Abstract
Mechanical ventilation systems, present in most buildings, generate a well-known broadband noise when functioning. However, with increasing awareness of the negative effect of low-frequency noise on humans, there is an augmentative requirement for ventilation systems that run quieter.
Installers traditionally use passive resistive silencers to attenuate duct-borne noise. Although passive silencers are very effective in attenuating noise over a broad frequency range, when required to attenuate low frequencies specifically, they tend to become large and bulky.
Good practices in controlling ventilation system noise
In general, the operation of ventilation systems is regarded as a noisy activity. Environment Protection Department 1 of Hong Kong offers a booklet describing good practices that designers and installers can follow while planning against these noise problems. It discusses the potential noise problems associated with ventilation systems and provides guidelines on measures that help with practical noise control. The guidelines are applicable to new designs as well as for retrofitting existing designs.
When planning against noise problems, the Environment Protection Department 1 suggests considering three factors—proper positioning of ventilation equipment, selection of quieter equipment, and conducting scheduled maintenance.
Positioning the ventilation equipment during installation plays a vital role in determining the noise level at sensitive receiver locations such as schools or residential buildings. The Environment Protection Department 1 suggests placing the equipment preferably within a plant room that has thick walls, or at a much larger distance from the receiver, or behind some large enough obstruction, such that it blocks the line of sight between the equipment and the receiver.
In case noisy equipment has to be necessarily placed near the receiver due to spatial constraints, installers must consider sufficient noise control measures in the design stage that prevents noise problems.
Quieter equipment tends to be more expensive on average. However, trying to reduce noise by modifications after purchasing low-cost equipment may turn out to be more expensive in the end than buying quieter equipment at the start. Manufacturers generally offer their equipment with a range of readily available noise control devices that deal effectively with noise problems—including noise level specifications in new orders is advisable. Equipment suppliers can then select appropriate equipment with optional noise control devices that will suit the acoustic requirements.
A regularly scheduled equipment maintenance program goes a long way in preventing increased noise from existing equipment. Equipment that is regularly operated and maintained has a better control on the level of noise and vibration. This may include lubricating various moving parts, replacing worn-out components, tightening loose parts, and inspecting equipment alignment. In their booklet, Good Practices on Vibration Systems Noise Control, Environment Protection Department 1 suggests measuring vibration at different frequencies to detect the causes of excessive vibration or noise of a machine.
Safety and health
Installers of ventilation systems need to be careful about acoustics in certain areas such as schools and laboratories where occupants must communicate and understand instructions clearly. Lewis 2 defines this as speech intelligibility. According to Lewis, this depends on two principle factors—background noise levels and the reverberation of sound within the area. Laboratories require good speech intelligibility that allows clear communication required for manipulation of chemicals and specimen. Impaired communication leading to mishandling may be dangerous, with potential for damage to humans, equipment, and instrumentation. Likewise, increased background noise in school classrooms is often associated with disruptive student behavior.
According to Lewis, 2 health implications may also arise from poor speech intelligibility—occupants in noisy spaces may suffer from vocal strain, poor concentration, and fatigue. Although they come with a cost or space penalty, installers often use fibrous materials for treatment of both rooms and inside ducts and silencers to control noise.
Before trying to control the noise levels, it is important to quantify the background noise. According to Lewis, 2 engineers commonly use the noise criteria method defined by The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). There are other methods also including those defined by RC, dBA, NCB, and RC Mark II. Although all methods express measurements in decibels, knowledge of noise pressure level octave band spectrum is also important.
Lewis 2 suggests avoiding high-pressure drops anywhere in the ventilation system, as these not only cause noise regeneration of mid- and high frequencies but also cause loud low-frequency resonances. Most often, these are difficult or impossible to control.
Active noise control
The interior surface of ducts using resistive passive silencers usually has a covering of porous, sound-absorbing material. Wavelength of the incident sound primarily governs the noise attenuation, together with the size of the air passage, the length of the silencer, and the flow resistance and thickness of the sound-absorbing material.
