Sound & Waves
From the hum of a tuning fork to ripples on a lake, waves are everywhere—carrying energy, transferring momentum, and encoding information in rhythmic motion. In this chapter, we explore the physics of mechanical and acoustic waves using tools you already have: a smartphone, a microphone, and a bit of curiosity.
Sound waves are longitudinal pressure waves that travel through a medium, and their properties—frequency, amplitude, wavelength, and speed—can be measured, visualized, and even felt. With the help of smartphone apps and sensors, students can turn simple setups into powerful demonstrations of resonance, interference, standing waves, and harmonics. Whether you’re analyzing the beat frequency of two notes or mapping out an echo to measure the speed of sound, each experiment makes the invisible world of vibration visible and audible.
Many of these activities use everyday objects—bottles, straws, knives—but they lead to surprisingly deep insights. The chapter is designed to build intuition and appreciation for how sound is produced, transmitted, and perceived. More than just a study of noise, this is an invitation to explore the structured beauty of oscillations and discover that physics, quite literally, resonates.
Sound Propagation & Perception
Loudness and Decibel Measurements (WAVE-01)
Sensors Used: Microphone
What’s Measured: Sound intensity (decibels) as a function of distance
Description
A smartphone can be transformed into a powerful acoustic tool simply by installing a sound level meter app. In this experiment, students use their phone’s built-in microphone to measure how sound intensity, expressed in decibels (dB), changes with distance from a source. Set up a speaker or second phone emitting a constant tone, then position the measuring phone at various distances along a hallway or across a room. By recording the loudness readings at each location, students can begin to observe how sound diminishes—ideally following the inverse-square law, where intensity is inversely proportional to the square of the distance (1/r²).
However, this is also where the real world intervenes: reflections, absorption by surfaces, and the acoustics of the space may cause deviations from the theoretical model. This makes the activity not only a demonstration of sound propagation, but also a rich entry point into environmental acoustics. Students can analyze how architecture and materials influence sound decay, while also gaining a practical sense of how logarithmic decibel scales work. The experiment offers an elegant blend of physics, math, and real-world relevance—all from the palm of your hand.
Measuring the Speed of Sound via Echo (WAVE-02)
Sensors Used: Microphone, Timer
What’s Measured: Time delay between sound and echo
Description
This experiment turns your smartphone into a modern acoustic ruler, using echoes to calculate the speed of sound. In a large space with a flat reflective surface—such as a hallway, stairwell, or even an exterior building wall—generate a sharp sound like a hand clap or short burst from a tone generator app. Using your phone’s audio recorder or a sonar-style app, capture the moment of the sound and its echo. The key measurement is the time delay between the initial sound and its reflection returning from the surface.
With that delay and a known distance to the wall, you can apply the formula c=2d/tc = 2d / tc=2d/t, where ddd is the distance to the wall and ttt is the round-trip time, to estimate the speed of sound in air. You can try different distances, environments, or temperatures to see how sound speed varies.
As a real-world extension, watch a lightning strike and count the seconds until the thunder follows—about three seconds per kilometer. Both versions of the experiment highlight how sound travels through air, bounces off surfaces, and delivers information about distance, echo, and environmental conditions.
References:
[1] “Echolocation,” https://www.thenakedscientists.com/get-naked/experiments/echolocation
Doppler Effect Using Two Smartphones (WAVE-03)
Sensors Used: Microphone, Speaker
What’s Measured: Frequency shift due to motion (Doppler shift)
Description
The Doppler effect—familiar to anyone who’s heard a siren change pitch as it moves past—is a striking demonstration of wave physics and motion. This experiment brings it into the classroom using just two smartphones. One phone acts as a sound source, playing a steady tone (for example, 4000 Hz) through a sound generator app. The second phone, equipped with a frequency analysis or tuning app, becomes the receiver.
As you move one phone relative to the other—either by walking toward it, away from it, or placing it on a cart—you’ll hear and see a measurable pitch shift. The faster the relative motion, the more dramatic the shift in frequency. This shift is governed by the Doppler formula, allowing you to estimate the speed of motion based on the frequency change.
