Designing Low Noise Cooling Systems

Cooling systems for larger electronic equipment dissipating several kilowatts of electrical power naturally emit more noise than those of smaller equipment. Comparison, however, of the noise of cooling systems in equipment with comparable electrical power dissipation shows a spread in noise levels exceeding 20 dB(A). Since there is little difference in the noise level of fans and blowers of similar design, the spread in system noise must be due mainly to system design. Some design considerations are discussed below.

Air Flow, Distribution, and System Pressure

The greater the air flow and pressure of a given system, the greater its noise. Total air flow can often be minimized by carefully distributing flow. This is done by placing circuit boards with highest power levels where the air stream has highest velocity. Baffles can be used to concentrate air flow on higher power devices and heat sinks to improve transfer of heat to the air stream. Pressure can be minimized by controlling the spacing of circuit boards and avoiding changes in air flow direction. Air inlet and exhaust ports must not be constricted.

Air Mover Geometry, Speed, and Mounting

After fans/blowers with obvious design deficiencies (e.g., upstream motor mount struts, venturi discontinuities) are rejected, there is little difference in noise among fans of similar size with similar flow-pressure (performance) characteristics. Because acoustical noise is primarily a function of fan or blower speed, a large air mover running at a low speed will emit far less noise for the same flow and pressure, than a smaller one at high speed.

• In multiple fan applications, lower noise will be achieved by using a small number of large fans rather than a large number of smaller ones.
  
  
• Poorly designed mounting plates serve as amplifiers of solid- borne motor noise.
  
  
• Any close obstruction upstream of a fan will increase noise. Tolerable size and distance from blade to obstruction depend on the size and speed of the fan.

For least noise, an air mover should be selected and its speed set such that it will provide the required air flow and pressure at about 70% of free air flow on its performance curve. Choosing a high pressure air mover (blower) in a low pressure application will result in significantly greater noise. Conversely, a low pressure air mover (fan) heavily loaded in a high pressure application will increase noise.

For more information: see Cooling System Design Tips.

ADAPTIVE COOLING

A very powerful means of controlling noise, as well as temperature, is with a SmartFan speed controller. Under normal conditions air movers run at low speeds, reducing noise levels by as much as 15 dB(A). Full cooling capacity is available and automatically applied as needed.

How Much Noise?

Many factors affect noise exposure and annoyance such that it is not possible to set simple limit levels for all computer and business equipment. Some factors are entirely determined by product design while others are beyond the designer’s control. Some of the most important factors are discussed below, followed by a table with suggested maximum noise levels.

• Proximity is important for equipment placed closer than 3 feet to an operator. Beyond that distance in typical office environments, the reverberant field dominates. An operator is more tolerant of noise, such as that from a keyboard, over which the operator has direct and immediate control. Because cooling system noise is not immediately controllable, it is less tolerable.
  
  
• Tones and impulsive noise are more objection-able than steady and random noises. Noises which continue through the work day are more annoying than those lasting for only an hour or two.
  
  
• A private office or conference room must be held to a much lower noise level than an open office. Higher levels are acceptable in computer machine rooms.
  
  
• The absorption characteristics of typical offices, machine rooms, and other spaces are generally known and can be taken into account in product design.
  
  
• When it is known that several machines will share an office or machine room space, the installation density (number of square feet of floor area allotted to each) should be considered.