subject to small errors or unknowns. Paragraph
85 in the N&V manual discusses this situation as
document is given in paragraph 3-4d of the N&V
it relates to the quality of the final answer. In sum-
manual. The use of approved ear plugs or ear muffs
mary, it states that the data and procedures have
is mandatory for personnel in engine rooms during
been found to produce satisfactory results in many
engine operation. Signs specifying the use of hear-
different situations and applications, but that un-
ing protection devices should be placed at each en-
usual circumstances statistically can produce unex-
trance to the engine room. Typically, well-fitted
pected results. Unexpected results can be avoided
ear plugs or ear muffs have insertion loss values of
or minimized by encouraging a slightly conserva-
about 15 to 20 dB in the 63- to 250-Hz bands, rising
tive approach in acoustical designs. Design decision
with frequency to about 25 to 35 dB in the 1000- to
arising out of the use of several of the data forms
8000-Hz bands. Poorly fitted devices may have
(app. A) are often based on the following four cate-
only 10 to 15 dB insertion loss values. When used
gories used to describe the relative reliability or
in series, ear plugs plus ear muffs can increase the
confidence level of the acoustical design. The de-
IL by about 10 dB over that of either ear plugs or
signer should weigh carefully the applicability of
ear muffs alone.
these four categories to any particular evaluation.
3-10. Nondisturbing warning and paging
a. "Preferred". The design equals or surpasses
systems.
the requirements of the analysis in all frequency
bands.
Outdoor audible paging systems are frequently an-
noying to neighbors. Indoor paging or warning sys-
b. "Acceptable".
The design produces no more
tems frequently are so loud that they contribute to
than the following noise excesses above the design
the hearing damage problem, or they may be so
goal: 4 dB in the 31-, 63-, and 125-Hz bands, 3 dB
quiet that they cannot be heard in a noisy engine
in the 250-Hz band, or 2 dB in all the higher fre-
room. Consideration should be given to the use of
quency bands.
one or more of the following nondisturbing warning
c. "Marginal". The design produces one or more
or paging systems: flashing lights (possibly coded
of the following noise excesses above the design
to convey special meanings), "walkie-talkies" for
goal, in any or all frequency bands: 5 to 7 dB in the
outdoor personnel, "beeper" paging systems for
31-, 63-, and 125-Hz bands, 4 to 6 dB in the 250-Hz
outdoor or indoor personnel, limited power and
band, or 3 to 5 dB in all the higher frequency
directivity for outdoor loudspeakers, and automatic
bands.
shut-off of outdoor paging systems at nighttime.
d. "Unacceptable". The design produces noise
excesses above the design goal that are higher in
311. Quality of analysis procedure.
any frequency band than those values listed for
A detailed acoustical evaluation brings together
"marginal" in c above. It is strongly recommended
that an "unacceptable" design not be permitted.
large amounts of data, each component of which is
31 6
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