Understanding the acoustic properties of flooring - Interface Europe Ltd, t/a Interface


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Welcome everyone. My name is Mark Griffin, technical product manager for Interface in EMEA. I'm also the person who wrote this CPD. This will be a 35 minute seminar after which there will be time for listeners to answer a small set of questions on what has been presented.



Our seminar today is about understanding the acoustic properties of flooring and the impact flaws could have on the overall acoustic experience of space. Excessive noise is one of the top five complaints in the workplace and with the rise of more open plan environments this will only increase as we create more spaces designed to promote creativity and collaboration.


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At Interface we know that our physical experiences impact on how we feel. In 2015 we commissioned a global study looking at the importance of well-being in the workplace and how incorporating things like biophilia and well designed acoustic spaces, is crucial to maximising productivity and creativity. The acoustic requirements of an interior space will always vary from project to project, so this seminar is positioned to demonstrate how decisions made about flooring could impact the overall acoustic experience.


Further, we also hope this will bring some clarity to the claims being made by manufacturers when promoting acoustic solutions.


The seminar will cover:

  • The basics of acoustics,
  • Acoustic tests for flooring,
  • Sound absorption including reverberation time,
  • Impact sound transmission 
  • Recap on the key points.


Introduction to Interface Europe Ltd

Now before we get into the seminar, I want to give you a brief summary of Interface. Interface is a worldwide leader in the design and production of modular flooring products that combine beauty with functionality and environmental credentials, to help businesses and organisations bring their design vision to life. We are a global company and employ just over 3000 people in 110 countries. Our aim is to enhance interiors with innovative design solutions that are also sustainable. Interface Europe limited is part of Interface Inc which was founded in 1973.


Every decision our founder Ray Anderson and the Interface business has made since the mid-1990s, in the design and manufacture of our products is intended to bring us one step closer to our sustainability goals. Interface was one of the first companies to publicly commit to sustainability, when it made its mission zero pledge in 1994.


Mission zero represents the company's vision of becoming totally sustainable and ultimately restorative by 2020. It influences every aspect of the business and inspires the company to continually push the boundaries to achieve its goal. Since then Interfaces developed and implemented radical innovations taking the company beyond the realms of what was thought possible and making significant progress towards its goal. The company has now announced a new mission climate take back, which aims to reverse the effects of climate change through new innovation, new business models and new ways of thinking about carbon. Interface is being widely recognised for his achievements in sustainability and was named third in the world in 2016, GlobeScan Sustainability leaders report.


Interface is the only business to have consistently placed in the top four companies globally since the report was launched in 1998. Interface is two European manufacturing facilities in Scherpenzeel Holland and Craigavon Northern Ireland which collectively produce around 14,000,000 square metres of carpet tile per year.


Learning Aims

As already mentioned, the aim of this seminar is to increase your knowledge and understanding of the acoustic properties of flooring and the impact floors could have on the overall acoustic experience of a space. To do this we will look at the acoustic test with flooring explaining how testing is done and also what the results mean.


The measurement of sound absorption will be explained so you are aware the 18 measurements are taken across a wide frequency range and it is the values at each of those frequencies which describe absorption behaviour of the material.


The unit of measure will be looked at and how to actually understand what the full range of results mean. Further we will look at classifying the measured results to provide a very general indication of sound absorption.


Next it is looking at reverberation times to understand how sound absorption affects the acoustics within a space and the subsequent intelligibility of speech. After that we will cover sound insulation and specifically impact sound transmission between rooms as that is a key acoustical benefit of flooring, specifically soft floors. Again the terminology and measurements will be explained so as to understand better what the results mean.


And in the end, we hope that this will bring some clarity to the acoustic properties of flooring, so it is easier to identify and select the floors required when designing a space for acoustics. Further to better understand acoustic claims of flooring made by manufacturers.


The Basics of Acoustics

Acoustics is the study of sound and sits within the wider subject of well-being. Good acoustics are essential to productivity and creativity in the workplace, so when designing any space acoustics should be an important consideration. Poor acoustics can limit communication, reduce efficiency and affect people's concentration and behaviour. Now poor acoustics of space are generally due to long reverberation times and this refers to the period of time it takes in seconds for a sound to become inaudible, or from noises outside entrance space due to poor sound insulation.


The actual acoustic quality of a space typically depends upon its size and shape, so for example the room usually becomes more reverberant with increasing height. The choice of materials for the surfaces of the space, absorbing surfaces not just for the floors but the ceilings, the walls, curtains and so on as well as furniture and people present in the room and all help to lower sound reverberation.


