Section 1. What is sound?

Sound is a form of energy produced when an object vibrates and creates a sound pressure wave that moves through a medium (i.e. solid, liquid or gas). 

Audience waves observed in stadiums are similar to this in that there is a disturbance with people moving up and down as the wave travels around the stadium. Each individual still remains in their seat and is not carried around the room as the wave travels, in much the same way that particles of water do not move along the path of a sound wave.

Source: Sound image

When we hear something, we sense the vibrations or changes in pressure by structures in our ears, which send electrical signals to the brain for interpretation. While certain animals can ‘hear’ sounds, other animals, such as fish and invertebrates, are sensitive to the particle motion of sound waves. 

Noise vs. Sound

Physicists use both ‘noise’ and ‘sound’ to refer to vibrations we hear with our ears. The difference in the choice of a term however, depends on the perspective of the listener and the acoustic signal of interest. 

A ‘sound’ becomes a ‘noise’ when it interferes with the reception or transmission of the acoustic signal of interest or disturbs a listener (i.e. an unwanted or disturbing sound). For example, a humpback whale singing might be a beautiful sound of interest to many, however, for someone who is listening for underwater earthquakes, that background singing might be considered a ‘noise’.

How sound travels

There are two types of waves: 1) transverse and 2) longitudinal. Sound travels as longitudinal waves in which the motion of the medium’s molecules is parallel to the direction of the energy transported. 

As the wave travels, it alternates phases of:

1. Compression – areas of high pressure where the molecules are pressed together and
2. Rarefaction – areas of low pressure where the molecules are spread apart.

Watch this – video on how a sound wave travels

Animations by Dan Russell.

Anatomy of sound waves

The basic components of a sound wave are wavelength, frequency, and amplitude.

Wavelength is the distance between two successive peaks in a propagation wave and is related to the speed at which sound travels. 

Frequency is the number of waves that pass by a specific point per second and is measured in Hertz (Hz). It is also commonly referred to as the ‘pitch’ of a sound. A high-frequency sound wave is associated with a high pitch and a low-frequency sound wave is associated with a low pitch.

Add a sound sample example for both types

Dosits frequency video with example

Amplitude relates to changes in pressure and the amount of energy a sound wave carries at a specific location. It is the maximum amount of displacement of a particle in a medium from its resting position and is commonly referred to as the ‘loudness’ or ‘softness’ of a sound. For example, as the amplitude increases, a sound is perceived as getting louder and softer as the amplitude decreases. 

(Add images and breakdown of all components and audio samples)

Intensity

As the amplitude of a sound increases, its intensity increases. The intensity is the average amount of energy transmitted through a unit area per unit time (measured in decibels (db)), in the direction the sound wave travels. Sounds with higher intensities are perceived to be louder.