engine manufacturers are represented in the data.
assumed to include also the auxiliaries and append-
At the time of the noise measurements, about 55
ages connected to the engine. For small engines,
percent of the engines were in the age bracket of O
the air intake noise is taken as a part of the casing
to 3 years, 32 percent were in the age bracket of 3
noise since it is relatively small and close to the en-
gine and would be difficult to separate, acoust-
to 10 years, and 13 percent were over 10 years old.
ically, from engine noise. For larger engines, in-
b. Objective: noise prediction. The purpose of
take noise is easily separated from casing noise if
the study was to collect a large quantity of noise
the inlet air is ducted to the engine from some re-
data on a broad range of engines and to set up a
mote point. Most large engines are turbocharged;
noise prediction scheme that could fairly reliably
that is, the inlet air to the engine is pressurized to
predict the noise level of any engine, on the basis
obtain higher performance. A typical turbocharger
of its design and operating conditions. This predic-
is a small turbine in the intake path that is driven
tion method could then reapplied to any engine in
by the high-pressure exhaust from the engine. Spe-
an installation, and its noise could be estimated and
cial blowers are sometimes used to increase the
taken into account in setting up the design for the
pressure and airflow into the engine. In d, e, and f
facility--all without anyone's actually having
below, sound power levels (PWLs) are given for
measured the particular engine. The prediction
the three basic sources of engine noise The N&V
method performs very satisfactorily when tested
manual (paras 25 and 53g) shows how to use
against the 50 engines that were measured and
PWL data.
used in the study. For three groups of engine cas-
ing noise data, the standard deviation between the
d. Engine casing noise. The estimated overall
measured noise and the predicted noise was in the
PWL of the noise radiated by the casing of a
range of 2.1 to 2.5 dB. This finding shows that the
natural-gas or diesel reciprocating engine is given
engines themselves are fairly stable sound sources
in table 21. This PWL may be expressed by equa-
and that the prediction method reflects the engine
tion 21:
noise parameters quite well.
c. Engine noise sources. Typically, each engine
has three principal sound sources: the engine cas-
ing, the engine exhaust, and the air inlet. The en-
where LW is the overall sound power level (in dB
relative to 10-12 W), "rated hp" is the engine manu-
gine exhaust, when unmuffled, is the strongest
facture's continuous full-load rating for the engine
source, since it represents an almost direct connec-
tion from the cylinder firings. The engine casing
(in horsepower), and A, B, C, and D are correction
radiates noise and vibration caused by all the inter-
terms (in dB), given in table 21. In table 21,
"Base PWL" equals 93 + 10 log (rated hp).
nal components of the operating engine, and is here
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