NOISE
TREATMENT STEP BY STEP
Increasing demands for
low noise levels either enforced by laws and regulations or by customer
demands, cause bus manufacturers to look deeper into the possibilities
of reducing both the external and internal noise of their vehicles.
The
main noise sources are the engine and its accessories, the transmission,
the exhaust system, the hydraulic system, the cooling fan and the tires;
in some cases also the fan in the air conditioning system has to be
considered. The relative contribution from the different sources varies,
depending on the type of bus and the driving conditions, as well as
on the location inside the bus.
Noise
from the different sources are transmitted to the interior of the vehicle
in five different ways:
• Airborne sound transmitted through gaps in the panelling, poorly fitting
hatches and cable pass troughs.
• Airborne sound transmitted through the body panels, which creates
structure-borne sound in the panels. The panels then create
airborne sound at the opposite side.
• Airborne sound, which creates structure-borne sound, but the structure-borne
sound is transmitted to another location, where it radiates
as airborne sound.
• Sound primarily generated as structure-borne sound, which radiates
as airborne sound in the area around the point of excitation.
• Sound generated as structure-borne sound, but transmitted to another
location, where it radiates as airborne sound.
These
are “pure” cases. Normally they will all be presented in a bus application
to a greater or less degree. Transmission and radiation will also be
greatly influenced by resonance in the different structures involved.
To
obtain low noise levels inside the bus it is important that a systematic
approach is made and the following procedure is suggested:
Try to determine the contribution from the different noise sources to
the overall level inside the vehicle at the driving conditions of interest.
A sufficient estimation can often be made by listening to the noises
made by the travelling under normal operating conditions combined with
frequency analyses based in sound pressure and knowledge of the mechanical
features of the different components. The contribution from, for example,
the transmission and the hydraulic system can be estimated by comparing
a narrow-band analysis with calculated gear mesh and pump frequencies.
Determine the contribution from the different inner surfaces to the
vibration level of the bus. An estimation can be made by disabling one
of the sources such as the radiator fan and comparing the sound pressure
signature with and without the fan running.
When the contribution from noise sources and the inner surfaces have
been determined, the following measures should be taken:
1. Modify, if possible, the dominating noise sources so that the airborne
sound radiation and the vibration levels are
reduced.
2. Reduce the transmission of structure-borne sound and vibrations from
noise sources to the framework and body.
3. Reduce the radiation from dominating internal surfaces excited by
structure structure-borne sound.
4. Increase the airborne sound absorption inside the engine compartment.
5. Reduce the transmission of airborne sound through holes and fissures.
6. Reduce the transmission of airborne sound through body panels.
7. Increase the amount of airborne sound absorption inside the bus.
Putting
it into practice
1: Modifying the dominating noise sources
Modifying the noise sources
usually involves fairly extensive constructional changes. The noise
and vibration characteristics of a diesel engine can be influenced by
changing the cylinder pressure development and by modifying the engine
block. Treating the dominating noise sources can also reduce radiation
of airborne sound from the engine. For example, the radiation from sheet
steel components like rocker cover, oil sumps and front covers can be
reduced if laminated steel material is used instead if standard steel.
Laminated steel can also be used for close-fitting shields to reduce
the radiation from the side if the engine block.
Another important noise source
is often the rear axle, whose noise can be reduced by improving the
quality of the gears in the differentials. Fan noise can be reduced
by an improved aerodynamic blade design combined with a shroud with
a minimum clearance. 5 to 10 dB of unnecessary noise is often generated
by having the support struts for the fan shroud or fan motor directly
in the path of the air passing to the fan blades. This is noticeable
by very intense pure tones at the blade passing frequency. Mounting
the support downstream of the fan or using similar style fan blades
can attenuate this effect.
A reduction in the exhaust
noise can be obtained by using a more sophisticated silencer design
or by using a larger silencer. As far as tire noise and tire-related
vibrations are concerned, the tread pattern can play an important role
and there may be a gain by selecting the right type.