There are several variations: keep the source still and move the receiver, or rotate the source in a circular path to simulate orbital motion. You could also simulate the classic “ambulance effect” with a stationary mic and a moving tone source. Whether modeling planetary motion, binary stars, or emergency vehicles, this experiment makes the abstract concept of Doppler shift directly audible—and measurable.
Depending on what is known, you can use the measured frequency shift to either estimate the speed of the moving sound source or, conversely, infer the speed of sound itself, if you know the speed of the moving sound source. This flexibility makes the Doppler effect a versatile and powerful tool for exploring motion through waves.
Swapping Sounds: Exploring Binaural Hearing and Sound Localization (WAVE-04)
Sensors Used: Human hearing (no digital sensors required)
What’s Measured: Perceived direction of sound; spatial hearing effects
Description
Our ability to determine where a sound is coming from—whether a car to our left or a bird chirping behind us—relies on subtle differences in the time and intensity of sound arriving at each ear. This experiment turns that ability on its head. By constructing a simple device using two funnels and a pair of garden hoses, you can “swap” the ears—sending sounds from the left side to the right ear and vice versa.
A blindfolded participant wears this crossover hearing apparatus while another person creates sound at different locations around them. Despite the sounds occurring in real space, the swapped inputs confuse the brain’s natural localization system. Participants often misidentify the direction of the sound, pointing in the opposite direction or becoming disoriented. The effect is both amusing and striking, and it reveals just how dependent our auditory system is on binaural cues for spatial awareness.
This low-tech, high-impact demonstration shows how timing and phase differences between ears shape our sense of acoustic space, and it opens up broader discussions about perception, brain processing, and sensory illusions.
References:
[1] “Swapping Sounds,” https://www.thenakedscientists.com/get-naked/experiments/swapping-sounds
Sound Waves & Harmonics
Displaying Musical Frequencies with an Oscilloscope App (WAVE-05)
Sensors Used: Microphone
What’s Measured: Frequency and waveform of musical tones
Description
Sound becomes beautifully visible in this experiment, where students use a smartphone as both a sound generator and a waveform analyzer. One app generates pure tones—musical pitches like A4 (440 Hz)—while another app, acting as a digital oscilloscope, displays the corresponding waveforms in real time. What begins as an invisible hum suddenly appears as a crisp sine wave, a square wave, or a more complex shape depending on the sound’s harmonic content.
By adjusting the pitch and observing the resulting waveform, students can connect auditory experience to measurable frequency and waveform shape. This offers a compelling way to explore pitch, tuning, and the relationship between fundamental tones and their overtones. Watching the clean arcs of a sine wave transform into the jagged shapes of more complex tones provides insight into how musical instruments produce distinct timbres—even when playing the same note.
This hands-on visualization bridges physics and music, making abstract ideas like frequency, amplitude, and harmonics concrete and intuitive. Whether you’re analyzing the purity of a tuning fork or the complexity of a guitar note, the oscilloscope reveals the hidden structure of sound.
References:
[1] “Musical frequencies shown on a CRO,” http://practicalphysics.org/musical-frequencies-shown-cro.html
Exploring Acoustic Resonance with a Mug (WAVE-06)
Sensors Used: Microphone (optional)
What’s Measured: Resonant frequency and pitch variation based on excitation point
Description
A simple ceramic mug becomes a rich tool for exploring acoustic resonance when tapped in just the right way. In this experiment, students investigate how the vibrational modes of a mug change depending on where it’s struck—especially in relation to its handle. Using a pen or small object, they tap the rim of the mug at different angles and listen closely to the resulting tones.
Striking the mug directly opposite the handle typically produces a slightly lower pitch. That’s because the handle participates in the vibration, effectively adding mass and lowering the resonant frequency. However, when struck at a 45-degree angle from the handle, the vibration bypasses the handle’s influence, leading to a purer, higher-pitched sound. This difference illustrates how structural asymmetries and added mass influence the natural frequencies of objects.
