At a glance
- Medium-density houses present a range of design challenges – including acoustic performance.
- Many living in medium-density buildings raise noise-related issues as having a significant impact on wellbeing and quality of life.
- Acoustic performance needs to be considered early in the design process.
- Identifying sources of undesirable noise is critical.
- Involving acoustic design experts in the process will ensure effective solutions.
Increased pressure on Aotearoa New Zealand’s existing residential building stock, particularly the need for intensification in major urban areas, together with the challenge to improve housing affordability have seen significant growth in the construction of medium-density housing (MDH).
To create common terminology – for the industry and the public – BRANZ has defined MDH as multi-unit dwellings up to six storeys.
Noise-related design challenges
The growth of medium-density housing presents designers with new challenges, requiring thoughtful solutions for how people live in closer, denser residential environments.
Noise-related issues – not just between dwellings, but also external noise – have been rated by many living in MDH as the biggest negative associated with this form of living and as having a significant impact on their wellbeing and quality of life.
Considering acoustics
Acoustic performance is often not given enough consideration early in the design stage nor appropriately budgeted for.
The design and construction of an MDH building significantly affect its acoustic performance. Acoustic considerations should be prioritised early in the design process, alongside structure, aesthetics, internal environment and fire protection.
As many design practices don’t have acoustic design expertise, it is a good idea to use an acoustic consultant. These experts have a greater understanding of the many proprietary systems available for good acoustic performance and know how these can be effectively integrated into the design.
Identifying noise sources
While there can be several sources of undesirable noise, there are generally three main sources that can generate unacceptable noise and cause issues within MDH environments:
- External noise – noise created by occupants and heard through open windows or from balconies as well as noise from the surrounding environment. This often has a greater impact on dwellings at higher levels.
- Occupant-generated noise – talking, television and music all at various levels of intensity and footfall noise from tenancies directly above and banging doors.
- Noise from building services – air conditioning, lifts and plumbing infrastructure.
Defining sound transmission
Eliminating all sources of undesirable noise is not possible – the key is to understand how the sound is transmitted and what can be done with the building design and construction to minimise sound transmission and the impact of undesirable noise.
There are two main types of sound transmission:
- Impact sound – noise that is transmitted through the building structure from blows or vibrations. This covers such things as footsteps, doors closing/slamming, items being dropped on the floor, moving furniture and plumbing noise.
- Airborne sound – noise such as music, environmental sound (traffic, car horns and sirens) and voices from both within and outside the building.
Sound can move through the air, through building parts and even around barriers. To control noise, it’s important to figure out how sound travels and use building design and materials to block it. Not all building materials reduce noise equally – some are better at stopping certain types of sound than others.
Noise-reduction measures
There are two noise-reduction measures used in Aotearoa New Zealand – sound transmission class (STC) and impact insulation class (IIC). STC relates to the transmission of airborne noise and IIC to the transmission of impact noise (generally through a floor).
STC is basically the reduction of airborne noise in decibels (dB) provided by a structural element in the building such as a wall or floor. If a wall between units has an STC rating of 55, a noise of 75 dB on one side will be heard as only 20 dB on the other side.
This is a general guide to the acoustic performance of different STC ratings for building components:
- STC ≤30 – poor sound control (minimal privacy).
- STC 30–40 – you can hear people talking in adjacent spaces.
- STC 40–50 – you can hear raised voices in adjacent spaces.
- STC ≥50 – reasonable acoustic privacy.
- STC 60+ – quite good acoustic privacy.
Different building components manage sound transmission differently depending on sound frequency.
For example, a concrete wall and a light timber-framed, plasterboard-lined wall may have the same STC rating, but the concrete wall may not block high-frequency sound as effectively as the timber-framed wall and the timber wall may not block low-frequency sound as effectively as the concrete wall.
Defining the frequency of sound to be managed is therefore a consideration when choosing construction materials.
Building Code requirements
Clause G6 Airborne and impact sound calls for minimum noise insulation for inter-tenancy floors and walls of connected dwellings. These are:
- STC ≥55 for walls and floors
- IIC ≥55 for floors.
Many local authorities also have sound transmission requirements over and above Code requirements.
Post-construction, MDH buildings must also achieve a minimum level of acoustic performance through field testing. A 5-point leeway is allowed with field testing so the building must meet a field sound transmission class (FSTC) of ≥50 for walls and floors.
However, problems with unacceptable sound transmission can still exist with Code-compliant construction and buildings that have passed the FSTC test. It is advisable that designing to a higher STC level of STC 60 or more will likely provide good acoustic privacy in most real-world situations.
Acoustic design guidelines
While the type of MDH building will have specific requirements to manage sound transmission, there are some basic guidelines:
- Consider Code-plus construction to ensure acceptable acoustic performance is achieved.
- Don’t locate plumbing fixtures on inter-tenancy walls or run plumbing services through ceilings of a tenancy below.
- Locate relatively noisy areas such as kitchens and living rooms away from bedrooms of adjoining tenancies.
- Use hallways and cupboards as a buffer.
- Specify perimeter seals on doors and windows.
Material selection
Sound transmission reduction is more effective with building elements that have greater mass or with greater separation between elements. Basic guidelines include:
- use thicker or denser materials where possible
- use materials with higher mass such as concrete or masonry, which have better sound transmission reduction
- research to find the best timber flooring for multi-level buildings – there are some BRANZ-appraised systems being widely and successfully used in this situation
- use solid-core doors or proprietary acoustic doors
- incorporate carpet floor coverings where possible
- use soft-close mechanisms in kitchen and bathroom cabinetry.
Construction considerations
Following the consented documentation is fundamental to acceptable acoustic performance – variations or substitution can compromise acoustic performance.
On-site construction supervision is also fundamental to acoustic performance. Other trades must be well managed to ensure acoustic assemblies are not compromised.