Understanding Acoustic Ratings: STC vs RW vs DnTw

by Media Xpose

How do you control what you can’t see? This is often the challenge with sound: it behaves strangely and can be very complicated to track. As a result, the techniques and instruments used to track sound give complicated results. It’s all fine for an expert to point to a waveform and talk about decibels, but it still makes little sense to everyone else.

Fortunately a series of ratings have been developed to help make sense of sound insulation. Though these do not replace the insight of an acoustics professional, they give us a better grasp of what we want to know when selecting the right acoustic partition or door for our sound insulation needs or deciding on the best treatment for that troublesome noise.

When looking for products that insulate sound, there are three rating systems you will encounter:

STC or Sound Transmission Class

The first is the STC rating. This is a United States standard scale: the higher the number, the better that material can reduce sound by reducing the frequency and decibels.

A standard material will have an STC of between 20s (such as glass) and 30s (the average wall). Effective sound insulation appears at STCs around the 50s. STC testing standards are always updated, so an STC rating from two decades ago won’t be the same as that same rating today.

There is one catch with STC: it doesn’t cover low frequencies very well. A material can have a high STC number, but not really block sounds such as rumbling traffic, reverberating construction, or the droning hubbub of office voices.

Rw or Weighted Sound Reduction Index

Rw is the alternative to STC and used by most of the world. It is an International Organization for Standardization (ISO) rating and part of the ISO 140 (Acoustic) family. Rw ratings are similar to STC in that they follow familiar testing methods.

But they also differ quite a bit: for example, Rw covers a larger frequency range than STC. You can’t simply compare an Rw rating to an STC rating. Some professionals prefer Rw because it corresponds to the decibel scale. So an Rw 50 rating means you could expect the noise you want to ‘block’ to be reduced by 50 decibels.

DnTw – onsite sound insulation

The DnTw rating describes the acoustic performance of a completed part of a building. It is as simple as measuring the noise level in the source room minus the noise level in a receiver room (as the drawing above shows). This updated acoustic sound level testing method is an in-situ or onsite measurement of the Standardised Level Difference according to ISO 162283-1 2014, analysed according to weighted Standardised Level Difference, DnTw, according to ISO 717-1.

DnTw ratings are being specified more since products are being tested in situ and not in laboratories. In most buildings there will always be a degree of sound leakage through the structure, and therefore there are separate sound reduction indices for laboratory and on-site measurements.

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