Acoustic insulation

Difference between sound absorption and sound insulation

The internal attenuation of sound is done by sound absorption. This is to improve the sound quality in a room. Here, you can think of acoustic panels and absorption plates, for example.

Sound insulation is used to prevent ambient noise from outside. This includes casings, retaining walls, floating floors, cavity filling or ground plates.

Acoustic insulation further explained

Acoustics is the science concerned with sound. Insulation refers to counteracting or separating. Think of a wall that has to separate cold air from outside and warm air from inside in winter.

Acoustic insulation is therefore specifically aimed at counteracting sound. Which acoustic insulation should you choose? And what is the difference between sound insulation and sound absorption? To answer the questions, we will first briefly discuss the types of sound.

Acoustic insulation

Sound is complex and has different forms in different positions. In most cases, counteracting noise is done to prevent or remove noise nuisance. How you block sound (better) depends on the type of sound. As well as the distance sound travels and the obstacles it encounters on its way to the receiver.

In order to arrive at the right solution, we would like to explain a few concepts.

We distinguish two types of noise: structure-borne noise and airborne noise.

Structure-borne noise

If a sound source causes a surface or material to vibrate by contact, this is referred to as structure-borne noise. A good example is a mezzanine floor that is set in motion by someone walking across it. This can cause such a vibration that it can be heard under the floor. Another example is the wheels of a car, which transmit vibrations to the bodywork through contact with the road.

You can acoustically insulate structure-borne noise by using materials that absorb the energy from the vibrations. We call these materials vibration insulation or anti-drumming. This effectively decouples the source to avoid resonances. For example, by installing vibration insulation under a wooden floor, the floor no longer comes into contact with the subfloor and this prevents resonances. By processing anti-drumming in the body (for example in car doors), the energy of the vibration that has already occurred is absorbed. This prevents the surface from vibrating again.

Airborne noise

Airborne noise is the sound or vibration of a source that can be directly perceived as sound. Think of voice or radio noise, often not necessarily annoying. In an annoying form, you can think of the sound of a pump or the sound of moving cars.
The acoustic insulation of airborne noise is done through a combination of mass and absorption. Mass is especially important. Think of a (decoupled) double plaster wall (mass) and a cavity filling (absorption) to keep out the TV noise from the neighbours. We also call this a mass-spring construction.

Another good example is acoustic sandwich panels to build enclosures around machines. These panels are both insulating (blocks sound) and absorbent (dampens sound).

Acoustic insulation: the difference between acoustics and insulation?

It should be evident that acoustic insulation can be done in different ways. In other words, the way in which you do this depends mainly on the type of sound and the situation. There is no point in putting vibration insulation on your partition wall if you want to block out the sound of your neighbour’s TV. For this, you need mass and absorption. Often in the form of a retaining wall.

Sound insulation materials are in essence meant to keep sound in its place. They ensure that sound is blocked. This way, the sound stays inside or outside a room. A heavy casing around an engine, pump or generator, for example, must ensure this. The weight of a casing plays a major role in blocking out noise. That is why we recommend reinforcing casings with ground plates.

How does sound absorption play a role in acoustic insulation? With a heavy casing around a machine, we keep the airborne noise in place. After all, it can not escape; we keep it (mostly) inside the casing.

However, the noise level within the casing remains very high because it cannot escape and reflects against the casing. To reduce this noise level, we have sound-absorbing materials. They lower the noise level inside a room.

This prevents reverberation or sound box formation internally. As a result, this is also favourable for the insulation value of the casing. Sound can therefore penetrate the casing even less easily.

Acoustic insulation is thus the counteracting of sound. One way is by absorbing resonances from contact sounds through vibration insulation. The other is by blocking airborne noise through a solid construction in combination with absorption.

Grilles, silencers and ventilation

Weak spots play a major role in blocking airborne noise. No matter how heavy a casing may be, a small hole in the construction is an enormous sound leak.

Especially if a casing or enclosure around a machine needs adequate ventilation. For example, a grille or ventilation can form a sound leak. This is why it is important to use acoustic grilles and silencers when constructing sound barriers and casings, for example.

Ruben de Visser

Acoustics specialist

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