Students may use a tuning app or spectrogram on their phone to measure the exact frequencies, but even without sensors, the pitch differences are clearly audible. This hands-on activity not only reveals the physics of everyday materials, but also draws connections between acoustics, design, and the underlying vibrational structure of familiar objects—proving that physics can live in your kitchen cabinet.
References:
[1] “Sounds from a Mug,” https://www.thenakedscientists.com/get-naked/experiments/sounds-mug
Creating a Straw Oboe to Explore Sound Production (WAVE-07)
Sensors Used: Microphone (optional), frequency analysis app
What’s Measured: Frequency, pitch variation
Description
With nothing more than a drinking straw and a pair of scissors, students can build a simple wind instrument that offers deep insight into the physics of sound. By flattening one end of the straw and cutting it into a triangular shape—forming two vibrating “lips”—they create a crude but functional double-reed, much like the kind found in oboes or bassoons.
Blowing through the modified end causes the reed to vibrate rapidly, setting the air inside the straw into resonance. The result is a buzzing tone that varies in pitch depending on the straw’s length. As students snip away sections to shorten the straw, the pitch rises—demonstrating the inverse relationship between the length of an air column and the frequency of the sound it produces. Lengthen the straw, and the pitch falls accordingly.
This playful experiment is remarkably rich in physics: students can explore concepts like resonance, harmonic series, and the mechanics of reed-based sound production. With a tuning or FFT app, they can even visualize the fundamental frequency and overtones. Whether used in physics class, music class, or a curious afternoon at home, the straw oboe turns everyday materials into a resonant learning experience.
References:
[1] “Straw Oboe,” https://www.thenakedscientists.com/get-naked/experiments/straw-oboe
[2] “Straw Oboe,” http://www.exo.net/~pauld/activities/RAFT/strawoboe.html
Exploring Sound Production by Blowing Across Bottle Openings (WAVE-08)
Sensors Used: Microphone (optional), frequency analyzer app
What’s Measured: Resonant frequency, pitch variation
Description
Blowing across the top of a bottle doesn’t just make a sound—it opens a doorway into the physics of resonance. In this classic experiment, students explore how pitch is shaped by the volume of air inside a container. When you blow gently across the bottle’s mouth, the air inside vibrates, producing a musical note. This note corresponds to the resonant frequency of the air column enclosed by the bottle.
By adding or removing water, the effective length of the air column changes. More water means less air space, leading to higher-frequency (higher-pitched) tones. Conversely, emptying the bottle lowers the pitch by increasing the air column’s length. This direct, audible relationship between air volume and frequency makes the bottle an elegant model of an open-ended resonator.
Students can go further by using a frequency analyzer app to measure the exact pitch and track how it changes with each adjustment. Whether comparing bottles of different shapes or testing how temperature affects sound, this experiment transforms an everyday object into a hands-on lesson in the physics of sound.
References:
[1] “Blowing on bottles,” https://www.thenakedscientists.com/get-naked/experiments/blowing-bottles
Estimating the Fill Level of a Propane Gas Bottle Using Resonance (WAVE-09)
Sensors Used: Microphone, frequency analyzer or FFT app
What’s Measured: Resonance frequency shift
Description
A propane tank can be surprisingly talkative—if you know how to listen. This experiment uses acoustic resonance to estimate the fill level of a propane gas bottle, turning a simple knock into a diagnostic tool. When you gently tap the side of the tank, it responds with a low-pitched ring. But that pitch depends on what’s inside.
The denser liquid propane dampens the vibrations differently than the gas above it. As a result, tapping near the empty section produces a higher-pitched sound, while tapping near the filled portion yields a lower tone. With a smartphone and a frequency analysis app or FFT tool, students can record and analyze these subtle shifts in pitch.
