Sound suppression of engine noise

A method and apparatus for suppressing sound generated by engines or other noise sources, by utilizing at least two acoustical encasements separated by an air channel with air being drawn through the air channel to provide intake air to the noise source. Sound leaks through the intake vent are attenuated by the air channel. Entrainment devices or an auxillary fan may be used to compensate for loss of air intake due to pressure losses through the air channel. Also, the use of entrainment allows for a smaller exhaust vent which reduces the opening ratio in the encasement, thus reducing sound leaks through the exhaust vent. Additionally, sound absorbing or attenuating materials are placed inside the intake air channel, thus suppressing the sound emanating from the noise source. Also, the exhaust vent is lined with proper absorbing materials to further reduce sound coming from the noise source. Furthermore, the inside encasement is substantially isolated from the outside encasement.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to sound and more particularly to the suppression of 
noise generated by equipment such as, engines, generators, fans, 
compressors, etc. 
2. Description of the Prior Art 
It is well known that engines, compressors, fans, etc. can generate such 
noise that normal conversation near them is essentially impossible. 
Furthermore, such loud noise can impair hearing and cause phychological 
effects that reduce the efficiency of a workman. As a result of the 
findings from research in this area of technology, great interest is now 
being shown in various techniques for regulating sound emissions from a 
wide variety of operating equipment. Since man would benefit from the 
noise reduction of operating equipment, a measure of sound, weighted to 
the response of the human ear, was developed. The weighted scale is 
referred to as the "A" scale. The noise level is measured in decibels, 
weighted to the A scale and called dBA. Various experiments have been 
performed to determine the dBA values for different noise conditions. It 
was found that 130 dBA was the threshold of pain. A value of 50 dBA was 
found to be the average level for office conversation. Since it was 
necessary to drop the sound level of field equipment down to 50 dBA, the 
Environmental Protection Agency issued a temporary goal of 75 dBA for 
field equipment with an ultimate goal of 65 dBA. It is to the 65 dBA level 
and lower that this invention disclosure is directed. 
Several journals have published the results of acoustical suppression of 
operating engines. These results used the standard techniques of employing 
acoustical materials, of designing special mufflers, of designing 
silencers, and of using various vibration isolators. A large industry has 
developed around sound suppression, providing new materials and test data 
on the materials. A review of the literature shows that the materials and 
techniques of sound suppression center mainly around buildings and 
operating equipment found in buildings. It is relatively easy to suppress 
noise sources in a building because there is not a weight or volume 
restriction in general. In fact, one could build a room around a noisy 
engine if need be. The suppression problem is more severe for those noise 
sources that are in the field, such as an engine-generator set or 
compressor. 
One of the major difficulties in acoustically suppressing an operating 
engine is a resultant engine overheating problem. If an engine is 
completely encased so that no noise can escape, the engine can overheat 
easily. If holes are made for engine intake air and exhaust air, then 
sound can escape through these holes. Designers have used mufflers, 
silencers, curved ducts, new materials and a variety of other techniques 
to suppress some engines. Published results show that these techniques 
have suppressed some engines (air compressors) down to 72 dBA. 
Significantly lower values have not been published to the best of this 
inventor's knowledge. The military has developed sound suppressors for 
military engine-generator sets, but at the expense of heating up the 
surface of the sound suppressor. For reasons other than sound, a hot 
surface surrounding an engine-generator set is not advantageous from a 
military viewpoint as it can easily be detected by state-of-the-art 
thermal viewers.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
This invention may perhaps be best understood by making reference to the 
drawings. The use of two or more encasements with air spaces in between 
has the advantage of suppressing low acoustical frequencies. The 
inventor's own experiments, tests and measurements on engines show that 
considerable acoustical energy is found in the frequency range from 31.5 
Hz to 500 Hz. Laboratory data show that several acoustical materials can 
suppress the frequencies above 500 Hz down below 50 dB but do not do an 
effective job below 500 Hz. 
Published data show that the attenuation of sound, by encasing the sound 
source, is a function of the percentage of openings in the encasement. An 
encasement could have an actual attenuation of 60 dB for no holes in the 
encasement and a value of 25 dB for 50% opening. Therefore, it is very 
important to reduce the percent of opening. The basic invention visualized 
herein uses a very narrow slot, lined with acoustical material, as the 
intake vent to the inside volume where the noise source is located. This 
narrow slot reduces the opening ratio. The use of entrainment devices or 
fans inside the encasement, draws more air through the intake slot and 
this helps to cool the noise source. Additionally, entrainment devices 
permit the use of a small exhaust vent which reduces the opening ratio, 
thus decreasing the sound leak through the exhaust vent. 
