by Perit Joe Formosa and Perit David Bonello
Acoustic comfort is essential for the quality of life of residents in domestic buildings. Noise nuisance may cause numerous health problems such as sleep disorders, stress or loss of concentration.
This article attempts to raise awareness of the importance of sound insulation in building design.
Physiologically, noise is generally an unpleasant or uncomfortable auditory sensation. Acoustical comfort is characterized by the absence of unwanted sound and opportunities for acoustic activities that may annoy people.
It is important to observe that acoustic comfort for a person is related to the person both as a receiver of sound but also as a source of sound.
It can be annoying to be exposed to noise coming from a neighbour, but it can be equally annoying to know that one’s own activities may be heard by other people and may cause annoyance to them. Poor sound insulation between dwellings may cause restraints on home activities and may cause social conflicts.
An ever-increasing population density reduces distance between people. This makes noise suppression yet more important.
Families with small children may have serious problems if neighbours react against noise coming from the children’s natural activities. On the other hand, children may be restricted to perform activities to avoid causing frustration and conflicts between neighbours.
Over the last few decades, housing trends in Malta have changed drastically with detached and terraced houses converted into apartment blocks thereby increasing population density. More residents are living surrounded by neighbours on all sides of their dwelling unit. Apart from this, the building fabric has changed from robust stone masonry walls to lighter building materials with inferior acoustic properties such as hollow concrete blocks.
the building fabric has changed from robust stone masonry walls to lighter building materials with inferior acoustic properties
In the building design process, acoustics are not given the same attention as structural strength, fire safety, thermal insulation, energy consumption, day light and visual aesthetics. Since sound insulation requires space to accommodate it in as well as additional costs, clients and particularly developers are reluctant to lose a few square metres of internal floor area and internal height. This also makes it harder to justify an increase in building costs yet properties with inadequate sound insulation are being sold as ‘luxury residential units.’
Legislation could play a key role in providing acoustic comfort by introducing building regulations that cover this important aspect of building design as is the case in most European countries. The following two figures show the minimum acoustic requirements for airborne and impact sound insulation for various European countries as at 2017. Malta alongside Cyprus, Luxembourg and Macedonia have no minimum standards as shown in Figure 1.
Noise Sources
Sound transmission is a complex issue. At the outset, a distinction needs to be made between noise nuisance which is the result of anti-social behaviour such as a noisy neighbour, or neighbour noise associated with normal day to day living such as talking, watching television, walking (footfall noise) or carrying out normal household activities. Where it is established that the noise complaint has been generated by normal neighbour noise, the likelihood is that the separating party wall or floor has poor sound insulation properties.
Estimated Equivalent DnT (Airborne) and LnT (Impact) sound in dB (Decibels)
There are four sources of noise in the building acoustics domain: airborne noise from external sources such as voices or traffic in the street; airborne noise from internal sources such as conversations, television, etc.; impact noise such as falling objects, footfall noise, etc. ; and equipment noise such as noise from elevators, pipe work etc. Impact noise is the result of sound travelling through a solid structure as a mechanical vibration which travels appreciable distances causing surfaces along the way to vibrate. Vibrating surfaces are nothing but inefficient loudspeakers.
Sound can be heard by direct emission through air gaps or it can be transmitted directly or indirectly (flanking) through the building fabric. It is therefore vital that party walls and party floors separating dwellings units and common areas are airtight and therefore masonry joints should be properly sealed with mortar and plastering. Routing of pipework and deep recessed installations should be planned away from party walls, and electrical sockets positioned back to back avoided.
The best recommendations to control sound is to handle noise at its source. Soft materials such as curtains and sofas absorbing sound before reaching the building fabric. A resilient flooring such as carpet absorb airborne sound but is also effective to reduce impact noise such as footsteps.
the heavier and thicker the material, the better the sound insulation
Sound Insulation
In the case of solid walls and floors such as concrete floors and stone masonry walls, the acoustic performance depends on density: the heavier and thicker the material, the better the sound insulation.
