The acoustic signals of a moving road vehicle are generated mainly by the move- ment of various components, such as a power unit and tyres but also others like an exhaust system [Thomas and Wilkins, 1970; Srour and Robertson, 1995; Wu et al., 1999]. Signals of the current research’s interest are vehicle exterior signals rather than the interior ones when it is assumed a vehicle is moving within the legal speed range. Note that under such an assumption, consideration of the aerodynamic noise generated around the vehicle body is not usually required [Sandberg and Ejsmont, 2002]. Also the horn and/or burglar alarm sound as well as any additional sound sources that may be equipped in a vehicle (e.g. for warning, entertainment, or mar- keting purposes), are outside the focus of this research.
With these assumptions, acoustic signals of a moving vehicle are generated by the vibration caused in the engine itself, the resonance of the exhaust pipe as the waste gases pass through, the tyre friction noise as well as vibration of the rest generated both directly and indirectly mainly but not only, by the above three notable sound sources within a vehicle [Thomas and Wilkins, 1970, 1972; Karlsen et al., 1995; Srour and Robertson, 1995; Nooralahiyan et al., 1998; Wu et al., 1999; Wang and Qi, 2002; Munich, 2004; Necioglu et al., 2005]. Since they involve multiple types of motions and materials, the characteristics of the resultant signals are not simple. This consequently suggests some complexity concerned with the current research in understanding the sound source attributes and their modifiers in terms of a recog- nition target.
(a) Power Unit
In the power unit of a common four-stroke petrol engine car, the core movement occurs in the crankshaft, which is connected to the rod bearing, then connecting rod and then the piston that moves up and down; causing the four main steps per- formed in one cycle of a four-stroke engine. The four steps are “intake”, “com-
3.1. SIGNAL GENERATION
pression”, “power”, and “exhaust”. The four-stroke is initiated from two cycles of the crankshaft’s movement triggering the piston to go down and up twice. During intake some petrol and air mixture is sucked in as the piston goes down. The petrol is brought in to the combustion chamber and gets mixed with the air. Then the mix- ture is compressed whilst the piston moves up. Next, a spark ignites the fuel and the combustion happens as the piston goes down for the second time, then the exhaust gas is discharged into the exhaust pipe to be thrown out to the exterior while the pis- ton goes up again to complete the four stages and be ready for the next cycle [Ofria, last accessed in 2009]. The noise generated by the power unit has a strong relation to the above operations. It is suggested that its main sources are caused either by the changing pressure of the combustion chamber and the mechanical movement of the various parts [Nelson, 1987] or the recurring ignition [Succi et al., 1999].
(b) Tyre Friction
In terms of the acoustic signals generated by the tyres, vibration caused by the rota- tional movement and the contact with the ground surface as well as the signals due to aerodynamic forces are the main source. Through modelling studies [Kujipers and van Blokland, 2001] and their empirical examinations [Kropp et al., 2000; Lui and Li, 2004], it was confirmed that the geometry between a tyre and the ground surface, of which the tyre has contact with, has some notable influence called the “horn effect” [Sandberg and Ejsmont, 2002; Cevher et al., 2007] on amplifying the sound radiated by the tyre. It has been recognised [Sandberg and Ejsmont, 2002] that at a higher travelling speed, sometimes the aerodynamic noise due to wind tur- bulence becomes more important, but often the distinction between that and the tyre noise is a challenging task.
Sandberg [Sandberg and Ejsmont, 2002] also pointed out that there are some dif- ferences between the dominant influential factors for power unit noise and that for tyre noise. The power unit noise is mainly affected by both the power of and the
3.1. SIGNAL GENERATION
revolving speed of the engine. On the other hand, for the tyre noise, the vehicle’s travelling speed as well as the combination of the tyre’s properties and the road surface condition are significantly more important. Also the forces caused by ac- celeration, breaking or cornering influence the latter.
It has been suggested that while the tyre friction noise increases proportionally to the travelling speed, the acoustic signals generated by the power unit are independent of speed variation [Sandberg and Ejsmont, 2002]. Consequently, a combination of the lower vehicle travelling speed and the higher engine repetition speed usually results in the signals generated by the power unit becoming greater than the tyre friction noise. On the other hand, at higher vehicle moving speeds, the lower engine repetition speed and the lower engine power can lead to domination by the friction noise [Nelson, 1987].
In terms of the speed at which the tyre friction noise becomes more dominant in acoustic signals generated by a moving vehicle, Rustighi [Rustighi et al., 2008] stated the threshold as at 40km per hour based on [Sandberg, 2001], whereas some others [Christou and Jacyna, 2005; Jacyna et al., 2005; Cevher et al., 2007] have said it is at around 50km per hour. However, it should be noted that some [Nelson, 1987; Sandberg and Ejsmont, 2002] have also pointed out that it can depend on multiple factors, including the ground surface conditions. Sandberg and Ejsmont showed, in addition, a discussion on how each sound source contributes towards the overall acoustic signals of moving vehicles, and also how that depends on types of vehicles, together with how all these have changed over the last several decades as a result of the progresses seen in the technology, legislation and the market trend.
In terms of the level of noise produced by vehicles in different classes, furthermore, it may be worth noting here that the acoustic signals generated by a heavy vehicle are often louder than that of a small car [Nelson, 1987; Sandberg and Ejsmont, 2002].
3.1. SIGNAL GENERATION