It is well known that acoustical energy can be absorbed and turned into 
heat energy by acoustical materials. The amount of absorption of a 
material depends on its physical structure and its thickness. Polyurethane 
can have an absorption coefficient of 0.04 at 200 Hz for 1/4 inch 
thickness and a value of 0.20 for a 1 inch thickness. In addition, the 
absorption coefficient for the frequencies below 500 Hz can be 
significantly increased by a proper choice of facing for the walls of the 
encasement. A more dramatic absorption effect can be obtained by using a 
thin metal sheet with holes. By properly selecting the number and size of 
holes, it is possible to resonately absorb certain frequencies. Therefore, 
it is possible to insert into the intake air channel a thin metal sheet 
with properly selected holes to selectively absorb certain frequencies 
from the noise source. This is particularly attractive for the absorption 
of low frequencies. 
Materials which are good absorbers are not good reflectors of acoustical 
energy. The best acoustical suppressor would be an encasement with no 
holes that reflects all sound inside the encasement and permits no sound 
to penetrate outside. This is not practical for field operating equipment. 
Therefore, a combination of absorption and attenuation is used for 
suppression. "Sandwich" materials that combine absorption and attenuation 
are the most practical materials for suppression. For example, a typical 
sandwich may be compared to two layers of polyurethane separated by a thin 
sheet of lead. The utilization of two enclosures of sandwich material, 
separated by an air space has the advantage of significantly increasing 
the total absorption coefficient for frequencies below 500 Hz. For 
example, 1 inch of glass fiberboard can have an absorption coefficient of 
0.03 at 125 Hz. With a 2 inch air separation, this value can be increased 
to 0.17. 
By properly selecting the materials, facing, geometry, resonate absorbers 
and air space between the two encasements of this invention, the 
attenuation coefficient at frequencies below 500 Hz can be dramatically 
improved. The attenuation of frequencies above 500 Hz is easily attenuated 
by the same materials. Therefore, acoustical suppression below 65 dB is 
assured and suppression below 50 dB is visualized. 
As previously noted, the prevention of engine overheating is most important 
and no acoustical suppression technique can be used if the technique 
overheats the engine. The inventor's data show that the double encasement 
does not overheat engines as long as fans, injectors or other entrainment 
devices are utilized. Auxiliary fans are the most useful but drain power 
from the engine. If the draining of power is of no concern, fans can be 
used to great advantage. The use of air injectors not only helps to draw 
in additional air but cools the exhaust gas. A good injector design can 
entrain over 10 times the amount of exhaust gas and thereby cool the 
exhaust gas and exhaust pipe. This eliminates the safety problem of hot 
pipes and gases. 
FIG. 1a shows the double encasement of this invention. Intake air is drawn 
through a narrow slot on the top of the outside encasement as shown in 
FIG. 1a. Sound from the noise source must travel through the narrow 
channel separating the two encasements, and exit through the slot. The 
attenuation of sound along this path is very high because of the path 
length and curvature of the walls. FIG. 1a shows the exit or exhaust vent. 
All exhaust gases and engine cooling air exit through this hole. The 
smaller the hole, the better the attenuation of sound. This hole has a 
silencer built into it. The exit duct has two bends with acoustically 
absorbing material on the inside surface. The combination of the 
suppressed duct and silencer suppresses the sound in the exhaust hole. The 
noise source itself is vibrationally isolated from the bottom of the 
outside encasement. FIGS. 1a and 1b show that intake air surrounds the 
entire inside encasement. Sound must travel through this channel. 
FIG. 2a shows a design for a small opening ratio. The auxiliary fan aids in 
drawing intake air through the louvers. This helps the cooling of the 
noise source. In addition, the fan permits the use of a small exhaust hole 
that helps attenuate the sound leaving the encasement. Furthermore, the 
fan mixes cooler air into the exhaust stream, thus lowering the exhaust 
gas temperature. The noise source is vibrationally isolated from the 
inside encasement and the inside encasement is isolated from the outside 
encasement. Clearly, this design can suppress the sound level down to very 
low levels by increasing the fan capacity and reducing the opening ratio 
as long as the fan itself does not contribute to the noise. FIG. 2b shows 
an auxiliary fan in the exhaust vent. The vent is designed to provide a 
180.degree. turn for the sound waves. A 180.degree. turn could add 
non-acceptable back pressure in the exhaust stream. The auxiliary fan 
provides a make up pressure for the 180.degree. turn and provides the 
necessary pressure to draw intake air through the louvers. The use of this 
fan assisted 180.degree. turn can suppress the sound significantly and 
cool the exhaust stream. 
FIG. 3b shows a design for an engine that uses air entrainment to provide 
the make up pressure for the intake air friction losses. The exhaust pipe 
can be made into a nozzle as part of an air injector. The air injector 
acts as a jet pump and draws additional air through the intake slots. The 
air ejector is shown in FIG. 3a. The exhaust gas vent is lined with 
acoustically absorbing material and a silencer. The engine cooling vent is 
used to entrain air also. This vent is lined with acoustically absorbing 
material and a silencer. Both entrainment devices are used to provide make 
up pressure inside the inner encasement. This in turn provides the 
necessary intake air to cool the engine. 
FIG. 4 shows a resonant absorber placed in the space between the two 
encasements. The thickness, number of holes and size of the holes can be 
selected to resonately absorb frequencies. This design can be used to 
resonately absorb the difficult low frequencies of operating engines.