Track Decay Rate (TDR) Measurement Method for Reactive Damping by Tuned Mass Damper (TMD) Wilson HO 1 Jabez Innovation Limited Unit 601, 6/F, Block A, Shatin Industrial Centre, 5-7 Yuen Shun Circuit, Shatin, N.T., Hong Kong Marco IP 2 , Yi-Qing NI 3 The Hong Kong Polytechnic University, Hong Kong Branch of National Rail Transit Electrification and Automation Engineering Technol- ogy Research Center, Hong Kong

ABSTRACT TDR measures the rate of vibration decay along rail in dB/m. Higher TDR leads to lower noise radi- ation. However, new railways are often having low TDR due to the use of resilient fasteners (to pro- vide vibration isolation between rail and supporting structure for ground borne noise concerns), which leads to high noise radiation. Rail damper is used to increase TDR (thus reduces railway noise), where TMD is an efficient damping mechanism dissipating the vibration energy of the rail. TMD provides reactive damping force, maximised after a few cycles of oscillations. TMD force is stronger with continuous excitation than impulse excitation. For convenient purposes in the industry, TDR measurements are primarily conducted by impulse method, which do not allow sufficient time to include reactive TMD force. Therefore, impulse excitation TDR is smaller than continuous excita- tion TDR. Continuous excitation TDR is considered to reflect more of the real case of wheel/rail interaction excitation during train running. Besides, TDR in terms of time decay in dB/s is an alter- native approach for evaluating noise performance of the rail. This paper presents TDR measurement results of short rail (~6m) with resilient fastener support in a laboratory setting.

1. INTRODUCTION

Track decay rate (TDR), in dB per m (dB/m), is widely adopted by the railway industry to indicates how far the vibration of certain frequency could travel along a rail [1]. Low TDR allows accumulation of vibration energy and leads to high rail vibration and excessive rail noise. It is affected mainly by the trackform design. Rail dampers could be installed on the rail to improve TDR. TDR improvement is one of the parameters during rail damper selection.

Shearing Tuned Mass Damper (STMD) reported 2-3dB/m TDR improvement at its tuned fre- quency in both vertical and lateral direction and its noise reduction could achieve as high as 6dB(A) in-saloon [2]. Due to reactive mechanism of tuned mass damper with ~90 degree phase lag before responding to vibration, STMD damping effect may not be fully accounted for in the current TDR measurement method with single impulse excitation source [1]. In the real situation with multiple wheel excitation on a rail, the rail vibration gradually increases before reaching the maximum, and this allows sufficient time for the TMD reactive response.

1 who@jabez.hk 2 kkmip@polyu.edu.hk 3 yiqing.ni@polyu.edu.hk

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In this paper, TDR improvement due to STMD was measured in a laboratory and compared with typical impulse vibration excitation and prolong vibration excitation to allow STMD damping power to fully developed.

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2. STMD DESCRIPTION

Shearing tuned mass damper (STMD) is a type of rail damper to mitigate noise emitted from rail by dissipating its vibration energy. Latest STMD have the effective range of 40Hz to 1000Hz [3] and could be tuned into specific resonance frequency for extra noise reduction. STMD has reported to able to suppress corrugation growth [4-6]. Through the reactive operation of the oscillator at ~90 degree phase lag, the TMD damping effect is equivalent to many times of the original damping in the track system. In this measurement, the STMD were tuned at ~600Hz resonance frequency.

Figure 1: Rail Damper Design

3. METHODOLOGY

TDR measurements are conducted based on BS EN 15461:2008 [1]. The standard specified the rail to be excited by single impulse and widely adopted by the railway industry. Additional measure- ments were repeated with 4 second prolong vibration excitation and compare the TDR improvement after STMD installation between different vibration excitations at vertical direction.

3.1. Laboratory Setup

The TDR measurements were conducted in a laboratory setup with a 6m long UIC60 rail section fastened on resilient baseplates with a stiffness of ~22MN/m. Vibration excitation, either with shaker or impact hammer, were applied at the mid-span of second rail mounting spacing, as shown in Figure 2 below. Assigning the spacing where vibration excitation was applied as “0”, accelerometers were attached at the mid-span under rail of 0, 1, 2, 4 and 6 th spacing. Each spacing is 0.6m apart.

