The modern world is a noisy place. We’re bombarded with auditory stimulus in pretty much every waking moment. That doesn’t stop when we sleep either. In fact, our ears don’t actually sleep, rather it’s our brain that takes a break from processing sound. Ears have the smallest bones and smallest muscles in the human body and we put them to work constantly. What does this have to do with noise and sound transfer in ventilation systems and why is it important? The answer is ‘quite a lot’ and it’s important because your hearing health affects your physical, mental and financial health.
The real issue for hearing health is the cumulative noise load on your ears throughout the day. This is what can do the damage. It means a ventilation system with poor acoustic insulation and significant sound transfer might be the final note in a symphony that’s one ‘movement’ too far. It’s the sound equivalent of the straw that breaks the camel's back.
Why Noise & Sound Transfer Issues in Ventilation Matter
Before we get into the finer details of noise in ventilation systems, it’s important to quantify the effect of sound on our lives. More specifically, what happens when sound affects our hearing health. Poor hearing health is wrongly perceived as a problem of age. Yet 1 in 10 millennials already have hearing issues, while the figure for Gen X is 1 in 14. In the United States, an astounding 1 in 4 workers suffer from tinnitus.
Issues of poor hearing health are directly related to depression, cardiovascular disease, diabetes and more. It’s a very good predictor of overall physical and mental health, and this extends to financial health too, with one study showing individuals with untreated hearing loss typically have annual incomes that are US$10,000 per year below those with normal hearing.
With the proliferation of earbuds and our constant ‘noise loading’, a relatively modern phenomenon is ‘listener fatigue’. This is that unexplained tiredness which kicks in at the end of the day. Which is precisely when an annoying noise from a poorly designed ventilation system is likely to announce itself.
That’s why it’s really important to get the acoustics of your ventilation system perfect. Not doing so ruins evenings, disturbs sleep and might just make you a little more grumpy than you (and those nearby) would like.
How Noise & Sound Transfer Occurs in Ventilation Systems
The noise reaching your ears from ventilation systems can be traced to one (or more) sources. The ventilation system itself will generate some sound, with the The dB level reached depending on the system used and how it was installed. Ideally, it should be around 40dB, that’s about the same noise level as a quiet library and not much above a whisper. A ventilation system at 50dB is on the same level as a moderate rainfall, with 60dB being about the same as a dishwasher. We’d suggest that’s more than enough to be annoying and any ventilation system that reaches 60dB has issues.
The second way a ventilation system can introduce noise is through transference. The ducting acts as a passage for noise to escape from one room to another. That might be handy for covertly listening to phone calls but a late night action movie in the living room that infiltrates your bedroom will be problematic. And possibly alarming depending on what’s happening on screen.
The third and final route for noise is from the outside. This can be through the ducting, a hole in the building facade where the cowl or louvre is located and there’s no insulation, or simply through a window left open to ventilate the space.
How to Silence Ventilation Systems
The methods used to keep the noise down can be classified as either product-led or design-led.
Product-led solutions include the use of specific products which have been designed to damper and reduce any noise from either the mechanical ventilation system, from transference through the ducting, or noise which passes through the gaps in louvred ventilation solutions.
Silenced Fans
These differ from regular fans inasmuch as they have specific acoustic benefits which work to dampen noise without affecting the airflow. A silenced fan will pack the metal casing with sound absorbing mineral wool. This dampens any air inlet, outlet and breakout noise. The supply or extract air is then passed over a perforated inner core and air noise is absorbed through the perforations, into a 50mm wall of sound dampening mineral rock-wall.
Inline Duct Silencers
An inline duct silencer will reduce noise transmitted inside ventilation ductwork by attenuating the sound. Sound travels in waves, each carrying a little bit of energy. Sound attenuation occurs when you disrupt those waves and reduce the energy, therefore reducing the sound (dB) that is transferred from point A to point B. We have had considerable success dampening sound using inline duct silencers consisting of outer and inner flexible spiral seam air ducts. They are made from aluminium and filled with non-flammable sound-absorbing material. The internal surface is perforated and covered in a protective coating to prevent any fibre from blowing out.
Acoustic Louvres
The blades of acoustic louvres are filled with a high-density mineral wool, with the rear faces perforated. They effectively work by creating an alternative path for the noise source and reducing the energy in the sound wave, preventing noise from reaching your ear. They are so effective we have installed acoustic louvres in highly noise sensitive developments such as Burwood Hospital. In that example, they were used to keep noise inside a plant room whilst maintaining the significant air flow needed to keep machinery cool. Acoustic louvres are well suited to industrial buildings in a built environment where noise levels must meet specific standards.
Design-led solutions
With an understanding of the principles of acoustic transmission, it's possible to incorporate certain design elements which will work to reduce noise and sound transference. Sound has two basic forms; acoustic energy and mechanical energy.
Acoustic energy is what we experience every day as sound waves (the vibration of air) is transformed by the tympanic membrane in the ear and processed into audible ‘sounds’ by the brain. Reducing noise from acoustic energy means slowing down or absorbing the sound waves. This might be achieved by:
Using short lengths of flexible duct in conjunction with long ruins of solid duct, providing ‘breaks’ which interrupt the sound waves and help dissipate the energy (and sound) they carry.
Use acoustically insulated ductwork.
Ensure that any centralised ventilation system has self-closing backdraft dampers in the circuits so that when the fan is not running there is no clear air passage from one room to another.
Mechanical energy is the source of acoustic energy. Anything that vibrates creates acoustic sound, so the design-led solution here aims to reduce the sound at its root cause. In other words, reduce the potential for vibration by:
Ensuring all fans are mounted on proper anti-vibration mounts (not just screwed to the timber house framing).
Fans are located in areas which are less susceptible to vibration (i.e. not an unstable wall).
We’d also caution against the use of direct “air transfer” systems, where air is blown from one room into another (quite common in older houses with the wood burner in one central room).
Finally, where natural ventilation devices are being used, a design-led solution would call for double glazed louvre windows. Yes, when they are open noise can obviously still pass through, but when they are shut they have excellent acoustic properties. And being a louvre, they have significantly more open area than a conventional top hung window and can thus be made much smaller whilst still providing good ventilation.
We should make it clear it’s never a choice of either a product-led or design-led solution. Rather, the best solution will use a blend of both. It will take into account the specific needs of the project, such as the rate of ventilation required, then incorporate products and a design to best reduce any noise and sound transference. The needs of a factory are different from a high density housing complex.
The only common element in any solution is the need to reduce sound. The benefits of doing so are quite remarkable and the perfect ventilation system would be ‘felt and not heard’. That’s something we can help with.
