George Maling, Noise Control Center and David M. Yeager, Motorola Corp

Introduction

Air-moving devices (AMDs), such as fans and blowers used for cooling in electronic systems invariably generate acoustical noise which must be considered if people are exposed to the emitted noise. In this article, we discuss quantitative measures for the description of noise, design guidelines for the selection of low noise air movers, upper limits for the noise emissions of computer and business equipment and measurement procedures which may be used to evaluate the noise radiated by electronic equipment.

Important Terms

Sound pressure and sound power are two important terms in acoustics and noise control engineering. A complete glossary of terms in noise control engineering is available [1].

Sound pressure is the rms value of the small variations above and below atmospheric pressure that constitute a sound wave. The human ear is sensitive to sound pressure, and it is the quantity measured by a sound level meter. Sound power is the energy per unit time emitted by a sound source and is a good measure of noise source emission. It is usually calculated from measurements of the sound pressure taken around a source. The sound pressure level and sound power level are corresponding logarithmic quantities. The unit of sound pressure level is the decibel, and the unit of sound power level is either the decibel or the bel [1].

A hemi-anechoic room is a special test room which has a hard floor, while other room surfaces are highly absorbent of sound. Such a test room is useful for measurement of sound pressure levels and determination of sound power levels.

All of the surfaces in a reverberation room are highly reflective of sound, thus making it useful for determining of sound power levels.

Measures of Noise Emission

The sound power level [1,2] is widely used to characterize the noise emissions of electronic equipment such as computers and business equipment. While the sound pressure level at a given position is easily measured with a standard sound level meter, the sound power level is highly dependent on the position of the microphone and the acoustical characteristics of the room in which the equipment operates. For these reasons, standard methods have been developed [3-5] for determination of the sound power level of both air movers and complete systems.

From an acoustical viewpoint, the most important consideration in air-cooled systems is the selection of one or more AMDs to provide cooling air. The type of AMD used for cooling depends on the volumetric air flow required and the static pressure rise across the AMD required to force air through the system. General design guidelines can, however, be given for the selection of AMDs.

Design Guidelines

The following guidelines should prove to be useful in the design of cooling systems for electronic equipment.

1. Choose the thermal design point appropriately, taking proper account of thermal and acoustical effects. Small changes in electronic case temperature requirements can result in dramatic changes in AMD noise emission levels. A system designed to withstand environmental extremes (room temperature and air density) can usually benefit considerably from an adaptive cooling design in which AMD rotational speed is controlled by an on-board controller and thermal sensor. In many cases, an adaptively-cooled system can provide better thermal protection under extreme conditions while having reduced noise emissions under more "typical" conditions.

2. Design the system to be cooled to have the lowest possible static pressure rise for the required air flow. A low static pressure rise indicates that the AMD can operate at a low tip speed, resulting in a low noise level. The static pressure rise across a system is caused by several sources of resistance, such as the devices being ventilated and finger guards which may be required for safety. If unnecessary sources of resistance can be eliminated, the air flow will increase. It should then be possible to reduce the tip speed of the device to obtain the desired air flow at a lower noise level.

3. Select the operating point for a centrifugal blower so that it operates near its point of maximum static efficiency, considering the required air flow rate and the pressure drop through the system. Operation away from the point of maximum static efficiency should be in the direction of lower static pressure rise and higher air flow.

4. Select a point of operation of a fan that is away from the best efficiency point in the direction of higher air flow and lower static pressure rise. Small fans are often unstable when operated at air flow rates less than the air flow rate at the best efficiency point. They are often very noisy under conditions of high static pressure rise and low air flow rate.

5. Select a fan or blower with a low sound power level and avoid AMDs that have high level peaks in their one third-octave-band sound power spectrum. Such peaks usually indicate the presence of discrete frequency tones in the spectrum. Such tones can be difficult to eliminate and generally are a source of annoyance.

6. Select a fan or blower having the lowest speed and largest diameter consistent with the other requirements.

7. Minimize system noise levels by designing the system so that obstructions are not present within one fan diameter of the inlet to axial-flow fans [6], so that the airflow into the inlet of axial-flow fans is as spatially uniform as possible. Avoid the direct attachment of the AMD to lightweight sheet metal parts.

8. Mount axial-flow fans so that the air-flow direction is towards the equipment being cooled. Pulling air over equipment being cooled usually causes undesirable turbulence at the fan inlet and produces an increase in noise level.

