Engine Noise Level Guide
SOUND AND NOISE
Sound consists of pressure
waves traveling through
the air (or water, etc.). Sound
pressure waves can be
described by their frequency
and amplitude. Noise is
unwanted sound, usually
consisting of many pressure
waves at different frequencies
and amplitudes.
FREQUENCY
“Frequency” refers to the
number of pressure waves
per second. It is usually
reported as “Hertz” (Hz), which
means cycles per second. The
human ear can usually
detect frequencies from about
20 Hz to 20,000 Hz.
AMPLITUDE (DB AND DB(A))
“Amplitude” refers to the
pressure level of the sound
wave. Since sound pressure
variations are extremely
small and cover a very wide
range, they are usually
measured on a logarithmic scale
called Decibels (dB),
instead of conventional
pressure units like psi.
Since the human ear has
different sensitivities at
different frequencies, a 50 dB
sound at 200 Hz would
not sound as loud as a 50 dB
sound at 2000Hz. For
that reason noise measurements
are usually reported
in dB(A). The “A” refers to a
set of weighting factors
based on the sensitivity of the
human ear at each
frequency. There are other
weighting systems such
as dB(B) and dB(C), but most
machinery and vehicle
sound regulations are in
dB(A).
Zero dB(A) approximately equals
the lowest possible
pressure wave audible to the
human ear at each
frequency. Each increase in
amplitude of 6 dB
represents a doubling of sound
pressure level. Using
the “A” weighting system, a 50
dB(A) sound at 200 Hz
should sound approximately as
loud as a 50 dB(A)
sound at 2000
Hz.
Sound levels from typical
sources are shown in Figure
50-1.
ADDING SOUND LEVELS
Since the Decibel scale is
logarithmic, Decibels can’t
be added directly. When adding
sound levels the
loudest sound dominates. Adding
additional sound
sources that are not as loud
have relatively little effect.
The following chart can be used
to add decibel levels
from different sources or at
different frequencies. Use
the chart to add two decibel
levels at a time. If you
have to add three or more
sources, add any two, then
add that total to the third,
etc.
To use Figure 50-2, first
determine the difference
between the two values being
added. Subtract one
value from the other to find
the difference, locate the
difference on the horizontal
(bottom) axis of the chart,
draw a straight line up to the
curve, then over to the
vertical (side) axis of the
chart to fi nd out how many
decibels to add to the higher
of the two original values.
For example, if you were adding
an 84 dB(A) source to
a 90 dB(A) source, the
difference would be 90-84=6.
From the chart you can see that
for a difference of 6
decibels you should add 1
decibel to the highest of the
two levels, so you would add 1
dB(A) to 90 dB(A) for a
combined level of 91 dB(A) for
both sources. If you are
adding two equally loud
sources, say 90 dB(A) each,
the difference would be zero,
and you would add 3
dB(A) to 90, for a combined
level of 93 dB(A) for both
sources.
DISTANCE EFFECTS
Decibel level drops off rapidly
with distance. Exactly
how much depends on how much
the ground and other
close objects reflect or absorb
sound. In a free field
(no absorption or reflection),
sound will drop off by 6
decibels for each doubling of
distance from the source.
You can use this to estimate
the effect of increasing
or decreasing the distance to
the noise source. For
example, a noise source of 90
dB(A) at 7 meters would
be about 84 dB(A) at 14 meters,
or 96 dB(A) at 3.5
meters.
“ENGINE NOISE” SOURCES
Several different noise sources
contribute to what
people sometimes consider
“engine noise.”
The noise levels reported on
the back of each engine
performance curve are only the
noise radiated directly
off the bare engine surfaces.
They are averages of
several microphones located 1
meter from the engine.
They do not include noise from
the exhaust system,
fan, etc.
Engine surface noise may not be
the largest noise
source. Exhaust noise is
frequently higher, and fan
noise can be, in some
installations.
Other signifi cant noise
sources can include the air
intake, drive train,
hydraulics, tires, etc.