By comparing frequency readings at different heights along the bottle, it becomes possible to estimate the boundary between gas and liquid—and thus gauge the remaining propane. It’s a clever, non-invasive way to explore how sound interacts with materials of different densities, and a practical example of using physics to solve everyday problems.
Resonance and Harmonics in Metal Objects (WAVE-10)
Sensors Used: Microphone, camera (optional high-speed video), tone generator
What’s Measured: Resonant frequencies, overtone structure, vibrational patterns
Description
Resonance isn’t just for strings and air columns—it’s alive in the solid world, too. This experiment explores how metal objects like table knives, hacksaw blades, and glassware vibrate, resonate, and produce complex harmonic structures. Using only a smartphone and a few common tools, students can observe and analyze these effects in a hands-on, highly visual way.
Start by striking a table knife while holding it at various points—near the handle, near the tip, or suspended in the middle. You’ll notice how the sound changes dramatically depending on where it’s held, revealing how the placement of your fingers affects the vibrational modes. A clear, ringing tone indicates freer vibration (fewer nodes introduced), while a dull sound suggests suppressed motion.
Next, clamp a hacksaw blade to a bench or table so that one end extends freely. Use a tone generator app on your phone to slowly sweep through low frequencies (100-300 Hz), and listen for a peak in vibration—that’s the blade’s resonant frequency. You can feel it with your fingers or pick it up with a smartphone microphone and a spectrum analyzer app. The resonance can be loud and surprising for such a simple setup.
For deeper analysis, strike a bell, a rod, or a wine glass and use a spectrogram or FFT app to examine its overtone series. Most objects don’t just vibrate at one frequency—they produce multiple harmonics simultaneously. These combinations are what give each object its unique “timbre” or sound character.
To add a visual layer, try filming the hacksaw blade’s motion with your phone’s high-speed video mode. You may catch a blur or even distinct nodal patterns depending on the frame rate and lighting. For better visibility, place a small reflective sticker near the blade’s tip to trace its motion more clearly in the footage.
This unified experiment invites students to explore resonance, damping, harmonic complexity, and vibrational physics—all with common objects and digital tools. It’s where sound, sight, and structure come together in one elegant demonstration of wave behavior in solids.
References:
[1] “Harmonic Knives,” https://www.thenakedscientists.com/get-naked/experiments/harmonic-knives
[2] “Hacksaw Blade Resonator,” Practical Physics (archived), https://web.archive.org/web/20181022215430/http://practicalphysics.org/Hacksaw-blade-resonator.html
Understanding Sound Production in Musical Instruments (WAVE-11)
Sensors Used: Microphone
What’s Measured: Frequency spectrum, pitch, overtone structure
Description
Musical instruments produce rich and varied sounds, but at their core, these sounds are just vibrations — of strings, air columns, or surfaces — shaped by physical structure and material. This experiment invites students to explore the physics behind musical sound by listening, recording, and analyzing a range of instruments or sound-producing objects using their smartphone microphones and an FFT (Fast Fourier Transform) or spectrum analyzer app.
Start by recording simple sources such as a tuning fork, a plucked guitar string, or a blown recorder. Then move on to more accessible objects — rubber bands, bottles, spoons — and even the human voice. When you view these recordings in the frequency domain, you’ll see not just a single pitch, but a structure of overtones layered above the fundamental. These overtone “fingerprints” give each instrument its characteristic timbre and are what distinguish a flute from a violin, even when they play the same note.
You can go further by comparing instruments made from different materials, or by altering tension or length in stringed or wind instruments. This experiment provides a gateway from waveform to waveform analysis — showing how pitch, timbre, and resonance are not just heard, but seen. It’s where physics meets music in a deeply intuitive way.
References:
[1] “Musical instruments,” http://practicalphysics.org/Musical-instruments.html
Interference & Standing Waves
Two-Tone Beat Frequency (WAVE-12)
Sensors Used: Microphone
What’s Measured: Interference pattern (beat frequency), audio waveform
Description
When two sound waves with slightly different frequencies combine, they create an auditory phenomenon known as a beat frequency — a slow, rhythmic pulsing caused by the constructive and destructive interference of the waves. This experiment lets you explore this beautifully simple acoustic effect using just two smartphones.
