• Samantha Hathway

What is an Acoustic Rating?

An acoustic or sound rating is used to judge and limit how noise travels within an environment. Around the globe, countries have different rules and guidelines for monitoring sound transmission and noise levels. Acoustic ratings are used to judge factors such as sound insulation, noise levels and absorption coefficients, ensuring building work remains within regulatory guidelines. In this post, we’ll focus on how these acoustic ratings are used within the construction industry and demystify some common terms you might come across during the planning and build phase of commercial and residential properties.

But before we delve too deep, it’s important to understand a bit more about sound – how it’s measured and how it travels….

What is sound?

Sound isn’t just noise, and it’s not just something you hear. Sound is actually a vibration that travels as an acoustic wave through mediums such as solids, liquids and gas. The way that we hear is actually how we receive these waves, and how the vibrations are interpreted by the brain. Sound is measured in decibels (dB), ranging from the barely perceptive rustling of leaves at 10 dB to a potentially deafening gunshot at 180 dB. Rather than being linear, the decibel scale is logarithmic, so a sound that’s 10 dB higher than another is actually 10 times as loud. Because we don’t hear all sounds and frequencies, human hearing is measured using what’s known as an A-weighted scale, or dB(A).

Why do we have acoustic ratings?

Although how we hear can be very subjective – what’s loud to one person can be fine for another – it’s well documented that living with continuous excessive noise is detrimental for your health. For this reason, there are set guidelines drawn up to ensure the health and wellbeing not only for workers but also the residential and commercial environment that surrounds a construction site. This is one example of acoustic ratings, to assess airborne sound transmission within and around a building structure.

One key way that noise levels can be minimised is via sound insulation, and to assess the effectiveness of such materials, there are several acoustic ratings. Here are the main ones you’ll encounter:

Standard Transmission Class

Just as building regulations vary across the world, so do acoustic ratings for sound insulation. Within the USA, the Standard Transmission Class (STC) is used to assess how well a building partition attenuates sound. Measured in decibel reduction, the STC tests frequencies between 125 Hz - 4000 Hz. The drawback with the STC is that it doesn’t capture low frequencies, i.e. those below 125 Hz, so a building with a high STC can still have issues from traffic, construction or home cinema speakers.

The Weighted Sound Reduction Index

The more popular rating that’s used by most of the world is the Weighted Sound Reduction Index, or Rw. This is the system we’re familiar with in the UK, and forms part of the international standard ISO 140 legislation.

The Weighted Sound Reduction Index is used to measure the sound insulating ability of individual building structures. This includes doors, windows, walls, ceilings and floors. Simply put, it expresses the difference between the sound intensity of hitting one side of the structure and the resulting sound recorded on the other side of the structure. It’s expressed in decibels, so increasing Rw by 1 translates to a reduction in noise of 1dB. Thus, the higher the Rw, the better the structure is at blocking airborne sound transmission, and this means that it provides greater sound insulation.

When it comes to construction, however, it’s not just Rw that must be taken into consideration. Given that the Rw is a laboratory-based measurement, there’s another equally important rating, the Weighted Level Difference, or Dw. This on-site sound reduction index measures the sound insulation of a partition, post-construction. The reason both indexes are so important is that lab results don’t always replicate identically in a real-world setting. This can be due to factors such as flanking via common constructions or weaknesses, room volume, partition size, etc.

You can usually expect a 5-10 dB reduction between the lab-based Rw and real life Dw (or greater for high levels of flanking or poor workmanship), so it’s important not only to model sound reduction indexes before construction but also to monitor sound levels and acoustic ratings during and post-completion of a build, and this is where the services of a specialist such as an acoustic consultant are so vital.

Sound Absorption Coefficients

When it comes to the acoustic properties of materials, another important rating you’ll come across is the Sound Absorption Coefficient. This measures the absorptive properties of specific materials, and is given as a number between zero, for a fully reflective surface that provides little to no sound insulation, to one, for a fully absorptive surface that literally absorbs the incident sound waves and allows little to reflect off the material. This rating is used to predict sound reverberation in a space that has not yet been built, allowing designers and architects to adequately control noise and reverberation within a room.

The absorption coefficients are expressed by frequency. For example, at 125 Hz curtains will only achieve a coefficient of 0.04, whereas a 4 inch thick fibreglass board can achieve a coefficient of 0.99 at 1kHz. These values are typically plotted on a graph to produce an absorption curve. From this, materials are rated from A (highest absorption) to E (lowest sound absorption), in accordance with the BS EN ISO 11654.

So as you can see, in order to protect us from living and/or working in a harmful noisy environment, there are many acoustic ratings that must be abided by during the construction process. In order to ensure your building project remains compliant with UK regulatory standards, contact us today. Our team of acoustic consultants will ensure you follow the latest legislative guidelines, and can help with every phase of the construction process. From planning applications to the design and build, and acoustic testing on completion, we’re with you every step of the way. We pride ourselves on providing straightforward advice, always ensuring that you understand where there might be an issue, and how this can be solved. For specialist advice in London, contact us on 020 7859 4530 and for the South East, contact us our Reading office on 0118 207 7324. Alternatively, you can contact us by email at

8 views0 comments