Experimental study of the effect of viscoelastic damping materials on noise and vibration reduction within railway vehicles

Abstract

Interior noise and vibration reduction has become one important concern of railway operating environments due to the influence of increased speeds and reduced vehicle weights for energy efficiency. Three types of viscoelastic damping materials, bitumen-based damping material, water-based damping coating and butyl rubber damping material, were developed to reduce the vibration and noise within railway vehicles. Two sleeper carriages were furnished with the new materials in different patterns of constrained-layer and free-layer damping treatment. The measurements of vibration and noise were carried out in three running carriages. It is found that the reduction effect of damping treatments depends on the running speed. The unweighted root-mean-square acceleration is reduced by 0.08–0.79 and 0.06–0.49 m/s² for the carriage treated by bitumen-based as well as water-based damping materials and water-based damping material, respectively. The first two materials reduce vibration in a wider frequency range of 63–1000 Hz than the last. It turns out that the damping treatments of the first two reduce the interior noise level by 5–8 dBA within the carriage, and the last damping material by 1–6 dBA. However, the specific loudness analysis of noises shows that the noise components between 125 and 250 Hz are dominant for the overall loudness, although the low-frequency noise is noticeably decreased by the damping materials. The measure of loudness is shown to be more accurate to assess reduction effect of the damping material on the acoustic comfort.

Introduction

Modern trains are expected to have a higher level of comfort for the passengers in order to keep or increase its commercial competitiveness. Several investigations have identified noise and vibrations as key factors to high comfort. For lightweight vehicles, interior noise and vibration increase sharply due to the reduction in transmission loss of the car body and the rise in air-dynamic noise and rolling noise. Unfortunately, the subject of noise within railway vehicles has been paid less attention in recent years than external “environment” noise such as rolling noise. When the new type of luxury sleeper carriage, traveling at speed of 160 km/h, is designed and manufactured in China, the comfort closely related to internal noise and vibration is one major concern. The question arises: what is the most appropriate and low-cost method to provide the comfortable riding environment with low internal noise?

One of the major problems is to prevent noise and vibrations generated by exterior sources, e.g. the wheel–rail rolling noise and the braking noise. The interior noise inside a railway coach is composed of air-borne at middle and high frequencies and structure-borne sound below 250 Hz. With a trend towards lighter trains the structure-borne sound will increase. There is a conflict between light weight structures and low levels of noise and vibrations. It has been proven difficult to achieve a satisfactory comfort level without adding mass to the structure. Many passive and active methods have been developed to improve the riding comfort. Some of them have been used for many years while others are still quite new or under investigation.

For a good vibration environment it is important that the interaction between different parts of the vehicle is kept as low as possible for all frequencies [1]. Traditionally, this has been done by separation of fundamental frequencies of bogie, car body, floor and passenger chair. This method is only applicable to case where there are distinct resonance frequencies. This method cannot reduce the internal noise level. Floating floor construction investigated in the ship and aircraft industry [2], [3], [4] has been used to obtain a vibration isolation of the inner floor so as to reduce the acoustical power radiated by the floor and transmission of air-borne sound. The floating floor constructions have been primarily used to minimize the transmission of noise into the compartment. It is interesting to note that although a floating floor construction can effectively reduce vibrations in the high-frequency region, there is also an increase of vibrations in the low-frequency region near the fundamental frequency of the isolation arrangement. Typically, this is the frequency range considered with regard to comfort-related vibrations. Thus, the use of a floating floor may decrease the vibration-related comfort level. There is a need for modification of current vehicle structure to install the floating floor. Koo et al. [5] described the vibration and noise reduction by application of low-noise wheel for motor and trailer cars in the electric multiple unit system. In spite of its promising potential of noise reduction for railway vehicles, the endurance and safety of the low-noise wheel are yet to be verified. As far as the reduction of internal structure-borne noise in the frequency range of 20–250 Hz is concerned, a few methods mentioned above are neither efficient nor suitable.

