When it comes to the materials used to absorb acoustic energy, which is essentially reducing sound, you want to be able to refer to something that clearly shows how well it’s able to do this. This is where sound absorption coefficient comes in; you’ll often find it mentioned alongside soundproofing materials used for walls, ceilings, and even flooring materials, usually for communal spaces such as offices, transport hubs, and schools.
Architects, interior designers, musicians, and even students may find themselves coming across references to sound absorption coefficient, so this blog will serve as a guide to understanding this term, and why it matters.
What is sound absorption?
Sound absorption stops a sound wave in its tracks, interrupting its travel through the air and absorbing it into a dense material. A sound wave is transmitted, and whilst part of it is reflected by the material and part of it passes through it, the rest of the material has the sound wave transferred to it.
Materials with good sound absorption are vital in spaces that are prone to inviting a lot of noise – usually ones that will see lots of people gather together, such as offices, and public spaces like stations, libraries, hotels, places of worship, museums, galleries, concert halls, and theatres. They can also be useful within homes, to ensure peace and quiet both inside and out.
There are several materials known for their sound-absorption properties; the most common examples include:
- Acoustic foam: Available in various shapes, such as wedges to properly get into those nooks and crannies, acoustic foam is generally made from polyurethane.
- Mineral wool: Commonly used within ceilings, walls, and floors, mineral wool isn’t just good at absorbing sound; it’s a dab hand at keeping you warm too with good thermal credentials.
- Fibreglass: This stuff is lightweight and good at absorbing sound waves of varying frequencies, making it a versatile material for sound absorption; you’ll often find it within acoustic panels, as well as insulation.
Understanding sound absorption coefficient
Sound absorption coefficient, which is used to rate the efficiency of a material at absorbing sound, is the ratio of absorbed energy to incident energy. It’s represented by α. For example, if a material can fully absorb acoustic energy (or soundwaves, in other words), then α = 1.
The sound absorption abilities of materials will be changed by different frequencies. This means that some materials absorb certain sounds better than others, and to show this, sound absorption coefficient frequency curves are used. However, these can be a little complicated to compare. Instead, the average sound absorption coefficients are often used, which is the average value of a material’s absorption at various frequencies.
The ability of a certain material to absorb sound is described by sound absorption A; this is calculated by multiplying the surface area of the material by its sound absorption coefficient.
The surface area and sound absorption coefficient of the ith wall are represented by si and αi, respectively.
The transmission coefficient, shown as τ, is the ratio of energy that passes through a material to the energy that hits it. In engineering, the ability of a material or building element to block sound is measured by the sound reduction index, also known as sound transmission loss (STL). This is expressed in decibels, which is a term you’re more likely to recognise.
How is sound absorption coefficient measured?
So, how exactly do we reach the numbers required to complete those equations? Well, this is generally done in one of two ways: the reverberation method, and the impedance tube method.
The reverberation method
The reverberation method involves calculating the sound absorption coefficient using reverberation time measurements; this is because reverberation time is ‘inversely proportional’ to absorption; this means that the better sound absorption is, the lower the reverberation time. In order to measure how good a material is at absorbing sound, the reverberation time is measured – this means we’ll find out how long a sound lingers – within a room both with and without the chosen material. From here, the sound absorption the material is capable of can be calculated.
The impedance tube method
To measure sound absorption coefficient using the impedance tube method, the material being tested is placed at one end of a tube. Sound waves are then sent through the tube, and by cleverly measuring the difference between the highest and the lowest pressure points in the tube, and comparing the incoming and reflected sound waves, the sound absorption properties of the material can be calculated.
Factors that affect sound absorption coefficient
Of course, there are a number of factors that play a part in the efficiency of a material at absorbing sound. These include:
Material properties
This is an obvious one; the makeup of the material you’re testing will greatly impact its sound absorption coefficient, which includes its thickness, density, and surface finish. Not every thick, dense material is going to absorb sound, though. For example, concrete would reflect sound, not absorb it.
Frequency dependency
As we’ve alluded to, sound absorption has a lot to do with different sound wave frequencies; not all soundwaves are the same frequency – far from it! How well a material absorbs sound depends on how heavy it is for its size – in other words, its mass density. Take a porous material, for example; if it gets denser, it will absorb low-frequency sounds better than high-frequency sounds.
Environmental conditions
Temperature, humidity, and location are all environmental factors that can influence measurements of sound absorption coefficient.
Practical applications
To appreciate good sound absorption coefficients, we should talk about the kinds of places that such materials should be considered and installed. Below are the main areas in which you’d expect acoustics to play a role, and therefore, where you could find materials with sound absorption coefficient measurements.
