FAQs - Acoustics & Suspended Ceilings

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1 - Which Armstrong ceilings provide good acoustic performance?
This depends upon your definition of 'good' and whether it is sound absorption or sound attenuation that is your main interest. All Armstrong ceilings can provide acoustic control with absorption values from 0.10 to 1.00 Alpha w, and attenuation values of up to Dfcw 44dB.
If you describe your concerns to your ITS group, they will be able to advise further on suitable products to meet your requirements. Also, our mainline brochure provides detailed acoustic performance for all our popular products.
2 - I see a reference to NRC in Armstrong literature; what does this stand for?
Noise Reduction Coefficient is a method for providing a single number rating of sound absorption. It is defined in ASTM C423 as the arithmetical average of the measured sound absorption coefficients for the four one-third octave band frequencies centred at 250, 500, 1000 & 2000 Hz, rounded to the nearest 0.05. This US based system has been used extensively in Europe for many years but is now becoming less popular with the advent of the Weighted Sound Absorption Coefficient αw.

3 - So what then is Alpha w (αw) as I thought this does the same thing as NRC?

Alpha w (weighted sound absorption coefficient) is also a method for converting a wide frequency based range of sound absorption coefficient values into a single number but this is done using a curve fitting process. Although more complex to derive, Alpha w is considered to be more representative of how the human ear interprets sound. The method is fully described in EN ISO 11654 and has now become the preferred European unit for making a simple and rapid comparison of sound absorption performance.
4 - Is there a direct correlation between NRC & Alpha w?
No. Although they are both single number descriptors of sound absorption neither one can be deduced from the other as there is no direct relationship between them.
5 - For ceilings, what is 'Absorption Class' and how is it connected to Alpha w?
This is a system for the classification of Alpha w values which is defined in EN ISO 11654. Because it groups consecutive Alpha w values together into six broad-bands (each successively identified as A to E & 'Not classified'), it is not so precise and therefore provides less useful detail than when individual Alpha w values are specified or selected.
6 - For ceiling tiles, how is Sound Absorption used?
When applied to room surfaces or objects, sound absorptive materials reduce the reflection of sound that strikes them which helps to make a space seem less “echoey” or “lively” or, more technically, less reverberant. The ceiling plane is often the only one of substantial size, and relatively unobstructed, where sound absorption can be introduced. However sound absorptive materials are most effective at controlling reverberant sound when distributed between several room surfaces or objects rather than just being applied to one.
7 - How can I stop a room being "echoey" ?
Echoes are discrete sound reflections from a distant surface which, if they are of sufficient intensity and time delay, can be heard distinctly from the direct sound, ie you hear the same sound twice in quick succession. The expression “echoey” is often used to describe the sound heard in an enclosed space which is particularly reverberant or 'lively'. This is actually the wrong use of the term as, perhaps surprisingly, echoes are a rare phenomenon in most normal sized and occupied enclosed spaces. However excessive reverberation and noise can be controlled by the introduction of sound absorptive treatments, such as suspended acoustic ceilings.
8 - What is reverberation time and how do Armstrong ceilings influence it?
This is the time, in seconds, required for reflecting or reverberant sound in an enclosed space to decay to one-millionth (equivalent to a drop of 60 dB) of its original energy level after the cessation of the sound source.
It is the most common, and easily obtained, measurement or predictor of a room’s potential sound quality. The reverberation time (RT) for any enclosed space will be influenced by the room's volume and how much sound absorption (which controls the reflection of sound) is present. Increasing the volume will increase the RT while increasing the amount of sound absorption will lower the RT. Because Armstrong suspended ceilings can provide a substantial surface area and can provide more or less sound absorption depending upon the product chosen, they can significantly influence the RT of a space. However any room will have an optimum reverberation time (RT) requirement depending upon its use and size and whether the main activity is speech or music based. Providing too much sound absorption, and hence having a very low reverberation time (RT), can be just as acoustically damaging and undesirable as having insufficient sound absorption when an excessively long reverberation time (RT) will result.
9 - What is the effect of sound absorption on noise levels?
Rooms with reasonable amounts of sound absorptive finishes appear quieter and less frenetic than those with little or no sound absorptive treatment. If the amount of effective sound absorption in a room is doubled (or halved), the noise level will be reduced (or increased) by 3dB (Decibels). However, it should be considered that a change of 3dB will only just be detected by the human ear, while a difference of 5dB is necessary to be really noticeable. In addition, sound absorptive treatments that are applied to the boundary elements (walls, ceilings, floors etc.) of a room, do not have any significant effect at enhancing the element’s sound reduction properties, ie when sound transmits through it from one adjacent room to another.
10 - How can I calculate the reverberation time of space?
By using a mathematical model based upon the 'Sabine' formula which takes into account the significant surfaces of a room, their respective sound absorption coefficients and the room dimensions. The acoustic module in "Estimate", provided by Armstrong to registered users, will enable a simple indicative calculation to be made. Alternatively, your ITS group will be able to do a more detailed calculation which also considers specific user criteria.
11 - What is a sabine and why would you use it?
