Why Is Ventilation Noise Control So Confusing
By Paul Leadbeater, Commercial Technical Sales Manager, October 2014
How often do Consultants and Contractors look at manufacturers’ catalogue data and fully understand the information supplied and how it relates to the job they are working on? In-duct noise levels, open inlet noise, dBA figures in free field and even estimated NR levels within a room, SWL – Sound Power Levels and SPL – Sound Pressure Levels. Is it all designed to baffle you with science?
As an introduction from a manufacturer’s perspective, let’s start with the industry standard ‘Breakout Sound Pressure Level’. All equipment manufacturers show a Breakout Sound Pressure Level (dBA at a distance of 3 metres) tested in Free Field Conditions. This is based on a theoretic infinite space with a unit 3 metres away, so that all the noise you hear comes directly from the unit and there is no reflective sound. Free Field Environment and the ‘A’-weighted Sound Pressure Level can be calculated for any distance. This is a theoretic estimated level and should not be used in highly critical noise areas.
It can also not be replicated by an on-site test. I remember a number of wonderful factory visits to Chiller manufacturers over the years trying to prove the tested data. Units were wheeled out of the factory gates with the highly reflective factory walls behind the unit, a noisy pump set supplying the unit, hidden behind a pair of wooden louvre doors, planes flying overhead and lorries leaving and entering the car park. Great days in the late 1990's and early 2000's where we interpolated the data, made allowances for all the background noise levels and threw in a safety factor hoping it covered all eventualities.
Fast forward 10 years and we have the advent of stringent noise specifications: BB93 for classrooms looking to achieve 35dBA within the room. Many schools designers don’t want to increase the external boundary condition hence the new plant needs to be designed to 10dBA less than the existing noise level.
Still I digress from the real issue. Unfortunately a great number of people are confused by this manufacturer’s ‘free field’ data and expect it to achieve the tested figure on site. This is never the case and noise problems occur, whether that is ducted noise or, more commonly, breakout noise from the unit to the room below.
For example, with Heat Recovery Units in Classrooms you are generally only 1.5 metres away from a unit below a false ceiling. Dependent upon the position of the unit and the acoustic properties of the ceiling, the on-site correction from the Free Field 3 metre figure can be between 8 – 14dBA. When dealing with manufacturer’s free field data, this equates to selecting a unit with a maximum catalogue breakout level of 27dBA @ 3 metres, and often slightly less, to guarantee hitting this stringent 35dBA requirement.
Nuaire looked at this from a different perspective by carrying out independent tests. Nuaire’s original schools unit, the XB55 Heat Recovery system, was independently tested by BSRIA to achieve BB93 noise levels of 35dBA and below based on BB101 air flow requirements (Clause 1.5.2). This unit has now been superseded by Nuaire’s award winning XBC range (CIBSE Energy-Efficient Product of the Year 2014) which offers up to 90% heat exchange efficiency and the quietest breakout noise levels in the industry. This independent test is detailed within our new XBC catalogues and details the unit, ductwork, silencers and grill arrangement measured during the BSRIA test. If these guidelines are followed, Nuaire guarantees achieving the BB93 noise level requirement.
The main cause of noise issues for ceiling void units tends to be breakout noise between the unit and silencers via flexible connectors or transformation sections. As an industry, we spend lots of money building double-skinned units to reduce the noise breakout from the fans, and this is then negated by someone installing a flexible connector between the HRU and the silencer, causing all the noise to break into the room below via the ceiling void. The HRU manufacturer is told their units are too noisy or the attenuator manufacturer is told his attenuator is undersized, however it is the installation that is wrong.
Nuaire’s award winning XBC units are selected with double-skinned matching attenuators ensuring that this weak point in the system is adequately attenuated. This further guarantees achieving the noise levels (as per the BSRIA test). As part of my role as Consultant Sales Engineer, I often arrange factory visits with my customers listen to the guaranteed low noise level solution. As with all low-noise requirements (35dBA and below), I would strongly recommend seeking specialist acoustic assistance to guarantee your installation meets the required specification or choose a guaranteed solution. Or alternatively speak to the people with the guarantees to back up their products. And remember, it can be at least 10 times more expensive to rectify acoustic problems than prevent them.