In multi-storey buildings where mechanical plant is located adjacent to, above, or below occupied spaces, structure-borne noise frequently becomes the dominant acoustic design challenge. Airborne noise from plant rooms can be addressed by the same acoustic treatments applied in industrial settings: barriers, enclosures, and attenuators on inlet and discharge paths. Structure-borne noise is more difficult to address because it arrives at the receiver through the building structure itself, bypassing most conventional airborne control measures entirely.

The physics are straightforward: vibrating machinery transfers energy into the structure through its mounting points, and that energy propagates as structure-borne waves through slabs, walls, and columns until it is re-radiated as airborne noise within occupied spaces. Concrete and steel are highly efficient transmission paths for low-frequency vibration energy, and the distances over which structure-borne noise can travel before decaying to insignificant levels are often larger than building occupants would expect.

Selecting vibration isolation mounts is necessary but rarely sufficient. The performance of an isolation system depends on interacting factors that must be considered together, and a design that addresses any one of them independently risks falling well short of the design intent.

Natural frequency and isolation efficiency

The theoretical basis of vibration isolation is well established. A resilient mounting system reduces the force transmitted from a vibrating machine to the supporting structure by a factor that depends on the ratio of the excitation frequency to the natural frequency of the mounted system. Above the natural frequency, isolation efficiency improves with increasing frequency ratio. Below the natural frequency, the mounting system can amplify transmission rather than attenuate it.

For effective isolation at the dominant excitation frequencies of HVAC plant, chiller compressors, or pumps, the mounted natural frequency must be substantially lower than the lowest significant excitation frequency. A typical 1500 rpm fan has a fundamental excitation at 25 Hz. To achieve useful isolation at this frequency, a mounted natural frequency of around 5 to 7 Hz is needed. This typically requires air-spring or wire-rope isolators, which provide significantly lower natural frequencies than rubber mounts at equivalent load.

The natural frequency of the mounted system depends on both the spring stiffness of the isolators and the mass being supported. Isolator selection cannot be completed without confirmed equipment mass data, including operating fluid weights where applicable. Provisional mass data leads to provisional natural frequency estimates, and the margin of error can be large enough to reduce isolation performance materially at the target frequencies.

Structural discontinuities and flanking

Even a well-designed isolation system can be undermined by flanking transmission paths, where vibration bypasses the isolation mount and re-enters the structure through rigid connections. The most common flanking paths in plantroom installations are pipework connections, conduit, and ductwork that bridges rigidly across the isolation plane.

Flexible connections in pipework and ductwork at the equipment connection point are standard practice, but they are not always adequate. The flexibility of a flexible connector under operating pressure and flow conditions may be substantially lower than its nominal static flexibility, reducing its effectiveness as a vibration break. The length and orientation of the flexible section relative to the dominant vibration direction also influence performance.

Electrical conduit rigidly attached to an isolated equipment frame and running through the slab or wall represents a direct structural bridge. Routing conduit off the isolated frame and providing a flexible loop before the rigid structural penetration is a detail that is routinely missed until the system is commissioned and measured.

In buildings with transfer slabs or shared structural elements between the plant area and adjacent occupied floors, the structural path analysis becomes more complex. Vibration energy injected into a slab at one point redistributes through the plate structure and may re-radiate at locations well removed from the injection point. Structural analysis to map the expected transmission paths is valuable in these cases.

Interaction between isolation and room acoustics

A correctly isolated system that delivers the expected reduction in structure-borne vibration may still not achieve the target noise level in the receiving room if the room’s acoustic characteristics amplify the re-radiated noise. Low-frequency sound is particularly susceptible to room resonances. In small to medium-sized rooms, peaks in sound pressure level at specific frequencies can be significant, and a structure-borne noise level that would be acceptable in a large, acoustically treated space may be unacceptable in a hotel bedroom, office, or apartment where a particular frequency component is amplified by the room response.

Room modes below 100 Hz fall within the range of the primary excitation frequencies of most mechanical plant. Acoustic treatment of the receiving room, including absorptive ceiling or wall finishes and floating floor construction, can address the room response component of the problem. These are measures of last resort. Resolving the transmission path is always preferable to treating the receiving room, but it is sometimes necessary where structural redesign is not feasible.

Multidisciplinary coordination as a design requirement

Effective vibration isolation in multi-storey buildings requires coordinated input from structural, mechanical, and acoustic engineers at the design stage, and a shared understanding of the constraints and objectives at each stage of procurement and construction.

Structural engineers need to understand where vibration injection is expected and where receiving-room criteria are most demanding, so that slab stiffness and continuity can be considered in relation to transmission. Mechanical engineers need to understand that equipment mass, connection detailing, and pipework routing all carry acoustic consequences. Acoustic engineers need access to structural drawings, confirmed equipment schedules, and early-stage plantroom layouts rather than receiving them after the coordination model is finalised.

Plantroom acoustic treatments, including acoustic enclosures, lined panels, and attenuators on intake and discharge paths, address the airborne component of plant noise. For the structure-borne component, the design solution lies in the mounting system and the structural connection detailing. Both need to be specified with the same rigour. AcousTech’s plantroom acoustic assessment work considers both airborne and structure-borne transmission, supporting the coordination process that effective isolation requires.

Vibration isolation in multi-storey buildings is a systems problem, not a product selection exercise. The performance of any isolation design depends on the natural-frequency tuning of the mounted system, the integrity of the isolation plane against flanking transmission, and the response of the receiving structure and rooms to any energy that does arrive. Achieving the design intent requires early coordination between structural, mechanical, and acoustic disciplines, accurate equipment data, and detailed connection design that treats every rigid penetration of the isolation plane as a potential performance risk. Where these conditions are met, effective isolation of mechanical plant in mixed-use and residential buildings is achievable. Where they are not, the shortfall typically shows up at commissioning, when the options for correction are limited and expensive.

Talk to the AcousTech team about your project.