To box or not to box? Low carbon acoustic solutions for performance spaces

When a development contains one or more spaces which are highly noise-sensitive, and/or generate very high noise levels, a commonly-adopted solution is to construct “box within box” designs.

Shown below, schematically, the principle of box-in-box construction is to introduce an air gap around the “internal box” (containing the performance space) which serves to significantly reduce the transmission of noise and vibration both in and out of the space.
Of course the “base” of the box cannot float – so it is typically supported by an array of spring isolators, constructed from either steel springs or rubber. When these isolators receive incoming vibration, they deflect, and much of the incident energy is transformed from kinetic into heat.

Traditional box-in-box structural isolation

While an established and effective method, the environmental consequences of providing “doubled” structure should be obvious. A box-in-box design requires twice the quantity of structure as a “traditional” build-up, requiring internal box “sides”, “bases” and “lids” in addition to their external counterparts.

This is exacerbated by the fact that many performance spaces like concert halls and theatres are large-volume spaces requiring large structural elements (columns, beams, trusses)to provide clear sight lines across long spans. It is environmentally unfortunate that the steel and concrete typically required in this double-structure are materials high in embodied carbon.

An alternative approach (where the site footprint permits) is to essentially construct each performance space as a separate building. By creating a structural isolation break around the perimeter of the entire primary structure, we can create a line across which vibration (and hence structure-borne sound) cannot pass.

Alternative primary structural expansion joint option

Often, these lines are simply a deliberate extrapolation of the expansion joints required in the structure regardless of performance aspirations. The extrapolation is the requirement for the joint to continue through ALL elements of connected structure. Where there is a continuous material connection, there is a bridge for vibration transmission. So, the expansion joints must continue not just through the primary slab, but through the structural walls and soffit.

Dubai Concert Hall (concept at competition stage)

While this can create some challenges around detailing, and may be problematic in conditions where the water table is high, the payback in terms of reducing embodied carbon can be enormous.

On a recent opera house scheme with two large auditoria, we saved almost 2 million tons of CO2e that would otherwise have been sunk into the additional “internal boxes” around the performance spaces by adopting this expansion joint design approach instead.