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Noise

What is noise?

Noise is any annoying, disturbing or unwanted sound.

Noise!Noise can have many effects on people. It can:

  • irritate people;
  • interfere with verbal communication;
  • reduce working efficiency;
  • disturb sleep;
  • damage hearing.

Sounds can be interpreted as music, a story, a joke, a warning or an endless array of other possible signals. Sound can often bring us great pleasure, but it can also annoy us, or those around us. People have varying tolerance levels for sounds. If the sound goes beyond the tolerance level, it is classified as noise, but this depends on the characteristics of the particular person as well as the sound. For example, a group of people might enjoy playing loud rap music at 3a.m. The same sounds might not be appreciated by their neighbours! Likewise, a crying baby needs to able to make its parents hear when it requires feeding other members of the family might be annoyed by the same sounds.

Where does noise come from? Almost every aspect of modern life generates noise industrial processes, construction, office work and even home and leisure activities. Industrial production and construction are rapidly increasing in many countries and this brings with it an increase in industrial or 'occupational' noise. If we expose ourselves to extreme noise levels, then damage to our hearing can easily occur. Ultimately this could lead to severe deafness and to exclusion and isolation from society. Therefore, it is essential that we take good care of our hearing.

 

 

Characteristics of noise

Noise or sound is created by alternate compression and decompression of particles of the air. This causes the air pressure to fall and rise in the form of waves. Frequency (pitch) and intensity (loudness) are the two characteristics of sound.
 
Frequency

Frequency is the number of times that the air is compressed and decompressed in a second, and is measured in cycles per second, or Hertz (Hz). Low frequency produces a low pitched, bass sound. High frequency produces a high pitched, whistle sound. Human ears respond to frequencies between 20Hz and 20,000Hz. The human voice produces frequencies between 500Hz and 2,000Hz. Below 20Hz and above 20,000 Hz sound cannot be heard but it can still be harmful. The ear is most sensitive to sounds between 1000 and 4000Hz.

Frequency range of human hearing

The infra-sounds and the ultra-sounds, although not heard by humans, produce feelings of nausea, giddiness and headache in people exposed to them. Infra-sounds are the most subtle and can occur in aeroplanes and other vehicles. Noise from heavy road vehicles can penetrate the walls of houses.
 

  Try this 500Hz sound  

Click on to hear sounds at different frequencies.

They are pure tones or sine waves. The last sound increases from 20Hz to 20,000Hz and is called a 'swept sine wave'. 

2,000Hz sound
20-20,000Hz sound
Intensity 

Intensity is the amount of sound energy reaching the eardrum, and is measured in decibels (dB). Zero decibels is the faintest sound that the average person can hear, and the loudest sound may be more than a million times this level. This is because the intensity of sound does not rise gradually - each increase of 10 decibels means that the intensity increases 10 times.  For example, a 10 decibel sound is 10 times louder than a 1 decibel sound; a 20 decibel sound is 100 times louder than a 1 decibel sound (10x10); 30 decibels are 1000 times louder (10x10x10), and so on. The ear responds to intensities from 0 to 140dB. Discomfort is felt at 120dB, and near 140dB, pain is felt with possible rupture and permanent damage to the eardrum. 

Some noise intensity measurements are given in a measure called 'dB(A)' which takes into account the frequency sensitivity of the ear. dB(A) measurements are obtained on a noise meter which weights the different frequency components. For example, the noise intensity for a circular saw, and the noise intensity inside a bus have the same value when expressed in dB. But the noise from a circular saw contains a greater proportion of high frequencies, and is more annoying than the noise inside a bus. Therefore, the circular saw has a higher dB(A) value.

European law now requires manufacturers of tools and appliances to state the noise emission levels of their products. This allows customers to choose quieter machines and gives the manufacturers some incentive to create quiet designs.

Some examples of noise levels are shown below.

Decibels (dB(A)) Exposure level Common noise sources
140 Harmful to hearing Jet engine 25m away; Shotgun blast
130 Threshold of pain Jet takeoff 100m away
120   Disco; Propeller aircraft
110 Possible hearing loss Live band; Jet takeoff 600m away
100   Electric mower; Pneumatic drill; Tractor
90   Open top car ride on motorway; Food blender; Heavy traffic 5m away
80 Very noisy Alarm clock; Sink garbage disposal
70 Upper limit for hearing conversation Vacuum cleaner; Private car
60   Conversation at 1m; Singing birds
50 Quiet Light traffic 30m away; Quiet office
40   Dripping tap; Library
30 Very quiet Soft whisper at 5m
20   Broadcasting studio
10   Leaves rustling
0   Threshold of hearing

 

Anatomy of hearing anatomy of the ear
Hearing is the function of the auditory system, which is organised to detect several aspects of sound, including pitch, loudness and direction.

A sensation of hearing is caused when sound waves pass through the ear passage, resulting in the vibration of the eardrum. This, in turn, activates a lever system of three small bones - the hammer (malleus), the anvil (incus) and the stirrup (stapes), situated in the air-filled cavity of the middle ear. The stirrup sets up vibrations in the fluid-filled inner ear and the mechanical vibrations are transformed into nervous impulses, which travel to the auditory cortex in the brain and are recognised as sound.

