Introduction to noise

Sound is the transmission of waves of energy in an elastic medium, such as air. It is a sequence of small and rapid variations in pressure that causes the molecules to vibrate. The interaction of the vibrating molecules sets up a wave motion that transmits the pressure variation away from the vibrating source. When these pressure changes are detected by the ear and transmitted to the brain they are perceived and then interpreted as sound. This is the process of hearing which is critical to communication.

Noise is described as unwanted sound either because it is too loud, or its nature is displeasing. What is considered ‘sound’ to one person can very well be ‘noise’ to somebody else. Anyone who is exposed to excessive noise is potentially at risk of developing hearing disorders with associated loss of communication skills, and possibly other health issues. The higher the level of noise, and the longer individuals are exposed to it, the greater their risk of suffering harm from the exposure.

Individuals are exposed to noise in their daily lives through environmental noise (alarms, sirens, traffic noise etc). Noise in the workplace is termed occupational noise. Millions of workers worldwide are exposed to occupational noise levels that puts their hearing health at risk. Excessive noise is considered an occupational hazard with many adverse effects, not only to the workers exposed through noisy operations but also to those around them. The effects of exposure are dependent on three characteristics of the sound i.e., frequency, loudness, and duration, and when these are poorly controlled this can lead to temporary or permanent hearing loss and can impair worker’s efficiency. It is to be noted that environmental noise contributes significantly to hearing losses in individuals and therefore not all hearing loss is occupational.

The characteristics of sound

Human awareness of sound is the function of three measurable qualities- decibels (dB), frequency, and duration. They are important when assessing occupational exposure and in identifying methods of control to prevent hearing loss.

  1. Decibels (dB) refers to the amplitude or the height of the wave and determines the intensity or loudness of the sound. The amplitude is quantified by measuring the pressure changes as sound waves are transmitted. Sound is measured from a reference value of 0 dB which is the lowest level of sound a normal healthy young person can hear.
  2. Frequency refers to the number of waves of vibration that can occur in one second and is often referred to as the pitch- the higher the frequency the higher the pitch- and is measured in units called Hertz (hz). Hertz is a measure of the number of wave lengths in one second. The range of frequencies that humans are capable of hearing generally falls within the range of 20-2000Hz. The human voice can generally produce sounds in the range of 250-8000hZ. The majority of sounds we can hear are between 20 and 8000Hz. It is interesting to note here that certain sounds are heard at pre-determined frequencies. Vowels are heard at low frequencies and consonants are heard at higher frequencies. This becomes relevant when discussing noise induced hearing loss (NIHL) as the damage associated with this loss will begin at higher frequencies e.g., 4000Hz which will interfere with speech perception.
  3. The third characteristic is duration, or how long an individual is exposed to the sound, which is an important factor in determining the impact that noise will have on hearing. The longer a person is exposed to loud noises the greater the risk of hearing damage. For this reason, noise in the occupational setting is rated as the actual level of the noise over a time interval or time weighted average (TWA). In industry this time interval is 8 hours (a normal work shift).

Noise is a common hazard and is present to some extent in almost all workplaces. It is the most prevalent health hazard in industries such as forestry, entertainment, manufacturing, agriculture, shipbuilding, textiles, mining and quarrying, food and drink, wood working, metal working and construction.

Even though it is considered the most preventable occupational hazard-significant numbers of workers still suffer from occupational NIHL.

Noise exposure can also lead to accidents due to loss of concentration, misunderstanding oral instructions and the masking of sounds of warnings and/or approaching danger.