By semi-active control of an orifice installed between the air spring and the auxiliary air chamber, the vertical vibration on the car body floor has been effectively reduced [6]. The active method by employing elastically suspended mass is described by Holst [7]. A semi-active control strategy using MR damper is proposed to decrease the vibration acceleration on the floor in the vehicle [8]. However, the advance of active and semi-active control technology in vehicles and commercial airplanes has been slow because of the high costs and complexity of the internal sound field. The practical application of these technologies in railway vehicles is still in the beginning stages.

Passive damping using viscoelastic materials is simpler to implement and more cost-effective than semi-active and active techniques. Passive damping has been dominant in the non-commercial aerospace industry since the early 1960s. Advances in the material technology along with newer and more efficient analytical and experimental tools for modeling the dynamic behavior of materials have led to the surface damping treatments in automotives, commercial airplanes and railways [9], [10], [11], [12], [13], [14], [15], [16], [17]. Although the viscoelastic damping materials have been widely used in the fields mentioned above, the comprehensive applications for internal vibration and noise control in the car body of the railway vehicle were rarely reported in public and have been almost confined to company brochures so that these case studies are not readily available. Suzuki [18], [19] introduced a method, which used viscoelastic constraint layers pasted partially on the outside sheeting of the car body. Based on the theoretical evaluation, it was found at the choice of the optimal length and appropriate characteristics lead to the maximum damping. These optimum parameters could give birth to the maximum improvement of riding comfort in a lightweight car body of a high-speed train. The full scale experimental results showed that the riding comfort level was improved by about 3 dB at 275 km/h. However, the experiments and analysis were limited to a low-frequency range of the riding comfort. The change in the internal noise by damping treatment has not been studied.

Considering that the properties of viscoelastic materials are significantly dependent on environmental conditions such as temperature, vibration frequency, pre-load, dynamic load, environmental humidity and so on, the proper surface treatment, dimension and appropriate characteristics of the damping material is of vital importance for the success of viscoelastic material in adding damping to the structure system. The viscoelastic materials have been used to enhance the damping in a structure in two most common ways: free (unconstrained) layer damping treatment and constrained-layer damping treatment. The damping material is either sprayed on the body and floor panels or bonded using adhesive to provide damping. When the base structure is deflected in bending, the viscoelastic material deforms primarily in the form of extension and compression. The degree of damping is restricted by thickness and weight of base and damping materials. The system loss factor in a free-layer system increases with the thickness, storage modulus, and loss factor of the viscoelastic layer. Constrained-layer damping treatment consists of a thin layer of damping material combined with a constraining layer of metallic foil. When the base structure undergoes bending vibration, the viscoelastic material is forced to deform in shear because of the upper stiff layer. The constrained-layer damping is more effective than the free-layer treatment since more energy is consumed and dissipated into heat within the viscoelastic layer.

In addition, there is tuned viscoelastic damper (TVD) similar to a dynamic absorber or referred to as tuned mass damper. TVDs are generally applicable to reduce vibration/noise with a single frequency or a narrow band of frequency. Even if they are designed to reduce vibration/noise frequency at a given frequency, several TVDs with different frequency range have a wide band effect. The TVDs are very sensitive to the expected operating temperature range and the glass transition temperature of the viscoelastic material. Any temperature change in the damping material caused by energy dissipation into the internal heating is sufficient to alter the dynamic stiffness. This may lead the TVDs to detune itself. This characteristic of the TVDs makes elastomeric materials for TVDs only used in the rubbery region where slight changes in temperature do not have significant effect on the stiffness.

This paper first investigates new viscoelastic damping materials and appropriate damping treatments in sleeper carriages of electric trains. Further, experimental method on the measurement of vibrations and noise within carriages is provided. The effect of the damping material on vibration and noise reduction within carriages is discussed in term of physical parameters such as accelerations and sound pressure level. Finally, taking into consideration the subjective sound comfort within railway vehicle, loudness evaluated gives more valuable insight into the influence of damping materials on internal noise.