Airborne and impact noise

What is sound?

Sound arises from vibrations of materials. These vibrations generate sound waves that travel through a medium and are eventually perceived by the human ear. The speed at which a material vibrates determines the pitch of the sound: faster vibrations produce higher-pitched sounds, while slower vibrations result in lower-pitched sounds. The number of vibrations per second is measured in hertz (Hz). A low frequency (e.g. 100 Hz) sounds low, while a high frequency (e.g. 1000 Hz) sounds high. For example, a typical male voice ranges between 110 and 440 Hz, while a female voice ranges between 200 and 900 Hz.

In addition to frequency, the amplitude—the extent of the vibration—is also important. The greater the amplitude, the louder the sound. Sound level is expressed in decibels (dB). A commonly used variant is A-weighted decibels (dB(A)), which adjusts the measurement to reflect the sensitivity of human hearing.

These two factors—frequency and sound level—together determine how we perceive sound.

What is airborne noise?

Airborne noise occurs when a sound source sets air particles into motion, creating vibrations that travel through the air as sound waves. Examples include voices, music, or traffic.

What is impact noise?

Impact noise arises when a sound source makes direct contact with a surface or structure, generating vibrations within the material. These vibrations then travel through floors, walls, or ceilings. Common examples include footsteps on a wooden floor or a washing machine vibrating against the floor.

What is the difference between airborne and impact noise?

The main difference lies in how the sound travels: airborne noise moves through the air, while impact noise travels through solid structures. In general, impact noise is more intense than airborne noise. To illustrate: you can't knock over a glass with your voice, but if the glass is placed on a vibrating washing machine, it might topple over.

How do you address airborne and impact noise?

Airborne noise can become echoey in spaces with hard materials, such as concrete, glass or stone. This is because the sound reflects off these surfaces. You can reduce the echo with sound-absorbing materials such as melamine foam or bubbled foam. The foam absorbs the sound waves, preventing them from reflecting back. As a result, the echo is reduced and the space sounds calmer.

If the airborne noise comes from outside or another room, then soundproofing is the solution. In this case, sound absorption does not work: the sound needs to be prevented from passing through, while sound-absorbing materials allow sound to pass but prevent it from reflecting back. Therefore, for soundproofing, you should use solid materials with mass, such as drywall or other dense, heavy materials. Start by sealing gaps and then add mass to the existing structure. The most effective method is to build an additional, decoupled wall or floor, such as a partition wall or floating floor. This is known as a mass-spring-mass system and provides a high level of sound insulation.

Impact noise requires a different approach. You want to address contact noise as close to the source as possible, before the vibrations spread through the structure. For this, it is necessary to apply vibration isolating materials. These absorb the vibrations, preventing them from spreading further through the building.