Architectural acoustics
There’s no doubt about it – acoustics within buildings matter; from large public spaces, to offices, and even our homes. Take theatres, concert halls, and auditoriums, for example; you know that hush you get when you walk in before the show has started, despite all the people making their way to their seats? It’s all to do with sound absorption. You’ll find that a mixture of sound absorbent and reflective surfaces are employed in accordance with the room’s shape, so when performers are on stage, the sound reaches the audience in the best way possible, without echoes.
The same applies for homes, offices, and libraries; architectural acoustics are used to keep any sound in the place it's made, so it doesn’t travel and disturb others. This could be in flooring, wall coverings, ceilings, and even furniture. No one likes to hear loud neighbours, whether they live next door to you, or sit close to you at work!
Industrial applications
Industrial spaces, such as factories, can be a noisy business, which is why acoustic measures need to be taken to control noise levels. This is to protect employee health more than anything; there are plenty of health and safety regulations around what is and isn’t tolerable for employees to work in, and when an employer should provide hearing protection. You may find partitions, acoustic ceilings, and acoustic wall coverings are all fitted to protect the hearing of those working within noisy industrial environments.
Consumer products
Carpets, curtains, and furniture designed with sound absorption in mind are all available to buy, all designed to make everyday living noise less of a disturbance to others, whether they live in the same property, or just nearby. If you’ve ever been the downstairs neighbour in a block of flats, you’ll appreciate what a godsend in sound absorption a layer of carpet can be!
How to choose the right materials for sound absorption
Believe it or not, there’s quite a lot that goes into choosing the right sound absorption for your space. The way you decide on the criteria you use may require you to explore the following:
Identify the type of noise
Higher frequency noise, such as people’s voices, will require a different approach than low frequency noises, such as music with heavy bass.
Know the space
The shape and size of the room will need to be carefully considered, as well as its purpose. What may work well for a 6-person office will likely not work for a 5,000-capacity theatre.
The properties of the materials themselves
It’s important to familiarise yourself with the definitions of words such as density and porosity. The denser the material, the better it is at absorbing lower frequencies, while lighter ones absorb high frequencies more effectively. Porous materials are, in general, better at absorbing sound full stop.
Aesthetics
Ceilings are a great example here; don’t underestimate the power of ceiling design. The good news is that opting for an acoustic ceiling doesn’t mean you need to compromise on it. Think hard about the kind of transformation your ceiling can achieve for you, and explore materials that not only effectively absorb the sounds within the space, but that will achieve the look you’re going for, too.
Installation
Who will be installing your sound absorbent materials, and what would suit the speed you need the project to move at? Some materials are easier to install than others, so consider whether you need materials that can be easily applied to walls or ceilings yourself (such as acoustic foam), or if you require a little help from the professionals.
The environmental impact
With sustainability now a pressing matter that few can ignore, chances are you’ll want to opt for eco-friendly materials if you can. If they fit the sound absorption you need, recycled cotton and cork are good choices for environmentally conscious projects.
Your budget
Of course, budget plays a part in any project. Some high-performance materials can be expensive, so consider the best option within your price range.
Future trends in sound absorption
In our opinion, the main trend within the world of sound absorption is undoubtedly sustainability; as we all strive for ways to achieve what we need to without harming the planet, much research is being done into how sustainable materials can absorb sound. The likes of bio-based polymers, natural fibres, recycled materials, porous metals, new composites, and smart materials are all being explored by sound scientists. As experts in the world of acoustic building materials, we’re excited to see what’s made available for use within projects.
Choose SAS for acoustic ceiling and wall panels
You might have guessed by now that when it comes to the acoustics of a room, SAS are the leaders in our industry; our acoustic ceiling products absorb sound using an open-cell porous material, which is faced with a perforated metal sheet. Whereas the majority of ceiling tiles rely entirely on the porous material behind the perforated metal to absorb the sound, our micro-perforated tiles are the exception.
As acoustic experts, we’ve developed a range of ceiling products, which range from suspended metal ceilings to baffles, ceiling rafts, nodal ceilings and more. Our experienced team pride themselves on creating totally bespoke acoustic solutions for any space, taking its purpose and shape into consideration, as well as your design vision and, of course, budget.
Every SAS client benefits from our unique design expertise, high quality manufacturing capabilities, and product integrity, as well as our specialist acoustic guidance. This includes advice on acceptable levels of acoustic privacy, reverberance, and the acoustic characteristics of various types of spaces. We’re confident about delivering exemplary acoustic solutions, with awards and accreditations to our name to bolster our knowledge, experience, and passion for transforming spaces through design that performs.
To learn more about the science of acoustics and its application within buildings, review our products' acoustic performance as well as our attenuation and absorption statements. Contact us today to discuss the acoustic requirements for your project.
