Main Difference – Sound vs. Noise
Sounds are ubiquitous. Sound plays an important role in our lives, allowing us to communicate, warning us of dangers and by providing us a with a means of entertainment in the form of music. Noise is a type of sound. However, the classification of a sound as noise is largely subjective. We could say that the main difference between sound and noise is that noise refers to undesirable sounds.
What is Sound
Sounds are essentially a vibrations. When sound propagates through a material, the molecules in the material oscillate to and fro. These oscillations are transferred from each molecule to its neighbours, and this allows sound to propagate. When the oscillations reach the ear or a sound-detecting device, it makes a membrane oscillate. When membranes in our ear oscillate, the brain can interpret the oscillations as “sound”. Since sound relies on oscillations of particles to propagate itself, sound cannot travel through a region where there is nothing to oscillate, i.e. sound requires a medium to propagate. For this reason, sound is classified as a mechanical wave. In this sense, “noise” is also a type of sound.
Sounds are merely vibrations, and at times they cannot be heard. Evelyn Glennie is a famous Scottish musician who is technically deaf, but perceives sounds through vibrations felt by her body. Humans are only able to hear sounds having frequencies of around 20-20 000 Hz. As we grow older, we lose our ability to hear some of the higher frequencies. Ultrasound, which is used in body imaging for medical purposes, have frequencies above the human hearing range. Some animals are able to hear infrasound, which are sounds with frequencies below the human hearing range. Some scientists speculate that these animals are able to sense natural disasters such as tsunamis and earthquakes before humans do, by listening to infrasounds emitted by these events.
Evelyn Glennie is a “deaf” Scottish musician who perceives sound through vibrations felt by her body
What is Noise
Noise is undesirable sound. If the frequency spectrum of sound is analysed, it would show that it consists of many random frequencies. This is in contrast to musical sounds, which show considerable peaks at fundamental frequencies and their overtones.
The distinction between noise and sound is strictly based on whether a given sound is desired. For instance, the ringtone of a mobile phone could be considered as “sound” by the owner of the phone, as it is important to them. To somebody else in the vicinity, the sound of the ringtone would be an undesirable noise!
Noise pollution is a serious issue in industrialised areas. Not only does noise have adverse effects on people’s physical and even mental health, it is even capable of changing animal behaviour. Over a long time, this could lead to significant changes in ecosystems around urban areas. Some effects of noise pollution can be reduced by imposing strict regulations on levels of noise that can be produced. In some countries, noise barriers are installed along highways to reduce the amount of noise that is heard by nearby inhabitants.
A noise barrier along a highway in The Netherlands
There is another usage for the term “noise”: when a signal is transmitted over a large distance, the signal becomes distorted due to external influences. The changes added to the signal in this way is also called “noise”. Often, when a signal is transmitted over a large distance, it is necessary to remove the noise and re-transmit the signal several times along the way. The type of signal determines how easy it is to remove noise from the signal. For instance, it is relatively straightforward to remove noise from digital signals, whereas it is more difficult to remove noise from analogue signals. The quality of a signal which has been contaminated with noise is expressed using signal-to-noise ratio (SNR). If the power of the signal at a given location is 
Difference Between Sound and Noise
Desirabilty
Sound is a type of mechanical wave that could be heard.
Noise is a type of undesirable sound.
Environmental Impact
Not all sound affects environment adversely.
Noise often has detrimental health effets on people and can change the behaviour of animals, leading to changes in ecosystems.
In signal Processing
Noise is used to refer to disturbances that contaminate a signal tha is being transmitted. In this context, it has no relationship to sound.
Image Courtesy:
“Evelyn Glennie at Moers Festival 2004” by nomo/michael hoefner (Own work) [CC BY-SA 2.5], via Wikimedia Commons
“Geluidsscherm langs A13 bij Overschie” by Michiel1972 at Dutch Wikipedia (Own work) [CC BY-SA 3.0], via Wikimedia Commons
