Sound Insulation Testing Explained

Airborne sound insulation relates to sound that travels via the air and through separating structures between rooms.  For example, airborne sound across a lightweight floor would pass through the plasterboard ceiling, into the floor void, through the floorboards and into the room.  It relates to noise sources such as speech, music, television sound…any noise source that radiates into the air, rather than directly into the structure.

The DnT,w + Ctr relates to a level of on-site sound insulation.  In simple terms, put a noise source in one room (the ‘source room’), measure the noise level, then go next door (the ‘receiver room’) and measure again, then calculate the difference.  Well actually, this would be a ‘D’.  A Dw is similar, but with the ‘w’ denoting a weighting across frequency which accounts for the way humans perceive different frequencies.  A DnT,w is a Dw which is then ‘normalised’ to account for the reverberation time of the room (the ‘nT’), recognising that the level difference is dependant on how reverberant the receiver room is, and that every room will have a slightly different reverberation time, hence we need to normalise the result to assess every sound test on a level playing field against a fixed target.  A DnT,w + Ctr then adds the addition of a low frequency correction (the ‘Ctr’), as low frequency noise is difficult to control and therefore the regulations add this correction to improve the standard of sound insulation against low frequency noise such as music.

Confused?  Let’s look at it really roughly.  Take a source level in one flat of 100 dB(A), then a receiver level in the neighbouring flat of 55 dB(A), the level difference is 45 dB, hence our sound insulation performance should be somewhere in the region of 45 dB DnT,w + Ctr…

The higher the DnT,w + Ctr, the better the level of airborne sound insulation.

What is Impact Sound Insulation and an L’nT,w?

Impact sound insulation relates to noise generated by actual impact on a structure.  Hence this typically is only assessed for intermediate floors, to consider noise such as footsteps, chair scrapes and any other object being dropped or impacting on the floor.

The L’nT,w is determined from measuring sound levels within a room where the floor above is being excited by an impact.  Your acoustic consultant will use what’s called a ‘tapping machine’ to do this, which drops a series of weights/hammers on the floor in regular succession at a fixed force (as if the acoustic engineer varied the force every time they test, they’d get completely different results!).

The lower the L’nT,w, the better the level of impact sound insulation.

What does Sound Testing actually involve?

Airborne sound insulation tests are conducted by placing a loudspeaker, or two loudspeakers in the ‘source’ room.  These loudspeakers play white or pink noise at a very high amplitude, to the tune of 95 – 110 dB(A) within the room (the acoustic engineer will be wearing hearing protection!).  The reason for using this type of noise, is that it is constant in amplitude, with an even spread of sound energy across frequency, keeping our test nice and consistent.  The engineer will use their sound level meter to make a series of measurements in the source room.  They will then go to the adjacent room (with the loudspeaker still on) and take a series of measurements within the ‘receiver’ room.