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Acoustic Noise Emission
Since chip dissipation levels and board densities are projected to increase,
it is clear that system dissipation levels will continue to rise well into the
next century. Unfortunately, there are no available projections for system level
dissipation growth rates. However, given the chip and board level trends discussed
above, it would be conservative to say that the dissipation levels will grow
by a factor of two in the next five-to-ten years. This would result in a 1.5
bel (15 dB) rise in noise emission level. At the other extreme, a four-fold
increase in dissipation would result in a 3 bel (30 dB) rise in level.
In general, system noise level increases of 1-3 bels would
be problematic. First, the increased noise would adversely impact many people,
and purchasers of the equipment could be expected to require manufacturers to
reduce noise emission. Second, many systems would no longer be in compliance
with national and international noise emission/immission requirements. If such
a scenario unfolds, noise emission will become a primary design issue, and significant
effort will be needed to prevent noise emission from becoming a limit on overall
system performance. The first systems where problems will occur will be those
where throughput demand is growing the fastest.
The trends discussed above apply primarily to equipment intended for indoor
installation. The situation for outdoor equipment is less clear. One problem
is that well-defined sets of outdoor noise emission limits do not exist, leaving
designers without appropriate benchmark information. ETSI and other standards
bodies are attempting to create a set of limits, but there are difficult technical
and political obstacles to overcome. A second problem is that there is very
little outdoor-product noise-emission data currently available. Thirdly, wireless
telecom equipment will be the dominant equipment type to be deployed outdoors
in the near term, and the designs are rapidly changing.
While these problems make it difficult to create a rational
prediction regarding outdoor equipment, there needs to be a general recognition
that when limits are set, they could be quite low. The primary concern that
will drive the limits down will be the possibility of installations on (or near)
apartment buildings, hospitals and other dwellings. As wireless cell sizes decrease,
the likelihood that equipment will be installed near sleeping areas will increase.
In the noise control arena, sleep interference problems generate the most vehement
responses. If the equipment radiates levels that are above nighttime ambient
sound pressure levels (typically 20-40 dB below daytime levels in densely populated
areas), there will be a steady stream of complaints. If the radiated noise contains
prominent tones, the tolerance will be even lower.
Issues such as these add a level of complexity for regulatory
bodies and manufacturers since wide variation in installation environments must
be considered. It is unclear how these issues will be resolved.
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