Today’s jargon buster is about sound insulation. Very broadly, the level of sound insulation of a partition indicates how well it stops sound from passing through. It’s used, for example, in testing party walls and floors to see if they are compliant with Approved Document E. It also comes into play when we consider speech privacy in offices, hospitals and schools. When people refer to “soundproofing”, sound insulation is generally what they mean.

Types of Sound Insulation – Airborne and Impact

Airborne sound, as you might expect, is sound transferred through the air – for example, someone’s voice, or music from a loudspeaker, sound from a television. This is defined as a variant of either Rw or Dw (see below for more information), and is usually specified as a minimum value which must be achieved.

Impact sound occurs when energy passes into a partition from an impact, and re-radiates as noise on the other side. The example of this you will likely be most familiar with is footsteps upstairs heard in the room below. Impact is a form of structureborne noise, which many of us have also experienced when neighbours decided to get the drill out and do some DIY too early on a Sunday morning! This is specified as a variant of Lw and, in contrast to airborne sound insulation, is specified as an upper limit which must not be exceeded.

So remember – with airborne sound insulation, the higher the figure the better the performance. But with impact sound insulation, the lower the figure, the better.

Specifying Sound Insulation: The Difference between Rw and Dw

Sound insulation is referred to in many different ways, depending on exactly what is being described. For example, DnTw,Tmf(max) is used in testing schools to refer to the on-site sound insulation measured, corrected for the mid frequency reverberation time (Tmf(max)) measured in the receiving room!

It’s generally the acoustic consultant’s job to worry about all of the flavours and variations of sound insulation, according to your particular project. But it is useful as a client to understand the difference between the “Rw” and “Dw” types of specification.

If you are specifying partitions using manufacturer’s data, for example selecting partitions from The White Book, the airborne sound insulation performance will be presented in terms of a laboratory tested Rw figure. The actual performance you get in your site-specific application will be affected by all kinds of things which your friendly acoustic consultant can help to assess, including the volume of the rooms, the area of the partition, reverberation time, background noise, and sound flanking.

Sound Flanking

So what is sound flanking? Well, that’s all the sound that transfers from Room A to Room B by a route other than directly through the partition. With a wall, sound might transfer through the junctions with the floor or ceiling slab or with adjacent walls, if not well detailed and sealed. It’s common on site to find a beautiful partition where someone has then drilled a large hole through for cables, which then hasn’t been sealed properly. Some flanking is inevitable – especially in a refurbishment – but it can be dealt with, with consideration.

Expanding foam is the bane of every acoustic consultant’s life – sorry, but it won’t stop the noise passing through, whatever it says on the can from a Well Known High Street DIY Store. Mass is the key to sound insulation (as well as the air gap between sides of a partition, for higher performance). Small openings need to be properly stopped up with mineral wool and mastic – larger openings may need a cover plate to make them good. Ductwork passing from room to room is also a common issue – crosstalk attenuators can be used to stop ducting acting like an old fashioned “speaking tube” and allowing speech to transfer directly from room to room.

Make an impact!

Very rarely, impact sound might be a concern for walls (e.g. balls hitting a sports hall wall next to a classroom). Sensible design should mean that such adjacencies don’t occur. But impact occurs all the time in floors. It’s less of an issue where there is a soft floor covering such as carpet, but in domestic situations often leads to complaints when carpet is replaced with wooden or laminate floors.

Resilient underlays are widely available commercially to help combat this issue. It’s also worth considering the overall construction of a floor and the ceiling below – a mass-air-mass construction, possibly with the ceiling resiliently hung, will typically perform better than a single layer floor (unless that single layer floor is a LOT of concrete).

So you want to achieve decent sound insulation?

Follow the golden rules:

  • Get some help on specification from an acoustic consultant – it’s not just about the wall/floor.
  • Build it properly – sound can be like water, it’ll trickle through any holes you leave.
  • Throw that expanding foam in the bin…please?
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    Thanks to Fran for his amazing cartoons. Look out for more coming soon!