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.

By Bradley N. Rohloff, PE, LEED AP, Harley Ellis Devereaux, Southfield, Mich. July 17, 2015

Learning objectives

  • 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.

Sound pressure and sound power

The sounds we hear are caused by sound pressure. It is due to fluctuation in atmospheric pressure that acts on the human eardrum. Sound pressure is dependent on the surroundings, therefore making it difficult to measure sound levels of the equipment, or sound power.

The sound power and sound pressure relationship can be described with an example. Consider a 5-hp fan motor. A 5-hp motor is a definable measure that can be used to compare one motor against another. This is the equivalent of sound power. However, it is not possible to know whether a 5-hp fan is sufficient to provide cooling/ventilation unless the temperature of the space is known. The temperature of the space is equivalent of sound pressure. If the fan is providing ventilation/cooling for a 2,000-sq-ft building, it will most likely be large enough to provide a comfortable temperature. If the fan with the 5-hp motor is used in a 130,000-sq-ft space, it is unlikely to provide comfort to the occupants. In each situation, the same size fan (sound power) provides very different results (sound pressure level).

Room criteria (RC)

ASHRAE’s favored method for determining sound levels is by room criterion, or RC, curves. The RC curve system was developed to overcome the shortcomings of the noise criteria (NC) system. The RC system adds the 16.5- and 31-Hz bands to deal with low-frequency sound and the 800-Hz octave band is dropped from the NC curves. The RC curve accounts for acoustically produced vibration in light building construction. In this article, we will address noise with the RC method described herein (see Figure 2).

Noise criteria (NC)

Noise criteria, or NC, curves are the most common standard in the United States for rating indoor noise from HVAC systems. NC curves were developed to take into account human response to sound pressure levels at different octave bands. NC curves are based on the 63- to 8,000-Hz octave-band values. The criteria curves define the limits of the octave band that must not be exceeded to meet occupant acceptance in a space. One issue with the NC method is that it does not evaluate low-frequency sounds below 63 Hz, which can be the most troublesome and most difficult to attenuate (see Figure 3).

Basic acoustical design techniques

Based on the 2007 ASHRAE Handbook–HVAC Applications, when selecting fans, pumps, and other related mechanical equipment and when designing air-distribution systems to minimize sounds transmitted from system components to occupied spaces, engineers should consider the following:

  • Design the air-distribution system to minimize flow resistance and turbulence. High flow resistance increases required fan pressure, which results in higher noise being generated by the fan, especially at low frequencies. Turbulence also increases flow noise generated by duct fittings and dampers, especially at low frequencies.
  • Select a fan to operate as near as possible to its rated peak efficiency when handling the required airflow and static pressure. Also, select a fan that generates the lowest possible noise at required design conditions. Using an oversized or undersized fan that does not operate at or near rated peak efficiency can substantially increase noise levels.
  • Design duct connections at both the fan inlet and outlet for uniform and straight airflow. Both turbulence (at fan inlet and outlet) and flow separation at the fan blades can significantly increase fan-generated noise. Also, turning vanes near fan outlets can also increase turbulence and noise, especially if airflow is not sufficiently uniform.
  • Select duct silencers that do not significantly increase the required fan total static pressure. Selecting silencers with static-pressure losses of 0.35 in. of water or less can minimize regenerated noise from silencer airflow.
  • Place fan-powered mixing boxes associated with variable-volume air-distribution systems away from noise-sensitive areas.
  • Minimize flow-generated noise by elbows or duct branch takeoffs whenever possible by locating them at least 4 to 5 duct diameters from each other. For high-velocity systems, it may be necessary to increase this distance to up to 10 duct diameters in critical noise areas. Using flow straighteners or honeycomb grids, often called “egg crates,” in the necks of short-length takeoffs that lead directly to grilles, registers, and diffusers is preferred to using volume extractors that protrude into the main duct airflow.
  • Keep airflow velocity in ducts serving sound-sensitive spaces a low as possible by increasing the duct size to minimize turbulence and flow-generated noise.
  • Duct transitions should not exceed an included expansion angle of 15 deg, or the resulting flow separation may produce rumble noise.
  • Use turning vanes in large 90 deg rectangular elbows and branch takeoffs. This provides a smoother directional transition, thus reducing turbulence.
  • Place grilles, diffusers, and registers into occupied spaces as far as possible from elbows and branch takeoffs.
  • Minimize use of volume dampers near grilles, diffusers, and registers in acoustically critical situations.
  • Use barriers near outdoor equipment when noise associated with the equipment will disturb adjacent properties. In normal practice, barriers typically produce no more than 15 dB of sound attenuation in the medium frequency range. To be effective, the noise barriers must at least block the direct line of sight between the source and receiver.
  • Vibration-isolate all reciprocating and rotating equipment connected to a structure. Also, it is usually necessary to isolate mechanical equipment in a basement, directly below a tenant space. It may be necessary to use flexible piping connections and flexible electrical conduit connections for pipes and ducts connected to the equipment.
  • Vibration-isolate ducts and pipes, using spring and or neoprene hangers for at least the first 50 feet from vibration-isolated equipment.

Table 1 in the 2007 ASHRAE Handbook—HVAC Applications lists several common sound sources associated with mechanical equipment noise. Anticipated sound-transmission paths and recommended noise-reduction methods are also listed. Airborne and/or structure-borne sound can follow any or all of the transmission paths associated with a specified sound source

Bradley N. Rohloff is an associate in mechanical engineering at Harley Ellis Devereaux.