Oct 2020

Speed Of Sound & Sound Waves: Understanding Key Concepts

October 12, 2020

Did you know that sound travels all around you in waves? This is true – sound waves travel throughout the atmosphere, allowing us to use it in many different ways. For instance, communicating with one another, navigation, music, and many more!

To understand more about sound, we need to understand how it came to be and the key principles of sound. First, you need to understand the basics of sound waves!

Sound waves

Sound waves are what makes up sounds that we hear. It is created when an object’s vibration propagates a wave through a medium from one place to another. A sound wave in this instance can be triggered by a vibrating string on a guitar, a vibrating fork, or a person’s vocal cords, upon which the sound wave travels from one place to the other through the medium that is air.

However, sound waves are unable to travel through a vacuum, unlike electromagnetic waves. It requires a medium for transmission. Any medium with particles that can vibrate is able to transmit sound.

Unlike electromagnetic waves which are made up of crests and troughs, sound waves are made up of compressions and rarefactions:

  • Rarefaction is a region of the wave where the molecules are have more space to expand, resulting in molecules that are farther apart.
  • Compression is a region of the wave where molecules are closely packed together.

Production of sound waves

We will be taking a further look at how sound waves are produced, using the example of a loudspeaker. Firstly, when a person speaks into the loudspeaker, the soundwaves they produce cause the diaphragm of the loudspeaker to vibrate. This in turn causes the medium that it is passing through, such as layers of air, to vibrate. The transfer of energy occurs like a domino effect, causing subsequent layers of air to vibrate. Due to the vibration, alternative rarefactions and compressions are produced. Through the vibration, longitudinal sound waves are produced.

Longitudinal waves

Longitudinal waves are waves where the motion of all individual particles of the medium is in a direction that is parallel to the direction of energy transport. Sound waves in a fluid medium are longitudinal waves as the particles in the transported medium vibrate in a parallel direction of the sound wave.

To better interpret a sound graph, here are the key definitions you should note:

  • Wavelength: Distance between two successive rarefactions or compressions
  • Amplitude: Pressure change at its maximum
  • Frequency: A measurement indicating the number of rarefactions or compressions in a fixed period of time.

Formula for frequency

f = V / λ

(f – frequency, V – velocity of the wave,  λ – the wavelength of the wave)

Speed of sound

In different mediums, the sound of speed varies. They travel the fastest in solids as the particles are closely packed together. Here are some examples of the speed of sound in different mediums:

  • Speed of sound in steel: 5000m/s (solid state)
  • Speed of sound in water: 1400m/s (liquid state)
  • Speed of sound in air: 330m/s (gas state)

Two physical conditions greatly affect the speed of sound in gas:

  • Humidity: Higher humidity = higher speed
  • Temperature: Higher temperature = higher speed

Direct method for calculating the speed of sound in air

It is possible to calculate the speed of sound in air by measuring it directly as follows:

  • Person A carries a pistol and person B carries a stopwatch
  • Person A and person B stand in an open field, their distance is measured using a measuring tape
  • Person A fires off a pistol
  • Person B will start the stopwatch upon the flash of the pistol, and stop the stopwatch when the sound goes off
  • The distance of person A and person B is d, the interval of time is t
  • Speed = d / t (d – distance, t – time of interval)

There are ways to improve this direct method of calculating the sound of speed in air. One improvement is to repeat the experiment several times, taking the average value of time to calculate the average speed of sound. Another improvement is to have person A and B swap positions to reduce any wind effects in the calculation.

Loudness and pitch 

Loudness and pitch are characteristics of sound that can help us to determine if a sound is unpleasant or pleasant.

  • Loudness is related to the amplitude of a sound. The larger the amplitude, the louder the sound will be.
  • Pitch is related to the frequency of a sound wave. The lower the frequency, the lower the pitch will be.

Echo in sound

Echo is created when a sound is reflected off of flat and hard surfaces, like a distant cliff or a large wall. The laws of reflection also apply to sound waves.

Through echoes, we can use echolocation to map out the locations of objects and measure distances. For example, echolocation can be used to measure the depth of an ocean by calculating the time lapse of the signal transmission and reception of the reflected signal (the echo). It can also be used to detect shoals of fishes and their positions.


Ultrasound is defined as a sound wave with frequency of over 20 kHz. This makes it inaudible to the human ear, as our audible frequency range lies between 20 Hz to 20 kHz. (Sounds with frequencies below the limit of the human’s range of audibility are called infrasounds.)

Although inaudible, that doesn’t mean ultrasounds are not of any use! In fact, their high frequency makes them useful for a range of applications, including:

1. Quality control

Ultrasound is used to detect levels of liquid, powder, or any other material that is found in a container. A pulse of ultrasound would be transmitted at a fixed height from a sensor. If the level of the contents in the container are lower than normal, the time between the reception of the ultrasound pulse and transmission would be increased, and the container is rejected.

2. Pre-natal scanning

Ultrasound is used to scan and examine the foetus’ development through ultrasound pulses sent into the body by a transmitter. The echoes will reflect any surfaces within the body, and the depth of the reflecting surface within the body would be known. This is how a real-time image of the foetus is produced.

Advantages of ultrasound in medicine

There are also many key advantages of ultrasound in medicine. Here are 2 examples:

  • Audible sound waves: Due to ultrasound’s shorter wavelength, there is less diffraction. This can lead to the detection of smaller objects, and allow for sharper images
  • X-rays: X-rays can damage body cells, but ultrasound is less hazardous as it has lower energy, and does not have any adverse effects


There are so many exciting and educational aspects to discover when it comes to sound! After all, sound is all around us and everywhere, such as the music we listen to and the sounds of people speaking. The applications, the examples, and problem-solving concepts of sound can get complicated, but don’t fret!

You can always enrol in a physics tuition class and seek the help of an experienced and professional physics tutor! Not only can they provide you with the best guidance, but they can also help you practice your weaker topics so that you can grasp them well. Get started with physics tuition today!

WhatsApp chat