Impact sound insulation is the most important acoustic property to be considered for designing floors that serve as party structures
In the absence of appropriate local building regulations for sound control, reference is made to the UK building regulations Document E: Resistance to the passage of sound (2010) which sets minimum requirements for sound insulation. According to the code, the resistance to airborne sound between walls separating purposely built dwelling houses and flats shall be a minimum of 45dB (43dB if the separating rooms have different functions).
The minimum required mass per unit area to achieve these results is 415kg/m2. This could be achieved with 190mm solid concrete with a density of 2200kg/cm or 230mm soft stone with a density of 1810kg/cm, both applied with a 13mm lightweight plaster (10kg/ m2) on both faces.
Nowadays, it is very common to build separating party walls in 230mm thick hollow concrete blockwork (HCB) with unfilled voids. The blocks are lighter and easier to handle on site, easier to plaster and reduce the total mass on foundations in comparison to solid walls. The main purpose of the holes is to increase the thermal insulation at the cost of reducing density and consequently significantly detrimental to the acoustic properties. A 230mm HCB wall weighing 304kg/m2 will not achieve the required density and it may require two leaves of 150mm HCB to achieve a mass of 458kg/ m2 .
Acoustic performance can be optimised by providing two solid layers separated by a cavity which may be filled with acoustic insulation such as mineral wool
In many occasions, it may not be practical or cost effective to increase mass to deliver the required sound insulation. Acoustic performance can be optimised by providing two solid layers separated by a cavity which may be filled with acoustic insulation such as mineral wool.
This kind of build-up respects the so-called “mass-spring-mass” principle: the first solid serves as a mass reflecting part of the noise and allows the rest to pass; the remaining noise is transmitted into the acoustic insulation which absorbs sound; the second solid reflects part of the noise back into the insulation (which absorbs more noise) and finally transmits the attenuated noise into the adjoining room.
The sound properties of a plastered 230mm HCB can therefore be enhanced by fitting an independent lightweight panel composed of two layers of gypsum finished plasterboard with a cellular insulation core. An air gap of 35mm should be left between the composite panel and the core wall.
Impact sound insulation is the most important acoustic property to be considered for designing floors that serve as party structures
The plasterboard should have taped and sealed staggered joints and should be fixed to the wall with small resilient metal ties or by an independent floor to ceiling supporting frame leaving an additional 10mm gap with the masonry core.
“Acoustic performance can be optimised by providing two solid layers separated by a cavity which may be filled with acoustic insulation such as mineral wool”
Impact sound insulation is the most important acoustic property to be considered for designing floors that serve as party structures. Airborne sound insulation will normally be enough if the requirements for impact sound insulation are met.
According to Document E, the maximum impact sound that can pass through a floor separating dwellings is 62dB. The resistance to airborne sound and impact sound in floors also depends mainly on the mass per unit area of the floor. Additional resistance to impact sound can be achieved by inserting a resilient layer between the finished floor and the concrete base or by an independent suspended ceiling with absorbent material.
The suspended ceiling may consist of 2 layers of plasterboard with staggered joints (20kg/sm) sealed properly around the perimeter and an absorbent layer of mineral wool (minimum density 10kg/cm) laid in the cavity formed above the ceiling. The ceiling should either be supported by resilient hangers attached directly to the floor or by independent joists fixed to the surrounding walls. In both cases a clearance of at least 100mm should be left between the plasterboard and the underside of the base floor.
Conclusion
The above party wall/slab detail plan and cross section captures the above specified wall and floor construction suggestions. As this detail is an adaptation of the UK Building Regulations detail to suit Maltese building standards, sound testing is required to establish the actual sound reduction values.
In order to establish a standard for both airborne and impact sound insulation requirements, testing of party wall and party slabs with different configurations of sound insulation is required. A balance between adequate sound insulation, simplicity of detailing and cost needs to be found in order to make the proposition of sound insulation appealing to clients and developers.
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