Designing out noise

Building acoustics can affect building users’ health and productivity, so controlling noise from HVAC systems is key, says Graham Laws.

Loud noise is an unavoidable part of modern life, from the alarm going off to the sound of the neighbours’ TV at night. In fact, according to a survey by the Department for Environment, Food & Rural Affairs, 48 per cent of people in the UK reported their home life was spoilt to some extent by noise.

Buildings are a major contributor to this widespread issue – not just during their construction, but also in their day-to-day operation. As awareness of the problems grows, architects are becoming increasingly focused on improving acoustics.

Noise and vibration in building services are caused by a wide range of mechanical and electrical systems.

From fan coil units, variable air volume systems, and grilles and diffusers to rooftop units, chillers, compressors and condensers, pumps and stand-by generators, boilers and cooling towers, systems can cause significant noise pollution and make the workplace uncomfortable and therefore less productive.

On the back of increased requirements for energy efficiency, buildings are now better insulated thermally and acoustically. However, this further drives the need for ventilation and thermal management, which can in turn create noise.

Common issues

Breakout noise is sound moving through walls of ductwork and into the space around it. Noise travelling through ductwork (‘in-line’ noise), can create a significant background nuisance if not properly managed. This can raise stress levels, harm communication and concentration and increase fatigue. A poorly-designed acoustic solution can potentially exacerbate issues by causing regenerative noise and affecting airflow.

An additional issue is cross-talk, whereby speech privacy is compromised by room-to-room noise transmission via the ventilation system. A good example is in an office building where employees on one floor might hear conversations via the return grille.

Meeting noise levels

Different building types present unique challenges, whether they are offices, schools, hospitals or commercial structures. In the education sector, acoustics are paramount, and there will be both separation requirements and background noise level requirements for a project to achieve. These may be provided as a single-figure rating or noise rating (NR) value, and it’s vital to know rooms’ function and the desired speech privacy level.

These levels are used in the measurement of noise from mechanical sources such as air conditioning systems in spaces like hotels, cinemas and schools. Most buildings will have an ‘in-room’ NR level requirement, based on the room’s function.  For example, typical noise rating values for a concert hall or church would be NR25 while a school or hotel would be NR35, significantly lower than an engineering environment for example.  To ensure these levels are achieved, a given Sound Power Level (LwA) for the HVAC unit is required.

Noise transmission

In terms of system design, air velocity within a duct system is a significant factor in noise levels. Transition pieces, bends, dampers, grilles and diffusers can create regenerated noise. This can be reduced by limiting the air velocities within the duct system, by easing transitions in the system design and by ensuring that internal acoustic treatments contain chamfers where appropriate to reduce the turbulent effect of sudden changes in of opening or direction.

When two adjacent or closely positioned rooms are sharing the same ductwork, sound travels within ducts and will decrease the room-to-room sound insulation dramatically. This can impact speech privacy and negatively affect the working environment.

Since vibration and sound from HVAC operation can transmit to the building and cause problems, isolating systems from the structure is one solution. This can be done via isolation mounts or through flexible linkages from the structure via the noise source and the duct runs.

Sound control

When it comes to HVAC noise control treatments, a range of solutions are available, tailored to project-specific requirements. Components manufactured from open-cell acoustic foam for example are designed to ease airflow and lower regenerative noise, with lining treatment engineered to deliver a low Sound Power (LwA) figure. Modern solutions can offer exceptional noise absorption, yielding reduced airborne noise at inlets and outlets, and lowering case-radiated noise.

Lining kits from Siderise for example are Class 0 fire-rated to EN 13501-1 to comply with Building Regulations Approved Document B. Solutions are supplied as a pre-cut kit, backed with modified acrylic adhesive for rapid installation.

Remedial treatment

For an existing building with an ‘in-line’ noise problem in ductwork, open-cell melamine resin foams are an easy and cost-efficient way to introduce noise control.  At the terminal ends, where air is being delivered to the occupied spaces, the grille can be removed to install the foam inserts. Inherently flexible, they are ideally suited to retrofit installation, particularly when there is minimal access or if work has to be carried out with minimum disruption.

For ‘duct noise breakout’, another solution which contributes well to quiet air-handling, is externally-applied acoustic treatments. Easy to install without affecting the HVAC system, they reduce breakout noise through a combination of acoustic absorption and sound reduction via an inherently damped flexible mass membrane.

Promoting comfort

We live in a very noisy world, but by proactively considering potential acoustic problems at the design stage of an HVAC system and adding sound control measures to minimise unwanted noise, building owners can benefit from a quieter system which can go a long way towards promoting comfort and productivity. Solutions are available to provide a holistic treatment for all areas of HVAC noise, for a peaceful, stress-free and more productive future.

Graham Laws is business development officer at Siderise Group