Acoustic ratings

What are Acoustic Ratings?

Someone asked me what an acoustic rating was on Quora, so I’ve had a go at simplifying some of the terminology here!

Answer: There are many number of acoustic ratings, which are often different depending on the guidelines provided in your country.

Some common ratings in acoustics would be:

Sound insulation/reduction – There are any number of ratings used in sound insulation, which are used to specify the reduction in sound pressure through a partition (such as a wall or floor).

In the UK the figures used depend on the usage, but laboratory tested figures are specified as Rw (before construction) and on-site test values are noted as Dw (after construction).

Other variations of the values are used for testing performance of partitions (such as DnT,w in hospitals, or DnT,w+Ctr in residences).

The sound reduction provided by glazing is often given as Rw, while natural ventilators are often quoted as Dne,w.

Noise Ratings – In the UK noise rating curves (called NR curves/ NC curves are used in USA) is often used to specify noise limits for industrial machinery, or other things that may have the majority of its energy in low frequency bands.

See the image and link below from Engineering ToolBox for more information.

NR – Noise Rating Curve

Noise rating
NR Curves

Room Acoustics

I guess some other ratings could be used in room acoustics:

T60 – The time taken for sound pressure level in a room to decrease by 60dB.

More practical values used are the T20 and T30, which extrapolate the time taken to reduce by 20/30dB. It is rare that a full T60 is measured in normal spaces, as you would need a very loud source, or a highly reverberant space!

Absorption coefficients – These values are used to provide information on the absorptive properties of a certain material. This allows for the prediction of reverberation times in a space that has not yet been built.

Simple predictions can be made using classical formulae, and more complex calculations can be made using 3D modelling software and a number of algorithms. See below from my company Prism Acoustics:

absorption rating
3D noise model

There are many, MANY more acoustic ratings, but hopefully some of these will give you somewhere to start!

Check out some more of our noise blog posts here, or head back back to the home page

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