Sound Absorption and Sound Transmission — What’s the Difference

In solving acoustical noise challenges, we are often asked, ‘What’s the difference between sound absorption and sound transmission?’
To understand how sound absorption and sound transmission differ, let’s look the definition and examples of each term.
Sound absorption refers to the elimination of reverberation within a room. This can be achieved by using panels to soak up the sound waves before they can reflect off the hard surface behind them and bounce around the space. This will lower the level of background noise in a given space, but generally will do nothing to stop the sound leaking into different rooms in the building.
Sound transmission refers to sound which penetrates the dividing walls or floors between rooms. This is separate from reverberation as an issue.
Two different products and approaches are required for the two different issues.
Sound absorption products are installed on hard surfaces such as walls and ceilings. The fact that they are installed on the exposed areas of existing surfaces makes them easier to install without requiring renovation.
Products that reduce sound transmission generally need to be installed behind the walls, floors or ceiling, and are not visible within a room. The general idea is to create a ‘room within a room’. This could include creating a substantial cavity (such as 15-20cm) between the two adjoining spaces and fill it with heavy insulation to prevent the sound penetrating. This is difficult and expensive to retrofit to an existing structure and requires significant renovation.
To put the difference into perspective, you can drop the noise level in a busy restaurant by 5db to 10db with a very good sound absorption treatment. This is a significant and noticeable reduction to those in the room (the decibel scale is logarithmic, so every 10db is twice as much sound). But when people talk about ‘soundproofing’— like you might see in a recording booth or home theatre — they are generally looking for a reduction of about 40db between rooms. You cannot achieve this kind of result with sound absorption alone — only take the edge off.