Play a steady tone (e.g. 440 Hz) from one phone using a tone generator app, and a second tone (e.g. 444 Hz) from another. As the sounds overlap in space, you’ll hear a rhythmic “wah-wah” sound — the beat frequency — which occurs at the rate of the difference between the two tones (in this case, 4 Hz). The effect is most noticeable when both phones are placed side-by-side or directed toward a common listening point.
To take it a step further, use a microphone and an oscilloscope or spectrogram app to visualize the waveform or amplitude modulation. You’ll be able to see how the signal swells and fades as the waves go in and out of phase. It’s a clean, compelling way to demonstrate wave interference and a foundational principle of acoustics — all with equipment already in your pocket.
Standing Waves in Tubes: Visualizing Resonance with Sound (WAVE-13)
Sensors Used: Microphone, Camera (optional)
What’s Measured: Resonant frequencies, standing wave patterns, node spacing
Description
Standing waves aren’t just an abstract idea — they can be seen and heard with surprising clarity in everyday objects. This experiment demonstrates how longitudinal standing waves form in a tube or pipe when sound is introduced at just the right frequency. With a smartphone, a speaker, and a simple tube (such as a cardboard mailing tube or plastic pipe), you can bring these patterns to life.
Place the tube vertically and use a speaker or tone generator app to play a steady tone near its opening. Sweep through frequencies slowly and listen for resonances — specific frequencies where the sound suddenly becomes louder or richer. At these points, standing waves form inside the tube. If you place lightweight beads (like Styrofoam or rice) inside, they’ll visibly bounce at the antinodes — points of maximum vibration — while remaining still at the nodes.
You can also record the audio using your smartphone microphone and visualize the peaks with a spectrogram or FFT app. These visual tools let you see the resonant peaks corresponding to different harmonic modes. For a hands-on extension, use a measuring tape to mark node spacing and calculate the wavelength and speed of sound.
Whether you’re hearing the clear amplification or watching the beads dance to hidden waves, this experiment offers a vivid demonstration of acoustic resonance, harmonics, and the physics of musical instruments — with just a tube and a phone.
References:
[1] “Longitudinal standing waves,” http://practicalphysics.org/Longitudinal-standing-waves.html
Interference of Sound Waves (Young’s Fringes) (WAVE-14)
Sensors Used: Microphone, Human Ear
What’s Measured: Sound intensity variation due to interference
Description
When two sound waves meet, they can interfere — sometimes amplifying each other, other times cancelling out completely. This experiment lets you hear and map that interference pattern, much like the famous double-slit experiment with light, but using nothing more than two speakers and your ears (or a smartphone microphone).
Begin by placing two identical speakers side by side and playing a pure tone from both at the same time. As you walk slowly in front of them, you’ll notice that the sound seems to swell and fade — growing loud, then soft, then loud again. These loud and quiet spots are the result of constructive and destructive interference: regions where the waves align or cancel out due to path differences.
To sharpen the effect, block one ear with a finger and move laterally across the wave field. You’ll be able to “feel” the wave pattern with remarkable clarity. For an extra demonstration, have a student stand at a quiet spot — a point of destructive interference — then cover one of the speakers with a sound-absorbing material, like a cushion. The sound will instantly become louder, as the interfering wave has been removed.
For a more quantitative approach, use a smartphone microphone and a sound intensity meter app to track the variation in sound level as you move across the interference pattern. This experiment elegantly demonstrates wave superposition, interference fringes, and phase relationships — all in a way that’s immediately audible.
References:
[1] “Young’s fringes with sound waves,” http://practicalphysics.org/youngs-fringes-sound-waves.html
Seeing Sound Waves
Visualizing Tuning Fork Vibrations (WAVE-15)
Sensors Used: Camera (high frame rate), optional laser and mirror setup
What’s Measured: Oscillation patterns, frequency, waveform
Description
Sound is invisible, but with a bit of creativity, you can make its motion visible. A tuning fork provides the perfect entry point into this hidden world of vibration.
