Impact Noise vs Airborne Noise: What’s the Difference?

Inside most homes, sound reaches us in ways we rarely think about.

Voices from another room, music playing upstairs, or the familiar sound of footsteps crossing the floor above all travel through a building slightly differently.

In building acoustics these sounds are usually grouped into two categories: airborne noise and impact noise.

The distinction is important because it explains why some sounds fade quickly while others seem to travel straight through floors and ceilings.

Airborne noise: sound moving through air

Airborne sound is the type of noise most people instinctively understand.

When someone speaks or music plays in a room, pressure waves move through the air until they reach a surface such as a wall, ceiling or floor.

When airborne sound meets a surface several things can happen:

• some of the sound reflects back into the room
• some is absorbed by the material
• some passes through the structure

Because airborne sound must interact with multiple layers of building material, it tends to lose energy as it travels.

This is why voices coming from another room often sound muffled rather than perfectly clear.

Impact noise: sound entering the structure directly

Impact noise behaves very differently.

Instead of travelling through the air first, impact sound begins when physical force is applied directly to the building structure.

Examples include:

• footsteps on timber floors
• objects dropped onto a surface
• chairs or furniture moving across floorboards

When a foot strikes a floorboard the energy transfers immediately into the floor structure. The boards flex slightly and pass that energy into the joists beneath them.

At that moment the sound has become structural vibration rather than airborne sound.

Why suspended timber floors transmit vibration

Suspended timber floors are particularly good at carrying impact noise.

Several structural characteristics contribute to this behaviour.

• timber floorboards flex slightly under load
• joists act like beams transmitting vibration
• the cavity beneath the floor allows energy to move within the structure

Once vibration enters the joists, it can travel surprisingly far before eventually becoming audible sound again in other parts of the building.

This is one reason footsteps from an upstairs room often sound louder than voices from the same space.

The structural pathway behind this behaviour is explained in our guide to how sound travels through a suspended timber floor.

Why frequency matters

Not all sounds behave the same way inside buildings because different sounds vibrate at different frequencies.

Lower-frequency sounds tend to travel further through building structures, while higher frequencies lose energy more quickly.

This is why deep bass sounds from music systems often carry through buildings more easily than higher-pitched sounds.

The same principle applies to footsteps. Heavy steps tend to generate lower-frequency vibration which can travel further through the floor structure.

The role of the floor cavity

The cavity beneath suspended timber floors also influences how sound behaves.

This space allows ventilation beneath the structure, but acoustically it can also allow vibration to move and reflect inside the floor assembly.

This is explored in more detail when looking at why empty floor voids make sound worse.

Managing sound inside floors

Because airborne noise and impact noise behave differently, improving acoustic comfort inside homes usually requires addressing both types of sound.

Airborne sound is often reduced by increasing the density or mass of building structures.

Impact vibration is typically managed by interrupting the pathway through which vibration travels.

This is why solutions such as acoustic underfloor insulation focus on absorbing vibration inside the floor assembly itself.

By introducing absorptive material within the cavity beneath suspended floors, less sound energy continues travelling through the building structure.

Frequently asked questions