How people use the space, for example in a concert Hall it is important that the reverberation time is correct so music sounds good. This means a longer reverberation time than the speech, which is best understood without echo, meaning a shorter reverberation time for speech in an office.


To help demonstrate this point we have the following sound files for speech and music. There are two reverberation times 0.46 seconds and 2.1 seconds and consider the difference they make to both speech intelligibility and music quality. Clearly speech is more intelligible with a shorter reverberation time but then what about music. Music sounds better and cleaner crisper with a longer reverberation time so it’s true that not one solution fits all and different spaces will have different acoustical requirements according to what they are being used for.


Consideration should be given to this at the design stage and so selecting the right materials to give the desired acoustical outcome for the space. Knowledge of the acoustic properties of materials would therefore be of use and laboratory tests can be done to determine what they are. 


Acoustic Tests

Moving onto acoustic tests, when sound hits a material part of the energy is reflected another part is absorbed in the material and the third fraction is transmitted. The amount of sound energy passing through the surface and the transmission, depends upon the sound insulation properties of the material. The amount of sound energy retained in the material depends upon the sound absorption properties of the material. Carpets provide acoustic benefits in dealing with structure borne sound caused by footfall so transmission and a degree of additional sound absorption within a space. So that is what carpets or floors are tested for sound absorption and impact sound transmission.


Now we have a short video to explain a little more about these tests.


We've been doing sound absorption testing and that is testing a variety of things such as carpet tiles. We're testing to see how they affect the reverberation time of a particular area. The reverberation time is the time it takes for sound levels to get from one level and drop down to another, so its a decay rate.


The way we do the sound absorption coefficient test is we would measure an empty room for the reverberation time, and we would record that at six different microphone positions with two sound sources. To get a good certainty in the measurements we measure 5 times so we're absolutely clear on what the results are. The test of the empty room i.e. no sample being installed is a reference point and then we're using that reference to determine the sound absorption coefficient of the sample that’s been installed in the room. We've got something called the real time analyser which both generates an output to the loudspeakers that are in the room and it will also record the output of the microphones. It’s got some clever software that can take a real time signal in the time domain and convert it into the frequency domain and that displays the data in 1/3 octave bands.


Typically we would use filtered white noise and that's basically just a flat frequency response. Then we would install the sample typically on the floor of the reverberation room and then we repeat the measurement with the sample inside the room. The way we obtained the sound absorption coefficient is through calculation using the two measurements, so we know that the sound absorption coefficient is of that sample that's been installed in the room.


The second test today was a measurement of impact improvement which uses a known sound source i.e. a tapping machine that is placed on the floor above the receiving room.


We measure the impact noise transmitted into the receiving room and then we would repeat the measurements with some kind of floor covering, be it laminate flooring carpet tiles. The impact improvement is the difference between the bare floor measurement and the sample on the floor.


Comparing the measurements for the bare floor and the sample installed on the same floor allows us to determine if there is a reduction in the amount of noise transmitted into the room below. In general floor coverings improve the amount of noise reduction between the floor and the receiving room below.


Sound Absorption

So let's start with sound absorption also known as room acoustics because it relates to sound within a space. We already said the amount of sound energy retained in the material depends upon the sound absorption properties of that material, and clearly soft surfaces whatever they may be will absorb more sound than hard surfaces. And it's not different with flooring from the graphic. You will know the sound waves are being absorbed by the textured loop al camping but then reflect back off the hard floor. This is the reverberation room we saw in the earlier video to measure the sound absorbency of a material to reverberation method is used. The room itself has hard surfaces and not parallel walls to give long empty room reverberation times. There are 11 plywood panels suspended from the ceiling which act as diffusers I.e. to scatter the sound field and making random.


The room is excited with random noise played into the room by a loudspeaker positioned in the corner of the room. The sound is then monitored at each of 6 microphone positions. Measurements are made on the rate of decay of sound with the room empty and then with the test sample installed. From a change in reverberation time with a test sample installed the sound absorption properties of that sample are determined.


Measurement of Sound Absorption

Since the sound absorption of a material will vary with frequency and the test method specifies a frequency range of 100HZ to 5,000HZ. During testing measurements are taken at 18 different frequencies within this range and the values at each of those frequencies will fully describe the absorption behaviour of the material, i.e. to what extent and at what frequencies the material absorbs sound. The unit of measure is called a sound absorption coefficient, alpha S which describes the absorbed energy per square metre.


