, reflections flatten the curve at larger distances.

What changes

What you change: distance from the source
What you measure: sound level and intensity
What you keep the same: source volume
meter height and orientation
background noise

Common misconceptions

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

Doubling the distance halves the loudness; it quarters the intensity.
Sound fades because it is used up; the energy is spread over a larger area, not lost, until absorption acts.

Safety card

Low riskLearners carry it out

Hazards

loud source near the ear

Controls

keep levels below 85 dB at 1 m

Note

Record the activity in RiskAssess (https://www.riskassess.com.au/) and follow the Science ASSIST risk management information sheet (https://asta.edu.au/resource/ais-risk-management-and-risk-assessment/).

Curriculum references

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

Physics Stage 6 Syllabus (2017), current: Year 11 until the end of 2026, Year 12 until Term 3 2027PH11-10 PH11/12-4
Physics 11-12 Syllabus (2025), not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027, first HSC examination 2028PY-11-02 PY-11WS-05
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities