A bubble without noise

This activity allows students to understand the principle of destructive interference used in active noise reduction headphones. It develops the ability to visualize complex wave phenomena.

Have you ever wondered how noise-canceling headphones manage to create silence in a noisy airplane cabin? The answer lies in one of the most elegant phenomena in physics: wave interference. When two identical waves meet in opposite phase, they can cancel each other out completely, producing silence from sound. This principle, first described by Thomas Young in 1801 for light waves, applies equally to sound waves. In everyday life, we experience interference without realizing it: the strange acoustic effects in certain rooms, the dead spots in a concert hall, or the fluctuating volume of a car stereo at highway speeds. Active noise cancellation technology exploits destructive interference by generating an anti-noise wave that mirrors incoming sound but with inverted phase. Modern headphones achieve remarkable noise reduction of up to 30 dB using tiny microphones and fast signal processors. In this experiment, students recreate the fundamental principle behind this technology using three smartphones, discovering firsthand how two sound sources can create zones of silence and zones of amplification.

Activity overview:

The student explores the phenomenon of sound interference using three smartphones: two simultaneously emitting a pure 680 Hz sound and a third measuring sound intensity. By moving the measuring smartphone between the two sources, the student identifies the areas of constructive (amplified sound) and destructive (attenuated sound) interference and then makes the link with the active noise reduction technology of modern headphones.

Level:

High school

FizziQ

Author:

Duration (minutes) :

45

What students will do :

- Observe and identify zones of constructive and destructive interference between two coherent sound sources
- Measure the spatial distribution of interference fringes and relate it to the wavelength
- Understand the relationship between wavelength, frequency, and the speed of sound
- Explain the operating principle of active noise cancellation technology
- Develop experimental skills in acoustic measurement using smartphone sensors

Scientific concepts:

- Wave interference
- Constructive and destructive overlap
- Wavelength
- Anti-noise
- Active noise control

Sensors:

- Microphone (sound level meter)
- Sound frequency analyzer

Material needed:

- Three smartphones including at least one with the FizziQ application
- A quiet space with few echoes
- A tape measure to measure distances
- FizziQ experience notebook

Experimental procedure:

Open the FizziQ application on all three smartphones. On two of them, go to the Synthesizer tool and set the frequency to exactly 680 Hz on both devices.
Place the two emitter smartphones on a flat surface, approximately 50 cm apart, with their speakers facing upward. Ensure they are at the same height.
On the third smartphone (the measuring device), open FizziQ and select the Sound Level (dB) sensor from the measurement tools.
Start the sound emission on both smartphones simultaneously. Verify that both are producing a steady 680 Hz tone.
Hold the measuring smartphone at the same height as the speakers, midway between the two sources. Record the initial sound level reading.
Slowly move the measuring smartphone along the line connecting the two sources, pausing every 2 cm to record the sound level.
Note the positions where the sound level is maximum (constructive interference) and where it is minimum (destructive interference).
Measure the distance between two consecutive minima. This distance should be approximately equal to half the wavelength (about 25 cm for 680 Hz).
Record at least 10 measurements at different positions between the two sources in your FizziQ experiment notebook.
Now move the measuring smartphone perpendicular to the line connecting the sources, starting from the midpoint. Observe how the interference pattern changes.
Try changing the frequency to 440 Hz on both emitters and repeat the measurements. Compare the spacing of the interference fringes.
Record all your data and create a graph of sound level versus position in your FizziQ notebook.

Expected results:

Students should observe clear alternation between zones of high sound intensity (constructive interference, where the sound level approaches the sum of both sources) and zones of low intensity (destructive interference, where the sound level drops significantly, potentially by 10-20 dB). The distance between consecutive minima should be approximately 25 cm for 680 Hz, corresponding to half the wavelength. When the frequency is changed to 440 Hz, the spacing between fringes should increase to about 39 cm. The interference pattern will not be perfectly regular due to reflections from walls and furniture, background noise, and slight differences between the two smartphone speakers. Students should expect measurement noise of ±2-3 dB and may find that the minima are not as deep as theoretically predicted.

Scientific questions:

- Why does the distance between interference fringes change when you modify the frequency of the emitted sound?
- What would happen if the two sources emitted slightly different frequencies instead of identical ones?
- Why are noise-canceling headphones more effective against low-frequency sounds than high-frequency ones?
- How does the interference pattern change if you increase the distance between the two sources?
- What role does the speed of sound play in determining the interference pattern?
- Why is perfect destructive interference difficult to achieve in a real room environment?

Scientific explanations:

When two sound waves of the same frequency meet, they superimpose according to the principle of interference. This superposition can be: 1) Constructive, when the maxima of the waves coincide, doubling the amplitude and quadrupling the sound intensity; 2) Destructive, when the maximum of one wave coincides with the minimum of the other, ideally leading to complete cancellation of sound.

For a pure sound of 680 Hz, the wavelength λ is approximately 50 cm (λ = c/f, where c is the speed of sound, approximately 343 m/s at 20°C). Interference follows a precise spatial pattern: the distance between two consecutive destructive interference zones is λ/2, or approximately 25 cm for 680 Hz.

This phenomenon is exploited in active noise reduction technology. Noise-canceling headphones use microphones to pick up ambient sounds, then instantly generate a sound wave of the same amplitude but in opposite phase (shifted by 180°).

The superposition of these two waves creates destructive interference which significantly attenuates the noise perceived. This technology is particularly effective for low-frequency, relatively constant sounds (such as the roar of an airplane engine), but less so for high-pitched or sudden sounds due to the system's responsiveness limitations.

The proposed experiment presents a static version of the phenomenon: the two sound sources create a fixed interference field in space, with alternating areas of reinforcement and cancellation. In contrast, active reduction headphones must dynamically generate this interference field in real time to adapt to constant variations in the sound environment.

The main challenge of this technology lies in the speed and precision with which the system can analyze the incident sound and produce the corresponding anti-noise, with a delay typically less than 1 ms.

Extension activities:

Frequently asked questions:

Q: The sound level does not seem to change much as I move the phone between the sources. What is wrong?
R: Make sure both smartphones are emitting at exactly the same frequency (680 Hz) and at similar volume levels. Also ensure you are moving slowly and in a straight line between the two sources. Reflections from nearby walls can wash out the interference pattern, so try the experiment in a larger, open space.

Q: Why do I not get complete silence at the destructive interference points?
R: Perfect cancellation requires two waves of exactly equal amplitude, perfectly matched frequency, and no reflections. In practice, the two smartphone speakers will differ slightly in output, and room reflections create additional waves that prevent complete cancellation.

Q: Can I use Bluetooth speakers instead of smartphone speakers?
R: Bluetooth introduces a variable delay (latency) that can shift the phase relationship unpredictably. It is better to use the built-in speakers of the smartphones to maintain coherent emission.

Q: Why is the spacing between minima about 25 cm and not 50 cm?
R: The distance between consecutive minima equals half the wavelength (λ/2), not the full wavelength. For 680 Hz, λ = 343/680 ≈ 50 cm, so λ/2 ≈ 25 cm.