Measuring the Speed of Sound in Water (without owning a submarine)

Measuring the speed of sound in air is a classic: two microphones, a clap, a ruler, and a small argument about who started the stopwatch too late. Water, however, is a different beast. Sound travels much faster in water than in air, so the time differences you’re trying to measure are tiny. That doesn’t make it impossible — it just means we need methods that don’t rely on someone’s thumb hovering over a phone timer like it’s the Olympic 100 m final.

Below are three practical approaches, from “school-lab achievable” to “this feels like we’re doing proper marine science”.

Method 1: Time-of-flight with two underwater microphones (best and most direct)

Idea

Send a sharp sound pulse through the water. Record when it arrives at two sensors a known distance apart. The speed is:

v = \frac{d}{\Delta t}

Where:

$d$ = distance between sensors (m)
$\Delta t$ = time delay between the two recordings (s)

What you need

A long water tank / trough / (in a pinch) a swimming pool
Two hydrophones (underwater microphones)
- If you don’t have hydrophones, you might hack it with waterproofed microphones in sealed bags, but reliability varies wildly.
Something to record both channels at once:
- A 2-channel audio interface into a laptop works brilliantly
- Some data loggers can do it too
A sharp sound source:
- Two pieces of wood tapped together underwater (clappers)
- An old school bell ( what we used)
- A short ultrasonic “ping” if you have a signal generator + transducer

Procedure

Mount the two hydrophones in line, separated by a measured distance (say 0.50 m to 2.00 m).
Make a sharp sound pulse near one end (closer to hydrophone A).
Record both traces.
Measure the time shift between the first big peak on channel A and channel B.
Calculate $v$ .

Tips to make it work

Use a bigger distance if you can.
Example: if $v \approx 1500\ \text{m s}^{-1}$ , then over 1.0 m the delay is only about 0.00067 s (0.67 ms). That’s measurable… but you need decent sampling.
Set audio sampling to ≥ 44.1 kHz (better: 96 kHz).
Keep sensors at the same depth to reduce odd paths and reflections.
Do it in the middle of the tank, away from walls, or you’ll get echoes bouncing about like a pinball machine.

Typical result

Fresh water at room temperature is usually around 1480–1500 m/s.

Main uncertainties / errors

Reflections from walls and the surface (extra peaks)
Distance measurement (especially if sensors aren’t truly in line)
Temperature (sound speed changes with temperature)

Method 2: Echo (SONAR-style) using a single sensor (simple concept, fiddlier in practice)

Idea

Make a sound pulse and measure the time until the echo returns from a reflecting surface at distance $L$ :

v = \frac{2L}{t}

Because the sound goes there and back.

What you need

A hydrophone (or transducer) and recorder
A flat reflecting target (a metal plate works well)
A way to know $L$ accurately

Procedure

Place the reflector at a known distance $L$ from the sound source/sensor.
Make a sharp pulse.
Measure the time to the echo peak.
Compute $v$ .

Why it’s trickier

Echoes can overlap with the original pulse in small tanks.
Multiple reflections can confuse which peak is “the” echo.
Works better in a long tank or a pool.

Method 3: Resonance in a water-filled tube (clever, but equipment-sensitive)

This is the “do it like the air resonance tube experiment… but underwater” approach.

Idea

Drive sound at a known frequency $f$ and find standing-wave positions in a column of water. Then:

v = f\lambda

If you can measure the wavelength $\lambda$ in water.

Challenge

Generating and detecting clean standing waves in water is harder than in air, and reflections/attenuation can be awkward. This is more of a “physics club project” than a quick GCSE practical — but it’s a brilliant extension activity if you have ultrasonics kit.

What to record in your results table

For time-of-flight (Method 1), a neat table looks like:

Sensor separation $d$ (m)
Time delay $\Delta t$ (s)
Calculated speed $v$ (m/s)
Water temperature (°C)
Notes on reflections / signal quality

Repeat for several distances and average the result.

Safety and sanity notes

Electricity + water: keep interfaces/laptops well away and use long leads.
Don’t smash glass tanks with enthusiastic spoon percussion.
If you use ultrasonics: it’s generally safe at typical lab levels, but don’t drive powerful transducers in a way that heats water or stresses equipment.

A nice “why it’s faster in water” paragraph (for the write-up)

Sound travels faster when particles are more strongly coupled (stiffer medium) and slower when the medium is more compressible. Water is far less compressible than air, so disturbances pass along more quickly — even though water is denser

24 February 2026