Performance Door Design - The Basics of Sound Reduction - Lorient Polyproducts Ltd

The CPD seminar covers:

• The nature of sound - examining both airborne and structural transmission of sound.
• Regulatory requirements and British Standards that relate to acoustic performance.
• Test procedures and interpretation of test reports.
• Effective design of door assemblies for acoustic performance, including door construction and the influence of sealing systems.
• Design conflicts between acoustic performance, durability and ease of operation of the door.
• Independent accreditation.

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Introduction

The demands of the various approved documents of the building regulations are placing an increased burden on the door assembly, which has been fire and smoke rated for many years but now many have to be acoustically rated too. As well as having to comply with document M, for ease and access to the building, which is having to make us work with lower operating forces from the door closer and so friction from the sealing system has to be avoided at all costs.

What is Sound?

Sound is a sensation perceived by the human ear and caused by the vibration of air particles giving rise to rapid fluctuations in air pressure, and we can instantly recognise this by looking at what speaker cones do in a sound system. In simple terms the speaker cones vibrate the surrounding air particles, causing pressure fluctuations which are instantly detected and interpreted by the ear. Pressure fluctuations tend to be in the form of longitudinal waves as opposed to transverse waves. The transmission of sound is as a result of its movement through a medium and the medium can be anything other than a vacuum. But for our purposes within buildings, it is either airborne or structure born.

Structure-borne Sound

All of these noises are transferred through the vibration of solid items such as walls, floors, service pipes, etcetera, but this subject is vast and outside today's presentation. We will be confining this presentation to airborne sound, as this affects the door assembly directly.

Airborne Sound

Door leaves respond to airborne sound and as the air molecules vibrate and oscillate, they caused the door leaf to vibrate in sympathy. Airborne sound is transferred by the oscillation of air molecules and is always in the form of a longitudinal wave. A repeated sequence of compressions and rarefactions in the air sets up a wave, a sound wave, which is directly related to the sound source. It is possible to represent the exact characteristics of sound by means of a wave diagram. In this diagram the positive peaks represent compressions and the negative troughs represent rarefactions in our longitudinal wave.

Frequency is very important and is always measured as the number of isolations or repetitive cycles per second, which is represented by the number of peaks per second and so we have frequency as the number of cycles per second and the unit of frequency is known as the Hertz. 1HZ equals 1 cycle per second, this is a very low frequency, because in terms of everyday acoustics we usually work in the range of 50 to 5,000HZ.

Sound Energy

Sound is a form of energy and is described as having a power which is measured in Watts but for acoustic purposes we don't measure sound in Watts because the human ear has characteristics that are sensitive to both frequency and sound power. For example a sound that is clearly audible at a frequency of 1000HZ, might be inaudible at 100HZ. So instead we work with sound pressure, which takes this into account. The unit of sound pressure is officially the Bell but in practice it is more convenient to use the term decibel which is 110th of a bell and is denoted as dB.

dB is the unit used in the comparison of two pressure levels relating to sound intensities, for example when we talk in terms of dB reduction from one side of the acoustic door, to the other. Sometimes however it makes more sense to talk about how the human ear responds to the loudness of the sound it hears. To do this we use a measure called a weighted decibels commonly abbreviated to dB(A).

Here are some examples.

Human beings are highly adapted to the physical phenomena of light, heat and sound. Although our sensitivity varies widely, the human ear can detect levels as low as 20 dB(A), the rustle of leaves and tolerate intense noises for short periods without any ear protection, such as a jet engine at around 120 dB(A). But it should be noted that the decibel scale is not a straight line arithmetic relationship, it's log arhythmic and as such the change from 20 dB(A) to 120 dB(A) is not 10 times louder than 20 dB(A) the jet engine is 10,000 million times more intense than the rustle of leaves. But whilst these figures are interesting, we should not always be concerned about keeping out loud noises, we should continue to be aware of the importance of quiet noise, and our duty of care where privacy is concerned, such as in the doctor's surgery.

Door Related Considerations

Door leaves respond to airborne sound, and not structure borne sound. And we expect them to be effective sound barriers when a door leaf vibrates in sympathy with the sound wave, sound pressure fluctuations are transferred from one side of the door leaf to the other. The door core plays a vital part in achieving the right level of acoustic performance, and they are rather like fire and smoke seals, in that they are very door dependant. A very dense or moderately flexible door leaf is harder to set into vibration, therefore, is a good acoustic barrier. A less dense or excessively stiff door leaf is a poor acoustic barrier. Around the perimeter of the door leaf there will be gaps which act as straight through pathways for sound waves offering practically no resistance at all, even with quite deep door stops or rebated edges.

