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Decibels - What Are They?!

Decibels are a strange system, and they can quite easily confuse our Customers. Importantly, they are not ‘linear’; they use the ‘logarithmic scale’. What this means in practice is that decibels cannot be added or subtracted in a linear way, so 50 dB + 50 dB does not equal 100 dB, it equals 53 dB. If that sounds complicated, decibels are actually intended as a way to simplify things!
Linear scale graph
Logarithmic scale graph

Origin

Decibels were developed by Bell Systems labs in the US (named after Alexander Graham Bell), to be able to describe a large variation in numbers. Well, they first developed the Bel, and then divided that system even further, in to tenths of a Bel, or deci-bel. This also explains why the decibel is annotated as ‘dB’ with the capital letter B.

Decibels can be used to describe anything really – where a base or reference value is provided. For instance, the height of a building could be described in decibels. Assuming the building is constructed entirely of brick, from top to bottom, the reference level would be 1 brick course. So if the building has 100 courses of brick to the roof level, we could say the building is 10 x Log (100/1), which is 20 dB. Therefore, the building is 20 dB tall. Of course, we don’t need to do this, as counting the number of courses is easy to do.

A Huge Range of Numbers

In acoustics, we use the decibel scale from 0 dB to around 120 dB to describe differences in sound pressure level relative to the lowest level that human hearing can detect. The alternative would be to present this scale as 2 x 10-5 or 0.00002 Pascals (the lowest level our ears can detect) up to 20 Pascals (‘the threshold of pain’), which is an absolutely huge range of numbers. Hopefully you see now why we use decibels!

Have a look through the gallery below for examples of different sounds – or noises – at different decibel levels.

< 20 dB

The lowest levels of sound that the human ear can detect.
As close to absolute peace as we can get...

20 - 30 dB

A breeze through the trees on a spring morning...
Lovely...

30 - 40 dB

The background sound level in your living room with your windows closed.
Before you put the TV on, mind.

50 - 60 dB

The background sound level in a central city garden.
Usually due to noise from traffic.

60 - 80 dB

The range of the human voice, from quiet conversation to SHOUTING!
At 1 m from the listener.

80 - 100 dB

A lawnmower or vacuum cleaner.
80 dB is actually the lower limit for Health and Safety exposure to noise. Anything above this, and depending on how long you are exposed to that level, you would need to start thinking about wearing hearing protection.

100 - 120 dB

A rock concert - or standing close to a loud speaker at a gig.
This is a very high level of sound, and even short-term exposure to this noise level can cause permanent damage to your ears.

> 120 dB

A jet aircraft taking off 100 metres away.
This is also called the 'threshold of pain'. Exposure to levels above this can damage your eardrums.

To find out what differences in sound level might ‘seem like’ to you, take a look at our Knowledge Article ‘5 dB Improvement? Sure, I know What You Mean!‘.

Trust the experts and get a quote to soundproof your home today.

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