Sound perception is a very personal thing and clients will have widely different ideas on appropriate sound levels. Some people tend to play the music they like at levels which other people find painful and annoying.

Loud, high frequency nose is considered to be more harmful to hearing while low frequency noises such as a bass guitar or traffic noise is more difficult to reduce the impact of.

Sources of noise

The most common sources of noise in a house are:

• noise generated outside the site such as traffic, trains, neighbours, aeroplanes, and schools
• external noise from within the site such as natural events like wind around the building or rain on the roof and also for items such as heat pumps and  water pumps
• internally generated noise such as loud conversations, washing machines, dishwashers, stereos and TVs, and air conditioners
• impact noise through the structure such as footsteps (particularly on stairs) and boisterous children’s games
• noise from services such as the toilet or water drainage or from vibrations caused by clothes dryers.

Sound paths

Sound:

• can follow a direct path between the source and the receiver
• can be deflected to the receiver by bouncing off one or more surfaces
• reflects more off hard surfaces than soft ones
• can pass over or around obstructions or through small gaps
• can travel through structural elements by causing the material to vibrate.

The weakest link

Sound can pass through very small holes, gaps and cracks. This means that tiny weak points in expensive sound-stopping can destroy its effectiveness. Such breaches can include: lightweight doors; open ceiling spaces above walls; gaps around doors; keyholes; gaps or cracks around partitions; loose-fitting opening windows; power, computer and phone outlets; light switches; and plumbing and heating pipes. When sound barriers are used, all of the ways in which sound can get through must be considered.

For external walls where windows provide passive ventilation, the need for ventilation must be balanced with the need to reduce noise – open windows do not reduce noise from outside.

  

Reverberation

When sound continues to reflect off surfaces after it has stopped at its source, this is described as reverberation. The ‘reverberation time’ is the period during which the reflections continue to occur. A long reverberation time (3 seconds or more) can mask the original sound, creating background noise and potentially making conversation difficult.

Sound reverberates more readily from hard surfaces such as painted walls and vinyl floors. It does not reverberate from soft surfaces such as carpets, curtains and soft furnishings. Rooms with lots of sound-absorbent material seem quieter than rooms with hard surfaces.

Controlling noise

Noise control needs to take account of noise from outside the home, noise from inside the home including impact noise, and rates of reverberation.

When planning a building, the effect of noise can be reduced by:

• increasing the distance between the noise and the location where it will be heard e.g. locating the building as far as possible from a noisy street frontage
• providing a buffer space or spaces between quiet spaces within the home for example a wardrobe space between bedrooms and between noisy and quiet spaces
• grouping noisy or quiet activities together
• locating quiet spaces furthers from external noise sources such as roads. Providing a buffer space can also assist
• locating doors and windows to avoid direct and flanking sound paths for example off-setting the door location down a corridor
• for noise from outside incorporating mass into external walls or by incorporating heavyweight fencing or earth mounding into the design
• minimising the size of or avoiding windows that face the noise source.

Building sound control into the structure of a home can be expensive, so the best approach where possible is to control sound at source.

Measuring sound stopping

The amount of noise that a wall, floor, door or sheet of glass is capable of stopping is expressed as a sound transmission class rating (STC). The higher the STC number, the greater the expected noise reduction. For example, walls with a rating of:

• 30 dB have little sound-control effectiveness
• 35 dB allows normal conversation to be heard
• 40 dB allows loud conversation to be heard
• 50 dB will stop the sound of a loud conversation
• 55 dB is the inter-tenancy wall requirement under NZBC G6.

Other sound ratings include:

• Noise Reduction Coefficient (NRC) – this rates the sound absorption of a building element
• Impact Insulation Class – this rates the degree to which impact sound is lessened.

Where specific sound control performance is required such as an inter-tenancy wall a specific acoustic wall and floor construction system must be used – refer to specific manufacturers literature for the construction and materials specification details. Current requirement under NZBC Clause G6 Airborne and impact sound for inter-tenancy separations are:

• for wall and STC of not less than 55
• for floors an IIC of not less than 55