Pitch and loudness of sound – comparing two sounds on an oscilloscope
Integrated ScienceLight & SoundAges 12–13
Use it with your class: students open it from a link or a QR code, with no account needed.
Next in Light & Sound
A tuning fork or a string vibrates with a frequency f and amplitude A chosen by the student; an oscilloscope shows the waveform, and scales show the loudness (dB) and pitch of the sound. A higher frequency gives a higher pitch and a larger amplitude gives a louder sound; switch on comparison mode to show two sounds on the same screen. A Play button plays the real sound, and the range audible to the human ear, 20 Hz – 20,000 Hz, is marked. Two more screens: Timbre compares the waveforms and harmonic spectra of instruments playing the same note (with a mystery instrument to identify), and Sound level puts a meter at a chosen distance from one or two sources, with absorbers, L = 10·lg(I/I₀), the 1/r² law and daily noise-exposure limits.
Lesson: Loudness and pitch of sound
What it shows
Pitch depends on frequency and loudness on amplitude: a tuning fork or string vibrates while an oscilloscope traces the waveform, with T = 1/f and λ = v/f. Instruments playing the same note repeat at the same fundamental frequency f₁ but mix in harmonics 2f₁, 3f₁… in different proportions, which gives each its timbre. Sound level in decibels is L = 10·lg(I/I₀) with I₀ = 10⁻¹² W/m²; from a point source I ∝ 1/r², so L falls by 6 dB when the distance doubles, and two identical sources add 3 dB. Above 85 dB the recommended daily listening time halves for every 3 dB.
How to use
On Pitch and loudness, press Compare pitch or Compare loudness and drag across the oscilloscope to count oscillations. On Timbre, set the Fundamental frequency f₁, pick Sound A and Sound B, compare the waveforms and spectra and press Play A or Play B; choose Mystery instrument for Sound B and identify it. On Sound level (dB), set the source level, Sources and an absorber, drag the meter or the distance slider, press Record measurement and answer the Tasks.
Parameters you can change
- Frequency of sound 1 10–30000 Hz
- Vibration amplitude of sound 1 0.05–5 mm
- Sound source Tuning fork, String
- Compare with a second sound
- Screen Pitch and loudness, Timbre, Sound level (dB)
Questions to explore
- How does the oscilloscope trace of an 880 Hz sound differ from that of a 440 Hz sound with the same amplitude?
- Why do a flute and a violin playing the same note sound different, and what is the same about them?
- By how many decibels does the sound level fall when you move from 2 m to 8 m from a loudspeaker?
