Acoustic insulation: construction solutions and verification

DB-HR requirements are expressed with standardised indices worth knowing how to read. For airborne noise between rooms, the index is the weighted standardised level difference, known by its symbol DnT,A and expressed in A-weighted decibels: the higher, the better the insulation. The document's central reference, the one governing collective housing, is the requirement between different occupancy units: as a general value, the protection between one dwelling and another, or between a dwelling and common or activity areas, must reach at least 50 A-weighted decibels in the cited index, with reinforced levels against plant and activity rooms.

For impact noise, the index changes logic: what is measured is the noise level arriving in the protected room, the standardised impact sound pressure level, symbol L'nT,w, and the lower, the better. The document's general reference places the admissible maximum at 65 decibels in protected rooms against adjoining rooms of other units or zones.

Against exterior noise, the façade requirement depends on the site's day noise index, the area's acoustic map, with increasing insulation values in noisier environments; and the document completes the picture with the noise and vibration limitations of the building services and the reverberation time in defined rooms. The data comes from the municipal acoustic map before the façade is chosen.

The physics explaining acoustic failures fits in one idea: sound does not go through the partition, it goes around the building. Between two adjacent dwellings, energy travels through the separating element and also through every element flanking it, floor slabs, façade, abutting partitions, and through the parasitic paths, chases, boxes, ducts. That is why the required index is measured between rooms and not on the element: the performance belongs to the complete system with its junctions.

This physics has two design consequences. The first is that solutions are conceived as assemblies, separating element plus flanking conditions, and that is how the document's simplified option catalogues them. The second is that decoupling, interrupting the rigid bridges along which vibration travels, is as important as mass, and its humble materialisations, the perimeter resilient strips, are critical items and not accessories.

Airborne noise between units. Two families dominate the separation between dwellings. Masonry solutions with linings, a heavy leaf providing mass plus self-supporting linings with mineral wool providing the spring, and double-frame lightweight solutions, two independent plasterboard leaves with absorbent in the cavity. Both reach the requirements comfortably when executed in full; the choice between them is a trade-off of thickness, weight, schedule and trade more than of performance. In cost terms, the acoustic differential concentrates in the linings, the strips and the absorbent, with the lightweight double frame as the reference for the best performance-to-thickness ratio.

Impact noise. The floating floor is the canonical solution: a resilient layer, the impact-damping membrane, under the screed, decoupled from the slab and the perimeter walls. Its cost per square metre is contained and its effectiveness depends entirely on the continuity of the decoupling: the rigid skirting or the service penetration bridging the membrane locally cancels the performance.

Façade. The façade's acoustic performance is decided by its openings: the glass, with asymmetric and laminated compositions as the characteristic upgrade, the joinery and its airtightness, and the roller shutter boxes, the classic weak point. The blind wall is rarely the problem; the acoustic specification of the joinery, expressed with its own index, is the item to watch in noisy locations.

Building services. Resilient plinths and supports, flexible connections, silencers in ducts and the position of machine rooms complete the repertoire, with the design rule already stated in the general article: noisy rooms are placed away from protected ones from preliminary design, because a position error is not corrected with material.

The simplified option applies the document's tabulated solutions: combinations of elements and flanking conditions whose complete adoption demonstrates compliance without calculation. It is the route of ordinary housing, fast and safe if respected in full. The general option calculates the expected insulation between rooms from the elements and their junctions, with the document's official tool as the reference instrument; it is the route of singular projects and optimisations, and it demands real acoustic expertise.

In both cases, the document's justification sheets record the result in the report, and the circle closes where it must: on site. Acoustic execution control, strips in place, chases sealed, membranes continuous, and in-situ measurement at completion, a growing practice already required by some administrations and developers, are the only guarantee that the index on paper is the index of the building.

As an indicative order of magnitude, the specifically acoustic items sit between 1 and 3 percent of the PEM (presupuesto de ejecución material, the direct construction cost) in ordinary multi-family housing, the range already given in the general DB-HR article. The cost structure has a particularity that budget analysis must respect: the bulk consists of differentials and humble items, strips, membranes, absorbents, seals, precisely the candidates for silent trimming on site. The rule of the method in the article on the cost of CTE compliance applies here with special force: every solution justified in the report must have its measured item, because the saving of one resilient strip is paid for in a neighbour dispute.

The first is compensating flanks with mass: fattening the partition while chases, back-to-back embedded boxes and rigid skirtings bridge everything gained. Flanking transmission is resolved in the detail rather than through thickness.

The second is adopting the tabulated solution by halves, the separating element without its flanking conditions, losing the simplified option's coverage. The simplified option only covers the complete solution it describes.

The third is specifying façade joinery without an acoustic index in noisy locations, entrusting the performance to the luck of supply. The index is stated in the item like any other performance.

The fourth is measuring the package by materials and not by systems, with quantities forgetting strips, seals and trims, the real cost of the performance. The item describes the complete system, including its accessories.

The fifth is giving up the final measurement in demanding projects: it is the only test answering the question the user will ask every night. The test is programmed before handover rather than after it.