Solving noise, vibration problems in HVAC systems
Vibration, sound, and noise can be mitigated by designing HVAC systems properly, which may include isolation pads, flexible pipe connectors, or other means.
- Understand the factors that may cause noise or vibrations in HVAC systems.
- Learn various methods to mitigate vibration, sound, or noise problems.
In the HVAC industry, most sound or noise is generated via rotating equipment and air and fluid movement through ducts and pipes. This movement creates vibration, sound, or noise. Technically, sound is a wave of mechanical energy that moves through matter. Noise is undesirable sound or sound without value. In this discussion, we will use sound and noise synonymously.
Vibration in its simplest form can be considered an oscillation or repetitive motion of an object around an equilibrium position. In the HVAC industry, sound is not generated without some form of vibration from equipment. Although sound is not present without vibration, there can be vibration without sound noticeable to the human ear.
Therefore, the best way to reduce sound is to limit the vibration produced by mechanical equipment. Examples are rotating shafts or gears, thermal processes such as combustion, or fluid dynamic means such as airflow through a duct or fan interactions with air.
Understanding vibration and sound
Control of HVAC system sound and vibration are of equal importance, but measurement of vibration is often not necessary to determine sources or transmission paths of unwanted sound or noise. Because vibration is the source of noise from HVAC systems, management of those conditions is imperative to a quiet design. System design that neglects to properly address vibration may result in malfunctioning components, noise, and, in some cases, catastrophic failure.
There are two facets of vibration management: isolation and damping. Isolation is the prevention of vibration from entering the system and dissipating it by changing kinetic energy of vibration into a different form of energy, such as heat.
Vibration isolation systems for mechanical components require some amount of damping. Damping dissipates mechanical energy from the system and attenuates vibrations more quickly. Without damping, these systems may vibrate for some time before coming to rest. The fluid in automotive shock absorbers is a kind of damper, as is the inherent damping in elastomeric (rubber) equipment mounts described below. This energy is converted to heat in the shock absorber or rubber mounts. There are also pads made of neoprene or cork used in equipment mounting that can be identified as damping devices. These two forms of vibration management are different from each other, but are often used in conjunction with each other to achieve the desired performance. In this discussion, vibration damping will generally fall under the category of vibration isolation.
Sound vibrations act on the inner ear as “pressure waves,” which our mind translates as music or someone’s voice. The amplitude of the sound wave represents the loudness and is measured in decibels. The louder the sound, the larger the amplitude or decibels (see Figure 1).
A- and C-weighted sound pressure
A-weighted sound pressure (measured in decibels; dBA) has been used for 60 years as a single-number measure of the relative loudness of noise, specifically for outdoor environmental noise standards. It is popular because it is a single number that most sound meters include. A-weighted is “corrected” to more closely resemble the hearing characteristics of the human ear. The C-weighted curve (dBC), which is more sensitive to low-frequency sound, contributes less to the overall sound level than dBA. The human ear has a relatively poor sensitivity to low-frequency sound in the 20- to 30-dBA range.
When attenuating sound for an outdoor installation of an air-cooled chiller, the manufacturer-supplied decibel rating would be compared to design decibel level at the design distance from the source. If the design level is exceeded, attenuation will be required. Acoustic screen walls or manufacturer-supplied attenuation enclosures can be added, or relocating the chiller farther from the sound-sensitive area may be the answer.