Limit Values for Sound Power Levels

The wide variety of electronic equipment cooled by AMDs makes it impossible to specify a set of limit values for sound power levels which are applicable to all equipment. However, there are two sets of requirements which have been adopted in Europe for data processing equipment. In Germany, the German Institute for Quality Assurance (RAL) has been authorized to grant the environmental logo Blue Angel to certain products, including workplace computers and workstations consisting of a system unit, keyboard, and monitor. These environmental requirements are mostly concerned with recycling; there are, however, acoustical requirements specified. A "non-official" translation of the acoustical requirements into the English language reads:

"In accordance with section 3.2.5 of ISO 9296 [7], the "declared sound power level, L_WAd, of the components, measured when idling and multiplied by 10, must not be more than 48 dB(A). In other conditions of operation (access to diskette or hard disk) the maximum value must not exceed 55 dB(A). The measurements are to be performed in accordance with DIN EN 2 7779."

The multiplication by 10 mentioned in the above paragraph is required for the conversion from bels (as specified in ISO 9296) to decibels. The measurements are to be made by an independent "test house", or by an applicant whose test facilities and procedures have been certified by an independent certification body in accordance with a European standard EN 2 9000 or ISO 9000.

In Sweden, Statskontoret Technical Standard 26:3 specifies recommended upper limits for the noise emissions of computers and business equipment. These requirements are summarized in Table 1.

Table 1: Swedish recommended upper limits for declared sound power level values. Technical standard 26:3. Statskontoret, Swedish Agency for Administrative Development, Stockholm, Sweden. First day of validity: 1993-05-01. In case of conflict, the Swedish text prevails over the English text in the table.

Noise Measurement for Personal Computers

National and international standards have been developed for determination of the sound power level emitted by machinery and equipment [2]. The computer and business equipment industry has developed procedures which can be adapted for the measurement of the noise emissions of a wide variety of electronic equipment. While it is possible to determine sound power in ordinary rooms through the use of sound intensity [2], it is more common to install the equipment in a special test room, a hemi-anechoic room or a reverberation room. When one of these facilities is available, the following seven-step procedure is useful for determination of sound power level.

1. Install the personal computer system in test chamber. The test chamber may be a hemi-anechoic room or a reverberation room. Install the personal computer in accordance with ISO 7779 on the floor of a hemi-anechoic chamber or a reverberation room for determination of sound power levels.

2. Calibrate the measurement system and check the frequency response. Calibration and frequency response should be checked on a weekly basis, or just prior to and after a set of measurements is completed.

3. Measure the background sound pressure and determine the background sound power level. These measurements are made with the personal computer system turned off.

4. Power up and warm up the equipment so that all initialization is complete and it is operating in the steady state before acquiring any data.

5. Determine the personal computer sound power levels in idle and operating modes. See ISO 7779, Annex C, for further details of modes of operation.

6. Measure the personal computer sound pressure levels in idle and operating modes. Desktop units should be placed on a standard test table (see ISO 7779, Annex A). With the personal computer in the same modes of operation as in step 4 above, the by-stander and operator position sound pressure levels should be measured. Although sound power can be determined in either a reverberation room or a hemi-anechoic chamber, sound pressure determinations must be done in the latter (see ISO 7779, section 7).

7. Perform a discrete tone analysis at the operator position. The existence of discrete frequency tones in the spectrum often requires that an additional measurement be performed to assess the prominence of the tone(s). Tonal prominence is related to annoyance and the psychoacoustics of how discrete tones are perceived in the presence of noise. Annex D of ISO 7779 specifies the detailed procedure.

Conclusion

Designers of electronic systems cooled by air must always be conscious of the acoustical noise emitted by the system if people are exposed to the noise. The three elements required for a low noise design have been described in this article, selection of air moving devices and design of systems for low noise, quantitative limits on the noise emissions of equipment, and measurement procedures which can be used to determine the sound power level of the equipment.

Footnote: The material in this article is taken from a chapter in the Handbook of Thermal Measurements in Electronics Cooling, by K. Azar, CRC Press, 1996.

References
1.Glossary of terms used in noise control engineering, Noise/News International, 3, 161-168, 1995.
2.Beranek, L.L. and Ver, I., Editors, "Noise and vibration control engineering", John Wiley & Sons, Inc., New York, 1992, Chapter 4 , Determination of sound power levels and directivity of noise sources.
3.ISO 10302, Acoustics - Method for the measurement of noise emitted by small air-moving devices, International Organization for Standardization, Geneva, Switzerland, 1995.
4.ISO 3744: Acoustics - Determination of sound power levels of noise sources - Free field conditions over a reflecting plane, International Organization for Standardization, Geneva, Switzerland.
5.ISO 7779, Acoustics - Measurement of airborne noise emitted by computers and business equipment, International Organization for Standardization, Geneva, Switzerland, 1988.
6.Washburn, K.B. and Lauchle, G.C. 1988. Inlet flow conditions and tonal sound radiation from a subsonic fan, Noise Control Eng. J., 31, 101-110.
7.ISO 9296, Acoustics - Declared noise emission values of computer and business equipment, International Organization for Standardization, Geneva, Switzerland, 1988

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