Methods of Measuring and Processing Signals during Tests of the Exposure of a Motorcycle Driver to Vibration and Noise

The paper presents a new method of measuring a motorcycle driver’s exposure to vibration and noise. This method uses the simultaneous measurement of vibration and noise at the points of their direct influence on the motorcycle driver, and the measurement is carried out in real traffic conditions. The vibration of the motorcycle’s handlebars, footrest and seat as well as the noise reaching the beginning of the motorcyclist’s ear canal are recorded. These signals correspond to the direct impact of vibration energy on the upper and lower limbs as well as the torso of a motorcycle driver and the acoustic energy reaching the motorcyclist’s hearing organs. The paper also proposes a signal processing method which enables determination of vibroacoustic effects on the motorcyclist without frequency correction of signals and with taking into account the current standards, and therefore with the use of appropriate frequency corrections. The measurement section of the paper presents examples of results of measurements of the actual exposure of the motorcycle driver to vibration and noise.

The sources of vibration present in the means of transport include, inter alia, engines, drive train, suspension, tyre contact with the road pavement as well as wear and clearances [9][10][11][12]. Direct human contact with a vibrating machine can lead to changes in the human body, including long-term illness [1,2,[13][14][15]. The cause-and-effect relationship between vibration and illness is often not directly evident, e.g., due to the passage of time [1,2,16]. The exposure of humans and their organs to vibration depends on the area where vibration penetrates into the body [2,14,15].
Vibroacoustic interactions have a significant impact on assessment of the quality and comfort of means of transport. Research in this area is carried out as part of the development of means of transport [25,[45][46][47] and as part of the assessment of their impact on users [8,14,26,34,35,48]. Measurements of the so-called workplace exposure are also carried out in accordance with the guidelines set out in the relevant standards [15,49]. In the latter case, the vibroacoustic effects mainly affect the comfort of life and the condition of the transport infrastructure as well as infrastructure in the vicinity. In this case, the standards and test methods developed for this impact [50] are used and methods for its reduction are defined [39,[51][52][53].
Motorcycles belong to a group of means of transport whose negative impact on people and the environment is significant and can take on different formats. The research carried out in this area can be found, among others, in References [6,[54][55][56]. The direct impact of noise and vibration is related to the construction of motorcycles and the close proximity of the driver and passenger to the sources of vibration and noise [10,[57][58][59][60][61]. As presented in the papers [62][63][64], professional motorcyclists and frequent users of these means of transport are at risk of hearing loss. The studies presented in Reference [65] indicate that motorcycle users are very likely to lose their hearing even when using very noisy motorcycles only once. The problem of hearing loss and/or tinnitus is present in many motorcycle users [6]. The sources of this exposure are both engine and drive train noise, noise related to the aerodynamics of the motorcycle and its occupants, and air turbulence. Tests carried out in Reference [64] showed a much greater hearing loss in police officers using motorcycles on duty than in police officers performing typical office work.
Vibration interactions include primarily the transmission of vibration through the structure of the motorcycle from its sources, i.e., the engine, drive train and tyre-pavement contact, to the place of their reception, i.e., the upper limbs, lower limbs and trunk of the motorcyclist and passenger. The exposure to lower back pain is discussed in Reference [3] and problems with finger and shoulder diseases in References [61,66] The studies on exposure of motorcyclists to general vibration were carried out, among others, in Reference [57]. These studies assessed typical motorcycles used in Taiwan, which are often the main transport vehicle. These studies have shown instances where vibration can be exceeded, adversely affecting the drivers' health.
Motorcycle handlebar vibration tests carried out in Reference [67] led to the conclusion that prolonged use of a motorcycle may cause significant adverse effects on the drivers' health. In Reference [13], a high exposure of the health of motorcyclists to both hand-arm vibration and general vibration was demonstrated.
