The facts behind acoustic ratings

Having returned to one of our favourite topics (soundproof doors) we’re now going to revisit how the volume or loudness of sound is actually  measured.

The unit of measurement is called the Decibel, originally named in honour of Alexander Graham Bell.  Now let’s have a look at some examples, and how we define different noise levels using Decibels (abbreviation dB).

Decibel comparisons

The Decibel is a logarithmic unit which expresses the ratio between two values of a physical quantity, commonly being power and intensity.  Sound intensity follows an inverse square law from the source i.e. doubling the distance from a noise source reduces its intensity by a factor of four. On the scale, the smallest audible sound (near total silence) is set at 0 dB. A sound 10 times more powerful is 10 dB. A sound 100 times more powerful than near total silence, however, is 20 dB, and a sound 1,000 times more powerful than near total silence is 30 dB.

To put things into perspective, let’s now look at some examples of common sounds and how many dB they are:

• Near total silence (threshold of normal hearing) – 0 dB
• A whisper at 2 meters – 15 dB
• A normal conversation at 1 meter – 60 dB
• City traffic, from inside the vehicle – 85 dB
• A lawnmower – 90 dB
• A jack-hammer at 20 meters – 95 dB
• A car horn – 110 dB
• A rock concert or a jet engine – 120 dB
• A gunshot or firecracker – 140 dB
• Death of hearing tissue – 180 dB

Any sound above 85 dB can cause hearing loss.  Any sound above 140 dB causes immediate damage, and actual pain!  No more than 15 minutes at 100 dB without protection is recommended, and NO exposure without protection of more than 110 dB is advised.

If you would like some further information, here are three useful links:

Decibel comparison loudness chart (and other useful links)

Heath and Safety Executive on Noise

The Control of noise at work regulations

Recommended noise levels

So given all that, it’s clearly important to have guidelines on recommended (maximum) noise levels.  Here are some for a variety of workspaces:

Workspace	Recommended maximum noise level (dB)
Offices	40 – 45 dB
Large Offices	45 – 50 dB
Classrooms	40 dB
Large Lecture Rooms	35 dB
Music Room	30 dB

Here are some sound reduction guidelines:

20 dB	Normal speech will be readily audible
25 dB	Loud speech will be clearly audible
30 dB	Loud speech will be clearly audible under normal circumstances
35 dB	Loud speech will be audible but difficult to distinguish
40 dB	Loud speech will be faintly audible but cannot be distinguished
45 dB	Shouting will be audible but cannot be distinguished
50 dB	Shouting will be barely audible
55 dB	Shouting will not be audible

So how do we measure the noise reduction?

We’ve learned that the Decibel is a logarithmic unit, while the percentage scale is linear. Thus, a statement like “solution XYZ would reduce noise by N dB” is meaningless.  And to emphasise the point, the Decibel scale has no maximum value, so it’s impossible to calculate a percentage reduction anyway!

We use instead a ‘Weighted Sound Reduction Index’, Rw, which expresses the difference between the sound intensity hitting one side of a structure and the resulting sound measured on the other side, i.e. it rates the effectiveness of a soundproofing system or material. Increasing the Rw by 1 translates to a reduction of approximately 1db in noise level. Therefore, the higher the Rw the better the insulation provided by the soundproof door will be.

We’d love to help you

As we said last time soundproof doors are only part of the answer, albeit a very important part.  Here at Enfield Doors we have been manufacturing and installing them for many years for numerous satisfied customers – and we can give you expert advice and guide you through all of the regulations and complications that you need to be aware of.

If you would like further information on our company please visit our home page or go directly to our contact page when we will respond as quickly as possible.

As always we’ll be delighted to help and advise you.