However, in the lower frequency ranges, conventional passive silencers have very little effect on noise. This is because of low attenuation produced by passive silencers when the dimensions of the silencer are small compared to large acoustic wavelengths. Therefore, to be effective at low frequencies, the silencer dimensions also tend to be large and bulky, making them impractical for installation. This is where active noise control is more effective in controlling noise.
Although relying on adaptive digital signal processing, the active noise control technique may not achieve adequate levels of attenuation is the installation does not account for the physical factors degrading the system performance. According to Larsson, 3 these degrading factors can be removed by placing adequate emphasis on analysis and installation design. Such degrading factors include standing waves within the ducts, acoustic feedback between the reference microphone and the control loudspeaker, and flow-induced noise affecting the microphone signals.
Larsson 3 suggests the module-based approach when designing and installing active noise control systems. This approach installs microphones and loudspeakers in separate modules, basing them on standard duct parts, thereby reducing flow-induced noise. Based on measurements conducted in acoustic laboratory conforming to ISO standards, the attenuation an active noise control system produces is approximately 15–25 dB between 50 and 315 Hz, even when the airflow speed is close to 20 m/s.
According to Larsson, 3 an active noise control system applies the principle of destructive interference to attenuate noise. It actually produces a secondary sound field equal in amplitude, but totally out of phase with the original and primary noise field. This combination of the primary and secondary sound fields produces a residual sound field with decreased amplitude. The level of residual sound depends on the accuracy of the amplitude and phase of the secondary sound field generated.
The active noise control system generates the secondary sound field. An adaptive system is necessary to track and counter the changing nature of the primary noise over time. In its simplest form, a feed-forward single-channel active noise control system uses two microphones and a loudspeaker. One of the microphones acts as a reference for detecting the primary noise, while the other monitors the resultant residual sound field.
To be effective in larger enclosures and complex acoustic sound fields, active noise control systems usually employ a multi-channel system. This typically uses several reference sensors, multiple control sources, and a number of error sensors.
Applications of active noise control system
Larsson 3 offers two examples of applications of active noise control systems:
Airplanes use this system to reduce the noise inside the cabin that engine and propellers generate.
Duct applications use multi-channel active noise control systems to control the propagation of tonal components.
Murthy et al. 4 discuss the application of an active noise control system for reducing the noise from a radial fan. Ventilating systems usually have rotating fans that cause airflow in the ducts. Although the primary motive for the ventilation system is to provide comfort to its occupants within the building, the noise it generates reduces that largely. Therefore, attempts were made to reduce the noise using an active noise control system.
The active noise control system was applied to the radial fan positioned within an insulated box, and the combination sucked in air. Murthy et al. 4 made measurements on the power spectral density, sound pressure levels, and octave band measurements within the box, and inside the ducts leading to and from the box. The measurements furnished information on the frequency range of the generated noise. Using this information, the team was able to decide on the type of active noise control system necessary and the best position to install it for effective noise control.
Murthy et al. 4 conclude that a single-channel feed-forward active noise control system is adequate for reducing noise in a ventilation system where the duct has an airflow speed of 4–5 m/s. They were able to obtain attenuation between 20 and 30 dB for tonal components and 5 and 10 dB for broadband noise in the frequency range of 100–200 Hz.
Application of passive resistive silencers
The Paroc Group Oyj, Paroc, 5 offer an example of the industrial application of passive resistive silencers. They use PAROC InVent slabs for reducing noise in ducts where airflow is causing noise.
According to Paroc, 5 ventilation ducts usually carry sound and spread it from one room to another, while the insulation used outside the duct reduces the amount of noise breaking through the wall of the duct. Depending on the use case, a variety of slabs and wired mats are available to use as silencers for reducing the noise.
For instance, ducts carrying heated air are usually insulated when they pass through a cold area, such as the attic. Sound attenuation slabs also act as insulators. They can be used in place of regular insulators, thereby doing double duty and reducing noise while providing insulation.