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NOVA Acoustics

What’s the difference between Rw and DnTw? Laboratory Acoustic Tests Vs Field Acoustic Tests

When it comes to specifying which partition wall or floor design to choose for a project or which product supplier or specifiers data sheet to trust, there’s a lot of confusion and mis-application.

One of the most common points of confusion is the difference between Rw and DnT,w. Misunderstanding these two can lead to under performance or overspending, so here’s a brief guide to help you out.

Definitions

DnT,w is used in reference to on-site sound insulation and it describes the in-situ or onsite airborne sound insulating power of a building element. Because it’s on-site, DnT,w accounts for all physical channels of sound transmission, including through a partition separating two spaces and any flanking paths around it, for example, ventilation, external walls and ceiling voids.

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Rw, on the other hand, is the weighted sound reduction index in dB (decibels) and it describes the airborne sound insulating power of a building element. It is a laboratory-measured value as defined in ISO717 Part 1. It can apply to walls, ceiling/ floors, ceiling/roofs, doors, or windows. The higher the number, the greater the sound insulating power of the building element. It is measured over the frequency range 100 to 3150Hz Walls carrying this kind of value have been tested in isolation, without the on-site channels of sound transmission mentioned above.

Why have two measurements?

These two measurements exist to give construction contractors a more accurate idea of how different walls or floors will perform. The same wall measured in laboratory conditions will produce the same result every time, but when measured on-site, results will vary from one space to another. When converting laboratory-rated performance (Rw) to on-site (DnT,w), you have to take several factors into account, including the reverberation time and volume of the ‘receiving’ room, the dimensions of the separating partition and the potential surrounding transmission on-site from the aforementioned channels. This last factor is tied in with the construction quality of the site, and attention to corridor detailing. Unfortunately, because of the DnT,w’s potential to vary between partitions, there’s no simple formula for conversion. There are a number of assumptions that can be made and generally the difference between Rw and DnT,w can range between 5 – 10 dB. We have found that with masonry or concrete constructed party walls and floors that the difference will be closer to 5dB, however with lightweight timber and steel constructions the difference will be closer to 10dB.

What does this mean for me?

Ensuring you consult an acoustic consultant at the beginning of your project can save you costly errors in specifying the incorrect partition walls and floors for your project. NOVA Acoustics Ltd has put together a FREE sound insulation design advice section to our website HERE. This runs you through a myriad of different floor and wall systems that are designed to achieve Part E of the building regulations. We have also outlined common flanking issues and acoustic detailing are correct to ensure you get a consistent PASS when it comes to your sound testing.

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