If you work in construction, you've probably been there. It's coming towards the end of the project, and somebody says, "What's that humming?" or, "Why are those fan coils so loud?". Building services noise can be expensive and incredibly difficult to fix, but it need not be an issue in the first place. With proper design and oversight, these issues can be eliminated before they even arise. In this article, we'll go over our top ten tips for buillding services noise and vibration, focussing primarily on internal noise.
1. High quality Manufacturer's noise data is worth its weight in gold
Manufacturers of HVAC equipment, pumps and generators want to sell more product, and one way of winning that sale is by having the quietest unit on the market. Unfortunately, this can lead to some manufacturers obscuring the noise data of their units to make them appear quieter. For ducted systems, in-duct sound power levels are generally the most useful, along with casing radiated noise. Condensers, chillers or pumps might have a sound power level or sound pressure level (at a specified distance).*
All sound data should be accompanied by the 'spectral' sound levels (sound levels at individual frequency bands), ideally from 63Hz to 8kHz. A standalone dB(A) figure is rarely helpful without the spectral data.
*To understand the difference between sound power and sound pressure, think of a sound source as if its a lightbulb - the power is the total energy spent on producing sound[or light] (and is independent of its environment), whereas pressure is the sound[or light] level at a given distance away from the source (and is dependent on its environment including screening, room surfaces and so on).
Takeaway: Some standardised test procedures which usually indicate a reasonable level of confidence of the plant sound power level are, BS EN ISO 3741, BS EN ISO 3744, BS EN ISO 3746, BS EN ISO 9614, BS EN 13053. If your selection has been measured in accordance with one of these standards, you probably have some good data on your hands.
2. Don't bridge the vibration isolators
This is a bit like a parent telling a child, "Don't wear your muddy shoes into the house." No matter how many times you say it, it's going to happen again.
'Bridging' the isolation means to create a rigid connection between the 'isolated' plant item and the structure from which it is isolated. In theory, if a plant item is properly vibration-isolated, all its connections to the structure will be somewhat flexible, and the noise and vibration transmitted to adjoining spaces will be of no consequence. What often happens in practice is that somebody rigidly fixes a cable tray to both the structure and the generator, or somebody forgets the flexible connection to the exhaust, or somebody uses a uni-strut to support a pipe close to where it meets the pump.
Takeaway: If you are hearing a humming or noise which seems to be travelling along the structure, double check to see if your vibration isolation hasn't been bridged.
3. Always allow for attenuators
Too often have we seen an HVAC design which has not allowed sufficient space or pressure drop for attenuators. The required attenuation can vary widely on a case-by-case basis, and this goes to illustrate how important it is to get a view from an acoustician early on in the design (RIBA Stage 3, or even earlier for noise-critical projects).
Takeaway: In our experience, for small MVHR units serving individual apartments, an allowance of 30 Pa pressure drop and approximately 1200 to 1500 mm in length for an attenuator is usually a good starting assumption. For larger, central air handling units, a larger pressure drop might be suitable (perhaps up to 50 Pa), but substantially longer attenuators could be required (two metres or longer would not be unusual).
4. Be generous with grill sizings
It's no use providing attenuators and vibration isolation, and doing all the hard work to reduce noise at the source, when your grilles are undersized. As air is squeezed through narrow gaps at a high velocity, it creates turbulence, which manifests as noise, and you can end up with a constant hissing sound over an otherwise quiet system. Some grille manufacturers can provide sound power level data for their grilles when given the face velocities and grill dimensions.
Takeaway: As rule of thumb, limiting face velocities to 2.0 m/s is a safe assumption for most normal occupied areas, prior to co-ordination with the acoustic designer.
5. Turbulence is noisy
As air travels through a duct, it passes round bends, splits, and over dampers. Each time the air flow is interrupted or changes direction, turbulence can occur, and with turbulence comes noise. This can lead to issues with duct breakout noise, or simply increase the noise level coming out through the terminal units in a room.
Turbulence also reduces the effectiveness of noise control measures such as attenuators and internal duct linings.
It's worth noting that there is a balance to be struck, as bends and splits are also responsible for attenuation of noise at some frequencies, and so the ultimate gain versus loss will be dictated by balancing the regenerated noise with the attenuation provided. Generally speaking, regenerated noise from turbulence is more of an issue in noise-critical spaces, with exposed ceilings or close to the terminal units.