On the other hand, the vibration impact of motorcycle traffic on the surrounding environment is low, while an important role may be played by noise generated by these means of transport. As the authors' research presented in Reference [8] shows, the noise level is highly dependent on the type of motorcycle and the exhaust system used [45]. Examples of test results in Figure 1 indicate that the level of noise generated by motorcycles is similar or even higher than noise generated by passenger cars. The use of different types of exhaust systems in motorcycles significantly contributes to the change in the generated sound level [45]. The results of the tests in Figure 2 indicate that the motorcycle noise recorded near the area of the motorcyclist's head, regardless of the silencer used (R1-R4), was above 80 dB(A). Therefore, if a motorcycle is used for a longer period of time, it can affect the motorcyclist's hearing. The use of means of transport also involves the possibility of their involvement in various road situations. The research presented in papers [68][69][70][71] discusses how motorcyclists and other drivers perceive and behave in different situations on the road. The factors, presented in papers [72,73], that describe the sensory and cognitive visibility of motorcycles may also be disturbed by noise and vibration occurring in the means of transport. Noise and vibration to which motorcyclists are exposed can also reduce their concentration and alertness when driving a motorcycle, which can consequently lead to greater risk-taking by the drivers.
The paper presents a new method of measuring and processing signals during tests of a motorcycle driver's exposure to vibration and noise. The measurement method uses simultaneous measurement of vibration and noise in places where they directly affect the motorcycle driver, and in the signal processing method two ways of determining the effects of vibrations and noise on motorcycle drivers were proposed. The methods were tested during the measurements of the motorcyclist's exposure to vibration and noise in real traffic conditions, where the basic vibroacoustic effects on the motorcyclist were determined.

Method of Measuring the Impact of Vibration and Noise on a Motorcyclist
So far, the assessment of the vibration impact has been carried out with the use of devices which, in separate measurements, recorded significant vibration acceleration values in different locations on the motorcycle. The measurement of the noise reaching the motorcycle's hearing organ when travelling in real conditions-the measurement of the sound level under the helmet in the area of the auricle and the beginning of the external auditory canal-has not yet been analysed in detail.
At present, there are known devices for measuring vibration and noise of machines and devices, as well as their impact on humans, which are used to study the level of vibroactivity and noisiness and to analyse the technical condition and environmental hazards. Local vibration can be measured with single-axis and three-axis vibration acceleration transducers, which allow the vibration level to be determined depending on, for example, the current technical condition of the machine or device, but also, for instance, depending on the condition of the road surface. Noise measurements use microphones with sound analysers which enable, for example, the measurement of the sound level at workstations, and binaural microphones, which enable the measurement of sound directly reaching the human ear and which are used, e.g., to record music. The presented test methods have been described, among others, in PN-EN ISO 5349-1: 2004 [74], PN-EN ISO 5349-2: 2004 [75], PN-EN 14253 + A1: 2011 [76], ISO 2631-1: 1997 [77], PN-N-01307:1994 [78] and PN-EN-ISO 9612: 2011 [79].
As part of the paper a new method of measuring a motorcycle driver's exposure to vibration and noise was developed [80]. This method uses the simultaneous measurement of vibration and noise at the points of their direct influence on the motorcycle driver. The method was used to measure the motorcyclist's exposure to vibration and noise in real traffic conditions.
Parallel recording and analysis of vibration and noise using the proposed method enables global assessment of the motorcyclist's exposure to vibration and noise. By analysing, among others, time and frequency waveforms of the recorded values, it is possible to assess the process of influence of vibration and noise on humans, taking into account the propagation of noise and vibration energy and identification of places where this impact causes the greatest nuisance. Knowledge of this information can help to assess a motorcyclist's local and global exposure to vibration and noise, and is a valuable source of information on the vibroacoustic characteristics of the area where vibration and noise are transmitted, as well as on the possibilities of designing new solutions with reduced vibration and noise levels.
The method of measurement is based on the fact that, unlike the methods used so far, the actual assessment of the impact of vibration and noise on a motorcyclist is performed by using parallel measurements of vibration affecting the upper and lower limbs of a person and his torso together with internal organs in three mutually perpendicular directions, as well as the measurement of noise reaching the human hearing organ. The advantage of this method of measurement is the fact that on the basis of the recorded vibration and noise signals it is possible to conduct a parallel analysis of the recorded values, obtain characteristics of noise and vibration energy propagation to a person, and assess the level of global exposure of a person to vibration and noise, while taking into account their different propagation paths.
