Acoustics

According to New Zealand Building Code Clause G6 (Airborne and impact sound), floors above habitable spaces in multi-unit residential buildings must be designed to reduce noise going to the space below. This includes two types of sound: airborne and impact. While compliance is assessed based on the entire floor assembly, the choice of flooring and underlay plays a significant role in reducing noise, particularly impact sound.

Always check project-specific requirements against the latest Building Code provisions and relevant Acceptable Solutions and Verification Methods.

Airborne Sound Insulation

Airborne sound insulation refers to a floor’s ability to block sound transmitted through the air, such as voices or music. This is measured using the Sound Transmission Class (STC) rating, which shows the reduction in sound level (in decibels, dB) after it passes through the floor.

The higher the STC value, the better the floor blocks airborne sound.

Floors separating habitable spaces from other units in multi-unit residential buildings must achieve a minimum STC 55 according to the Building Code.

It is important to note that this rating is largely determined by the substrate and any ceiling cavity below, while the floor covering itself typically has minimal impact on airborne sound insulation. Acceptable Solution G6/AS1 considers a bare 150mm concrete slab sufficient to meet the STC 55 requirement.

Impact Sound Insulation

Impact sound insulation is a floor system’s ability to absorb sound caused by impacts, such as footsteps, dropped objects, or moving furniture. This is measured by Impact Insulation Class (IIC), which indicates how much impact noise is reduced as it passes through the floor.

The higher the IIC, the better the floor is at blocking impact noises.

The Building Code requires floors above habitable spaces in multi-unit residential buildings to achieve at least IIC 55. For reference, a bare 150mm concrete slab typically has an IIC of 25, which is well below the requirement. Additional measures, such as adding a ceiling cavity and a suitable floor covering, are needed to improve impact sound insulation performance. According to G6/AS1, installing carpet and carpet underlay on a 150mm concrete slab is enough to meet the requirement.

While the Building Code sets a minimum IIC of 55 for floors, this is only a baseline. In practice, higher IIC ratings are often specified for premium developments or noise-sensitive environments.

For instance, high-end apartments may target IIC 60-70 for better comfort and fewer complaints about footfall or furniture movement. Similarly, aged care facilities often require higher ratings to support patient well-being and privacy.

Impact Sound Insulation Requirements in Schools

Acoustic requirements in schools are not covered under the Building Code but are instead governed by the Ministry of Education’s Designing Quality Learning Spaces (DQLS) guidelines.

Under these guidelines, suspended floors in high noise spaces such as music rooms, or in spaces with hard floor coverings like technology rooms and laboratories, must achieve a minimum Impact Insulation Class (IIC) rating of 55 dB.

For suspended floors in all other areas, the minimum IIC requirement is 50 dB.

How to Determine Impact Sound Insulation

Impact sound performance of flooring is typically tested to ISO 10140-3. A tapping machine generates impact noise on the flooring in a source room, and microphones in a receiving room below measure sound transmission. Results are analysed using ISO 717-2, producing:

• Ln,w: Weighted normalised impact sound pressure level (lower = better)
• ΔLw: Improvement in impact sound insulation compared to a bare slab (higher = better)

IIC and Ln,w are both measures of impact sound insulation, but they are derived from different standards and should not be used interchangeably. While IIC measures how much impact noise is reduced as it passes through the floor, Ln,w shows how much impact noise is transmitted. Their scales work in opposite directions: a higher IIC means better impact sound insulation performance, while a higher Ln,w indicates poorer performance.

IIC can be estimated from Ln,w using the formula: IIC = 110 – Ln,w.

ΔLw, also referred to as impact sound reduction, represents the flooring’s contribution to impact sound insulation. It is used by acoustic engineers to predict the IIC rating of the full floor build-up.

Ceiling cavity design is equally important in improving the impact sound insulation of the floor assembly. For instance, carpet tiles with EcoBac backing glued directly to a concrete slab will typically achieve an IIC rating of around 63. This rating increases to around 70 when a 100mm suspended ceiling system without insulation is installed beneath the slab.

Impact Sound Insulation Performance of Belgotex Flooring

The table below shows the typical impact sound reduction (ΔLw) values for various Belgotex flooring categories. Refer to the product’s technical specifications for the exact values.

Enhancing Impact Sound Insulation of Timber Flooring

Engineered and hardened timber flooring on their own provide minimal impact sound reduction because of their hard and rigid structure.

To enhance acoustic performance, timber flooring can be installed double bonded on rubber underlay or floating on foam underlay such as Aqua Elite. Testing shows that floating installation on Aqua Elite delivers slightly better impact sound reduction compared to double bonding over a 5 mm rubber underlay.

Direct-stick installation of engineered or hardened timber flooring using a suitable 3-in-1 timber flooring adhesive can improve acoustic performance; however, it may not achieve the same level of impact sound reduction as installations using rubber or foam underlay. Refer to the adhesive supplier for details on the acoustic performance of their 3-in-1 timber flooring adhesives.