Speed Of Sound & Echo

Speed of Sound

In Air

The speed of sound in air is typically between 330 and 350 meters per second (m/s). This velocity can vary depending on several factors, including temperature and altitude. Interestingly, the speed of sound increases with the temperature of the air because warmer air has more energy, causing particles to vibrate faster. This means that at higher altitudes, where the air is generally cooler, the speed of sound decreases compared to sea level. Contrary to what one might expect, changes in atmospheric pressure have minimal impact on the speed of sound in air.

In Other Materials

The speed of sound is not constant and varies significantly across different mediums. It travels faster through solids than liquids, and faster through liquids than gases. This variation is due to the density of the molecules in each state of matter: molecules in solids are packed closely together, allowing sound waves to transmit more efficiently through the medium. In liquids, molecules are less tightly packed than in solids but are closer together than in gases, facilitating a faster speed of sound than in gaseous environments but slower than in solids.

MaterialSpeed of Sound (m/s)
Air (20°C)343
Water (25°C)1498
Seawater (25°C)1533
Ice ($\text{H}_2\text{O}$)3980
Speed Of Sound In Various Materials

Determining The Speed of Sound in Air

Calculating the speed of sound in air requires just two measurements:

  1. The distance between the sound source and the receiver.
  2. The time it takes for the sound wave to travel from the source to the receiver.

The formula to calculate the speed of sound is:

$$\text{Speed} = \frac{\text{Distance Travelled}}{\text{Time}}$$

Precautions Taken To Reduce Measurement Errors

To ensure the accuracy of the speed of sound measurements, certain precautions are necessary:

  • Exchange Positions: By swapping the locations of the sound source and the measuring device and repeating the experiment, one can mitigate the effects of wind, which can distort the speed of sound in air.
  • Multiple Trials: Conducting the experiment several times and calculating the average of the recorded time intervals helps in achieving a more accurate measurement of the speed of sound. This approach minimizes random errors and improves the reliability of the results.

Additional Considerations

  • Temperature Correction: Given that the speed of sound in air increases with temperature, it’s crucial to account for the ambient temperature during experiments. A standard correction factor can be applied to adjust the measured speed to what it would be at a reference temperature (usually 20°C).
  • Humidity Effects: The presence of water vapor in the air can also affect the speed of sound. Generally, sound travels faster in humid air than in dry air because molecules of water vapor have less mass than the nitrogen and oxygen molecules they replace.


Sound waves can be reflected by large, hard surfaces like buildings, walls and cliffs. Reflection of sound occurs just like the reflection of light.


Echo is a distinct, reflected sound wave from a surface.

  • A reflected sound can be heard separately from the original sound if the sound source is closer to the receiver while the reflecting hard surface is sufficiently far from receiver. Such reflected sound is called an echo.
  • Generally the reflected sound is not distinctly heard, as it follows so closely behind the original sound and prolongs the sensation of the original sound. This effect is called reverberation.
  • If the surface is rough, the incident sound waves are broken up and the original waveform is lost, thus no reflected sounds are heard. To reduce the effects of echo, walls can be roughened or “softened” (with padding) or covered with curtains and floors covered with carpets.
  • Principle of echo is used in echo sounder to find the depth of a sea or the location of shoals of fish. Echoes can be used to measure the speed of sound.

Note: Remember that the distance travelled by the sound is doubled for echo. (The sound “go there and come back”) For instance, if a sound wave takes 10 seconds to travel to the bottom of the sea and back, the total distance travelled is 2d, where d is the depth of the sea.

Hence, the velocity of the sound for echoes can be calculated by:

$$\begin{aligned} v &= \frac{\text{Total distance travelled by sound}}{\text{Time taken}} \\ &= \frac{2d}{t} \end{aligned}$$

Step-By-Step Guide To Determine Speed Of Sound In Air

You can try measuring the speed of sound in air using two microphones and a digital timer. This method relies on recording the time difference between when a sound is emitted and when it is received by each microphone.

Materials Needed

  • Two microphones capable of connecting to a digital timer or a computer with sound analysis software.
  • A digital timer or a computer with sound analysis software (such as Audacity).
  • A sound source (e.g., a speaker or a clap).
  • A measuring tape.
  • A quiet, large room or outdoor space (to minimize echo and interference).

Step 1: Setup the Equipment

  1. Position the Microphones: Place the two microphones at a known distance apart. Use the measuring tape to measure this distance precisely. For simplicity, you might start with them 10 meters apart, but ensure this distance is accurately measured.
  2. Connect the Microphones: Connect the microphones to the digital timer or the computer. If using a computer, open your sound analysis software and ensure it can record from both microphones simultaneously.
  3. Test the System: Do a quick test by making a sound near one microphone to ensure both microphones and the recording system are working properly. You should see the sound being captured in real-time if using software.

Step 2: Record the Sound

  1. Generate a Sound: Make a sharp, loud sound (like a clap or a balloon pop) equidistant between the two microphones. This ensures the sound waves reach both microphones at different times.
  2. Start Recording: Record the sound simultaneously on both microphones. If using a digital timer linked to the microphones, it should automatically capture the time difference. If using software, you’ll visually see the waveforms.

Step 3: Analyze the Data

  1. Identify the Time Difference: On the digital timer, read the time difference between when each microphone detected the sound. If using software, zoom in on the waveforms and note the exact times each microphone received the sound.
  2. Calculate the Speed of Sound: Use the formula $v = \frac{d}{t}$, where $v$ is the speed of sound, $d$ is the distance between the microphones, and $t$ is the time difference between the sound’s arrival at each microphone.

Step 4: Repeat and Average

  1. Repeat the Experiment: To ensure accuracy, repeat the experiment several times, recording the time difference for each trial.
  2. Average Your Results: Calculate the average time difference from all your trials and use this in the speed of sound formula to find a more accurate result.

Step 5: Consider Environmental Factors

  • Temperature: Note that the speed of sound in air varies with temperature. You might want to measure the air temperature during your experiment and adjust your calculations accordingly. The speed of sound increases by approximately 0.6 meters per second for every degree Celsius increase in temperature.
  • Humidity and Pressure: These factors also affect the speed of sound but to a lesser extent than temperature. For a high school or introductory college physics experiment, these can usually be neglected.

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