In the developed method, the measuring device used is characterised by the fact that it consists of a set of measuring instruments comprising two three-axis vibration acceleration transducers and a measuring pad equipped with one three-axis transducer, as well as binaural microphones and a data acquisition system. Transducer 1 is attached to the handlebars of the motorcycle, in the handle, in the area immediately adjacent to the position where the motorcyclist's hand is located. Transducer 2 is attached to the footrest in the area where the motorcyclist's foot is located. The measuring pad with Transducer 3 is mounted on the motorcycle's seat, in the place occupied by the motorcyclist during normal use. Binaural Microphones 4 are mounted in the area of the motorcyclist's auricle at the beginning of the external auditory canal under the helmet. Measuring Transducers 1 and 2, measuring Pad 3 and Microphones 4 are connected to the Data Acquisition System 5, in which the measured values can be recorded or transmitted to the next recording device.
The reference diagram of the mounted device is shown in Figure 3 in a system with and without the motorcycle driver. The proposed method is an optimal solution for assessing a motorcyclist's global exposure to vibration and noise, both in real traffic measurements and in laboratory conditions. It is characterised by full information on the characteristics of vibration and noise in areas where they affect people, which in turn makes it possible to obtain a full picture of the vibroacoustic hazards to which a motorcyclist is exposed. This method also makes it possible to eliminate errors resulting from vibration measurements which so far have been carried out separately and which do not take into account exposure to noise among the motorcyclist's health risks.

Application of the Developed Measurement Method
The measuring system selected for active experiments is presented in this section on the basis of the theoretical method for measuring vibration and noise affecting a motorcyclist in real traffic conditions, developed and presented in Section 2. Preliminary tests were carried out in order to validate the measuring system for road tests, and then measurements were made on selected motorcycle.
The test experiment was conducted on one tourist motorcycle equipped with an engine with a cylinder capacity of 599 cm 3  The research assumes that the influence of the motorcycle on the driver is a typical influence of a motorcycle on the motorcyclist and therefore does not require any additional ethical approvals.
The measurements were carried out using a measurement system consisting of: -two 3-axis PSB transducers, model 356A02, -two OKM II Rock-Klassik Studio Version binaural microphones made by Soundman, -measuring pad with a Svantek 3-axis transducer, type SV 39, -LMS Scadas XS recording system from Siemens, which enables simultaneous recording of the motorcycle's GPS position together with the dedicated measurement control software-installed on a tablet type portable computer. Figure 4 shows the diagram of the measuring system and Figure 5 shows the locations where the measuring system's elements were installed on the selected measurement motorcycle.

Signal Processing Method
The developed method enables the analysis of acceleration of vibration recorded in different locations on the motorcycle-points of direct contact between the driver and the motorcycle-and the level of sound pressure present in the motorcycle helmet at the beginning of the driver's ear canal. Since the impact of vibration and noise on people is one of the constantly recognised methods of research in the field of occupational health and safety, as well as medical research, it has been assumed that the signal processing methods should enable two types of analysis to be carried out. In the former case in the entire frequency band of the recorded signals not previously subjected to corrective filtering, and in the latter case for signals subjected to the frequency correction according  Figure 6 shows a diagram of the method of determining signals for further analyses. The most important symbols on the diagram: W h , W d , W k , W A -frequency weighing characteristics, x, y, z-vibration measurement directions, fft-fast Fourier transform, rms-root mean square of the signal.

Research in the Time Domain
The real levels of vibration and noise, which are present during the operation of a motorcycle and to which the motorcyclist is exposed, were analysed on the basis of the signals recorded during road measurements. In these tests, the directional distribution and the effective value of vibration at selected measurement points (their impact on humans), recorded at a constant travelling speed of 50 km/h, were evaluated. The levels of the sound in the helmet recorded at that time were also analysed. The results of these calculations are shown in Figure 7. These tests show that the greatest human exposure to vibration occurs from the handlebars to the upper limbs, smaller from the footrest of the motorcycle to the lower limbs, and the smallest from the seat to the torso (i.e., exposure to general vibration). The tests were carried out in three mutually perpendicular directions. When analysing the distribution of vibration energy in each of the directions considered, it can be observed that the highest vibration acceleration values were recorded in the Y and Z direction, i.e., perpendicular to the direction of motion. Direction X-parallel to the direction of motion-was characterised by the lowest level of vibration amplitude for interaction from the seat and handlebars.
