Acoustic isolation

Acoustic isolation is the first problem to solve in recording studio design, before any decision about interior treatment. Without a minimum level of isolation, external noise will contaminate recordings and the studio's sound will disturb its surroundings. The most careful acoustic treatment cannot compensate for poor isolation: they are two disciplines that operate at different levels and cannot substitute for each other.

What is acoustic isolation?

Acoustic isolation controls the transmission of sound between spaces: it prevents external noise from entering the recording room and prevents the sound produced inside from escaping to the exterior or to other parts of the building. Its goal is not to control how the inside of the room sounds — that is acoustic treatment — but to determine how much sound passes from one space to another.

The level of isolation is expressed in decibels (dB) and represents the difference in sound level between the source space and the receiving space. An isolation of 50 dB means that the sound leaving the room arrives in the adjacent space at 50 dB lower level. The higher that value, the better the isolation.

Achieving good isolation is, in construction terms, far more demanding than acoustic treatment. Absorptive panels can be installed and removed with relative ease; isolation involves permanent construction decisions that must be made before any building work begins.

The two sound transmission paths

Sound travels between spaces via two fundamentally different paths, and isolation must interrupt both simultaneously:

Airborne transmission

Sound travels as a pressure wave through the air and strikes building elements (walls, ceilings, floors). That pressure sets the element vibrating, causing it to act as a membrane and radiate new sound waves on the other side. The more rigid and massive the element, the less it vibrates under a given pressure and the less sound it transmits.

The mass law states that doubling the mass per unit area of a building element increases its airborne sound isolation by approximately 6 dB. Hence the importance of material thickness and density: a 20 cm concrete wall isolates far more than drywall of the same thickness.

Structure-borne transmission (impact noise)

Sound also travels directly through the solid material of the building without needing air as a medium. When a drum kit strikes the floor, the vibrations propagate through the concrete slab and reappear as sound in adjacent rooms, even if there is no air path between them. This type of transmission is particularly significant for bass and sub-bass frequencies and is far harder to control than airborne transmission.

The solution for structure-borne transmission is not mass but decoupling: introducing elastic elements (rubber, springs, viscoelastic materials) that interrupt the rigid vibration path.

The three principles of isolation

Every professional isolation system is based on the coordinated application of three principles:

• Mass: Heavy, dense building elements that vibrate little under sound pressure. Concrete, solid brick, multiple layers of acoustic plasterboard and heavy acoustic membranes are the primary materials. Mass is effective against airborne transmission, particularly at mid and high frequencies.
• Decoupling: Elastic separation between the studio's floating structure and the building structure. This is achieved using anti-vibration mounts, high-density rockwool under floating floors, elastic hangers for floating ceilings and wall studs with elastomeric pads. Decoupling is the only effective solution for structure-borne transmission.
• Sealing: Isolation is only as effective as its weakest point. Any crack, gap or unsealed penetration — however small — provides an airborne transmission path that can invalidate the entire system. All joints between elements must be sealed with acoustic mastic, service penetrations must be filled and the perimeter edges of floating walls and ceilings must be sealed with elastic materials.

The acoustic bridge: the enemy of isolation

An acoustic bridge is any rigid contact between the floating interior structure and the building structure. Through that contact point, vibrations travel directly via structure-borne transmission, short-circuiting the entire decoupling system. A single acoustic bridge can drastically reduce the performance of an isolation system that has cost thousands of euros.

The most common acoustic bridges are:

• A screw or nail passing through both layers of a double wall.
• An inner wall making contact with the original ceiling of the building.
• A pipe or duct penetrating the floating floor without isolation.
• Levelling mortar filling the gap beneath the floating floor without leaving an elastic separation.
• An electrical socket recessed in the inner wall that touches the outer wall.

Preventing acoustic bridges is the most critical task during studio construction. It requires continuous supervision and extreme attention to detail, because once the work is finished they are invisible and practically impossible to correct without demolition.

The room-within-a-room (box-in-a-box) technique

The highest-performing isolation solution for professional studios is the room-within-a-room (box-in-a-box) technique: building a second completely independent structure inside the existing space, with no rigid contact with the surrounding building.

Its three elements are:

• Floating floor: The first element to be built and the one that supports everything else. It consists of a slab or platform resting on elastic elements (high-density rockwool, anti-vibration mounts or Sylomer pads) that decouple it from the building's floor slab. The floating floor must be built with sufficient mass and rigidity so that the entire interior structure can rest on it without causing elastic collapse.
• Decoupled walls: Interior walls that rest on the floating floor and have no rigid contact with the building walls or the ceiling. They are typically built with metal studs, rockwool between layers and multiple sheets of acoustic plasterboard. The air cavity between the inner and outer wall, filled with absorptive material, adds both mass and decoupling simultaneously.
• Floating ceiling: Suspended from the floor slab above using elastic hangers or supports, or resting on the decoupled walls. It must have no rigid contact with the building ceiling. Its construction combines mass (multiple layers of plasterboard) and the decoupling provided by the elastic hangers.

The main drawback of this technique is the reduction in usable space: the double walls, raised floor and lowered ceiling consume between 20 and 40 cm in each room dimension. In small spaces, this can be decisive for the project's viability.

Weak points in isolation

The overall isolation level of a room is determined by its weakest element. Even if the walls have excellent isolation, a poor door or window will limit the entire system to that element's performance. The points requiring specific attention are:

Acoustic doors

A conventional door provides barely 20–25 dB of isolation. Studios requiring 50 dB or more of isolation need specifically constructed acoustic doors: heavier leaves (40–80 kg), perimeter seals with compressible gaskets that activate on closing, and an automatic drop seal that descends to seal the floor gap when the door closes.

Studios with high isolation requirements often install acoustic airlocks: two doors in series with a small air chamber between them, arranged so that both are never open simultaneously. The air chamber between the two doors acts as an additional sound trap.

Acoustic windows

Windows are the second most common weak point. The professional solution is asymmetric double glazing: two panes of different thickness (for example, 6 mm and 10 mm) separated by an air cavity of at least 10–15 cm, installed at slightly different angles to avoid resonance between the two parallel surfaces. Panes of different thickness have different coincidence frequencies, preventing both from falling at the same frequency and creating a dip in the isolation curve.

Service penetrations

All cable, pipe and duct penetrations through walls or the floating floor are potential acoustic bridges or airborne transmission paths. They must be resolved with:

• Flexible ducts: Instead of rigid penetrations, ventilation and HVAC ducts are connected using flexible sections that absorb vibrations before they can be transmitted to the structure.
• Duct silencers: Attenuate sound travelling through the interior of a ventilation duct from one room to another.
• Filled cable runs: Any chase cut into an isolation wall for cabling must be filled with acoustic mastic before finishing.
• Non-opposing electrical boxes: Recessed electrical fittings in isolation walls must not be placed back-to-back (one on each face of the wall), as this creates a direct transmission path. They must be offset both horizontally and vertically.

Bass isolation: the greatest challenge

Low frequencies are the greatest challenge in acoustic isolation for two fundamental reasons: they have very long wavelengths (a 50 Hz frequency has a wavelength of nearly 7 metres) and they transfer their energy very efficiently to building elements. Isolating low frequencies requires very high mass and very effective decoupling, which means far heavier and more substantial structures.

This is why isolating a drum studio is incomparably more demanding — and more costly — than isolating a voice-over studio. It is also why in home studios with limited budgets it is preferable to accept the isolation limitations in the bass range and adapt the type of recording to the achievable isolation level, rather than investing in an incomplete construction solution.