This coefficient ranges from 0.00 to 1.00 where 1.00 means that all sound is absorbed and 0.00 means that no sound is absorbed and therefore reflected. The Alpha S follows a linear scale and so a coefficient of 0.2 means that 20% of incident sound is absorbed. Generally the sound absorption properties of a carpet are better at high frequencies. Solving room acoustic problems with measurements should always use these measured values as acoustic problems tend to occur within small frequency ranges. Therefore knowledge of the full range of values helps to identify which materials can help at frequencies of interest.


The following video shows some typical activities within an office and the frequency ranges they cover. This gives the frequency some context and so highlights the need to understand the full range of measured values.


The Weighted Sound Absorption Coefficient

So to recap sound absorption is frequency dependent and for true acoustical design the full range of data, the measured Alpha S values are used by an acoustician to target particular frequencies of interest for the space they are dealing with.


However, for general and simplistic guide routine applications with low acoustical requirements a single number value may be used. In Europe this is the weighted sound absorption coefficient Alpha W, which is based on the measured Alpha S values. To calculate this the 18 alpha S values are averaged within an octave band and rounded to the nearest 0.05 to become a practical sound absorption coefficient Alpha P. The 6 alpha P values are then plotted against a reference curve on a graph. And the Alpha W is defined as the value of the shifted reference curve at 500HZ. This is then used to determine the sound absorption class.


On the graphic are the Alpha P values for a typical carpet tile with the reference curves for the Alpha W values and the sound absorption classes included. So again the scale is 0.00 to 1.00 where 1.00 means that all sound is absorbed. Hard floors will have an Alpha W of between 0.00 to 0.10 whereas the carpet tile or the PVC or bitumen backing will typically be 0.15 to 0.2 and so higher than a hard floor. Of course there are many different bracket options for carpet tiles designed for added under foot comfort and acoustic properties. Typically these provide Alpha W values of 0.25 to 0.30 and above.


So the Alfa W is based on an averaged rounded and weighted number and so should only be used as a very general rating of sound absorption. The standard for calculating the Alpha W is EN iso 11654 which clearly states, the single number rating is not appropriate when the products are to be used in qualified environments requiring careful the acoustical design by expertise. In such cases only complete sound absorption data as a function of frequency are satisfactory.


Sound Absorption Test Results

We now have a sound absorption test report. On the reports we can see the sound absorption coefficient. The sound absorption class. The sound absorption coefficients at each frequency the 1/3 octave, and the practical sound absorption coefficient at the centre octave. We can also see the shape indicator and the shape indicator means that the sound absorption coefficient at 1 or more frequencies is considerably higher than the values of the shifted reference curve and so the complete sound absorption curve should be used not just the Alpha W.


So let us now consider what effect the Alpha W or the Alpha S values of the flooring have on the acoustics of a room. To understand this we need to know the effect they have on the room’s reverberation time. Remember the acoustic quality within a room can be expressed by the reverberation time and put simply the reverberation time indicates the period of time it takes for a sound to become inaudible. Reverberation time has a direct effect on speech intelligibility as we discovered with the sound files earlier. With a longer reverberation time there can be an overlapping of words which reduces speech intelligibility making it difficult to fully understand what has been said. To demonstrate this let's listen to speech at 3 different reverberation times, 1.2 seconds, 0.8 seconds and 0.4 seconds.


In simple terms a longer reverberation time results in poor speech intelligibility generally the shorter the reverberation time the better the speech intelligibility, for example building bulletin 93 for schools specifies particular mid frequency reverberation times for teaching and study spaces i.e. less than 0.6 seconds in general teaching areas. Guidance documents for commercial buildings recommends the reverberation time for an open plan office should be less than 0.5 seconds.


Now the reverberation time of any room will actually depend upon:

  • The volume of the room,
  • The surfaces of the room,
  • The furniture or people in the room


And so knowledge of these factors enabled the reverberation time of the room to be calculated. For the surfaces the measured alpha s values at the different frequencies are used rather than the very general Alpha W. That's because sound absorption is frequency related as described already.


So for all surfaces used within the room that is the ceiling, plaster, wall panels, carpets, curtains and so on. There alpha S values are multiplied by the surface area they cover to provide their absorption at each frequency. The absorption of all surfaces within the room are then added together to give the total absorption units in meters squared for the room, and this will be each frequency.


