**3. Ride comfort evaluation**

Ride comfort, also called long-term comfort, of rail vehicles is affected by multiple parameters such as vibration, noise, temperature, smell, and visual stimuli. Derived from wheel-track interaction and rail motion, vibration is the critical factor affecting users' comfort and health, limiting their performance. Therefore, evaluating the vibration transmission in a rail environment is fundamental to quantify passengers' comfort levels and assess the harmful consequences of vibration [31–35].

Three main standards are fully dedicated and employed to evaluating passengers' comfort based on vibration analysis, ISO 2631, EN 12299, and Sperling ride index. The human body has its natural vibration mode, which affects the human vibration feeling. When this mode matches an externally induced vibration, resonance may occur, which, if absorbed, can lead to the physical stress of tissues and organs [10, 33, 35–39]. Furthermore, depending on the human tissue's characteristics, vibration with similar intensities but different spectral content will induce different dynamic responses in the human body. Thus, acceleration needs to be weighted based on human body sensitivity to obtain an index that can reflect the vibration feeling. Although different methodologies exist, the three methods mentioned above share the application of frequency weighting curves. Those capable of producing the highest effect are ranked with the highest weight, and other frequencies are attenuated based on their relative importance [10, 40]. All methods state acceleration measurements in three directions: vertical, fore-and-aft, and lateral.

It should be highlighted that, besides sharing the same assumptions and goals, the methods use different calculation techniques to evaluate comfort. Therefore, one method cannot be transformed into another just by analyzing its results. Instead, a complete analysis and correlation amongst indexes need to be performed [10]. ISO 2631, EN 12299, and Sperling ride index methods and their vibration evaluation techniques will be given. The present work followed the ISO 2631 standard approach. Thus, it is used as a reference methodology to identify rail track infrastructure sections and train maintenance needs will be demonstrated.

#### **3.1 ISO 2631 comfort evaluation**

ISO 2631 standard quantifies WBV regarding comfort, human health, and motion sickness. Comfort and health evaluations are related in many ways; thus, frequencies between 0.5 and 80 Hz are defined as the most relevant ones since, at this range, vibration affects the body as a whole, which can lead to discomfort and fatigue.

Acceleration measurements should occur on the vibration transmission interfaces: floor, seat surface, and seatback. Then, the root-mean-square (rms) acceleration is

calculated for each axis and the corresponding weighting curve is applied [36–38, 40]. The weighting process is calculated according to Eq. (1):

$$a\_{\mathbf{w}} = \left[\sum \left(W\_i a\_i\right)^2\right]^{\frac{\mathbf{w}}{\mathbf{h}}} \tag{1}$$

where *Wi* represents the weighting frequencies and *ai* the rms accelerations. Weighting curves application depends on the measurement location and purpose. The total vibration (*a*v) is achieved following Eq. (2):

$$a\_{\mathbf{v}} = \left(k\_x^2 a\_{\mathbf{w}x}^2 + k\_y^2 a\_{\mathbf{w}y}^2 + k\_x^2 a\_{\mathbf{w}z}^2\right)^{\ddagger} \tag{2}$$

where aw are the rms accelerations for each axis, and *k* represents the multiplying factor dependent on the measuring position, presented in **Table 1**.

Finally, based on *a*v, the discomfort is evaluated by a defined scale, **Table 2**, where accelerations higher than 0.315 m/s<sup>2</sup> are ranked as uncomfortable.

#### **3.2 EN 12299—mean comfort method**

The EN 12299 standard is a statistical method based on the rms method. The mean comfort is divided into two methods, the *standard method* and the *complete method*. The standard method only considers the floor vibration, whilst the complete method


**Table 1.**

*Frequency weighting curves and multiplying factors defined by ISO 2631 for comfort analysis of a seated passenger.*


**Table 2.**

*ISO 2631 comfort evaluation scale.*

uses floor and seat locations. Thus, the standard method is a simplification of the complete method.

The two method variations quantify the passenger mean comfort during a continuous 5 minutes run. Therefore, the measurement duration shall be a multiple of five, and a minimum of four zones traveled at constant speed must be accomplished to apply the method [11, 41, 42].

In opposition to the ISO 2631 method, the frequencies are initially weighted, and then the rms acceleration over 5 seconds is calculated for each axis. Finally, the 95th and 50th percentiles are determined for periods of 5 minutes, and the mean comfort index is obtained.

The mean comfort (*N*MVÞ is calculated following Eq. (3):

$$N\_{\rm MV} = 6\sqrt{\left(a\_{\rm xP95}^{\rm w}\right)^2 + \left(a\_{\rm yP95}^{\rm w}\right)^2 + \left(a\_{\rm xP95}^{\rm w}\right)^2} \tag{3}$$

where, *a*<sup>w</sup> P95 represents the 95th percentile of the weighted accelerations in the three directions, *x*, *y*, and *z*. The evaluation of *N*MV is defined based on a scale, **Table 3**. The scale considers values between 1 and 5, where a ride comfort index under 1 is considered a "very comfortable ride", and above 5 is considered a "very uncomfortable ride" [42].

This method presents some significant limitations; the use of the 95th percentile leads to data exclusion and the lack of possibility to correspond the track irregularity's location with the *N*MV values (the highest *N*MV values can occur during different 5 seconds time intervals). Moreover, measurements must occur at a constant speed for 5 continuous minutes, which is difficult to achieve during passenger service [42].

### **3.3 Sperling's method**

The special characteristic of Sperling's method is that the ride comfort index ð Þ *Wz* is evaluated individually for vertical and lateral directions. The calculation goes following Eq. (4).

$$\mathcal{W}\_{Zi} = \left[ \int\_{0.5}^{30} G\_i(f) \mathcal{B}\_i^2(f) \, \mathrm{d}f \right]^{\lessless} \tag{4}$$


**Table 3.** *EN 12299 evaluation scale.*

*Railways Passenger Comfort/Discomfort: Objective Evaluation DOI: http://dx.doi.org/10.5772/intechopen.111704*


#### **Table 4.**

*Sperling's method ride comfort evaluation scale.*

Where *Gi* corresponds to the double-side square acceleration [(cm/s<sup>2</sup> ) 2 ] and *Bi* represents the frequency weighting curve. As with the previous methods, the WBV level is evaluated based on a scale, **Table 4**. The passengers will not feel discomfort for values under 3 and will feel extreme discomfort for results above 3.5 [10].

This method is mainly applied to evaluate the vibration level of the vehicle rather than the users. Therefore, Sperling's method is specially used when comparing two or more train comfort rides. This method's major limitation is that vibration influences in different frequency bands and directions relating to sitting comfort are ignored [39].
