Acoustic Measurement

Acoustic measurement is given in terms of decibels (dB) which increase on a logarithmic scale, i.e. it cannot be manipulated in conventional arithmetical ways. (For example, 45dB + 45dB equals 48dB, not 90dB.) 

The human ear does not perceive all frequencies of sound to the same extent; to allow for this, a weighting correction is applied to dB measurements. Hence the weighting correction for doorsets is Rw.  Weighted apparent sound reduction index (Rw) is a single-number index which characterises the frequency-dependent airborne sound insulation performance of building elements.

Adding/subtracting decibels


If the levels differ by                            The following should be added to the higher level

0 to 1dB                                               3dB

2 or 3dB                                               2dB

4 to 9dB                                               1dB

10dB or over                                        0dB



If the levels differ by                             The following should be subtracted from the higher level

more than 10dB                                    0dB

6 to 9dB                                                1dB

6 or 4dB                                                2dB

3dB                                                       3dB

2dB                                                       5dB (approx)

1dB                                                       7dB (approx)



  • 34dB + 37dB = 39dB
  • 41dB – 36dB = 39dB


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