Engine Noise Level Guide

Hercules Generator Noise Level


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” 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” 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 



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 



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 



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 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: 


Generating less noise at the source, by specifying 

quieter engines, transmissions, tires, etc. 


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. 


Plastic foam, fabric, or other soft porous materials can 

quiet sound by absorbing some of the sound pressure 


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. 


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. 


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. 


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 



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). 



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 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, 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. 


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. 


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 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. 

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