Start with the simplest approach: strike a tuning fork and gently dip it into a shallow dish of water. You’ll see ripples erupt from the point of contact, revealing the mechanical vibrations transferred from metal to liquid. If your phone supports slow-motion video (ideally 240 fps or more), film the vibrating prongs directly. With a fork of low enough frequency (e.g., 100 Hz), you can actually observe the fork’s oscillations frame by frame.
To go further, try attaching a small mirror to one prong of the tuning fork. Shine a laser onto the mirror and reflect the beam onto a distant wall. As the tuning fork vibrates, the mirror moves, and the laser spot dances in a sinusoidal path. Introduce a rotating mirror into the setup, and you’ll stretch this pattern into a waveform that maps the vibration over time — effectively turning sound into a visible graph.
A similar approach works with loudspeakers. Connect a speaker to a tone generator and place a lightweight membrane or reflective surface (like a balloon or stretched plastic) in front of it. When the speaker plays a tone, the surface vibrates. Shine a laser onto it and observe the reflected beam flicker with the sound’s frequency. You can even use a strobe light to “freeze” the vibration.
Together, these techniques offer a powerful, hands-on demonstration of resonance, wave motion, and harmonic oscillation — making sound not only heard, but seen.
References:
[1] “Vibrations of a Tuning Fork,” https://av.tib.eu/media/30436
[2] “Seeing sound waves,” http://practicalphysics.org/seeing-sound-waves.html
Visualizing Sound Wave Patterns in Water (WAVE-16)
Sensors Used: Smartphone speaker or external speaker, camera (optional for video)
What’s Measured: Surface vibration patterns
Description
Sound waves are invisible, but under the right conditions, they leave stunning imprints on matter. In this experiment, sound becomes visible through its influence on water—specifically, by generating Faraday waves. These standing wave patterns emerge on the surface of a shallow water layer when it’s vibrated at the right frequency and amplitude.
To begin, place a thin layer of water in a shallow dish or tray and position a speaker nearby—but not in the water. A small portable speaker or even your smartphone speaker (if protected) can be used to deliver sound. Play pure tones using a frequency generator app and gradually increase the volume and frequency. As you tune through different pitches, you’ll see the water surface begin to shimmer, and at certain resonant frequencies, beautiful symmetrical patterns—rings, grids, or polygons—will emerge.
These ripples aren’t just pretty—they’re physics in action. The patterns represent the interaction between the driving force of the sound and the restoring force of surface tension, forming stationary waves where crests and troughs stand still. You can capture them with your smartphone camera, or even record video for frame-by-frame analysis.
This demonstration brings abstract wave concepts to life in a visceral, almost hypnotic way—transforming invisible sound into geometry you can see.
Visualizing Sound Waves with Chladni Plate Patterns (WAVE-17)
Sensors Used: Smartphone (audio source or video capture), optional external speaker or signal generator
What’s Measured: Mode shapes and vibration patterns
Description
Sound isn’t just something we hear—it can be something we see. The Chladni plate experiment is a striking way to make the resonant patterns of vibrating surfaces visible. By sprinkling fine sand on a metal plate and exciting it with sound, students can observe the formation of symmetrical, intricate patterns that reveal how the plate vibrates.
To perform the experiment, secure a thin metal or acrylic plate horizontally. Attach a speaker or a tone-generating device (such as a smartphone app or a piezoelectric transducer) to the plate. As the plate is vibrated at specific frequencies, standing wave patterns develop. The sand dances away from areas of strong vibration and settles along nodal lines—where the plate remains still—forming the characteristic Chladni figures.
Each frequency produces a different pattern, illustrating the complex vibrational behavior of two-dimensional systems. Using your phone to play tones and record video adds a layer of interactivity, making the experiment both accessible and mesmerizing.