A sabine (also known as the equivalent absorption area) is measure of sound absorption afforded by a material which is defined as the product of its exposed surface area S (m2), multiplied by its random incident sound absorption coefficient alpha s. However the sabine is also specifically used to describe the total absorption provided by individual discrete objects, such as an acoustic canopy, cloud or baffle, where all of its surfaces may be influentially providing sound absorption and the use of as would not be sensible or realistic.
Once the total sound absorption present in a room (from both planar surfaces and objects) has been calculated, an estimate can be made of the room's probable reverberation time. The installation of clouds and canopies in a reverberant space can significantly reduce the reverberation time and contribute to the reduction in background noise
12 - Why would I use Armstrong acoustic canopies instead of a traditional wall-to-wall ceiling?
Some trends in modern building technology, such as the use of concrete thermal slabs as heat-sinks, requires that the slab be exposed to the occupied space and therefore a continuous (wall to wall) ceiling, which could interfere with the airflow pattern, is not permissible. But the downside of not having an acoustic ceiling will probably result in higher reverberation times and increased noise levels above those which would be acceptable to the users. Also in many existing spaces, even though a continuous ceiling is present and has to remain in place for various technical reasons, it may provide insufficient sound absorption than is suitable for the activities undertaken. Therefore the installation of canopies in a reverberant space, in sufficient numbers and layout to satisfy both technical and aesthetic considerations, can significantly reduce the reverberation time and contribute to the reduction in background noise and improvement of aural comfort.
13 - What is the speech frequency range and is it defined in any official standard?
The speech frequency range is generally described as being between about 500Hz and 4000Hz. However it is not defined in any known national or international standard.
14 - What is the difference between sound absorption and sound attenuation?
Sound absorption relates to the control of sound reflections within a room while sound attenuation is associated with the control of sound transmission between adjacent rooms via a continuous suspended ceiling.
15 - Will materials that provide high sound absorption also provide high sound reduction?
Probably not. Materials that provide high levels of sound absorption are generally lightweight and porous which is the direct opposite of the qualities required for sound reduction ie massive and impervious.
16 - Would repainting an Armstrong mineral fibre or Orcal metal tile change its acoustic performance?
In terms of sound absorption there may be a small loss, depending upon the tile face pattern (fissures, perforations, scrim etc), the paint type used, and the thickness of the applied coat(s). It is unlikely that the ceiling's sound reduction or attenuation performance will be adversly affected but if the spaces where the ceiling has to be repainted are acoustically critical, then laboratory testing to assess any possible differences in acoustic performance should be conducted on repainted samples.
It should be noted that the repainting of ceiling tiles could also adversly affect their other technical performance factors such as fire reaction, sag, light reflectance etc and the implication of such possible changes needs to be considered. Finally it should be appreciated that the repainting of any tiles supplied by Armstrong will invalidate any warranty that was provided when the tiles were new.
17 - What is so special about the sound reduction performance of Armstrong suspended ceilings?
Armstrong suspended ceilings are one of few building products whose sound reduction can be measured in two entirely different ways. These are sound reduction index (R or SRI = vertical or single pass) which is measured in accordance with EN ISO 140 Part 3 and Normalised Level Difference for Ceilings (Dnc = horizontal or double pass) which is measured in accordance with EN ISO 140 Part 9.
18 - Is there a direct correlation between the Rw and Dncw tested values for ceilings?
No. Although there are some empirically derived relationships between the two different values for the same product, there are no theoretically based methods for deriving one value from the other. The overall process is far too complicated and it is certainly NOT the case that Dncw = 2 x Rw!
19 - I thought decibels were for measuring noise levels so why do you use them for ceilings?
The decibel is a unit used in acoustics to describe the magnitude of sound levels. These levels can either describe how loud something is (eg 85 dB due to a passing bus), or they can describe the ability of a product or system to reduce sound, eg a 35 dB suspended ceiling will reduce a sound level of 75 dB in one room down to 40 dB in an adjacent room. The bigger the number the greater is the sound energy level or sound difference involved.
20 - Is there a difference between sound reduction and sound attenuation?
Insofar that the terms 'reduction' and 'attenuation' both mean a decrease or lessening of something, then these expressions describe the same process and are usually interchangeable. In relation to the acoustics of suspended ceilings, 'sound reduction' is generally used to describe the 'single or vertical pass' decrease (typically from a ceiling cavity to a room below) while 'sound attenuation' is reserved for the 'double or horizontal pass' lessening in transmitted sound energy where the ceiling is continuous above two adjacent rooms.
21 - If a timber (or concrete) floor construction provides a sound reduction of Rw 35 dB, and one of your ceilings gives Rw 20 dB, what overall sound reduction would you expect if the ceiling is rigidly suspended 200mm below the floor?
The Rw of the total construction is unlikely to be more than about 40 dB ie an increase of up to only 5 dB on the floor construction. This is because the two separate elements are very close to each other and are rigidly connected together and therefore the sum of their two individual reductions cannot practicably be achieved.
22 - How can I improve the acoustic insulation through the floor/ceiling from my neighbour above?
This is not a problem that can be easily or simply solved by installing a further suspended ceiling below. It all depends upon the level of disturbance that is occuring and the type and construction of the existing floor and surrounding walls. Further advice should be sought from an acoustic consultant or specialist supplier of noise insulation materials.
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