In terms of acoustics, when we talk about noise what we generally mean is ‘extra’ sound of greater than usual volume. Often the extra sound is unwanted so activities are often described as being ‘noisy’. For instance, you might think other people are being noisy if you are not involved in their conversation, or the noise of roadworks outside your office could be described as noisy.
Sound is actually the vibration of matter which can be perceived by the sense of hearing. Usually vibrations travel through the air but they can also travel through gases, liquids and solids. When the vibrations reach the human ear, they are converted into nerve impulses that are sent to our brains, allowing us to understand what the sound is.
Sound can be measured scientifically in two ways. Intensity, or loudness of sound, is measured in decibels. Pitch is measured in frequency of sound vibrations per second. A low pitch, such as a deep voice or a tuba, makes fewer vibrations per second than a high voice or a violin.
The decibel (abbreviated to dB) is the unit used to measure the loudness of a sound. Decibels are a ‘dimensionless unit’ like a percentage (%). This means they are not an absolute measure themselves - they are used to describe a ratio between two quantities.
Our sense of hearing is able to cope with a huge range of sounds. Humans can hear everything from the rustling of trees to a jet engine. In terms of power, the sound of the jet engine is about 1,000,000,000,000 times more powerful than the smallest audible sound. Decibels are useful because they allow such large ratios to be expressed as a convenient small number.
On the decibel scale the smallest audible sound (near total silence) is 0 dB. A sound 10 times more powerful is 10 dB. A sound 100 times more powerful than near total silence is 20 dB.
The decibel was named after Alexander Graham Bell, the pioneer of the telephone, in 1923.
The following are decibel levels of common sources of noise. Remember these are average levels and that actual noise levels vary depending on the particular item and where you are in relation to it.
0 dB the lowest sound level a person with normal hearing can detect
20 dB(A) is a quiet room at night
60 dB(A) is ordinary spoken conversation
80 dB(A) is shouting
90 dB(A) is an underground railway
110 dB(A) is a pneumatic drill nearby
130 dB(A) is an aeroplane taking off 100m away
Experts agree that continued exposure to noise at or above 85 dB(A) over time can cause damage to hearing. Simply put, the louder you listen the shorter the time you should listen for.