In the next stage of the research signals recorded at various quasi-stationary values of motorcycle travelling speeds were analysed. During these measurements, the motorcycle travelled at speeds of 50, 70, 90 and 110 km/h in gear 2, 3, 4 and 5 respectively. The aim of these experiments was to analyse the influence of such factors as travelling speed, engine speed and motion resistance on vibration and noise affecting the motorcyclist. Figures 8 and 9 show values calculated without using correction curves as well as frequency corrected values. The general vibration acting on the driver's torso is significantly lower than vibration present on the footrest and handlebars. The vibration levels are in the range of 1.7-28.83 m/s 2 for the seat and 11.2-119.47 m/s 2 for the footrest and handlebars. With unadjusted measurements of the sound pressure level-linear measurement in the entire band-the value of noise increases non-linearly with the increase in motorcycle travelling speed and ranges from 98.18 dB to nearly 119.35 dB. When analysing the test results, it can be observed that in order to reduce the negative impact of the motorcycle on the driver the highest operationally viable gear ratio of the motorcycle gearbox should be used for propulsion as quickly as possible.
Carrying out vibration acceleration processing using appropriate corrective characteristics W d , W k and W h and application of weight coefficients k made it possible to calculate levels of vibration acting on the driver through the seat, footrest and handlebars of the motorcycle, and application of correction A enabled calculation of noise reaching his hearing organ.
The The values of local hand-arm vibration acceleration, i.e., values recorded on the handlebars, were each time higher than the daily mechanical vibration exposure limit value (A (8)limit = 2.8 m/s 2 ) and at a higher engine speed they were higher than the short-duration mechanical vibration exposure limit value (a w,30min, limit = 11.2 m/s 2 ). Measurements of the sound level in the helmet using correction curve A indicate that sound level 84 dB(A) is exceeded already at a motorcycle speed of 70 km/h. The recorded maximum values were above 94 dB(A).

Research in the Frequency Domain
The impact of vibration and noise generated by means of transport on users and the environment should also be considered in the frequency aspect. This approach allows identification on the basis of the recorded vibroacoustic signals, their main components and their frequency distributions. The assessment of the phenomenon of generation, propagation and influence of vibration and noise carried out based on this is the basis for conclusions on the impact of these signals on the health and comfort when using means of transport.
A frequency analysis of the influence of the change of motorcycle travelling speed (engine speed) on the structure of signal spectra and their influence on the motorcyclist was carried out. Signal processing was performed for signals that were not frequency corrected. The calculation used signals recorded at a constant motorcycle speed of 50, 70 or 90 km/h in the third gear, which allowed the use of low, medium and high internal combustion engine revolutions. The calculated spectra for the recorded signals are shown in Figures 10-13.
The determined signal spectra indicate that the dominant frequencies of vibration and noise signals are those associated with the combustion process in the engine. They occur in the frequency range above 100 Hz and increase with the motorcycle's speed and therefore with the engine speed. At a travelling speed of 70 km/h and 90 km/h, in the spectrum of signals recorded for the seat and footrest of the motorcycle, there are additional frequencies that are significantly lower than the dominant frequency. When analysing the noise spectrum, in addition to the combustion frequency in the engine, an increase in noise can also be observed at increasing travelling speeds in the low frequency ranges.    In further studies, the change in the structure of the spectra of the recorded vibration and noise was evaluated. This test type makes it possible to examine the main components present in the signals and their impact on motorcyclists at a variable motorcycle travelling speed (motorcycle engine speed). Time and frequency distributions of signals recorded during the acceleration of the motorcycle were calculated. Short-duration spectra were calculated using a fast Fourier transform (fft). The signals recorded during the rapid acceleration of the motorcycle from the speed of about 45 km/h to about 100 km/h in the third gear, frequency corrected and not, were used for the calculations. Figure 14 shows the calculated time and frequency distributions of vibration affecting the driver through the seat and lower and upper limbs in the Z direction, as well as the recorded sound pressure level in the helmet. Time and frequency distributions of vibration enable identification of changes in spectrum components depending on the motorcycle travelling speed and, therefore, its engine speed. In these distributions, a significant increase in signal amplitudes can be observed due to the combustion process and its harmonics in the range from about 100 Hz to about 250 Hz. The bands of these frequencies are predominant in the spectrum regardless of the measuring point and overlap in the selected speed ranges with the natural vibration of the selected organs of the human body.