Takeaway: Close to terminations or in spaces with exposed services, try to keep your air flow smooth and avoid siting dampers or VAVs close to terminations.
6. It's not cool to cross party walls
People enter an apartment, classroom or office through the front door, so why shouldn't services? It's all too common that we see ductwork and pipework passing through party walls, or walls separating class rooms or private offices. The primary concern here is that this immediately downgrades the performance of the separating wall. A secondary concern is that it also means a shorter duct route, so that any crosstalk which might occur receives less attenuation than it would do otherwise.
Takeaway: Best practice for routing services is to pass them through common areas (e.g. corridors), and into apartments, offices or classrooms over (or near enough to) the front door. The front door is an acoustic weakness anyway, so adding a penetration in the services void next to the front door is of less concern. In any case, all services penetrations should be adequately sealed with coated mineral fibre batts or plasterboard pattressing, depending on the situation.
7. Natural ventilation needs space
Many projects these days are looking to incorporate natural ventilation - either to eliminate the needs for mechanical ventilation and cooling entirely, or to provide more flexibility and reduce the on-going carbon emissions of future buildings. If we are to make a naturally-ventilated scheme workable, it needs to be quiet enough. The problem is that large ventilation openings allow a high level of sound to pass through. In order to attenuate this sound before it reaches noise-sensitive spaces, we need long duct runs, perhaps with attenuators, or internal acoustic linings, and perhaps with some bends. This applies equally to the discharge as it does to the supply air.
Takeaway: The key to attenuating noise in natural ventilation schemes is to allocate space for attenuators and/or long, labyrinth air paths, all of which which will gradually whittle down the sound energy as it travels through the duct towards the room.
8. Select plant quieter than the requirement
This point follows on from (1). We often see manufacturers make claims about the noise levels of their plant, which are usually based on favourable assumptions. For example, with fan coil units, a manufacturer may claim they can achieve a given Noise Rating (NR38, for example). However, these claims often make unrealistic assumptions about the installation, the room, the measurement procedure, and don't take into account that there may be multiple units in one room. Two NR38 fan coil units in one room could be NR41 when both are running!
Takeaway: Treat claims of a certain NR level with caution. If you have multiple units in one room, you may want to consider selecting units which are quieter than the requirement for that space.
9. If you have a noise problem, check the MEP commissioning
We sometimes find that a system has been suitably designed by the mechanical and acoustic engineers, but there are still exceedances of the noise requirements. Often, in these cases, the problem lies in how the system has been left following mechanical commissioning. This could mean that, during the acoustic commissioning, dampers which should be open are closed (causing turbulence and noise), AHUs are running at higher duties, or dust filters have not been cleaned, thereby forcing FCUs to work harder (and noisier). Alternatively, a noise issue it could indicate a fault or problem with the system which was not picked up during the MEP commissioning, or required the system to run at a higher duty to overcome it.
Takeaway: Always check that the MEP commissioning has been carried out, and that the systems are operating as designed, before undertaking acoustic commissioning, or troubleshooting noise problems.
10. Be clear about what 'design duty' means
We have found that the understanding of 'design duty' differs among practices. To one building services engineer, the design duty of a fan coil unit could be the expected day-to-day duty required for a large portion of the year, but another engineer could be designing to the worst case duty, which may only occur a few days a year. It's important to be clear about what these duties represent in your design, and provide these definitions to the acoustician, so that a discussion can be had about what implication this has for noise levels.
Are you designing to the worst-case, in which case, will the noise levels be quieter for the majority of the year, or will there be an exceedance on those hottest few days, if you are designing to a 'typical' day?
Takeaway: Be clear about the intention of your design duties, and what time periods or portions of the year they are likely to represent. If possible, provide two duties - one 'typical' and one 'worst-case.
Conclusions
We've seen ten points that building services engineers need to know to get the acoustics right first time. However, we couldn't possibly cover all potential issues in a bullet point list, and it's always worth getting an opinion from an acoustics expert. For acoustic advice on your MEP system, be it at design stage or completion, contact us today.
