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When considering the interior layout, designers should pay special attention to acoustic privacy.

There are several common sources of nuisance noise in medium-density housing environments, particularly apartment buildings and multi-unit dwellings. These include:

  • activities of other residents, such as loud conversations
  • televisions and loud music (particularly bass)
  • plumbing systems
  • heating, air-conditioning and ventilation (HVAC) systems
  • building services, such as lifts and vertical drainage pipes
  • exterior pedestrian and vehicle traffic
  • foot noise from the floor above, adjacent walkways or stairways
  • doors banging
  • appliance noise, such as washing machines.

The two types of sound transmission to consider in MDH buildings are:

  • airborne sound – noise originating in air, such as voices, music and vehicle traffic
  • impact sound – noise originating directly on the structure by blows or vibration, such as footsteps, moving furniture or knocking plumbing.

All building elements reduce noise to some degree, but some are much more effective than others. Some are also more effective at reducing airborne noise than impact noise and vice versa.

The degree to which a building element reduces sounds as it passes through is its sound insulation characteristic.

In order to better compare building products and materials, sound insulation is generally described using a single number. There are two complementary systems in common use in New Zealand – sound transmission class (STC) and impact insulation class (IIC).

STC ratings relate to the transmission of airborne noise, and IIC ratings relate to the transmission of impact noise.

These figures represent a summary of the element’s acoustic performance over a range of frequencies. While the figures are very useful, it’s important to remember that they do not provide a complete description of the element’s performance at all frequencies.

As a general guide, the level of acoustic privacy expected by an STC rating is:

  • STC <30 – poor sound control with little privacy
  • STC 30–40 – allows normal conversations to be heard in adjacent spaces
  • STC 40–50 – allows raised voices to be heard in adjacent spaces
  • STC >50 – provides a reasonable acoustic privacy.

The desire for amenity in MDH developments – the occupants’ ability to use and relax in their home without the influence of nuisance noise – means MDH needs good sound insulation.

The performance requirements of New Zealand Building Code clause G6 Airborne and impact sound sets minimum sound insulation requirements for dwelling units of:

  • STC ≥55 for inter-tenancy walls and floors
  • IIC ≥55 for inter-tenancy floors.

In addition to these design ratings, an MDH building must also achieve at least a minimum level of acoustic performance in field testing.

During field testing, the Building Code allows a 5-point leeway to take into account on-site issues such as flanking and build quality, so MDH buildings must meet a field sound transmission class (FSTC) of ≥50.

Acoustic design guidelines

When arranging internal spaces, simple design concepts can eliminate many noise issues.

Consider the use of each space, and prioritise the level of acoustic performance of each space within the dwelling. For example, position noisy living areas (living rooms, kitchens) away from noise-sensitive spaces (sleeping or study areas). Try to position low-amenity spaces (hallways and storage spaces) as a buffer between noisy and noise-sensitive areas.

There are several other simple design concepts to further limit noise including:

  • separating ground-level dwelling units by using garages as a buffer
  • not installing services (power outlets, plumbing fittings) on party or inter-tenancy walls
  • ensuring openings (doors and windows) have perimeter seals to minimise sound leakage
  • allowing adequate wall and floor thickness to accommodate acoustically designed partitions.

Other basic rules to mitigate noise from external sources include:

  • siting the building as far as possible from noise sources
  • using the building layout to position noise-sensitive spaces (bedrooms) away from noise sources
  • decoupling or separating each side of the inter-tenancy construction from each other – for example, using a double-stud system (where two frames are constructed separately with a gap) or a resilient mount system
  • providing quality perimeter seals on windows and external doors
  • using landscape features, ancillary buildings or acoustic walls to break line-of-sight sound paths from the source to receiver.
Med dens draft 19 v2

Increase acoustic privacy by breaking line-of-sight sound paths between source and receiver.

Limiting noise from services

Building services are some of the most common sources of nuisance noise in MDH buildings. Keep the following in mind to reduce noise from services:

  • Choose appliances and equipment for their quietness (supported by independent testing).
  • Mount equipment on resilient or isolation mounts.
  • Use flexible connectors at the junctions between fixed equipment and pipes and ducts.
  • Design and construct ductwork using simple layouts, silencers, smooth joints and transitions, long radius turns and calming chambers.
  • Use fans and impeller designs that are quiet and operate at low speed.

The following also apply to plumbing services:

  • Isolate plumbing fittings and pipes from the structure and fix pipes with resilient pipe clamps to reduce structure-borne noise.
  • Use copper and cast iron pipes for waste and flexible plastic pipes for water supplies.
  • Minimise the number of elbows, take-off points and wingbacks in the system.
  • Use a single long drop for vertical stacks or discharge pipes.

Materials selection

There are a range of factors to consider when selecting, installing or detailing noise control systems in wall, floor and ceiling assemblies.

As a general rule, reducing sound transmission requires greater mass or larger separation of elements. When the separation is fixed, increase mass by using a thicker version of the same material or specify a denser material, which will have a greater mass for the same volume.

When selecting materials to reduce sound transmission, consider the following:

  • Use thicker, heavier or higher-density building materials, where practicable.
  • Reduce stiffness by using flexible components such as resilient rubber mounts. Avoid stiff, lightweight materials.
  • Use solid-core doors with perimeter seals or proprietary acoustic doors.
  • Install the thickest underlay and carpet to floor surfaces (this typically improves IIC by ≥10).
  • Specify soft-close cupboards and drawers in kitchens to reduce impact transfer through walls.
  • Add cushioning or damping to floor systems to improve impact insulation.

Above all, always keep to the design specification for building elements that contribute to sound insulation, particularly for inter-tenancy walls. Variation or substitution of components from a proprietary system is not permitted, as an unauthorised change can compromise acoustic performance.

The tables in this BRANZ fact sheet indicate the typical acoustic performance of wall and floor elements used in MDH buildings. Always refer to the manufacturer’s guidelines and independent test results for actual acoustic performance figures.