EFFECT OF LOW FREQUENCY VIBRATION ON TRAIN PASSENGER COMFORT

Abstract

Trains are a preferred mode of transport for millions of people in India. The number of passengers who want to utilize their travelling time effectively for performing various activities like reading, writing as well as wc-Tking on computer is increasing day by day. Moreover, even though trains offer many facilities to the passenger in addition to comfort and safety, recent studies revealed discomfort of passengers due to interference in sedentary activities caused by vibration and noise aboard trains. In order to alleviate passengers' discomfort, it is imperative to study the effect of vibration and noise on comfort. Comfort may be broadly classified into two types: a) the sense of well being and b) ability to perform sedentary activities, also known as activity comfort. The influence of vibration on comfort depends on its direction, magnitude and frequency. Additionally, the effect of noise depends on duration of exposure and sound pressure level introduced. Moreover, the comfort is largely affected when the combination of environmental stressors such as vibrations, noise and temperature act simultaneously. Majority of published studies have considered either the vibration in single direction with its magnitudes and frequencies much higher than the real situations experienced in trains or noise alone. Further, such studies ;;re not available in context to train travel in India. This issue attains a greater significance since more than 13 million passenger travel every day by train in India, therefore it is obligatory to address their health and safety issues. Therefore, with an aim of investigating the effect of vibration and noise on train passengers' discomfort, it was decided to conduct a study under controlled laboratory conditions. In order to accomplish this aim, the following principal objectives of the study were fixed as: ➢ Design and fabricate a simulator chamber which is a mock-up of passenger train coach and capable of simulating controlled train environment such as vibration and noise etc., for conducting the investigations. ➢ Investigate the train passenger comfort by assessing discomfort in terms of subjective and objective measurements under different vibration and noise conditions. ➢ Investigate the effect of vibration on activity performance for different vibration conditions. iii Further, it is an arduous task to exactly recreate the real vibration and noise environment of trains in the laboratory. Hence, it was decided to choose the frequency of vibration as 5 Hz, where the first resonance of the body appears under vertical vibration and is responsible for more discomfort. The vibration magnitudes and noise levels were selected based on a previous study pertaining to Indian passenger trains. The present study considered four sinusoidal vibration conditions consisting of vibration in individual axes X, Y and Z and multi-axis vibration along with a control condition (no vibration). Acceleration magnitudes of 0.6, 0.9 and 1.2 ms-2 in each vibration condition and noise levels of 54, 62, 70 and 78 dB LAeq (weighted for one minute) were used. The simulator chamber, a mock-up of passenger train coach, is indigenously designed and fabricated in the Vehicle Dynamics Laboratory; IIT Roorkee It consists of three main systems namely, vibration platform, noise generator and temperature and humidity controller. The vibration platform is centrally housed in a chamber (room) with three electro-dynamic exciters attached in mutually perpendicular directions so as to provide excitations to the platform, either individually or simultaneously. The vibration platform was fabricated from corrugated steel sheets, on which two chairs, one table and a couch are rigidly fixed to perform experiments with maximum of three participants at a time. Noise generator consists of speakers and woofer to replay the recorded train interior noise. The temperature and humidity controller consists of a controller unit and ducts for supplying fresh air into the chamber and drawing out the circulated air. A pilot study was conducted to check the validity of vibrating platform. It was found that the vibration platform is capable of generating 4 to 32 Hz and 2 to 20 Hz frequencies under sinusoidal mode and random modes respectively in uni-axial as well as multi-axial directions at a maximum vibration magnitude of 6 ms-2 with a maximum pay load of 320 Kg which is equivalent to average weight of three persons. The noise, temperature and humidity levels that can be maintained in the chamber are 54 - 79 dB LAeq, 17-35°C and 28-85% relative humidity (RH) respectively. Discomfort was assessed by measuring physiological and-psychological responses from 12 participants. The physiological parameters considered were Galvanic Skin Response (GSR), Mean Respiration Temperature (MRT), Pulse Rate (PR), Respiration rate (RPR), root mean squared Electromyography (r.m.s EMG). Parameters of Heart Rate Variability (HRV) such as sympathetic tone, vagal tone and LF/HF ratio were also iv measured. In total, 19 exposure conditions were considered. The test protocol consisted of '15 min' exposure to a randomly selected combination of vibration and/or noise conditions to each participant sitting in upright position which was followed by 10 min recovery period. The results showed a considerable increase in activation of sympathetic system and reduction of parasympathetic system in the range of 0.6 -1.2 ms-2, thereby indicating discomfort among the participants. However, from the control condition to 0.6 ms-2, activation of parasympathetic system and reduction of sympathetic system suggested existence of comfort among the participants. The results of statistical analysis revealed insignificance between exposure conditions and most of the physiological variables. The psychological responses were measured by taking subjective rating of discomfort. It evaluated by marking on a seven point rating scale with two end values at '1' and '7', marked as no discomfort and extremely discomfort respectively for 14 psychological symptoms. On the whole, lower ratings by the participants showed that there was minimal discomfort among the participants. Activity discomfort was assessed using FiVslaw in an experiment in which 11 participants performed a 'pointing and clicking' task on laptop using a computer mouse while exposed to various vibration conditions. A combination of four vibration directions and three vibration magnitudes and a no vibration (control) condition yielded 13 conditions which were fixed as exposure conditions. A program code in Labview was developed to generate the task whereir: a white button of invariable size and a yellow button of different sizes were generated on the laptop monitor at random locations. The task required participants to move the cursor from white button to yellow button and click within its boundary using a standard optical mouse. Time taken to move the cursor and number of errors committed while performing the task were considered as the indicators of discomfort. The program also calculated the number of times clicking was done outside yellow button boundary during the task execution (error) and movement time of cursor from white to yellow buttons in seconds. It was revealed that both the vibration direction and magnitude considered in the study, affected the pointing and clicking task. Further, it demonstrated that certain sizes, distances and angle of approaches affected the performance. Multi-axis vibration and higher vibration magnitudes showed more discomfort for task performance. From the results, it was concluded that physiologically and psychologically, the human performance was influenced by different vibration directions, magnitudes and noise levels that were considered in this study.