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Science 7–10 · Year 9

Pitch and loudness: measuring frequency and amplitude with a phone spectrum app

Physical sciences — Waves and motion: waves, sound and light (NSW Stage 5 focus area)

Practical, model not builtLow risk

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The idea

The pitch of a sound is set by its frequency and the loudness by its amplitude, and both can be read from a microphone trace.

What you need

  • smartphone with the phyphox app (Audio Spectrum, Audio Scope, Audio Amplitude and Tone Generator experiments), 1 per group
  • tuning forks 256 Hz, 440 Hz and 512 Hz, 1 each
  • rubber striking pad, 1
  • a second phone as a tone source (and a musical instrument if available)
  • metre rule, 1

How to do it

  1. Open Audio Spectrum, strike the 440 Hz fork and hold it 5 cm from the microphone; read the peak frequency. Repeat for 256 Hz and 512 Hz and compare with the stamped values.
  2. Open Audio Scope with the 256 Hz fork sounding; read the time between successive peaks (the period) from the trace and calculate 1 / period.
  3. Open Audio Amplitude; strike the fork softly and then hard and record the amplitude reading for each. Then play a steady 1000 Hz tone from the second phone at a fixed volume, hold its speaker 10, 20, 40 and 80 cm from the microphone and record the amplitude each time; a steady tone is used because a fork's sound dies away while it is being moved.
  4. Use Tone Generator on a second phone to play 200, 400, 800 and 1600 Hz; describe the pitch and check each on Audio Spectrum.
  5. Sing or play a note and find its frequency. Use the tone generator to find the highest frequency each learner can hear; do not use it to find the lowest, because a small phone speaker gives out very little sound at low frequencies, so the result would measure the speaker rather than the ear.

What you should see

The spectrum peak lands close to the stamped fork values (256, 440 and 512 Hz), within the frequency resolution the app shows. The scope trace of the 256 Hz fork repeats every 3.91 ms, and 1 / period returns 256 Hz; the 440 Hz fork repeats every 2.27 ms. A harder strike raises the amplitude without moving the frequency peak. The amplitude reading falls as the tone source moves away. A small source in open air would halve it for each doubling of distance; in a room, echoes and the phone speaker sending more sound in some directions than others make the measured fall differ, so the learner looks for the pattern rather than exact halving. Each doubling of the tone generator's frequency sounds one octave higher, and the highest frequency heard differs from learner to learner (the phone speaker can also limit how high a tone it produces).

What changes

What you change
source frequency (Hz), strike strength, or distance from the microphone (cm)
What you measure
measured frequency (Hz) and amplitude (relative or dB)
What you keep the same
  • same phone and app settings
  • quiet room
  • source held at the same orientation

Common misconceptions

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

  • A louder sound has a higher frequency.
  • Pitch and loudness are the same property.
  • The microphone measures the sound's speed.

Safety card

Low riskLearners carry it out

Hazards

  • high volume from the tone generator near ears

Controls

  • phone volume no more than half
  • no headphones for the tone source

Note

No hazardous chemicals or naked flames are used. Complete the school's risk assessment for the activity before the lesson; the NSW Department of Education Science safety and compliance page points to CSIS 1.7 (Risk assessment – a pre-requisite for risk control) for how to carry it out.

Curriculum references

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

Sources

The pages the author read to write this activity.

  1. curriculum.nsw.edu.au/learning-areas/science/science-7-10-2023/outcomes
  2. curriculum.nsw.edu.au/learning-areas/science/science-7-10-2023/content/stage-5/faafd3c3df
  3. phyphox.org/experiment/audio-spectrum
  4. phyphox.org/experiment/audio-scope
  5. phyphox.org/experiment/audio-amplitude
  6. phyphox.org/experiment/tone-generator
  7. www.iop.org/sites/default/files/2020-05/Teaching-sound-waves.pdf

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