Amplitude

The amplitude of a wave is the maximum value of the deviation between the oscillating physical quantity and its equilibrium position.

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How to measure it in class

With the FizziQ app, students can visualize the amplitude of a sound in real time using the sound recording module.

Steps:

Open FizziQ and select the Sound level or Oscillogram module to display the signal captured by the microphone.
Produce a soft sound (whispering) then a loud sound (hand clap) at the same distance from the smartphone.
Observe on the screen the difference in the height of the oscillations: the loud sound produces much higher peaks than the soft sound.
Conclude that the amplitude of the signal is directly related to the perceived sound volume. Students can record their measurements in FizziQ’s experiment notebook.

Scientific activities on this topic

Several experiments easily performed with a smartphone, a tablet or a computer allow you to measure and understand the amplitude of a sound signal.

Learn more

A bit of history

The notion of amplitude was formalized in the 17th century with Christiaan Huygens’ work on pendulums. He observed that the height of a pendulum’s swing determines the energy of the motion. This idea was later extended to all waves.

Amplitude and energy

The energy carried by a wave is proportional to the square of its amplitude. Doubling the amplitude of a wave quadruples its energy. This is why a high-amplitude earthquake is far more destructive than a low-amplitude one.

Orders of magnitude

The pressure amplitude of a sound at the threshold of hearing is about 20 µPa. At a rock concert, it can reach several pascals. The amplitude of an ocean wave ranges from a few centimeters (calm sea) to more than 15 meters during a storm.

Complementary experiments

The experiment Chloe at the concert allows students to study how sound amplitude varies with distance to the source.

Formula

For a sinusoidal wave: y(t) = A × sin(2π × f × t)

Meaning:

A: maximum amplitude (unit of the oscillating quantity, for example Pa for sound)

f: frequency of the wave (Hz)

t: time (s)

y(t): value of the oscillating quantity at time t

Application examples

The volume of a speaker increases when the amplitude of the electrical signal sent to the loudspeaker increases
Ocean waves: a big wave has a much greater amplitude than a small ripple
Earthquakes: the amplitude of seismic waves determines the intensity of the shaking felt
A tuning fork struck hard produces a high-amplitude sound, struck gently a low-amplitude sound
The volume setting on a smartphone changes the amplitude of the audio signal sent to the earphones

FAQ

Q: What is the difference between amplitude and frequency? A: Amplitude measures the intensity of an oscillation (loud or soft sound), while frequency measures the number of oscillations per second (high or low pitch). These are two independent characteristics of a wave.

Q: Does amplitude change the pitch of a sound? A: No. Amplitude changes the volume of the sound (loud or soft), not its pitch. Pitch depends on frequency. An A at 440 Hz remains an A, whether played loud or softly.

Q: How do you measure the amplitude of a sound with a smartphone? A: With FizziQ, you can display the oscillogram of the sound captured by the microphone. The amplitude can be read as the maximum height of the signal peaks from the rest line.

Q: Does amplitude decrease with distance? A: Yes. The amplitude of a wave decreases as you move away from the source. This is attenuation. For sound, the sound intensity decreases with the square of the distance.

Q: What unit is used for amplitude? A: The unit depends on the oscillating physical quantity. For a sound wave, the overpressure is measured in pascals (Pa). For a mechanical wave, amplitude is expressed in meters (m).

Frequency - Period - Sound Level - Decibel - Sound Intensity - Oscillogram