Noise Reduction in Home Health Care Product DesignNoise Reduction in Home Health Care Product Design

Medical devices are allowing patients to live longer, healthier lives,and remain in their homes when they're ill. But noise from medical devices isone complaint often cited by patients. Noise, then, can be viewed as a productdefect, albeit not one that the Food & Drug Administration (FDA) wouldrecognize.

Among the growing list of home health care medical devicesincreasingly coming to market, CPAP (Continuous Positive Airway Pressure) units- a fairly simple device used to treat sleep apnea - are becoming more and morecommon. Sleep apnea is a condition in which the individual fails to get enoughoxygen while asleep. The CPAP unit is profiled in this article to illustratehow and when medical product designers and manufacturers should consider noisein the design process. Devices to control sleep apnea, such as the CPAP, use ahose and mask to deliver pressure to the respiratory tract, thereby preventingoxygen deprivation.

As much as the snoring associated with sleep apnea might annoy aspouse, CPAP devices are also a source of complaints. The two biggestobjections are the discomfort or claustrophobia of wearing a mask to bed andthe noise the unit makes. Noise complaints can simply take the form of "tooloud," but they can also involve the quality of the sound. When noise ispresent, hissing, whining, clicking or tonal sounds are readily apparent anddisconcerting to the listener, especially since the device is typically placedon a bedside table in an otherwise quiet environment.

CPAP noise is a combination of air flow and motor noise. With anothersleep apnea device, known as BiPAP units (Bilevel Positive Airway Pressure),valve clicking can also be audible. These noise sources can be minimized andthe quality of the overall sound enhanced; however, these goals are bestaccomplished by including noise as a priority during the design process.

Targeting the Source

Good practices exist for designing both quiet products and productswith superior sound quality. A simplified approach is to picture noise as acombination of a source mechanism, a transmission path and one or more surfacesradiating sound. A typical CPAP device contains a motor, a fan, a flow pathwithin the unit, a hose and a mask fitted to the patient's face. The sources inthis case are the motor and the fan, both of which can create noise andvibration. Two transmission paths exist: an airborne path, in which noise iscreated by pressure fluctuations in the air itself, and a structure-borne path,whereby vibration forces from the motor and/or fan excite vibration at themounting location. Vibrating surfaces can then radiate noise like smallloudspeakers. Radiating surfaces are most likely the flat outer panels that aregood radiators of sound, excited by the vibration traveling from the mountingjuncture.

For BiPAPunits, impacts from the valves follow a similar vibratory path through thestructure. The operation of the valves can also produce airborne noisedirectly. In both types of devices, exhalation air near the mask can be asource of continuous noise, while an ill-fitting mask can cause short durationnoise bursts as the air pressure momentarily lifts the mask from the face,creating a small opening similar to releasing air from the neck of a balloon.

Understanding the Cause

Source noise can have various exciting mechanisms including electricalexcitation and mechanical components. DC brushless motors, reputed to be veryquiet, can create noise due to poor bearing tolerances and fromnon-uniformities in their rotation created by the electrical excitationwaveform that drives the motor. The fan itself must be selected so it operatesnear its point of maximum static efficiency. The fan impeller and shroud,unless carefully designed, can cause pulsations at the rotation frequency andespecially at the blade passage rate. Blades themselves have structuralresonances that can be excited by the interaction of air moving across theirsurfaces.

The airborne and structure-borne transmission paths must be consideredseparately. In designing the airflow path, a balance is needed between acircuitous path that hinders sound from leaving the unit, and a path of lowresistance that allows the fan to operate at a slow speed. The flow ducts can belined with FDA-approved sound-absorbing foam, but in order to be effective, thefoam will, by necessity, occupy a substantial portion of the interior volume.In some medical devices, designers have set aside a small piece of real estate onthe device where an inlet or outlet silencer can be incorporated, should areduction in noise through the flow path be found to be beneficial once aprototype is built and tested.

In comparison with the flow path, the transmission of vibrationthrough the structure can be much more difficult to control, particularly asthe market pushes medical devices to be smaller and lighter. In a unit such asa CPAP device, the motor and fan will have little mass, making isolation atlower frequencies (for example 60 Hz) difficult, due to the required softnessof the supporting isolators. Lightweight structures often lack stiffattachments points, which are needed to design an effective isolator.Structural ribbing at mounting locations can be used to stiffen attachmentspoints, and new moldable foams can provide a distributed spring which makeslocal housing stiffness less important.

When noise reduction is examined according to its various elements - noisesource, transmission path and radiating surface - it becomes clear thatfactoring these into the design process in its early stages offers the bestopportunity to produce a quiet product. Acoustical noise and vibrationconsultants can assist designers and manufacturers with noise reduction on a finishedproduct; however, after the design is complete and the unit has gone intoproduction, the noise control options become much more limited.

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