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The main noise reduction 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.
from the different sources are transmitted to the interior of the vehicle
in five different ways:
• soundproofing foam transmitted through gaps in the panelling, poorly fitting hatches and cable pass troughs.
• sound proofing material transmitted through the body panels, which creates acoustic material sound in the panels. The panels then create soundproofing foam at the opposite side.
• soundproofing foam, which creates sound proofing material, but the sound proofing material is transmitted to another location, where it radiates as acoustic material.
• Sound primarily generated as sound proofing material, which radiates as soundproofing foam in the area around the point of excitation.
• Sound generated as sound proofing material, but transmitted to another location, where it radiates as soundproofing foam.
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.
obtain low noise reduction 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 reduction sources to the overall level inside the vehicle at the driving conditions of interest. 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 reduction sources and the inner surfaces have been determined, the following measures should be taken:
1. Modify, if possible, the dominating noise reduction sources so that the airborne sound radiation and the vibration levels are reduced.
2. Reduce the transmission of sound proofing material and vibrations from noise reduction sources to the framework and body.
3. Reduce the radiation from dominating internal surfaces excited by structure sound proofing material.
4. Increase the soundproofing foam absorption inside the engine compartment.
5. Reduce the transmission of soundproofing foam through holes and fissures.
6. Reduce the transmission of soundproofing foam through body panels.
7. Increase the amount of soundproofing foam absorption inside the bus.
it into practice
1: Modifying the dominating noise reduction sources
Modifying the noise sources usually involves fairly extensive constructional changes. The noise reduction 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 control sources can also reduce radiation of soundproofing foam 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 sound proofing 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 control source is often the rear axle, whose sound proofing can be reduced by improving the quality of the gears in the differentials. Fan noise control can be reduced by an improved aerodynamic blade design combined with a shroud with a minimum clearance. 5 to 10 dB of unnecessary sound proofing material 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 control can be obtained by using a more sophisticated silencer design or by using a larger silencer. As far as tire sound proofing 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. For more information on sound proofing material click here.
2: Reducing the transmission of sound proofing material and vibrations from noise control sources to the framework and body.
Transmission of sound proofing
noise control and vibrations from the noise control 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 control-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 sound proofing source and the framework, such as pipe work on gear linkage.
3: Reducing the radiation from dominating internal surfaces excited by sound proofingsound.
Even if the sound proofing sources are modified and the transmission of sound proofing material 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 sound proofing control, it is probable that resonances will occur, causing increased radiation of soundproofing foam. The vibration amplitude and therefore the sound radiation at the resonant frequencies can be reduced by increasing the internal Sound proofing materia losses if the surfaces themselves. For more information on noise reduction click here.
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 sound proofing material , such as plywood or core board. When the sound proofing 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 sound proofing material 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 ?LV can be estimated using the formula: In a case when the loss factor is increased 10 times, the reduction will therefore be 10 dB.
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 sound proofing material such as aluminium sheets with a self-adhesive viscoelastic layer.
4: Increase the soundproofing foam 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 sound proofing material specially designed for engine compartment applications is Acustimet™ , an All Metal Self Supporting Sound Absorbing Panel Without Any Fibres.For more information on noise control click here.
5: Reduce the transmission of soundproofing foam through holes and fissures.
Airborne noise control transmitted through holes and fissures often destroys the effect of an otherwise extensive noise control 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 sound proofing 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.
In case of acoustic material 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, acoustic foam, 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. The improvement can be show to be: 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 control at lower frequencies. Increased damping will reduce the amplitude of the resonant vibrations and increase the sound reduction index, industrial noise reduction.
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 sound proofing material four times its weight combined with a traditional Acoustical insulation sound proofing material 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 acoustic insulation 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 sound proofing material between the panels should not be too stiff. Heat insulation materials with closed cells should not be used to replace the absorbent sound proofing material and sound control , as they normally have much higher dynamic stiffness, causing a considerable increase in the resonant frequency.
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.
For more information on acoustic material click here.
By making a systematic approach and considering the noise control reduction control aspects at the vehicle design stage, it should be possible for bus manufacturer to reduce the noise control reduction 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.