Minimizing acoustical noise in electronic systems

Introduction

Air-moving devices (AMDs), such as fans and blowers used for cooling inelectronic systems invariably generate acoustical noise which must be consideredif people are exposed to the emitted noise. In this article, we discussquantitative measures for the description of noise, design guidelines for theselection of low noise air movers, upper limits for the noise emissions ofcomputer and business equipment and measurement procedures which may be used toevaluate the noise radiated by electronic equipment.

Important Terms

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

Sound pressure is the rms value of the small variations above and belowatmospheric pressure that constitute a sound wave. The human ear is sensitiveto sound pressure, and it is the quantity measured by a sound level meter. Soundpower is the energy per unit time emitted by a sound source and is a goodmeasure of noise source emission. It is usually calculated from measurements ofthe sound pressure taken around a source. The sound pressure level and soundpower level are corresponding logarithmic quantities. The unit of sound pressurelevel is the decibel, and the unit of sound power level is either the decibel orthe bel [1].

A hemi-anechoic room is a special test room which has a hard floor, whileother room surfaces are highly absorbent of sound. Such a test room is usefulfor measurement of sound pressure levels and determination of sound powerlevels.

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 noiseemissions of electronic equipment such as computers and business equipment.While the sound pressure level at a given position is easily measured with astandard sound level meter, the sound power level is highly dependent on theposition of the microphone and the acoustical characteristics of the room inwhich the equipment operates. For these reasons, standard methods have beendeveloped [3-5] for determination of the sound power level of both air moversand complete systems.

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

Design Guidelines

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

1. Choose the thermal design point appropriately, taking properaccount of thermal and acoustical effects. Small changes in electronic casetemperature requirements can result in dramatic changes in AMD noise emissionlevels. A system designed to withstand environmental extremes (room temperatureand air density) can usually benefit considerably from an adaptive coolingdesign in which AMD rotational speed is controlled by an on-board controller andthermal sensor. In many cases, an adaptively-cooled system can provide betterthermal protection under extreme conditions while having reduced noise emissionsunder more “typical” conditions.

2. Design the system to be cooled to have the lowest possible staticpressure rise for the required air flow. A low static pressure rise indicatesthat the AMD can operate at a low tip speed, resulting in a low noise level. Thestatic 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 forsafety. If unnecessary sources of resistance can be eliminated, the air flowwill increase. It should then be possible to reduce the tip speed of the deviceto obtain the desired air flow at a lower noise level.

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

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

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

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

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

8. Mount axial-flow fans so that the air-flow direction is towardsthe equipment being cooled. Pulling air over equipment being cooled usuallycauses undesirable turbulence at the fan inlet and produces an increase in noiselevel.

Limit Values for Sound Power Levels

The wide variety of electronic equipment cooled by AMDs makes it impossibleto specify a set of limit values for sound power levels which are applicable toall equipment. However, there are two sets of requirements which have beenadopted in Europe for data processing equipment. In Germany, the GermanInstitute for Quality Assurance (RAL) has been authorized to grant theenvironmental logo Blue Angel to certain products, including workplace computersand workstations consisting of a system unit, keyboard, and monitor. Theseenvironmental 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 “declaredsound power level, L_WAd, of the components, measured when idling andmultiplied by 10, must not be more than 48 dB(A). In other conditions ofoperation (access to diskette or hard disk) the maximum value must not exceed 55dB(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 forthe conversion from bels (as specified in ISO 9296) to decibels. Themeasurements are to be made by an independent “test house”, or by anapplicant whose test facilities and procedures have been certified by anindependent certification body in accordance with a European standard EN 2 9000or ISO 9000.

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

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

Noise Measurement for Personal Computers

National and international standards have been developed for determinationof the sound power level emitted by machinery and equipment [2]. The computerand business equipment industry has developed procedures which can be adaptedfor the measurement of the noise emissions of a wide variety of electronicequipment. While it is possible to determine sound power in ordinary roomsthrough the use of sound intensity [2], it is more common to install theequipment 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 isuseful for determination of sound power level.

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

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

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

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

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

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

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

Conclusion

Designers of electronic systems cooled by air must always be conscious ofthe 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 thisarticle, selection of air moving devices and design of systems for low noise,quantitative limits on the noise emissions of equipment, and measurementprocedures which can be used to determine the sound power level of theequipment.

Footnote: The material in this article is taken from achapter 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 controlengineering”, 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 bysmall air-moving devices, International Organization for Standardization,Geneva, Switzerland, 1995.
4.ISO 3744: Acoustics – Determination of sound power levels of noisesources – Free field conditions over a reflecting plane, InternationalOrganization for Standardization, Geneva, Switzerland.
5.ISO 7779, Acoustics – Measurement of airborne noise emitted by computersand business equipment, International Organization for Standardization, Geneva,Switzerland, 1988.
6.Washburn, K.B. and Lauchle, G.C. 1988. Inlet flow conditions and tonalsound radiation from a subsonic fan, Noise Control Eng. J., 31, 101-110.
7.ISO 9296, Acoustics – Declared noise emission values of computer andbusiness equipment, International Organization for Standardization, Geneva,Switzerland, 1988