To calculate the reverberation time a simple formula is then used. T is the reverberation time equals 0.161 multiplied by the volume of the room divided by the absorption units. This provides the predicted reverberation time at each frequency and so enables acousticians to determine what materials to use to achieve the desired acoustic quality. A floor therefore makes a contribution towards sound absorption within a space but to one extent depends upon many factors, not least the size and shape, volume of the room, the choice of materials for all surfaces, the type and spacing of furniture and how people will use the space. It is the combined effect of all of these that directly affects reverberation time and the acoustic quality of a room.


Sound Transmission

So now we move on to impact sound. The amount of sound energy passing through the surface depends upon the sound insulation properties of the material. This is often referred to as building acoustics as the aim is to minimise sound transmission between rooms sound insulation is there for a measure to separate rooms from unwanted sounds from adjacent rooms or indeed the outside.


From the graphic it can be seen that sound can be transferred in a number of ways, for example direct sound transmission through a partition wall as people talk. Impact sound transmission through the floor structure is another way so when people walk over the surface sound energy is transferred into the room below or in two adjacent rooms. Floors deal with impact sound insulation.


Measurement of Impact Sound Transmission

Impact sound measurements are recorded in the reverberation room on the roof of this chamber is a concrete floor slab over which a taping machine is used as the impact sound source. The sound pressure levels produced by the taping machine in the reverberation room below are measured both over the concrete base and then with the floor sample over the concrete. 


The difference in sound pressure levels shows the reduction in transmitted impact sound through the use of flooring, i.e. camper. The result is an impact sounding proven index Delta LW which is rated in decibels. The higher this figure the less sound is transmitted. So this figure relates to a reduction in transmitted impact noise between rooms, it does not relate to the acoustic quality within the room itself.


Reduce Noise By Impact Sound

Typically there aren't too many discernible differences in impact sound reduction between one carpet and another. The biggest differences are between a soft floor and a hard floor. The values for carpet tiles compare favourably with values for standard hard floors which can range between 1 DB to 6 DB. So there is a noticeable difference in impact sound reduction between a soft and a hard floor but differences between soft floors are perhaps more difficult to determine.


Carpets are specified because they deal with impact noise far better than a hard floor and that is a key acoustical benefit of a carpet. At this point we should also point out that there are acoustic underlays to be used beneath hard floors which can improve their DB ratings further. So for example a vinyl floor with an acoustic underlay could achieve a figure as high as 19DB.


In the UK building regulations, for residential buildings, floorings must meet a figure of 17 DB to be used in common areas such as corridors which clearly most carpets will achieve. As already mentioned, acoustic type underlays are also available for some hard floors to comply with this as well. So whilst the higher DB figure means less sound is transmitted consideration has to be given as to what figure is actually required to comply with recognised standards or recommendations from the acoustician for the room or building in question.


Further points to consider include site conditions, for example subfloor type as all testing for impact sound transmission is done over a concrete floor. So how do results relate to installation over raised access panels?


Key Learning Points

We now move on to the key points to remember. Flooring has an effect on both sound absorption room acoustics and sound insulation building acoustics and so the choice of flooring has a direct effect on the acoustics of a space. Sound absorption relates to the absorption of airborne sound and there are a number of surfaces and materials within a room that can absorb sound and flooring is one such surface.


The total absorption provided by all surfaces and materials affects the reverberation time, which in turn affect speech intelligibility. With a long reverberation time there is no overlapping of words which reduces speech intelligibility making it difficult to understand what is being said. It is therefore necessary to select materials to achieve the desired reverberation times, and so knowledge of a material’s acoustic properties across the full frequency range enables informed decisions to be taken.


Sound insulation relates to sound transmission between rooms and one way this can take place is impact sound transmission through the floor. The type of flooring therefore affects how much of the sound energy passes through it and an impact sound improvement index Delta LW is used to explain the reduction in transmitted impact noise. This is racing decibels DB and the higher this number the less noise is transmitted. So hopefully it is clearer as to how the right type of flooring can make a positive contribution towards the acoustic quality of a space. Of course selecting the right flooring has not always been easy as it can be quite difficult to fully understand what their acoustic results mean. The expectation is that this training goes someway towards addressing that problem.


Thank you for taking the time to listen to this seminar. For questions on anything you have seen today please contact Mark.Griffin@Interface.com. Thank you.

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