The oscillogram is the curve that represents the amplitude of a signal as a function of time. For a sound, it reveals the shape of the wave: a sinusoid for a pure tone, a complex but repeating pattern for an instrument’s note, an irregular trace for noise.
Discover FizziQ
How to measure it in class
On a smartphone, the oscillogram displays in real time the signal captured by the microphone: at regular intervals, the microphone measures the acoustic pressure and FizziQ plots these successive values along the time axis.
Steps:
- Open FizziQ and select the Amplitude (oscilloscope view) instrument among the microphone measurements: it is the one that draws the oscillogram of the sound, on a switchable time scale.
- Emit a pure tone (tuning fork, sustained note) and observe the sinusoidal shape of the trace.
- Pronounce a vowel and compare the complex yet periodic pattern obtained.
- Produce noise (crumpling paper) and note the irregular, non-periodic trace.
- Freeze the screen to read the period T on the time axis, by spotting a repeating pattern.
- Record and export the curve to measure the period and compute the frequency f = 1/T.
Scientific activities on this topic
- Timbre and harmonics
- Spectrum of sung vowels
- Tuning forks: pitch of A
- Harmonic and inharmonic sounds
See also the smartphone sensors guide.
Learn more: how it works
The microphone converts acoustic pressure variations into an electrical signal, sampled by the analog-to-digital converter at a regular rate, typically 44,100 times per second. Each sample is an instantaneous amplitude measurement; connecting these points reconstructs the wave’s curve. Since the sampling frequency is well above audible frequencies, the trace faithfully renders the shape of the sound. Unlike a laboratory oscilloscope, the smartphone’s oscilloscope only visualises the sound signals captured by the microphone: you cannot feed an electrical signal directly into it. On this curve, three pieces of information can be read directly: the shape of the wave (sinusoidal or complex), the amplitude (height of the oscillations, related to the sound’s intensity) and the period T, duration of a repeating pattern, from which the frequency follows.
Orders of magnitude:
- Common sampling rate: 44,100 Hz, that is, one point every 23 µs or so.
- Audible range: 20 Hz to 20,000 Hz, that is, periods from 50 ms to 50 µs.
- Tuning fork at 440 Hz: sinusoid of period T ≈ 2.3 ms.
- Male spoken voice, around 120 Hz: period close to 8 ms.
- Sampling theorem: you must sample at more than twice the highest frequency of the sound (at least 40 kHz for the audible range).
Formula
The frequency of the sound follows from the period measured on the oscillogram:
f = 1 / T, with f in hertz (Hz) and T in seconds (s), the period of a repeating pattern.
For example, a pattern of period T = 2.3 ms read on the curve gives f = 1 / 0.0023 ≈ 440 Hz.
Application examples
- Measure the period of a tuning fork and derive its frequency.
- Compare the waveforms of a flute and a guitar playing the same note.
- Visually distinguish a pure tone (sinusoid) from a complex sound (rich but periodic pattern).
- Show that noise presents no regular periodic pattern.
- Observe the attack and decay of a note (variation of amplitude over time).
- Relate the height of the oscillations to the perceived intensity of the sound.
FAQ
Q: What exactly does the height of the oscillations represent on the oscillogram? A: It represents the amplitude of the signal, that is, the extent of the acoustic pressure variations. The higher the oscillations, the more intense the sound, but this height is not calibrated in pascals.
Q: How do I measure the frequency of a sound on an oscillogram? A: Spot a repeating pattern, read its duration T (the period) on the time axis, then compute f = 1/T. A short pattern corresponds to a high-pitched sound, a long pattern to a low-pitched one.
Q: Why does noise not give a regular curve? A: Noise has no period: it results from the disordered superposition of many frequencies. Its trace is therefore irregular and shows no repeating pattern, unlike a musical sound.
Related concepts
Amplitude - Period - Pure tone - Complex sound - Fundamental frequency - Microphone - Spectrogram