The change of the spectrum structure in the low-frequency range-up to 80 Hz-is characterised by low energy values of vibration levels. As the speed increases, an increase in amplitude of these frequencies is also observed, but their value is significantly lower than the frequency of the combustion process in the engine. The signal processing carried out with the use of band-pass filters made it possible to analyse the exposure to vibration transmitted by the human body with general and local effects. In this case, increases in the amplitude of low-frequency vibration, and therefore an increase in exposure of humans to these effects, was observed. It can be noted, however, that the application of these filters leaves in the vibration signal a strongly energetic component associated with the combustion process.
Measurements and processing of noise signals not subjected to frequency filtration tests indicate strongly energetic changes in the amplitude of sound pressure in the low-frequency range. The amplitude value increases as the rotational speed increases. When using correction Filter A, this noise becomes significantly lower energetically in the low-frequency range and therefore, despite the increase in its amplitude, it does not create a significant nuisance for the motorcycle driver. However, local increases in sound amplitude can be observed in this signal at a higher engine speed, which is caused by the fact that filtration A leaves the components associated with the combustion process, i.e., frequencies above 200 Hz.

Conclusions
At present, means of road transport are subject to increasingly higher requirements in terms of minimising their negative impact on the environment and ensuring high comfort of use. These requirements mean that the development of means of transport requires the use of highly advanced design methods and complex systems for testing prototypes.
The emission of vibration and noise is among the significant impacts of means of transport on the environment and their users. In vehicles, this problem can be considered as an aspect of ensuring a sufficiently high level of comfort of use, and also as a factor contributing to health deterioration. While analysing the literature, it was found that motorcycles belong to the group of means of transport which have a negative impact on their users. The main problem here is the exposure of motorcyclists to vibroacoustic signals penetrating the human body through the upper limbs, lower limbs, trunk and hearing organs.
The method of measurement of vibroacoustic signals proposed in the paper, which takes into consideration general vibration, vibration affecting lower and upper limbs as well as noise present in the helmet, is a new approach to the possibility of assessing vibroacoustic effects on motorcyclists. It also allows a broader look on the motorcycle as an object that generates vibration and noise. The use of the measurement system proposed in the publication, which carries out the measurement simultaneously in various areas of the motorcyclist's exposure, enables a later analysis of specific cases of interactions. This type of measurement system also enables analyses with high sampling rates, which is particularly important when analysing fast changing phenomena. In this case, traditional signal analysers, such as those that determine the equivalent sound level or the maximum sound level, are not sufficient.
In the paper this method was used to evaluate the exposure of motorcyclists to vibration and noise and to compare the level of this exposure using different types of motorcycles. The developed method can also be used to evaluate the motorcycle itself as a source of vibroacoustic signals, the reduction of which is one of the most important tasks pursued in research and development centres dealing with the comfort of this type of means of transport.
The conducted identification tests of the proposed method of measurements in real traffic made it possible to record the waveforms of vibration signals of selected elements of the motorcycle and noise present in the motorcyclist's helmet. This therefore confirmed the practical utility of the method for testing motorcycles and their vibroacoustic impact on users in real traffic, as proposed in the patent application.
In connection with the information contained in numerous scientific papers on possible negative impacts of vibration and noise on humans in a wider frequency range than previously considered in the standards, the paper analyses signals in the entire frequency range of signals not subjected previously to the recommended corrections, and applies recommendations of standards for the relevant interactions. These studies show that vibration and noise not subjected to frequency correction were characterised by significantly higher values than frequency corrected signals. Consequently, significant vibroacoustic effects on motorcyclists occur in frequency bands which are omitted from the standards or which are in a near range of frequency bands recommended by them.
When comparing the test results, it can be concluded that the level of vibration affecting the driver through the motorcycle's seat is the lowest among those recorded. Vibration affecting the lower and upper limbs is significantly higher than for the seat. When analysing the vibration and noise values, it can also be concluded that the vibration levels of the seat and footrest do not exceed the limit values currently adopted in the standards, while the handlebar vibration and the sound level in the helmet may exceed these values. This depends mainly on the way the motorcycle is used and on its type.