the loudness of sound

Decibels (dB) is a logarithmic – not a linear – unit of measurement for the loudness of sound.

Normally a change of 1 dB is the smallest volume change detectable by the human ear. A reduction of 3 dB in noise level is equivalent to about 50% of the sound energy being removed from the system.

Regular exposure to noise at or above 90 dB causes permanent hearing damage.

Decibel Levels of Common Sounds

The harshness of sound

The “colour” or “harshness” of the sound is defined by the frequency of the sound. Frequency is the number of waveforms generated in one second. For example, if you hit the middle C key on a piano, the strings in the piano will vibrate back and forward around 261 times per second. Higher pitched notes have a higher frequency and lower pitched notes have a lower frequency.

The sensitivity of human hearing is frequency dependent. Human ears are most sensitive to the noise around 1,000Hz. Human ears are less sensitive to low frequency noise such as below 60Hz or extremely high frequency such as above 10,000Hz. As a result, noise level is adjusted to reflect the sensitivity of the human hearing and the adjusted noise level is termed dB(A), i.e., “A” weighted noise level.

Harsh noise: noise concentrating at around 1,000Hz and higher. This noise frequency tends to cause discomfort to human ears. A typical example is the high pitch noise of vacuum cleaners.

Tonal noise: when one single frequency noise is about 10dB(A) higher than the all the other frequency noise. Tonal noise is extremely unpleasant especially exposed over long period of time. A typical example is the “humming” noise from a transformer.

Examples of Frequency Levels

Industrial noise

Occupational Health and Safety Regulations in Victoria, Australia, limit workers’ exposure to 85 dB(A) averaged over 8 hours, or to any instantaneous noise in excess of 140 dB(C).