Noise is measured in decibels (dB), a logarithmic unit that expresses the ratio of the intensity and pressure of sound.
Normally, the human ear can hear sounds from 0 dB to 140 dB. Above 80 dB, hearing damage can occur depending on the length of exposure; sounds above 120 dB can be painful as well as dangerously damaging.
How we perceive changes in loudness is a subjective one that does not relate in a linear way to the actual increase or decrease in decibels.
Noise attenuation is achieved by an acoustic barrier material that both blocks and absorbs the noise. Installing the barrier as close as possible to the source of the noise maximises the sheltered area. In all cases, the reduction must be at least 10 dB to be perceived as making a significant difference to the level of noise.