This experiment bridges physics, music, and art—transforming sound into shape, and resonance into revelation.
References:
[1] “Amazing Resonance Experiment!” https://www.youtube.com/watch?v=wvJAgrUBF4w
Visualizing Vibrations Using a Rubber Sheet (WAVE-18)
Sensors Used: Smartphone (camera or video), optional speaker or signal generator
What’s Measured: Vibration modes and nodal patterns
Description
A stretched rubber sheet can serve as a surprisingly powerful tool for visualizing how waves move and interfere in two dimensions. Much like the surface of a drum, the sheet supports complex standing wave patterns when excited at the right frequencies. These patterns—comprised of nodal lines (where the sheet doesn’t move) and antinodes (areas of maximum vibration)—are physical manifestations of wave interference.
To perform the experiment, stretch a rubber membrane tightly over the mouth of a bowl or frame. Use a tone generator app on your smartphone connected to a small speaker or mechanical oscillator to vibrate the sheet. As the frequency is varied, certain resonant modes emerge, producing distinct symmetrical patterns. Sprinkle fine sand, rice, or powder on the surface to make the nodes visible as the material collects along stationary regions.
These shapes can be analyzed and compared to theoretical Bessel functions, which describe the vibration modes of circular membranes. Recording with your phone’s camera—especially in slow motion—makes it easier to capture and analyze these mesmerizing patterns.
This is a tactile, visual introduction to wave physics, revealing the hidden geometries that arise when sound and structure meet.
References:
[1] “Vibrations of a rubber sheet,” http://practicalphysics.org/vibrations-rubber-sheet.html
Advanced Concepts
Exploring Path Differences and Phase Difference in Wave Interference (WAVE-19)
Sensors Used: Microphones, smartphone oscilloscope or waveform recorder apps
What’s Measured: Phase differences due to path length variation
Description
This experiment dives into the heart of wave interference by demonstrating how differences in the distance sound waves travel can lead to observable phase differences. These phase shifts give rise to the familiar phenomena of constructive and destructive interference and serve as the foundation for much of wave physics—acoustic or optical.
Using a fixed-frequency tone from a smartphone speaker or signal generator, place two smartphones (or one smartphone and one external mic) at different distances from the source. Record the received sound on both devices. Alternatively, feed both microphones into a two-channel oscilloscope app to directly compare the waveforms in real time. You’ll likely see the sound waves arriving out of phase, depending on the difference in path length to each mic.
As you move one microphone closer to or farther from the speaker, the phase relationship between the two waveforms changes. At certain distances, the waves may be perfectly in sync (constructive interference), while at others they’ll cancel each other out (destructive interference). These phase flips occur whenever the path difference equals half a wavelength (λ/2), allowing you to measure the wavelength directly with a ruler.
This experiment can also be run in a more visual mode by having students walk through the room with their ears or a second phone, detecting “hot spots” and “quiet spots” of sound interference in space. It’s a tangible, measurable way to experience wave behavior—by ear, by ruler, and by waveform.
References:
[1] “Path differences and phase difference,” http://practicalphysics.org/Path-differences-phase-difference.html
Estimating Jet Engine RPM Using In-Flight Sound and Fourier Analysis (WAVE-20)
Sensors Used: Microphone
What’s Measured: Dominant frequencies in engine noise
Description
As your plane cruises high above the Earth, the steady hum of the jet engines isn’t just background noise—it’s a real-world signal carrying measurable data. In this experiment, you can use your smartphone to record that sound and estimate the rotational speed (RPM) of the engine’s internal components.
Sit near the wing during takeoff or cruising, and use a sound recording app to capture a short sample of the engine’s hum. Then, use a Fourier transform or spectrogram app to analyze the audio. You’ll likely see a set of clear frequency peaks—these correspond to the blade-passing frequencies of different engine components, particularly the N1 fan (the large front fan) and N2 core (the high-pressure compressor).
