Every mechanical floor in a tower sits directly above somebody's bedroom and directly below somebody's boardroom. The plant on it runs continuously, is bolted to a structure specifically engineered to be light and flexible, and radiates into a building where sound travels through concrete far better than through air. Yet vibration isolation is routinely specified as a line item — "spring isolators, 25 mm deflection" — copied between projects without anybody checking the one number that decides whether it works at all. And when it is wrong the result is not slightly worse: below a critical ratio, an isolator does not reduce transmission, it amplifies it. A 6 mm rubber pad under a 600 rpm machine delivers 29 % isolation; under a slower machine it makes things worse than a rigid mount.
1 · Why towers are acoustically unforgiving
- There is no buffer. In a low-rise building plant sits on a roof or in a basement with a car park between it and anyone who cares. On a mechanical floor there is one slab between a 150 kW machine and a paying tenant.
- The structure is efficient and therefore lively. Post-tensioned slabs and light long-span floors have low mass and low damping — exactly the properties that transmit structure-borne vibration well.
- The paths multiply. Every pipe, duct, cable tray, conduit and hanger crossing from a plant room is a flanking path. The isolators can be perfect and the noise still arrives through a rigidly clamped chilled-water riser.
- Nobody can fix it later. Once the plant is commissioned, the slab is cast and the risers are anchored, the remedies are limited, disruptive and expensive.
- Expectations are high. Residential and hotel floors in a landmark tower are specified at NR 25 or lower — a target that leaves almost no margin against a machine radiating 95 dB of sound power one slab away.
2 · The one equation that decides everything
A machine on isolators is a mass on a spring. Its natural frequency depends only on how far the isolator deflects under the load:
where \(T\) is transmissibility — the fraction of the disturbing force that reaches the structure — and \(f\) is the disturbing frequency, usually the running speed. The behaviour has three regions, and only one of them is useful:
- \(f/f_n < 1\) — the machine runs below resonance. The isolator does essentially nothing.
- \(f/f_n \approx 1\) — resonance. Transmission is amplified, potentially enormously. This is where equipment shakes itself and its surroundings apart.
- \(f/f_n > \sqrt{2}\) — isolation finally begins, and improves as the ratio grows. Practical design targets \(f/f_n \ge 3\), and \(\ge 5\) where the receiving space is sensitive.
3 · Interactive: isolation efficiency vs static deflection
Set the machine speed and the isolator deflection. The curve is the fraction of vibration transmitted into the structure; the peak on the left is resonance. Note where variable-speed operation puts you — a machine isolated correctly at 1,450 rpm may be sitting on the resonant peak at 30 % speed.
A 1,450 rpm pump on 25 mm springs gives 97.8 % isolation — a frequency ratio of 7.7, comfortably clear of resonance. Drop the deflection to 6 mm and it falls to 91.9 %; that sounds close, but the transmitted force has nearly quadrupled. Now drag the machine speed down to 400 rpm on the same 6 mm pad and the ratio falls below √2 — the mount amplifies. This is exactly what happens to a variable-speed machine at low turndown, and it is why the isolator must be selected for the slowest speed the drive will hold, not the nameplate.
4 · Flanking paths — where the isolation actually leaks
A perfectly isolated machine still transmits if anything rigid connects it to the structure. In order of how often they are the cause:
- Pipework. The largest and stiffest connection. Flexible connectors at the machine deal with the first metre; beyond that the first three to five hangers must be resiliently isolated with deflection matched to the machine's. Rigidly clamped risers carry pump vibration hundreds of metres.
- Ductwork. Flexible connections at the fan plus resilient hangers on the first section, and — critically — no rigid contact where the duct passes through the plant room wall. Pack the penetration resiliently and seal it acoustically.
- Conduit, cable tray and drainage. Small, stiff, easily forgotten, and often the last path left when everything else has been fixed.
- Inertia bases short-circuited by grout or debris. A base that has been grouted solid, or that has construction rubble under it, is not isolated at all. This is astonishingly common and it is invisible once the plant room is finished.
- Snubbers and limit stops touching in normal operation. Seismic restraints must have clearance in service; set hard against the base they are a direct rigid path.
Flexible connectors deserve a warning of their own: they are for vibration, never for correcting misalignment, and a connector installed in tension or offset transmits more than the rigid pipe it replaced.
5 · Interactive: plant-room level and the partition you need
Equipment sound power sets the level inside the plant room; the room's absorption modifies it; and the difference between that and the target next door is the transmission loss the separating construction must deliver.