(vertical)

0.6m

Figure 2: Schematic of Laboratory Setup

3.2. Measurement Parameters

TDR measurements were conducted with 4 types of vibration excitation, 1) single impulse by hammer impact, prolong white noise excitation by shaker for 4s. TDR of Each type of vibration ex- citation were compared between before and after STMD installation.

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Vibration level time history were recorded at each accelerometer from before the vibration exci- tation until reaching background for further analysis.

3.3. Data Analysis

TDR of each scenario is determined at each 1/3 octave band frequency from 250 to 4000Hz. For prolong excitation, data were extracted at the end of the excitation for STMD to fully response to the excitation ( Figure 3 ). TDR improvement due to STMD installation is determined by comparing the TDR of corresponding scenario with and without STMD. TDR in terms of dB/m was determined by the average vibration level decayed between adjacent accelerometer along the rail from the vibration source.

Analysing period

Analysing period

Vibration Level, 250-4000Hz

Vibration Level, 250-4000Hz

Prolong excitation

Impulse excitation

by shaker, 2-4s

by hammer

Time

Time

Figure 3: Schematic of Data Analysis Range

4. MEASUREMENT RESULTS

The measurement results are shown in Figure 4 . Both impact and prolong vibration excitation shows a significant improvement around the tuned frequency at 630Hz from the baseline. Prolong vibration excitation produce less data variation and lower TDR at 250Hz to 500Hz.

The TDR with prolong excitation was improved by 6dB near the tuned frequency of 630Hz, from 2dB/m to 8dB/m, when STMD are installed.

For impact excitation, TDR was improved by 4.5dB at 630Hz, from 2.5dB/m to 7dB/m.

5. CONCLUSIONS

STMD damping effect may not be fully accounted for in the current TDR measurement method with single impulse excitation source due to reactive mechanism of tuned mass damper with ~90 degree phase lag before responding to vibration. In the real situation with multiple wheel excitation on a rail, the rail vibration gradually increases before reaching the maximum, and this allows suffi- cient time for the TMD reactive response.

Laboratory TDR improvement measurement was conducted for STMD and compare with impulse vibration excitation by hammer and 4s prolong vibration excitation by shaker for vertical direction.

With prolong vibration excitation, TDR improvement was increased by 1.5dB/m over impact vibra- tion excitation.

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Vertical Decay Rate TSI Limit Impulse w/damper Impulse w/o damper 4s Shaker w/damper 4s Shaker w/o damper

10

9

STMD tuned at ~600Hz

8

Decay Rate, dB/m

7

6

5

4

3

2

1

0

250 500 1000 2000 4000

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1/3 Octave Band Frequency, Hz

Figure 4: Averaged TDR Measurement Results in Vertical Direction 6. ACKNOWLEDGEMENTS

The funding support from the Innovation and Technology Commission of the Hong Kong Spe- cial Administrative Region to the Hong Kong Branch of National Rail Transit Electrification and Automation Engineering Technology Research Center (K-BBY1). 7. REFERENCES

1. BS EN 15461:2008+A1:2010 Railway Applications – Noise Emission – Characterization of the

Dynamic Properties of Track Selection for Pass by Noise Measurement. 2. Ho. W., Soltanieh G., Wong P., Wong W., Ip M., and Tse D., Noise Reduction Effect by Damper

on Single Rail, Proceedings of the 27th International Congress on Sound & Vibration (ICSV27), (2021) 3. Ho. W, Ip. M, Soltanieh. G, Wong W. and Tse D., Groundborne Noise Reduction by Rail Damper

Effect on P2 Resoance, Proceedings of the 27th International Congress on Sound & Vibration (ICSV27), (2021) 4. W. Ho, B. Wong, D. England, A. Pang, and C. W. S. To, "Tuned mass damper for rail noise and

corrugation control", presented at the Acoustics 2012 Hong Kong, 2012. 5. W. Ho, B. Wong, D. Tsui, and C. Kong, "Reducing Rail Corrugation Growth by Tuned Mass

Damper", presented at the The 11th International Workshop on Railway Noise (IWRN11), 2013. 6. W. Ho, D Tse, P. Wong, W. Wong, M. Ip, G. Soltanieh, “Suppression of Corrugation Growth by

Rail Damper”, presented at the The 27th International Congress on Sound and Vibration (ICSV27), 2021