In practice, the following question often arises: if a soundproof door has been installed and the door frame acts as an acoustical bridge, is it necessary to provide evidence of impact sound insulation performance? This article explains what DIN 4109-1 has to say on the matter and what planners, building owners and tradespeople need to know.
What is impact sound – and what does it have to do with doors?
Impact sound is caused by direct mechanical action on a building component – typically by footsteps on a floor, which transmits its vibrations as sound into adjacent rooms. It is classified as structure-borne sound and differs fundamentally from airborne sound, which soundproof doors are primarily designed to reduce.
With doors, the issue of impact sound arises particularly when a timber door frame extends across an acoustically engineered joint in the floor or wall. This ‘bridging’ can theoretically allow sound transmission – and in practice occasionally leads to discussions between clients, designers and contractors.
What does DIN 4109-1 say about impact sound insulation for doors?
A client requested proof of impact sound insulation for fitted soundproof doors. Due to the structural layout, the timber surround frame bridged a joint designed for sound insulation, meaning that sound transmission could be demonstrated. Even if sound is indeed transmitted via this bridging, this does not constitute a defect.
In DIN 4109-1, Table 1, Row 4, there is a ‘–’ (dash) in the ‘Impact sound insulation’ column for doors. This means that no impact sound insulation values need to be specified or demonstrated for doors. In this respect, only the standard sound insulation rating for the door element applies – that is, airborne sound insulation.
Why isn’t there an impact sound rating for doors?
The reason lies in physics: doors are movable components that are not connected to the floor over their entire surface. Sound insulation against structure-borne noise, which is transmitted via fixed connections, cannot be meaningfully measured or standardised in the design of a door unit. The sound insulation standard therefore focuses exclusively on the airborne sound insulation value Rw for doors – that is, protection against noise that propagates through the air.
A wooden surround frame that bridges an expansion joint does, admittedly, create an acoustic connection. However, as the standard does not require an impact sound rating for doors, this situation does not constitute a defect in the door unit. Responsibility for connections free of sound bridges lies with the building design and structural work – not with the door manufacturer.
What does this mean for planning?
Although doors are not required to meet any specific impact sound insulation standards, there are practical considerations that planners should consider when planning acoustical details.
The door frame should not act as an acoustical bridge between acoustically separated structural elements. If the floor structure provides for impact sound decoupling – for example, through a floating screed – it must be ensured that the door frame does not exhibit rigidity and bridge this joint. The solution lies not in the door-set itself, but in the detail drawing of the frame installation: elastic back fill, clean separation joints and the use of suitable types of steel frame can prevent acoustical bridges.
Equally important is distinguishing between airborne noise and impact noise when dealing with complaints. If a resident perceives noise through a soundproof door, it must first be clarified whether it is airborne noise or impact noise. Impact noise, which is transmitted via walls or floors, is not an indication of a defect in the door-set – but rather of flanking transmission within the building.
Airborne sound, structure-borne sound, flanking transmission – an overview of the key terms
Airborne sound refers to sound that travels through the air – voices, music, road noise. Soundproof doors are designed to block precisely this type of sound transmission. The key performance indicator is the rated sound insulation index Rw, measured in decibels.
Do you have questions about the sound insulation of your doors, or would you like to determine the appropriate sound insulation class for a project? Our soundproof doors are tested as complete units in accordance with DIN 4109 – from the door leaf to the frame.