For any given door assembly and acoustic sealing system is absolutely necessary in order to optimize the doors performance. A sealing system may control other unwanted at the same time such as drafts, dust and fire and smoke. Fire and smoke protection is particularly important bearing in mind that the positioning of most acoustic doors in a building will be in rooms possibly leading onto an escape corridor. A single professional sealing system can be relied upon for all of these tasks. 

Test Proceedures, Performance and Classifications

We will now look at test procedures to see how the acoustic performance of a door is measured. Like fire tests and smoke tests, acoustic tests are carried out with a full sized door and frame assembly attached to a chamber and exposed to an environment, i.e. sound waves at different frequencies.

Acoustic Test Chamber

A test chamber consists of two rooms, a source room with a signal generator effectively allowed speaker. And the receiving room fitted with highly sensitive microphones, which act as the sound wave signal collectors. The specimen door, test door, is built into a standard dividing wall between the rooms. This equipment enables us to measure and analyse the effectiveness of the barrier, in reducing the transfer of incident sound from one side of the door assembly to the other.

A full size operational door including all of the appropriate hardware and perimeter sealing system is used. Typically, a single door assembly will take up to 30% of the wall construction, more for a pair of doors. Typically, the sound source is varied over a range of frequencies, this is because the different sound conditions will generate different responses from the acoustic door test specimen.

At the designated frequencies emitted from the source a difference in sound pressure reduction levels will be experienced in the receiving room. From the measurements then taken the sound reduction index (R) is derived and this is defined by the number of dB by which sound energy randomly incident on the acoustic door test specimen is reduced in passing through it. A diagram can then be built to show the effectiveness of the test specimen over the chosen frequency range.

Under the standard the frequencies range from 100 to 3150 Hertz, and the graph shows the variations in R, which is the dB reduction achieved by the door assembly, as the frequency is changed. Each door assembly will be characterised by its own graph like this one, but each will be inherently different.

In order to make easier comparisons between one door assembly and another it has been found useful to have a single figure rating, rather than a myriad of different graphs or performance figures at different frequencies. So, to do this we have a rating standard as detailed in BS EN ISO 717 -1 the standard contains a set of mathematical rules that weights the sound barrier performance of an assembly, according to the more commonly encountered frequencies. It uses the results of the BS EN ISO140-3 test to determine a single figure performance guide. This is known as the weighted average sound reduction index and is always denoted in terms of a number of dB, followed by the suffix RW. The RW figure doesn't tell us everything we need to know, but it does enable a quick rule of thumb comparison between various acoustic door performances.

Building Regulations - Approved Document E, England and Wales

The main reference point is approved document E, which came into effect in July 2003 and applies to England and Wales. There are separate building regulations for Scotland and Northern Ireland. For the first time the acoustic performance of door assemblies were detailed for a number of situations. The acoustic performance contained with approved document E is clearly stated as minimum requirements. Although they are widely interpreted as being absolute requirements. We have to remember that acoustic doors are not only required to deal with loud noises but also with soft noises.

Regulation E1 is titled "Protection against sound from other parts of the building or adjoining buildings" and clearly identifies the main requirements as “dwelling-houses, flats and rooms for residential purposes, shall be designed and constructed in such a way that they provide reasonable resistance to sound from other parts of the same building and from adjoining buildings”. This statement doesn't really tell us how to achieve reasonable resistance, so we need to look at other sections of approved document E, to find out how we can comply with the regulation. Most of the content of document E is devoted to wall, floor and ceiling construction, also known as robust details and the methods of achieving required acoustic performance.

Most of the content of document E is devoted to the wall, floor and ceiling construction also known as robust detail and the methods of achieving the acoustic performance requirement. But useful and relevant guidance is given to door assemblies in several places, under the heading corridors, walls and doors, with three main clauses to consider repeated in three sections of document E:

• New build or public and commercial buildings,
• dwelling houses and flats with a material change of use, such as a single house being converted into apartments,
• rooms used for residential purposes, this particular section is extremely wide ranging and includes care homes, student accommodation, and hotels or any other buildings where sleeping people lie, however hospitals are exempt.

Approved Document E - Clauses 2.25 / 4.19 / 6.5

On pages 18, 54 and 63 of the approved document we have identical guidance for separating walls. The separating walls described in this section should be used between corridors and rooms, in flats in order to control flanking transmission and to provide the required sound insulation. However, it is likely that the sound insulation will be reduced by the presence of a door. This statement sets the scene and draws attention to the important influence of the door assembly.