Q. Why does your voice sound different on a tape recording?
A. When you hear yourself speak you are hearing some extra vibration produced by the passage of sound waves through your skull. Your voice as played by a tape recorder is the way it sounds to someone else, who receives the sound waves only through the air.

Q. How can you tell the direction of a sound?
A. Depending on the position of your head, sound reaches the ear closest to its source about 1/1500 of a second sooner than the other ear. Also, the sound is a little louder in the closer ear. These differences are recognised and analysed by your brain to tell you where the sound is coming from.

 

The effects of noise

 
Legal requirements for noise

In the UK the Noise at Work Regulations limit the noise to which an employer can expose the employees. It is based on three Action Levels:
First Action Level: 85dB(A) for 8 hours per day. Employees can demand hearing protection.
Second Action Level: 90dB(A) for 8 hours per day. Employees must wear suitable hearing protection.
Peak Action Level: 140dB(A). This must never be exceeded.
If workers are exposed to noise for more than 8 hours per day, these levels must be reduced. Conversely, if workers are exposed to noise greater than these levels, then the duration of their exposure to the noise must be reduced.
The following are the recommended noise exposure levels in the UK:

Max noise level (dB(A)) Duration (hrs per day)
90 8
93 4
96 2
99 1

It is likely that these levels will be reduced in the near future due to new legislation called the Physical Agents (Noise) Directive from the European Commission. This will apply across the European Union.

Deafness
The main effect of noise is the development of deafness, especially when you are exposed to high levels of continuous noise, for example, during working hours. This may result in 'noise-induced occupational deafness'. There is no cure and no treatment for deafness. Damage to your hearing is permanent, and will never get better.

Occupational deafness is a gradual process of reduction of hearing ability associated with damage to the cochlea. Occupational deafness does not occur accidentally; it is a result of not reducing exposure to noise, for example, by moving out of a noisy environment or wearing hearing protection. The initial loss of hearing occurs at frequencies above those involved in speech (500-2,000Hz) and consequently considerable loss can occur before the effect becomes noticeable to you. The obvious danger is that considerable damage may have occurred to the hearing mechanism before you become aware of it, and in later life, this deafness will become worse because of the natural effects of the aging processes.

The adverse effect of noise on hearing depends on the characteristics of a sound:
  • its intensity - it is probable that sounds of less than 80dB intensity do not cause noise-induced deafness,
  • its frequency - tones of 1000, 2000, 3000 and 4000Hz frequency are considered to produce the greatest damage,
  • its duration - in terms of your exposure during the working day and during your working life. Your exposure should be more limited at frequencies and intensities that are likely to cause damage.

Noise and performance
Noise causes fatigue and loss of concentration and efficiency, resulting in a decrease in work output and an increase in the number of mistakes made. Work involving learning is more susceptible to disruption from noise than routine work. This could be of importance to a school leaver training on-the-job in a noisy work environment.

Speech comprehension
The comprehension of speech is affected by both the sound level of the background noise and the sound level of the voice itself. The average intensity of a human voice in a room at a distance of one metre lies within the following ranges:

conversation 60-65dB(A)
dictation 65-70dB(A)
calling out 80-85dB(A)

The general background noise level must be at least 10dB(A) below these levels if the sound of the voice is to be heard clearly.

 

Guidelines for noise levels

Noise generated by humans, such as voices, dropping of objects, noisy children, can be a strong irritant. Background noise, determined by the environment around you can also be an irritant. Proximity of schools, operation of street cleaning equipment or refuse collection, and traffic noise can all contribute to increased noise exposure. In 1994 it was estimated that 22% of residents in Europe (77 million people) were exposed to transportation noise of more than 65dB(A); this level is considered to be unacceptable. In general, different residential areas have different background noise and may range from 'very quiet suburban' to 'very noisy residential'.

Many leisure activities have become major sources of noise and hazards to hearing. Levels in night clubs can exceed 100dB(A). Most visitors are exposed to this only for a few hours per week, but DJs, musicians, bar and security staff are working in this all the time and may be at risk of damage to their hearing.

The World Health Organisation (WHO) provides some guidance on acceptable levels of noise in the community. 

Specific environment Health effects Target level (dB(A))
Outdoor living area Serious annoyance, daytime and evening 55
Moderate annoyance, daytime and evening 50
Indoor living area Speech intelligibility and moderate annoyance, daytime and evening 35
Inside bedrooms or hospital ward rooms Sleep disturbance, night time 30
Outside bedrooms Sleep disturbance (window open) 45
School classrooms Speech intelligibility, disturbance of information extraction, message communication 35
Hospital treatment rooms Interference with rest and recovery as low as possible
Industrial, commercial, shopping and traffic areas Hearing impairment 70
Ceremonies, festivals and entertainment events Hearing impairment 100
Public address Hearing impairment 85
Music through headphones Hearing impairment 85

Noise FAQs

Q. When should a display 'flash' and when should it 'beep'? Answer

 

Acknowledgments
Content: Neil Mansfield
Images: IMSI's MasterClips Collection