Examples of Measuring Sound Speed in Air: 3 Practical Examples

Let’s start with the three headline experiments you can actually run. Each example of a sound-speed measurement uses a different idea: timing echoes, timing two microphones, and measuring resonances in a tube. Together, these give you a nice set of examples of measuring sound speed in air: 3 practical examples that span home, classroom, and more advanced lab setups.

Example 1: Echo timing along a hallway or outdoor wall

One of the simplest examples of measuring sound speed in air is the classic echo experiment. You make a sharp sound, record it, and measure the time until you hear the echo from a distant wall or building.

Basic setup in plain language

You need:

• A long, straight distance (a hallway, gym, or outdoor wall). Think 100–200 feet if possible.
• A smartphone with a voice recorder or audio app that shows the waveform.
• A tape measure or a laser distance measurer.

How it works

Stand a known distance from a large flat surface. Clap your hands or pop a balloon and record the sound. On the waveform, you’ll see two big spikes: the original sound and the echo. The sound travels to the wall and back, so the total distance is twice the wall distance.

Speed of sound \(v\) is then

\[
v = \frac{2d}{\Delta t}
\]

where \(d\) is the distance to the wall and \(\Delta t\) is the time between the original clap and the echo.

Real-world example

Suppose you stand 150 ft from a concrete wall and record a balloon pop. In your audio app, the time between the original pop and the echo is 0.27 s.

• Total distance traveled: 2 × 150 ft = 300 ft
• Speed: 300 ft / 0.27 s ≈ 1,111 ft/s ≈ 339 m/s

That’s close to the accepted value at room temperature. If the air is a bit cooler than 68°F, you’d expect a slightly lower value, which you’re seeing.

How to tighten up this experiment

• Use multiple distances (100 ft, 150 ft, 200 ft) and plot distance vs. time/2. The slope gives the sound speed.
• Record several echoes at each distance and average the times.

• Measure air temperature; compare to the theoretical speed using the approximate relation from physics texts:

  \\[ v \approx 331 + 0.6 T_{\text{C}} \; \text{m/s} \]

  where \(T_{\text{C}}\) is temperature in °C.

Example 2: Two-microphone timing with a laptop or audio interface

For more precise timing, one of the best examples of measuring sound speed in air uses two microphones separated by a known distance. You record the same sound at both microphones and measure the tiny time delay between them.

What you need

• Two identical microphones (USB mics or a stereo mic pair)
• A laptop with free audio software such as Audacity
• A tape measure
• A sharp sound source (balloon pop, clap, or clicker)

Concept in practice

Place the two microphones along a straight line, say 3.0 ft apart. Put the sound source a few feet in front of Mic 1, directly along the line. When you pop the balloon, the sound reaches Mic 1 slightly before Mic 2. In the stereo waveform, you measure the time offset between the peaks.

If the distance between the mics is \(L\) and the time delay is \(\Delta t\), then

\[
v = \frac{L}{\Delta t}
\]

Concrete example

• Microphone spacing: 3.0 ft (0.914 m)
• Measured delay: 2.7 ms (0.0027 s)

Speed:

• In meters per second: 0.914 m / 0.0027 s ≈ 338 m/s
• In feet per second: 3.0 ft / 0.0027 s ≈ 1,111 ft/s

Again, that’s right where we expect sound speed at a mild room temperature.

Ways to expand this into more examples

These two-mic timing setups give several additional real examples:

• Indoor vs. outdoor: Repeat the measurement in an air-conditioned classroom and outside on a hot day. You should see a higher speed on the hotter day, matching the temperature dependence.
• Cold gym vs. warm lab: In winter, measure in an unheated gym (~50°F) and a warm lab (~72°F). Compare the difference to the theoretical change.
• Humidity comparison: Use a cheap humidity sensor and note that very humid air has a slightly higher sound speed. The effect is small but measurable with careful timing.

These variations turn this into one of the best examples of measuring sound speed in air when you want to show that sound speed is not a fixed magic number, but depends on real atmospheric conditions.

Example 3: Resonance in a tube with a speaker and microphone

The third of our examples of measuring sound speed in air: 3 practical examples uses standing waves in a tube. Instead of timing, you measure distances between resonances.

What you use

• A long tube (PVC pipe works well), at least 3–6 ft long
• A small speaker or phone playing a pure tone
• A microphone on a movable probe, or just your ear and a sound level meter app
• A signal generator app that can sweep frequencies

What’s happening physically

Standing waves form in the tube when its length matches an integer number of half-wavelengths. For an open-open tube, the fundamental mode is \(L = \lambda/2\). For an open-closed tube, \(L = \lambda/4\). Once you know the wavelength \(\lambda\) and frequency \(f\), the speed of sound is

\[
v = f \lambda
\]

A hands-on example

Imagine an open-open PVC tube that’s 1.50 m (4.92 ft) long. You sweep frequency until the sound is loudest (strong resonance) and find a peak at 115 Hz.

For the fundamental of an open-open tube:

\[
L = \frac{\lambda}{2} \Rightarrow \lambda = 2L = 3.0 \; \text{m}
\]

Then

\[
v = f \lambda = 115 \; \text{Hz} \times 3.0 \; \text{m} = 345 \; \text{m/s}
\]

That’s very close to the theoretical value at about 72°F.

More ways to use this resonance method

This resonance approach gives several examples include:

• Multiple harmonics in the same tube: Measure resonance peaks at the 1st, 2nd, and 3rd harmonics. Each gives an independent estimate of sound speed; you can average them.
• Different tube lengths: Use shorter and longer tubes and see that the speed of sound estimate stays consistent even though the resonant frequencies change.
• Open-closed vs. open-open: Cap one end of the tube and compare the pattern of resonances. You’ll need to use the quarter-wavelength formula, but the resulting speed should match.

