Impact sound

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Sound is transmitted in buildings by both air-borne sound and structure-borne sound.

Impact sound (or impact noise) is a form of structure-borne sound that occurs when an object impacts on another, resulting in the generation and transmission of sound. The structural vibration caused by the impact results in sound being radiated from an adjacent vibrating surface. NB structure-borne sound may also be generated by vibrating sources rather than impact sources.

A typical example of an impact sound is footsteps on a floor resulting in sound being transmitted through the floor construction and heard in the space below. Impact sound can travel through solid structures and through cavities.

Impact sound can be a form of noise nuisance, particularly in dwellings, and can contribute to health problems such as:

• Tension or anxiety.
• Decreased performance.
• Reduced productivity.
• Eardrum damage or hearing difficulties.
• Increased blood pressure or stress levels.
• Psychological damage.

The occurrence of impact sound is dependent on a range of factors, including:

• The force of impact.
• The characteristics of the surface that is impacted upon.
• The characteristics of the structure through which the vibration transmits.
• The characteristics of the radiating surface.

Impact sound can be prevented or reduced by:

• Carpets and pads – soft carpets and thick pad cushions can be particularly effective at reducing middle to high level frequencies, although they are less effective at isolating low frequency sound. It is important to consider whether carpets or pads might be removed in the future.
• Resilient underlay – can have a similar effect to carpets and pads. Generally, they are made from recycled rubber, rigid fibreglass, foam or other such materials. They absorb, or isolate impact energy and can be effective in minimising impact sound at middle to high level frequencies. If the underlay has significant mass, it may also provide sound insulation from air-borne sound.
• Resilient mounts – hanging resilient mounts, sound clips or spring ceiling hangers can reduce the transmission of impact sound.
• Soundproofing compounds – applying sound proofing layers can be effective in reducing impact sound. Typically the compound is applied between two rigid materials, such as subflooring. The compound dissipates the vibrations caused by sound waves as they move through the structure, and can reduce sound at all frequencies.
• A suspended ceiling system or raised floors can be used to improve impact sound insulation.

Building Regulations Approved Document E - 'Resistance to the passage of sound' sets minimum standards for impact sound insulation.

Impact sound transmission is typically measured in-situ with a tapping machine which uses steel-faced hammers to strike a test surface and generate sound in an adjacent space which can be recorded or monitored. This is useful only in giving an indication of the likely level of impact sound as it does not accurately represent the variety of impacts that might be experienced in practice.

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[edit] Related articles on Designing Buildings Wiki

• Airborne sound.
• Ash deafening.
• Audio frequency.
• British standards.
• Building acoustics.
• Building Bulletin 93: acoustic design of schools.
• Building regulations.
• Flanking sound.
• Noise nuisance.
• Reverberation time.
• Sound absorption.
• Sound frequency.
• Sound insulation.
• Sound v noise.
• Structure-borne sound.