Approved Document E - Clauses 2.26 / 4.20 / 6.6

Crucial to acoustic door assemblies is the following extract from the approved document E.

"Ensure that any door has good perimeter sealing including the threshold where practical and a minimum mass per unit area of 25 kilograms per square meter or a minimum sound reduction index of 29dB RW."

However, the statement does not tell us:

• what makes a good sealing system,
• that it is impossible to achieve a credible acoustic performance without sealing the threshold,
• the use of a door leaf with a minimum 25 kilograms per metre is no guarantee of an acceptable acoustic performance, particularly if the threshold isn't sealed.

While this information is well intentioned it falls short of being sufficiently specific for performance related considerations. What we require is a benchmark, happily this is provided in the latest statement 29dB RW, the above 2 values of 25 kilograms per metre 29 dB RW are not equivalents as implied. In practice it is not either or. Finally, the acoustic door assembly may also have to comply with approved document B for fire safety. However, there is a potential conflict between approved Docs B and E regarding the sealing of the threshold with the decision being clearly determined by approved document E.

Approved Document E - Clauses 2.27 / 4.21 / 6.7

Finally, the following is also stated, “noisy parts of the building should preferably have a lobby, double door or high performance doorset to contain the noise. Where this is not possible nearby flats should have similar protection”. By double doors they mean 2 doors in a series to reduce the sound transmission and not 2 doors side by side. This is quite common practice in Scotland but not so widespread in England and Wales. Approved document M details the recommended opening and closing forces very clearly, which makes the selection of the door closer and its adjustment absolutely critical. Therefore, the careful selection of the perimeter seal is vital in order to reduce the potential friction, that the door closer encounters in the final moments of latching the door shut, as friction must be illuminated or at least minimised at all costs. Specifiers should not just rely on a verified acoustic door assembly, with unverified seals because, average smoke seals will fall short of the required 29dB RW. A professional sealing system could be expected to provide full acoustic smoke and fire performance, which requires us to look at three individual test reports.

Approved Document E - Acoustic Design of Schools

We must not overlook the special provisions which have been made for the design of our future schools. Approved document E also covers acoustic conditions in schools. Section 8 of approved document E recognises building bulletin 93, the acoustic design of schools as the normal way of meeting requirement E4.

Building Bulletin 93 (BB93)

Building bulletin 93 states, performance standards for airborne sound insulation between circulation spaces and other spaces used by students, minimum sound reduction index RW little spaces except music rooms 30dB RW music rooms 35dB RW. This means that a classroom which leads onto an escape corridor will now have to be an FD30S with a 30dB RW. Whilst any music rooms may be an FD30S with a 35dB RW.

Interpretation of Test Reports

When we look at an acoustic report or a brochure from a door manufacturer, we need to be aware of certain things. Many acoustic tests are carried out on fully caulked doors where the door is rendered completely inoperable, as the air gaps around the perimeter are filled with a dense lead field putty. Alternatively, the tests may be carried out on assemblies which are tightly wedged shut, using sealed systems which are over compressed in order to maximise the results, but again the door is inoperable. These practices are justified by claiming to see what the doors construction was capable of achieving but the report should always indicate how the perimeter was sealed and particularly the threshold. Similarly seal manufacturers may resort to practices which artificially enhanced the performance of their products, such as undersized or over compressed and otherwise inoperable seals. On occasions it has been known that a 1 metre square acoustic panel has been used in the test, with three sides caulked into the wall and the 4th fitted with the seals exploiting the test parameters to the limit and achieving highly impressive but deceptive results.

Influence of The Door Leaf

So, what goes into the makeup of a good acoustic door assembly. Firstly, let's consider the influence of the door leaf. For a general purpose door i.e. the 29 dB RW that we're aiming for, the doors will be mainly solid core minimum 44mm thick. By solid core we mean laminated timber, extruded chipboard, chipboard or mineral composite, paper honeycomb doors are not able to give us the required acoustic performance.

For typical FD30 fire doors the density and stiffness will not vary enough to make a significant difference in performance. A good sealing system should achieve performance in the region of 31 to 35 dB RW, higher performance needs a specialist door leaf.

Further information can be obtained when the door is tested in its fully caulked condition showing the theoretical maximum that the door leaf is capable of achieving. The door is rendered completely inoperable as the gaps around the door’s perimeter, are sealed with a dense lead based putty. A good sealing system should come as close as possible to its theoretical maximum performance, without impeding its operation.

This graph shows the significant effect of the sealing system in relation to an unsealed door and a fully caulked door when tested across the frequency range, and then the measured improvement when using a selection of perimeter seals.

