The fundamentals of noise and tempory noise control

What is sound?

The basic definition of sound is airborne vibration. When objects move they create air pressure waves / vibrations similar to ripples on water, when these air vibrations reach our ear we hear the sound that has been created.

The intensity of the pressure of these sound waves is described in terms of decibels (dB) and the differences between sounds are caused by variations of intensity, pitch, and tone – the three components that sound is made up of.

Intensity

Sound is a wave and waves have amplitude, or height. Amplitude is a measure of energy. The more energy a wave has, the higher its amplitude. As amplitude increases, intensity also increases. Intensity is the amount of energy a sound has over an area. This is often commonly defined by how loud something may be.

Pitch

Pitch depends on the frequency of a sound wave. Frequency is the number of wavelengths that fit into one unit of time. Remember that a wavelength is equal to one compression and one rarefaction. Frequencies are measured in hertz. One hertz is equal to one cycle of compression and rarefaction per second. High sounds have high frequencies and low sounds have low frequencies. The human range is commonly given as 20 to 20,000 Hz, though there is considerable variation between individuals, especially at high frequencies, and a gradual decline with age is considered normal. Using a dBA-filter, the sound level meter is less sensitive to very high and very low frequencies, filtering out frequencies that are not audible to humans. Measurements made with this scale are expressed as dB(A).

Tone

​Another difference you may have noticed between sounds is that some sounds are pleasant while others are unpleasant. This is because they have a different tone, or sound quality. When a source vibrates, it actually vibrates with many frequencies at the same time. Each of those frequencies produces a wave. Sound quality depends on the combination of different frequencies of sound waves.

noiseThe decibel (dB) is a logarithmic unit used to express the ratio between two values of a physical quantity, often power or intensity. One of these quantities is often a reference value, and in this case the decibel can be used to express the absolute level of the physical quantity, as in the case of sound pressure.

Because sound is logarithmic not arithmetic, if you were to double the sound energy you only increase the decibel level by 3dB, please see the illustration opposite.

​Changing sound levels by 1 dB can barely be detected by humans and even a change of 2 to 3 dB will not make a significant difference. However due to the decibel scale being logarithmic not arithmetic a 5 decibel change is readily noticeable and a decrease of 10 dB is perceived as reducing the noise by half, therefore a reduction of 15-20 dB represents a dramatic change.

Sound is a sequence of waves of pressure which propagates through compressible media such as air or water. During their propagation, these waves can be reflected, refracted, or attenuated by the medium. All media has three properties which affect the behaviour of sound propagation:

  1. A relationship between density and pressure.
    This relationship, affected by temperature, determines the speed of sound within the medium.
  2. The motion of the medium itself, e.g., wind.
    Independent of the motion of sound through the medium, if the medium is moving, the sound is further transported.
  3. The viscosity of the medium.
    This determines the rate at which sound is attenuated. For many media, such as air or water, attenuation due to viscosity is negligible.

In any situation regarding the environment or the health and safety of personnel it is recommended that you follow a hierarchy of control similar to what is set out below.

  1. Eliminate or Substitute:
    a. Use quieter equipment or a different quieter process
    b. Engineering technical changes to reduce the noise at source
  2. Control at source:
    a. Utilise screens, barriers, enclosures or absorbent materials
  3. Safe work procedures:
    a. Utilise screens, barriers, enclosures or absorbent materials
    b. Improved ways of working to reduce noise levels
    c. Limit the time you spend in noisy area
  4. Personal Protective equipment:
    a. Ear defenders or plugs

​When you have fully assessed the options of elimination or substitution of the noisy practices, you will need to consider the most effective form of attenuation out of the three options available.

  1. Controlling the noise source.
    This normally the most recommended as it will give you the maximum attenuation possible.
  2. Creating a barrier within the sound path.
    In some circumstances it may only be feasible to install barriers within the sound path between the source and the receiver – block the line of sight. For this method to have maximum effectiveness they often need to be very large and will require a temporary works engineer to ensure the safety of the structure to which the acoustic barriers are secured.
  3. Screen off the receiver.
    If it is not possible to effectively block the line of sight between receiver and the source with the options mentioned above then you will need to look into the feasibility of screening off the receiver. This can have a negative impact as it will also block any visual view there may have been.

noise2​As you can see in the illustration opposite sound is similar to light and a barrier will create a shadow zone often referred to as the ‘acoustic shadow’. The closer you can situate the barrier to the noise source the more likely you are to block the line of sight which will increase the acoustic shadow and the more effective the attenuation.​