Trip-trap, who’s that tripping over my floor?

By - , Build 76

Impact noise on floors can be a real problem for those in multi-unit dwellings. Though various proprietary products are available to help deaden sound, as always, care is needed with installation.

The level of disturbance that impact noise can cause, such as from footsteps, a scraped chair or dropping an object on the floor, is related to the character and loudness of the noise. How this sound is transferred to the occupants below will depend on the floor construction and the type of floor finish.

How sound travels

Impact noise generally moves through the floor because the impact causes the floor to vibrate and it is this vibration that transmits the sound, like a drum. Once through, the sound becomes airborne again but with less energy.

The amount of sound absorbed by the floor depends on its mass and stiffness. For dense floors, such as a concrete slab finished with a hard surface such as ceramic tiles, the impact sounds are commonly described as a clicking sound or as a sharp tap. This is because the dense floor tends to allow higher rather than lower frequencies to pass through. However, with a lightweight timber- or steel-framed floor, impact noises are more commonly described as a thump or thud, because more low-frequency sound is transmitted. For lightweight floors most of the sound reaching the living space below is low frequency and is generally below the 250 Hz frequency minimum used to determine the IIC (impact insulation class).

The minimum sound insulation requirements for inter-tenancy floors of dwelling units set by the performance requirements of the New Zealand Building Code, Clause G6, is an IIC performance rating of at least 55.

Dealing with impact noise

There are a number of factors to consider when designing a floor that requires a specific IIC rating.

Impact noise is easiest to deal with if the design of the floor and ceiling below incorporate an acoustic break or separation. To achieve this requires either a:

• ceiling constructed as a separate entity from the floor above (in some systems ‘resilient’ supports may be used to support long-span ceiling framing from the floor above)

• floating floor constructed over the structural floor.

Also important is the finish used on the floor. Generally the softer and thicker the floor covering, the better the acoustic performance. This is because the floor covering cushions and absorbs sound.

The level of acoustic performance specified in the Acceptable Solution is more difficult to achieve when solid or framed floors are constructed without a break (i.e. floor and ceiling below are constructed as one element), particularly when hard floor finishes are used. For example, finishing a concrete slab with ceramic tiles directly adhered to the concrete does not change the IIC of the floor, but attaching tiles to a particleboard or plywood substrate will make it worse (reduce the IIC).

Proprietary products

A range of add-on options are available to improve the sound transmission performance of a floor finished with a hard surface. Most are specialised proprietary products and performance is dependent on meeting the manufacturers’ specifications.

Always check the availability and performance of products before finalising the floor design. Though the level of improvement may be small, it may it may be enough to meet the IIC requirement.

Sound-deadening mortars: These are proprietary specialised products applied to the floor before tiling. It is important to check specific manufacturer’s instructions as products may not be suitable for wet areas or for use over some substrates, such as particleboard.

Specialised adhesives: These will give a small improvement in the IIC for tiles and direct-stuck timber boarding.

Resilient pads, tiles or mats of cork or rubber: These specialised products can reduce the sound transmission when laid under the floor finish. The level of performance achieved depends on the particular product and its thickness. For example, test results from a US source have shown that adding 6 mm of a proprietary cork layer over a 150 mm concrete slab can nearly double the IIC of the floor (from approximately 27 to 50). Improvements for rubber over a concrete slab are in the order of 12 IIC points for 5 mm rubber and 15 points for 10 mm.

Proprietary pliable, nylon-filament composites: These consist of a three-dimensional geomatrix with a non-woven fabric heat-bonded to the upper surface, used between the structural flooring and a tile underlay such as fibre-cement.

Other proprietary options include sandwich construction and sheet underlays.

Resilient underlays

Where a resilient underlay is used it is important that:

• it is designed for use with the proposed surface finish, e.g. tiles

• the underlay is fully adhered to the flooring material

• the tile adhesive is compatible with the underlay to ensure tiles are fully adhered

• waterproofing of wet areas is maintained

• no mechanical fixings are used (they will adversely affect the sound control performance).

Cork is frequently used as an underlay under ceramic tiles but it needs special care if it is to work well. There are specific requirements for installing cork:

• allow the cork to condition to the moisture levels within the space (those that will occur when the building is occupied)

• ensure the surface the cork is being applied to is clean and dry

• unroll cork sheet with the curled side down

• adhere the cork with the manufacturer’s recommended adhesive

• tightly butt joints

• ensure there are no air bubbles under the cork and 100% adhesive contact between cork and substrate

• roll the cork with a 50 kg roller

• allow the cork adhesive to fully cure before installing tiles

• adhere ceramic tiles with an adhesive specifically designed for use with cork.

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Articles are correct at the time of publication but may have since become outdated.

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