Question

A student performs an experiment by ringing a bell in the air and in a pool and measuring the time it takes for a friend to hear the bell.

Answer 1

I need one quick clarification: is the friend listening with their ears underwater (both bell and friend submerged) or is the friend in air while the bell is under the pool surface? The explanation and what you should expect depend on that.

Below I cover the common possibilities and what you would measure.

1) Bell and friend both in the same medium
- If both are in air: sound speed ≈ 343 m/s (at 20 °C). Time = distance / 343.
- If both are underwater (freshwater at ~20 °C): sound speed ≈ 1480 m/s. Time = distance / 1480.
- Conclusion: sound travels several times faster in water than in air (roughly 4.3× at 20 °C). So for the same separation the friend will hear the bell sooner underwater. Example: at 10 m separation, t_air ≈ 10/343 ≈ 0.029 s (29 ms); t_water ≈ 10/1480 ≈ 0.0068 s (6.8 ms). Note: these intervals are small compared to typical human reaction times (~100–200 ms), so you’ll need recording equipment to measure the difference reliably.

2) Bell underwater, friend in air (or vice versa)
- Acoustic impedance mismatch at the water–air interface is very large, so most of the sound energy is reflected at the surface and only a tiny fraction transmits into air. The intensity transmitted from water into air is on the order of 0.1% (a drop of ~30 dB). That makes the underwater bell sound much quieter in air and possibly hard to hear.
- The arrival time of the transmitted component will be determined mainly by the travel time within water to the surface plus the (very short) travel through air; because sound is faster in water, that transmitted wave will generally arrive earlier than a purely airborne source at the same horizontal distance, but its amplitude is much smaller.
- In practice a listener in air may instead hear vibrations conducted through the pool structure (metal, concrete) or through bone conduction if they touch the pool; those paths can transmit energy faster or with different delays and can dominate what is heard.

3) Practical experimental issues and how to measure reliably
- Human reaction time is too slow to resolve millisecond differences. Use electronics: a synchronized pair of sensors (an air microphone and a hydrophone) or two microphones and a common recording device (high sample rate) to record the bell strike and the received waveforms, then measure arrival times.
- Keep paths simple and known (straight-line distances), minimize reflections and echoes (walls, pool bottom), and repeat many times.
- Be aware of coupling losses at interfaces. If you want a clear result comparing propagation speeds, put both bell and microphone in the same medium and compare air vs water measurements (same separation).
- Temperature affects speed: for air c_air ≈ 331 + 0.6·T(°C); for water c_water ≈ 1480 m/s near 20 °C (varies with temperature, salinity, pressure).
- If you measure a smaller or larger delay than expected, consider alternate transmission paths (structure, surface waves) and the low transmitted amplitude across the air–water boundary.

If you tell me which configuration you actually used (positions, distances, whether the friend’s ears were submerged, temperature, and how you timed the sound), I can calculate expected times and suggest the best measurement method for your setup.