The frequency f of these peaks can be related to the rotational speed using the formula: RPM = (f × 60) / n, where n is the number of fan or turbine blades contributing to that tone.
For example, on a Boeing 737, the N1 fan might produce a prominent tone at ~500 Hz during cruise. With 24 blades, this suggests: RPM ≈ (500 × 60) / 24 ≈ 1,250 RPM, which could be one harmonic of the full rotation speed. With harmonics and higher-order peaks, you may be able to estimate values closer to N1 ≈ 5,500 RPM or N2 ≈ 14,000 RPM.
This experiment connects spectral physics to modern aviation and turns a passive flight into an active exploration of real-world engineering through sound.
Capstone / Research Extension
Measuring the Speed of Sound in Air and Water Using Piezo Sensors (WAVE-21)
Sensors Used: Piezoelectric elements (or buzzers), smartphone oscilloscope or audio input app
What’s Measured: Time delay of sound signal across different media
Description
This hands-on experiment reveals the hidden speed of sound as it races through air, water, and even solid materials—using piezo sensors and a smartphone. The setup is simple but powerful: place two piezoelectric elements (commonly used as buzzers or contact microphones) at opposite ends of a plastic container. One piezo serves as the transmitter, driven by a signal generator (via phone or microcontroller), while the other acts as the receiver, connected to a smartphone audio input or oscilloscope app.
Start by filling the container with air and sending a sharp sound pulse. Measure the time delay between the sent and received signals. Then repeat the measurement with the container filled with water. The speed of sound in air (~343 m/s) is significantly slower than in water (~1493 m/s), and the difference in time delay becomes dramatically apparent over the same path length.
To refine accuracy, first place both piezos in contact (no travel distance) and record the baseline delay of your system—this accounts for electronic lag. Subtract it from subsequent measurements. You can then calculate the speed of sound using the basic formula:
v = d / t, where d is the distance between sensors, and t is the corrected time delay.
For advanced exploration, test other materials: measure the time it takes a sound pulse to travel through a metal rod, or use known tuning fork frequencies to estimate the speed of sound in iron (about 5130 m/s). Some tuning forks are labeled with both frequency and wavelength, which lets you back-calculate the wave speed.
This experiment beautifully connects electronics, acoustics, and material science—giving students a tactile way to measure invisible waves flying through different substances.
Acoustic Levitation: Floating Small Objects Using Sound Waves (WAVE-22)
Sensors Used: Microphone (for tuning), Camera (for observation)
What’s Measured: Levitation behavior, stability, standing wave position
Description
Acoustic levitation may sound like science fiction, but with a smartphone, a pair of high-frequency transducers, and a little patience, it becomes a hands-on demonstration of how sound can suspend matter in midair.
In this capstone experiment, you’ll set up two opposing speakers or piezoelectric elements, aligned precisely to face one another, and drive them with a high-frequency tone—ideally around 20 kHz, at the edge of human hearing. A tone generator app on your phone, paired with a small amplifier, provides the input signal.
When tuned correctly, the speakers generate a standing wave between them—regions where sound waves interfere constructively and destructively. At specific locations in this wave, the acoustic pressure is just enough to counteract gravity. Carefully place a lightweight particle—such as a styrofoam bead or grain of rice—into the wave’s path. If conditions are right, the particle will hover, seemingly defying gravity, locked in place by the invisible grip of the sound field.
Use a smartphone camera to film the levitation, observing how small adjustments in frequency or alignment affect the particle’s stability. Advanced variations include using multiple nodes for multi-particle levitation, or visualizing pressure zones by adding a bit of fog or dust.
More than a parlor trick, acoustic levitation reveals how pressure waves can exert real forces, and how resonance and symmetry create structures out of pure sound. This experiment not only connects wave physics to cutting-edge research in materials handling and non-contact manipulation—it also delivers a little magic, built entirely from sound.
References:
[1] “Acoustic Levitation,” The Naked Scientists, https://www.thenakedscientists.com/get-naked/experiments/acoustic-levitation