A 95 dB machine in a lightly absorbent plant room produces about 86 dB, and separating that from an NR 30 space needs roughly 55 dB of transmission loss — beyond a single blockwork wall and firmly into double-leaf or heavy composite territory. Line the plant room to α = 0.35 and the requirement drops by about 6 dB, which is often the difference between a buildable partition and an impossible one. Absorption inside the plant room is almost always cheaper than transmission loss in the wall, and it is the first move to make — but note that it does nothing at all for structure-borne transmission, which is the isolator's job.
6 · Interactive: variable speed as a noise control measure
Slowing a fan or pump reduces its sound power steeply — roughly 50·log₁₀ of the speed ratio for a fan. This is the most under-used acoustic tool in a building, because it costs nothing once the drive is there.
Running a fan at 70 % speed drops its sound power by 7.7 dB — close to halving the perceived loudness — while cutting shaft power by 66 %. That is a free acoustic result, and it argues strongly for selecting fans and pumps that will spend their lives at part speed rather than selecting tight to the duty and running them flat out. It also argues for oversizing the duct rather than the fan: lower velocity means less regenerated noise at every bend, damper and terminal, and regenerated noise is the one source a silencer cannot fix because it is created downstream of it.
7 · Installation & execution tricks
- Specify by static deflection, checked at minimum speed, and require the supplier to submit the selected deflection under the actual operating weight — not the catalogue rating.
- Inspect isolators under load, after commissioning. Measure the actual deflection of every spring; a mount that has not compressed is either wrongly selected or bottomed out, and both are common. This is a five-minute check that catches most failures.
- Check the base is free. Walk every inertia base looking for grout bridges, packers left in, debris under the frame and snubbers touching. Photograph it before the plant room is closed.
- Isolate the first five hangers on every pipe and duct leaving isolated plant, with the deflection reducing progressively rather than dropping to rigid at hanger two.
- Pack and seal every penetration resiliently — and make sure the fire-stopping detail chosen is also an acoustic detail, because a rigid mortar fire-stop is a perfect sound bridge.
- Line plant rooms before you thicken walls. Absorption is cheap, thin and effective on airborne level; use it first and size the partition on what remains.
- Commission acoustically, and to a written criterion. Measure NR in the sensitive spaces with plant running at design and at minimum speed, at night, and record it. Without a baseline measurement every future complaint is unanswerable.
- Watch out for tonal noise. Blade-pass and pump vane-pass frequencies are perceived far more strongly than broadband level suggests; if a tone is audible, a 3 dB overall reduction will not fix it — the fix is speed, blade count or a tuned attenuator.
8 · The design & installation checklist
- Set acoustic criteria per space (NR/NC) at concept, and make them contractual.
- Select isolators on static deflection, with f/fₙ ≥ 3 at the minimum operating speed.
- Use inertia bases for pumps and close-coupled machines, sized at 1.5–2× machine mass.
- Design every flanking path — pipes, ducts, conduit, trays, drainage, snubbers.
- Line plant rooms and size partitions on the resulting level, with the slab and the door treated as part of the envelope.
- Exploit part-speed operation as a design assumption, not a happy accident.
- Keep duct and pipe velocities down near sensitive spaces to control regenerated noise.
- Inspect deflections under load and photograph every base before closing out.
- Measure and record NR at handover, at design and minimum speed.
References & standards
- ASHRAE Handbook — HVAC Applications, Noise and Vibration Control chapter — transmissibility, isolator selection tables, plant room treatment and flanking paths.
- CIBSE Guide B4 — Noise and Vibration Control for Building Services Systems; and CIBSE Guide A for indoor design criteria.
- Institute of Acoustics / ANC guidance on building services noise, and BS 8233 Guidance on sound insulation and noise reduction for buildings.
- ISO 1996 and ISO 3382 series — measurement of environmental and room acoustic parameters; ISO 717 for airborne sound insulation rating.
- ASHRAE Design Guide for Tall, Supertall, and Megatall Building Systems, 2nd ed. — plant location and acoustic separation on mechanical floors.
- AMCA 300 / ISO 3744 — fan sound power determination; and Eurovent guidance on equipment sound data.
- SMACNA Seismic Restraint Manual — restraint and snubber arrangements compatible with vibration isolation.
- Beranek, L.L. & Vér, I.L. Noise and Vibration Control Engineering — theory of isolation, structure-borne transmission and room acoustics.