NOISE TREATMENT - GENERAL
Noise can be transmitted from
any noise-generating
component in the form of
“air-borne” noise or
“structure-borne” noise.
Air-borne noise is transmitted
directly from the surfaces of
the component through
the air to the ear.
Structure-borne noise is transmitted
through the engine mounts or
other solid connections
to the cab or chassis in the
form of vibration, then from
there it goes through the air
to the ear.
Most noise treatments work on
either structure-borne
or air-borne noise in one of
the following ways:
SOURCE REDUCTION
Generating less noise at the
source, by specifying
quieter engines, transmissions,
tires, etc.
SHIELDING -
A heavy wall that will not
vibrate easily, placed between
the noise source and the ear,
can help block the
pressure waves. This is what
concrete “noise fences”
along highways
do.
Heavy, solid, well-damped
materials (such as concrete,
lead, or heavy rubber) make the
best shields.
Lighter shield materials (such
as sheet steel) are most
effective when used in
combination with absorptive
material and/or
damping.
ABSORPTION -
Plastic foam, fabric, or other
soft porous materials can
quiet sound by absorbing some
of the sound pressure
waves.
Both absorption and shielding
are most effective on
high-frequency vibrations.
That’s why when a car with a
loud stereo passes your house,
you hear only the bass.
ISOLATION -
Rubber mounts can be used to
keep structure-borne
noise from being transmitted
from the engine or
other noise sources to cabs or
sheet metal that could
transmit the noise to the ear.
Any solid connection can
transmit structure-borne noise,
including throttle levers,
exhaust system brackets,
etc.
Isolating noise sources (such
as engines and muffl ers)
can be effective. But if the
operator is enclosed in a
cab, isolating the cab can
provide the best results.
STIFFENING -
When structure-borne or
air-borne noise is transmitted
to cabs, chassis or shields,
resonant vibrations can be
excited in sheet metal panels,
amplifying the noise.
Stiffening panels by adding
stamped-in or added-on
bases can help detune resonant
frequencies and
reduce
amplitudes.
DAMPING -
Sometimes resonant vibrations
in sheet metal panels
can be absorbed by adding
layers of damping materials
(such as rubber or tar-like
substances) to the panels.
This is why automotive
undercoating makes cars
quieter.
SEPARATION -
Dominant noise sources should
be physically
separated so they do not add
together. For example, if
the engine surfaces and the
exhaust pipe produce 90
dB(A) each, they will produce
93 dB(A) together. But
if the exhaust pipe is routed
to the opposite end of a
large machine, the noise at
either end will be close to
90 dB(A).
NOISE TREATMENT -
SPECIFIC
NOISE SOURCE
IDENTIFICATION
The most important rule in
noise treatment is to identify
the noisiest component, and
concentrate your control
efforts on it. Even if you
completely eliminate the
second or third noisiest source
it can’t have more than
a few dB(A) effect. If the
noise goes down 3 dB(A) or
more when one source is
eliminated, it is larger than
all other sources combined. A
reduction of 1 or 2 dB(A)
may also be significant if
there are many sources close
in amplitude.
You can identify the primary
noise source by
temporarily removing or
treating each source one at a
time. Fan noise is easy to
check by removing the fan
temporarily. To isolate
transmission or drive train noise,
disconnect the clutch. To
isolate exhaust or intake
noise, reroute them away from
the machine to check
their
contribution.
EXHAUST NOISE
Exhaust noise is the loudest
untreated noise source
on most applications and is
also the easiest to treat.
Standard mufflers can reduce
exhaust pipe noise by
10-15 dB(A) through absorption.
Quieter “residential”
mufflers are also available.
The best source of muffler
performance information is your
muffler supplier. The
exhaust pipe should direct
exhaust flow away from the
cab, the operator, and
bystanders’ ear level.
For ultra-quiet installations
it may be necessary to wrap
the muffler with
high-temperature (ceramic) fabric and
a sheet metal cover, to shield
and absorb air-borne
“skin-noise” from the muffler
shell.