This is one of the best examples of measuring sound speed in air for a physics class because it ties directly into standing waves, harmonics, and musical instruments.

Beyond the main 3: more real examples of sound-speed measurements

So far we’ve focused on examples of measuring sound speed in air: 3 practical examples that you can run with modest equipment. In real labs and classrooms, people push these ideas further. Here are additional real examples that build on the same physics.

Smartphone timing apps and distance measurements

Modern smartphones have good microphones and decent timing resolution. That’s led to a wave of classroom activities where students:

• Use an app to generate a sharp click from one phone.
• Record the click with a second phone at a known distance.
• Use the timestamped audio or a waveform view to measure time of flight.

In some high school and undergrad labs, instructors combine this with GPS-based distance measurements outdoors. While GPS introduces its own uncertainty, it’s a nice way to introduce measurement error and data analysis.

Classroom “human wave” demonstration with microphones

Another example of measuring sound speed in air uses a line of students each holding a phone or small recorder. An instructor fires a starter pistol or claps at one end of the line. Each phone records audio with its own timestamp. Later, students line up the signals in software and see how the sound front moves from one microphone to the next.

By plotting microphone position vs. detection time, the slope of the best-fit line gives the sound speed. This turns into a great intro to linear regression and uncertainty analysis.

Outdoor distance verification for sports or events

A more applied example of measuring sound speed in air shows up in sports timing and event monitoring. For instance:

• A track meet starter pistol is recorded at multiple microphones along the track to check synchronization.
• Fireworks shows or construction blasts are monitored with microphones at known distances to estimate sound speed and verify safety calculations.

These are less controlled than a lab, but they’re real examples of how the same physics underpins practical monitoring systems.

Why your measurements differ from the textbook value

Once you’ve run these examples of measuring sound speed in air: 3 practical examples (and the extra ones), you’ll notice that your numbers aren’t all identical. That’s not a failure; it’s where the physics gets interesting.

Temperature effects

As noted earlier, sound speed in dry air roughly follows

\[
v \approx 331 + 0.6 T_{\text{C}} \; \text{m/s}
\]

So between a chilly 50°F (10°C) and a warm 86°F (30°C), the speed changes by about 12 m/s (around 40 ft/s). That’s big enough to see in careful measurements.

Authoritative references like the U.S. National Institute of Standards and Technology (NIST) provide detailed equations of state for air and speed-of-sound calculations that agree with this simple formula in everyday conditions.

Humidity and pressure

Humidity slightly increases sound speed because water vapor is lighter than dry air. At typical indoor humidity levels, the effect is a few m/s at most. Atmospheric pressure, at the same temperature and composition, doesn’t change sound speed much, because both density and stiffness scale together.

Measurement limitations

Each of our examples of measuring sound speed in air has its own weaknesses:

• Echo timing: sensitive to reflections from multiple surfaces and timing resolution of the recorder.
• Two-mic method: requires accurate microphone spacing and synchronized channels.
• Resonance in tubes: needs accurate length measurements and careful identification of resonance peaks.

In 2024–2025, the trend in teaching labs is to lean heavily on digital tools—smartphone apps, free software like Audacity, and low-cost data-acquisition devices—to reduce timing error and give students direct access to raw waveforms. That aligns well with all the examples discussed here.

Putting it all together

Across these examples of measuring sound speed in air: 3 practical examples—echo timing, two-mic delays, and tube resonances—you see the same underlying relationship: distance, time, and wavelength are all just different ways of getting at the same speed.

If you’re teaching, these are some of the best examples to:

• Show that sound speed depends on temperature and, slightly, on humidity.
• Connect everyday experiences (echoes, musical instruments) to quantitative physics.
• Introduce data analysis, error bars, and model comparison.

If you’re learning on your own, start with the hallway echo, move up to the two-microphone setup, and then tackle the resonance tube. By comparing your results, you’ll not only get a solid estimate of sound speed in air—you’ll also get a feel for how experimental physics really works.

FAQ: common questions about examples of measuring sound speed in air

Q: What are some simple examples of measuring sound speed in air for a school project?
A: Three of the best examples are: timing echoes from a wall using a smartphone; using two microphones a known distance apart and measuring the time delay; and finding resonant frequencies in a tube to compute speed from wavelength. Variations include indoor vs. outdoor measurements and comparing warm and cold environments.

Q: Which example of a sound-speed experiment gives the most accurate result?
A: In most classroom settings, the two-microphone method tends to be the most accurate, because you can measure very small time delays with audio software and use relatively short distances. Resonance-tube methods can be equally good if the tube length and frequencies are measured carefully.

Q: How do temperature and humidity affect these examples of sound-speed measurements?
A: Temperature has the biggest impact; warmer air means faster sound. Humidity has a smaller effect, slightly increasing sound speed as humidity rises. If you log temperature and humidity for each run, you can compare your data to theoretical predictions from physics references and see that your results track the expected trend.

Q: Can I use a smartphone app as a valid example of measuring sound speed in air?
A: Yes. Many modern physics labs now incorporate smartphone microphones and timing apps because they offer surprisingly good time resolution. As long as you document your method, distances, and timing, a smartphone-based setup is a perfectly valid real example of a sound-speed experiment.

Q: Are these examples safe to perform indoors?
A: Yes, as long as you avoid very loud sound sources. Hand claps, small speakers, and balloon pops are generally safe in a classroom or hallway. Always follow your school’s safety guidelines and avoid any sound levels that could risk hearing damage.