Black Knight Secondary Glazing
Black Knight Secondary Glazing

Noise Reduction

You have probably heard the word decibel when people are talking about noise pollution levels. In reality what should be quite a simple thing to do is actually quite difficult.


In practical terms how we actually hear an increase in noise is important.

By using secondary windows with suitable specialist glass and fitting it is possible to achieve a noise reduction of 75%.


By using secondary glazing for noise reduction it is possible to achieve a good level of acoustic insulation but it is important to ensure that the gap between primary and secondary glass is adequate.


Noise reduction definitions

The decibel (dB) scale is used when measuring both the current levels of noise pollution and the achieved reduction after secondary windows are installed.



  • Decibels - a scale used for all noise – simulates what average human hearing will experience when subjected to different volumes, frequencies & types of noise.
  • dB Rw – how the results from a sound insulation test will appear - Rw represents the weighting of 16 individual frequencies to give a result as a single number.
  • Rw + Ctr – this scale is used for traffic noise - it takes the data from the  Rw testing and adds a scale that represents average traffic noise.

Unlike measurements that you may be familiar with decibels are logarithmic. If your O level maths seems like a distance memory then put simply the difference between a linear and a logarithmic scale is as follows:


LINEAR 1,2,3,4,5…..

LOGARITHMIC 1,2,4,8,16 …..


That means that halving or doubling the measured noise level is not the same as what we actually hear and so all measurements must be taken from a starting point. 


In general terms this means:


  • With a 10dB decrease in the measured noise you experience a sound that is 2 times quieter
  • With a 20dB decrease in the measured noise you experience a sound that is 4 times quieter
  • With a 30dB decrease in the measured noise you experience a sound that is 16 times quieter

Measuring noise reduction through windows

  • On a busy road the estimated road traffic noise level is 75dB
  • A typical existing timber window in the closed position will provide sound reduction of 25dB – you will still hear 50dB of noise.
  • Installing Black Knight with the optimum glass specification and cavity between glasses you will experience a further 20dB noise reduction  level.
  • The dB level that you actually experience is 30dB compared to an open primary window reading of 75dB and a closed primary window reading of 50dB

Although you have achieved a further reduction of 20dB by installing secondary windows you will actually reduce the noise by 75%


Take a look at some of our testimonials to see what real life customers think about reducing noise with secondary windows