Airborne sound that is reflected against hard surfaces, for example, a hood over a machine or the walls of a room, can be reduced by covering the surfaces with absorbents on the inside. Materials such as plastic foam, textile fibres, or metal. The sound absorbents convert the airborne sound into small amount of heat energy.

The absorption factor denotes how much of the incoming sound that will be absorbed.

One way to reduce the propagation of noise is to insulate the noise source using a wall or hood. The noise insulation capability of a wall increases as it weight increases. The insulation capabilities can also be further improved by using a double wall system. Insulation pads are available with and without a wear resistant surface.

The Transmission Loss DTL denotes the reduction expressed in dB as a wall passage.

Structure borne sound is vibration in solid materials, generated by a machine, for example. It can be damped by applying a pad on one side or by the sandwich method. In both cases the vibration energy is converted into heat.

The loss factor denotes how much of the vibration energy is absorbed.

Acoustical Terms
Frequency is measured in Hertz (Hz, the number of vibration cycles per second) and denotes the pitch of the sound. 100 Hz is a bass tone with a wavelength of 3 m whereas the wavelength for a frequency of 3,000 Hz is only 1 dm.

Decibel (0,1 Bel = 1 dB) is a unit used to express the ratio between two levels of sound. Besides sound pressure (LP), the following magnitudes are expressed in dB: sound intensity (LI), sound power (LW), equivalent average value (Leq). Zero Bel is defined as the lowest limit of audibility.

6 dB higher sound level means a doubling of the sound level.

Two independent sound sources of equal power, result in a sound level that is 3 dB higher
than a single source.

LP dB(A) is an average value across all frequencies in the audible range, adjusted to the sensitivity of the ear to the different frequencies. 


The Illustration shows sound pressure levels for various environments.


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


ε0 = Initial loss factor.

ε = Loss factor of damped structure.

Combined Loss factor,η
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


ε0 = Initial loss factor.

ε = Loss factor of damped structure.

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.

Sontech NoiseControl™ - Sontech AB · Energivägen 6
SE 19 637 Kungsängen · Tel +46 8 445 50 60 · Fax +46 8 38 47 99