Medical devices are allowing patients to live longer, healthier lives,
and remain in their homes when they're ill. But noise from medical devices is
one complaint often cited by patients. Noise, then, can be viewed as a product
defect, albeit not one that the Food & Drug Administration (FDA) would
recognize.
Among the growing list of home health care medical devices
increasingly coming to market, CPAP (Continuous Positive Airway Pressure) units
- a fairly simple device used to treat sleep apnea - are becoming more and more
common. Sleep apnea is a condition in which the individual fails to get enough
oxygen while asleep. The CPAP unit is profiled in this article to illustrate
how and when medical product designers and manufacturers should consider noise
in the design process. Devices to control sleep apnea, such as the CPAP, use a
hose and mask to deliver pressure to the respiratory tract, thereby preventing
oxygen deprivation.
As much as the snoring associated with sleep apnea might annoy a
spouse, CPAP devices are also a source of complaints. The two biggest
objections are the discomfort or claustrophobia of wearing a mask to bed and
the noise the unit makes. Noise complaints can simply take the form of "too
loud," but they can also involve the quality of the sound. When noise is
present, hissing, whining, clicking or tonal sounds are readily apparent and
disconcerting to the listener, especially since the device is typically placed
on a bedside table in an otherwise quiet environment.
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CPAP noise is a combination of air flow and motor noise. With another
sleep apnea device, known as BiPAP units (Bilevel Positive Airway Pressure),
valve clicking can also be audible. These noise sources can be minimized and
the quality of the overall sound enhanced; however, these goals are best
accomplished by including noise as a priority during the design process.
Targeting the Source
Good practices exist for designing both quiet products and products
with superior sound quality. A simplified approach is to picture noise as a
combination of a source mechanism, a transmission path and one or more surfaces
radiating sound. A typical CPAP device contains a motor, a fan, a flow path
within the unit, a hose and a mask fitted to the patient's face. The sources in
this case are the motor and the fan, both of which can create noise and
vibration. Two transmission paths exist: an airborne path, in which noise is
created by pressure fluctuations in the air itself, and a structure-borne path,
whereby vibration forces from the motor and/or fan excite vibration at the
mounting location. Vibrating surfaces can then radiate noise like small
loudspeakers. Radiating surfaces are most likely the flat outer panels that are
good radiators of sound, excited by the vibration traveling from the mounting
juncture.
For BiPAP
units, impacts from the valves follow a similar vibratory path through the
structure. The operation of the valves can also produce airborne noise
directly. In both types of devices, exhalation air near the mask can be a
source of continuous noise, while an ill-fitting mask can cause short duration
noise 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 electrical
excitation and mechanical components. DC brushless motors, reputed to be very
quiet, can create noise due to poor bearing tolerances and from
non-uniformities in their rotation created by the electrical excitation
waveform that drives the motor. The fan itself must be selected so it operates
near its point of maximum static efficiency. The fan impeller and shroud,
unless carefully designed, can cause pulsations at the rotation frequency and
especially at the blade passage rate. Blades themselves have structural
resonances that can be excited by the interaction of air moving across their
surfaces.
The airborne and structure-borne transmission paths must be considered
separately. In designing the airflow path, a balance is needed between a
circuitous path that hinders sound from leaving the unit, and a path of low
resistance that allows the fan to operate at a slow speed. The flow ducts can be
lined with FDA-approved sound-absorbing foam, but in order to be effective, the
foam 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 on
the device where an inlet or outlet silencer can be incorporated, should a
reduction in noise through the flow path be found to be beneficial once a
prototype is built and tested.
In comparison with the flow path, the transmission of vibration
through the structure can be much more difficult to control, particularly as
the market pushes medical devices to be smaller and lighter. In a unit such as
a CPAP device, the motor and fan will have little mass, making isolation at
lower frequencies (for example 60 Hz) difficult, due to the required softness
of the supporting isolators. Lightweight structures often lack stiff
attachments points, which are needed to design an effective isolator.
Structural ribbing at mounting locations can be used to stiffen attachments
points, and new moldable foams can provide a distributed spring which makes
local housing stiffness less important.
When noise reduction is examined according to its various elements - noise
source, transmission path and radiating surface - it becomes clear that
factoring these into the design process in its early stages offers the best
opportunity to produce a quiet product. Acoustical noise and vibration
consultants can assist designers and manufacturers with noise reduction on a finished
product; however, after the design is complete and the unit has gone into
production, the noise control options become much more limited.
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