The Hinge

Hinges act like air gaps allowing sound waves to pass through them, even quite small interruptions to the sealing system at the ironmongery positions, will lead to substantial transmission of sound waves, especially at medium to high frequencies. In order to optimize the acoustic performance, the perimeter seals must be continuous and uninterrupted at the ironmongery positions, i.e. The hinges on one side of the door leaf and the lock latch on the opposite side. Taking this precaution will simultaneously ensure the smoke barriers integrity.

The Perimeter Seals

This table clearly illustrates the unsuitability of the popular brush type of smoke seal as an acoustic seal. They are not only difficult to use around the ironmongery positions and having a brush pile, they are relatively porous and therefore unable to give the required level of acoustic performance. The DS is fitted into the frame, allowing one thin to be interrupted at the iron positions, whilst the other remains intact and continuous around the door perimeter ensuring, no loss of acoustic performance. The IS 1212 fits into the perimeter of the door stop, optimising smoke and acoustic performance integrity simultaneously, by ensuring complete continuity at the hinge and lock positions. Both options offer a hygienist wipe clean surface, offer almost no friction and are very durable.

What Makes a Good Acoustic Door Assembly

As we know in order to achieve a good dB rating and continuous seal is needed around the perimeter of the door. This particular seal is a combined acoustic fire smoke seal. It is fitted into either the door frame or door leaf along the two long edges and head of the door. The fins are elasto merrick and offer some of the lowest resistance for the door closes very strict opening and closing forces, under approved document M.

A design feature is that one fin remains intact and continuous whilst the other can be interrupted at the hinge and locking points on the door frame. Low friction and wipe clean, making it ideal for the healthcare sector, offers 35dB RW with IS 8010 or IS 8005.

The illustrated finesse seal is similar to the previous seal but has a unique benefit of clear elastomeric fins, that have been designed to be less obstructive in the door frame. It shares the same benefits of being a low friction and white clean seal. These types of seals are more tolerant to minor door warpage in everyday service, but they can be susceptible to fin damage, if the leaf to the frame gaps around the door are too tight.

Generally, it is advisable to have the threshold seal on the same plane as the perimeter seal, to avoid any stray sound leakage at the corners and at the door base. Even the smallest air gaps can result in a loss of 1 to 2 dB in overall performance.

It is also possible to have a combined compression and wiping seal, such as the one illustrated. The acoustic and smoke performance integrity has been simultaneously optimised because the seal is continuous at the hinge and locking positions and is fitted into the perimeter edge of the door stop. One of the main benefits of this seal is that it is friction free, the door only comes into contact with it when it is well past the lock strike plate, making it very durable and white clean and perfect for premises with very old or young generations. This particular seal has no intumescent protection at all and so if used on a fire door, it must have a separate intumescent fire seal.

The Threshold

Threshold seals are less well understood and are absolutely essential for effective acoustic performance, without one the required 29dB RW is not achievable. Here is an example requiring no electrical connections or raised threshold plate. Automatic drop seals allow easy access for wheel traffic and are completely friction free.

Design Conflicts

We need to be aware that the results obtained in the clinical environment of the laboratory, may well be higher than those achieved when the acoustic doorset is fitted on site, and therefore we should be aiming for an acoustic rating of 1 to 2 dB higher than required. There is always going to be a conflict between acoustic performance and the closing forces required to shut the door, and so it is crucial that the doors perimeter sealing system as the minimal effect.

Interference is unavoidable but must be at the lowest possible levels, but seals which do not provide some sort of interference will be poor acoustic or smoke barriers, but will yield a door that is easy to operate. The one off exposure to the EN test tells us nothing about the service ability or the long term performance of the seal. We need some sort of assurance because whilst the door leaf is inherently stable the sealing system is subject to immense wear and tear from continual use. Unless the sealing system is well designed and manufactured, the acoustic performance of the seal will deteriorate rapidly and will require continual replacement.

Independent Quality and Performace Accreditation

One independent third party accredited scheme is sponsored by the British Woodworkers Federation certifier, requiring as a minimum 100,000 open shut cycles on a full sized door assembly without any loss of performance. The seal should not add significantly to the opening and closing forces, within very strictly specified limits. Subject to a random factory audit production seals remain consistent with originally tested seals. This scheme follows similar lines to that of certifier but places much more emphasis on the products fitness for purpose as a building product, as well as looking at an isolated performance in the laboratory conditions of a test environment.

In the context of acoustic seals, the main difference would lie in the insistence of the BBAs monitoring of the service ability of the seals in an actual building. The BBA also require that products are periodically and randomly submitted for a complete reassessment.