2: Reducing the transmission of structure-borne
sound and vibrations from noise sources to the framework
and body.
Transmission of structure-borne
noise and vibrations from the noise sources of the framework are generally
treated using resilient mounts. The degree of isolation obtained is
often limited because the mounts cannot be made too soft, for durability
or installation reasons. Some vibration energy will therefore be transmitted
to the framework and in to the inner surfaces if the vehicle. To obtain
the desired effect if resilient mounts it is important to mount the
noise-making components on points of the frame where the mobility is
as low as possible, which means on points where the mass and stiffness
are high, such as the interconnecting points between beams.
Resonance, however, often causes an increase in mobility of the framework,
leading to an increased transmission. This can be taken care of by increasing
the structural damping in the framework, such as by using the constrained
layer damping technique which is described later. It is also important
to ensure that resilient mounts are not short-circuited by any rigid
connection between the noise source and the framework, such as pipe
work on gear linkage.
3:
Reducing the radiation from dominating internal surfaces excited by
structure structure-borne sound.
Even if the noise sources
are modified and the transmission of structure-borne sound and vibration
is reduced, some vibration energy will always be transmitted to the
interior surfaces of the bus. All these surfaces normally have a large
number of natural frequencies in the range of interest for noise control,
it is probable that resonances will occur, causing increased radiation
of airborne sound. The vibration amplitude and therefore the sound radiation
at the resonant frequencies can be reduced by increasing the internal
losses if the surfaces themselves.
The most important surface
regarding sound radiation to the interior is the floor and, in case
of rear- or front-mounted engines, the bulkhead. Most bus floor today
consists of some kind of wood-based constructional material, such as
plywood or core board. These materials have relatively small internal
losses when subjected to vibration, which means little ability to convert
mechanical energy into heat. This ability can be increased considerably
if the floor is built on the constrained-layer principle in which a
viscoelastic damping layer is introduced between two sheets of the constructional
material. When the material is subjected to bending wave vibrations
the viscoelastic layer is mainly deformed by shear, which leads to conversion
of vibration energy into heat by internal molecular friction. This in
turn, leads to reduced amplitude of vibration and a reduced radiation
of airborne sound from the floor panels.
The
improvement in structural damping when a viscoelastic damping layer
is introduced into a plywood construction is shown in the Figure below.
The internal losses represented by the loss factor are increased by
a factor of more than 10. In cases where a panel has several natural
frequencies within the range of interest and the excitation spectrum
is broad-banded, the reduction in vibration level ΔLV
can be estimated using the formula:
ΔLV
= 10log ε/ε0
dB
where
ε0
= Initial loss factor.
ε
= Loss factor of damped structure.
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Combined
Loss factor,η |
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Temperature,
ºC |
In a case when the loss factor is increased 10 times, the reduction
will therefore be 10 dB.
The increase in the loss
factor not only reduces the amplitude of resonant vibrations, but also
leads to a damping of propagating waves through the structure.
In the case of rear or front
engines the bulkhead is often made of steel or aluminium. The damping
can then be increased by using a laminate consisting of two metal sheets
with an optimised intermediate viscoelastic damping layer. When the
floor and where appropriate the bulkhead have been damped, it might
also be necessary to increase the damping of the other internal surfaces.
Critical areas have been shown to be the wheel housings and the internal
steps. When these are made of aluminium, fibre glass or steel, they
can either be made of a damped laminate or a damping layer and constraining
layer can be added to the existing structure, to form a sandwich construction.
The other internal surfaces
are normally made of thin sheet metal or laminates, which can be sufficiently
damped by using extensional damping layer such as self-adhesive damping
pads or lightweight constrained-layer type material such as aluminium
sheets with a self-adhesive viscoelastic layer.
4: Increase the airborne sound absorption inside
the engine compartment.