The muffler can also transmit
structural noise to the
cab or frame. Avoid bracketing
the muffler or exhaust
pipes to the cab or frame if
possible. If it’s necessary
to support the muffler on the
cab or frame, isolate the
exhaust system using flexible
exhaust connectors to
break the structural vibration
path, or use rubberized
exhaust pipe hangers such as
used on passenger cars.
FAN NOISE
Fan noise, due to a
large-diameter fan turning at
high rpm, can be greater than
noise coming from the
exhaust pipe or engine
compartment. Fan noise can be
controlled by following these
guidelines:
• Run the fan as slow as
possible. Fan tip speeds (fan
rpm x circumference) of 12,000
feet per minute or
less are recommended for quiet
installations. If the
fan is running over 16,000 fpm,
it may be the loudest
noise source on the
machine.
• Follow the fan application
guidelines in the Cooling
Section of this manual to
maximize fan efficiency. For
the same air flow, an efficient
fan can be run slower
than an inefficient fan. Large
fans at slow rpm are
usually quieter than small fans
at high rpm for the
same air flow.
• Air obstructions cause noise
when a fan blade moves
past them, particularly on the
inlet side. Keep the fan
at least 1/2 to 1 blade width
back from the radiator
and well away from engine
obstructions (such as
alternator pulleys, hoses,
etc.).
• Use shielding and absorption
to reduce fan noise at
the source.
ENGINE AIR-BORNE NOISE
Hercules engines are among the
quietest in the industry.
Generally speaking, engines run
somewhat quieter at
lower speeds, but other than
reducing speed, there is
very little you can do to
reduce engine surface noise at
the source.
The most effective way to treat
engine surface noise is
by using an enclosure lined
with absorptive materials.
Sound enclosures work best when
as much of the
machine as possible is
contained within the enclosure.
Ideally, the entire machine
should be enclosed. This
has the added advantage of
helping to silence any
other noise sources (such as
the fan, transmission,
etc.) that are also located in
the enclosure.
To provide effective shielding,
the enclosure should be
sealed as completely as
possible, except for air flow
openings. Openings for air flow
should be generous,
but they should be baffled to
direct air flow over sound
absorbing materials and away
from ear level.
The use of a blower fan should
be strongly considered
to control engine compartment
temperatures. Wrapping
and shielding of exhaust
components will also help.
Exhaust should be routed out of
the compartment along
with the cooling air flow so it
does not recirculate in
the engine compartment. With
blower fans (as shown)
it should exit in the front.
With suction fans, exhaust
should exit in the
rear.
With blower fans, the air
cleaner inlet can be taken
from within the engine
compartment. With suction fans,
it should be taken from in
front of the radiator, but within
the front sound
shield.
Care must be taken to make sure
the sound absorbent
materials used can tolerate the
high temperatures that
can be present in the engine
compartment. How high
the temperature will be depends
on your installation.
Exhaust gases or hot components
must be kept away
from any flammable sound
absorbent material. Sound
absorbent material should be
used cautiously below
the engine, particularly if any
oil leaks are present.
Oilsoaked absorptive material
can be combustible.
ENGINE STRUCTURE-BORNE
NOISE
Engines can also transmit
structure-borne noise from
vibration to frames and cabs.
Hercules fully balanced
4-cylinder engines produce
significantly less vibration
than competitive 4-cylinder
engines.
Well-matched rubber engine
mounts can also help
reduce structural noise
transmission. However, poorly
matched rubber mounts can be
worse than solid
mounts. Refer to the Engine
Mounting section of this
manual for mount design
guidelines. Remember that
other solid connections such as
rigid exhaust pipes will
prevent rubber mounts from
working properly, and will
transmit structure-borne noise
themselves.
AIR INTAKE NOISE
Air intake noise
is usually adequately muffled by using
a properly sized
canister type air cleaner. If you are
using a small or
“throw-away” type air cleaner and
air intake noise
is a dominant noise source, consider
changing to a
canister.
Call us now at :
Tel : 63-2-7359810,
63-2-4002650
Mobile :
63-090-88881668
Email : purepower@hercules-power.com
|