To
prevent an increase in the sound level inside the engine compartment
due to reflections at the boundary surfaces, sound absorption materials
have to be applied. A material specially designed for engine compartment
applications is Acustimet™ , an All
Metal Self Supporting Sound Absorbing Panel Without Any Fibres.
5: Reduce the transmission of airborne sound through
holes and fissures.
Airborne noise transmitted through holes and fissures often destroys
the effect of an otherwise extensive noise treatment. It is therefore
important that the number of openings is reduced to an absolute minimum
and that they are carefully sealed by such things as heavy rubber gaskets.
Critical areas are the doors, especially when they are situated close
to any of the noise sources like the engine and rear axle. By using
double gaskets and sealing brushes it is possible to reduce the transmission
to an acceptable degree.
6: Reduce the transmission of airborne sound through
body panels.
In
case of airborne sound transmission the most important surfaces are
the floor and the bulkhead. Wood-based materials like plywood are relatively
light and stiff, which makes them very suitable as constructional materials
for buses, but at the same time these properties are negative from an
acoustical point of view. Due to their relatively high stiffness-to-weight
ratio, these materials deviate from the acoustic mass law giving an
increase is sound reduction index of 6 dB per doubling of mass and by
6 dB per doubling of frequency. The reason is a phenomenon called coincidence,
which causes a deviation from the mass law curve at and above the critical
frequency. At and above the critical frequency, the transmission dominates
by resonant vibrations, which means that an increased loss factor is
also beneficial regarding the sound reduction index. The improvement
can be show to be:
ΔRV
= 10log ε/ε0
dB
where
ε0
= Initial loss factor.
ε
= Loss factor of damped structure.
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Transmission
Loss, dB |
|
Temperature,
ºC |
When
plywood panels are mounted on a steel framework the fundamental natural
frequencies of the panel often fall in the region of interest for noise
control. This means that resonance will probably also cause an increase
in the transmission of noise at lower frequencies. Increased damping
will reduce the amplitude of the resonant vibrations and increase the
sound reduction index.
In some cases the sound reduction
index has to be increased even further. This can be obtained by combining
the viscoelastic damping layer with a heavy barrier mat. Another traditional
possibility for increasing the sound reduction index is to use a double
wall construction. This is automatically achieved by the use of Acustimet
Metal. Laboratory test have shown that it has superior insulation properties
to a barrier material four times its weight combined with a traditional
foam absorber system.
In some rear-engine buses
good result have been obtained by using laminated steel in similar double
wall constructions. To maintain a high sound reduction index of the
double wall it is, however, important have as few connections as possible
between the outer and inner panels of the wall. It is also important
that the absorbent material between the panels should not be too stiff.
Heat insulation materials with closed cells should not be used to replace
the absorbent material, as they normally have much higher dynamic stiffness,
causing a considerable increase in the resonant frequency.
7: Increase the amount of airborne sound absorption
inside the bus.
When airborne sound has entered
the interior of the vehicle it will be reflected to a degree depending
on the absorption characteristics of the boundaries. In many cases,
especially in city-buses, the boundary surfaces are hard and non-absorptive;
in addition the seats are covered with an impervious surface, which
leads to an increase in sound level, due to reflection.
Means of increasing the absorption
ought to be considered because the A-weighted noise spectrum inside
the bus often dominated by frequencies in the region 500 to 2000 Hz.
In that region good results can be obtained with absorption materials
of moderate thickness. The possibilities of using more porous seat coverings
should also be considered.
One surface that can be made
absorptive without great difficulties is the roof. But by using the
Acustimet Metal with an airspace behind it a decorative tough and hygienic
sound absorbing panel can be effected.
By making a systematic approach
and considering the noise control aspects at the vehicle design stage,
it should be possible for bus manufacturer to reduce the noise level
both inside and outside the bus without too drastic or too heavy treatments.
Sontech
has its own independent acoustical service to provide advice and acoustical
survey to aid the manufacturer in its design of the future vehicles.
Please contact your representative if you would benefit from our advice.
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