The mechanisms of tire/road noise have been explored for decades. Various research studies identified an extremely complicated mix of tire/road noise mechanisms (Sandberg and Ejsmont, 2002). The main distinct mechanisms identified from previous studies are listed in Table 2.1. There is generally no disagreement on the existence of such mechanisms, although the relative importance of their contributions is disputable. It is important to note that the term “tire/road noise mechanisms” is rather wide to cover the phenomena related to both the generation and propagation of noise. The noise generation mechanisms mainly include impact- induced vibration, friction/adhesion-induced vibration and air pumping effect, while the noise amplification mechanisms involve horn effect and acoustic resonance. The relative contributions among these mechanisms may vary depending on tire types,
46
pavement surfaces and operating conditions. This section briefly reviews the major mechanisms and provides some fundamentals on tire/road noise.
2.2.2.1 Impact-Induced Vibration
The structure-borne noise in tire-pavement interaction comes from tire vibrations. The main mechanism responsible for inducing vibrations in tire walls is the sudden displacement of a tread element, in relation to its “rest position” in a rotating tire, when it impacts the road surface (Sandberg and Ejsmont, 2002). At the leading edge, tread element is pushed in and at the trailing edge it is pressed out.
These vibrations radiate sound at frequencies generally below 1000 Hz (Descornet and Sandberg, 1980; Nilsson et al., 1980), and radiation is concentrated at the area around contact patch (Kuijpers and van Blokland, 2001; Yum et al., 2006) due to the damping of tire carcass. A rotating tire will be subjected to deflection around its circumference even if the tire and pavement are smooth. The existence of separated tread elements causes a disruption to the smooth displacement at the contact patch edges, leading to a “tread impact” mechanism. On the other hand, pavement surface texture, especially the macrotexture, also applies impacting on the tire tread with an equal importance, which could be named “texture impact”. It was highlighted by Sandberg and Ejsmont (2002) that impacts on tire tread by pavement texture or tread elements create substantial vibrations, emitting much higher noise than the case with smooth tire tread and smooth road surface.
The impact-induced vibrations contain both radial and tangential components (Larsson et al., 1999). In the case of “texture impact” (see Figure 2.16), the tire tread is displaced by an asperity at a certain angle relative to the road surface. The impact force is therefore neither perpendicular to the road nor radial to the tire. Similarly, tire tread is displaced both radically and tangentially for the “tread impact” as well. The relative magnitude of each component depends on the texture level, tread pattern and tire radius, while the frequency depending on the distance between tread elements or
47 texture asperities and the vehicle speed. In the direction perpendicular to tire middle plane, literal vibration also happens when tread and pavement texture are not uniform in the literal direction, but it is much lower than those in other directions (Bergmann, 1980; Dugan and Burroughs, 2003). It was found that differentiating pavement textures in the literal direction may be unnecessary in the tire/road noise modeling (Hamet and Klein, 2000; Wullens et al., 2004).
Besides tire tread, tire sidewalls also vibrate and radiate noise (Nelson, 1986;
Ruhala and Burroughs, 2008). Sidewall vibrations were found to be critical in the 500-1000 Hz frequency range (Donavan and Oswald, 1980; Eberhardt, 1984) and most significant near the trailing edge of contact patch (Phillips et al., 1999). Its excitation may come from circumferential tube resonances and tread block vibrations (Ruhala and Burroughs, 1998; Dare, 2012).
2.2.2.2 Friction/Adhesion-Induced Vibration
Besides the impact effect, tire vibrations are also induced by friction and adhesion effects at the tire-pavement interface. There are two major friction/adhesion- related mechanisms, both of which are related to pavement microtexture. The first mechanism is "stick-slip", in which tangential stresses in the rubber-road interface are built up and released. The second one is a "stick-snap" phenomenon due to adhesive bonds between tire tread and road surface, which are broken when rubber is pulled away from road surface. The contribution of these mechanisms to overall emission is low-to-moderate, with the exception of certain abnormal tire/road combinations (e.g.
sticky rubber tire and painted smooth pavement) (Sandberg and Ejsmont, 2002).
Stick-Slip Mechanism
The stick-slip mechanism can be visualized when considering a tire in acceleration (Dare, 2012). As a tread block enters the contact patch, it sticks to the pavement initially. As the wheel continues to rotate, tire tread element accumulated a
48
potential energy when pulled against the vehicle travel direction, until shear forces built up in the tread exceed the static friction forces. At this moment the tread element suddenly "slips" across the pavement to a position it can “stick” again. This process is repeated, resulting in tire vibrations. Another vibration source occurs as a tread element exists the trailing edge of contact patch (Wozniak and Taryma, 2004). It is exposed to accumulated shear stress, which is abruptly released when the friction force suddenly diminishes. The stored potential energy is partly converted to kinetic energy, leading to the free vibrations of tread elements. The friction-induced tread vibrations occur mainly in the tangential direction.
Stick-slip noise source exists at both the leading and trailing edges of contact patch (Richard et al., 1998), and within the patch as well. The frequency of the stick- slip noise is usually around 1-2.5 kHz, depending on normal force and vehicle speed (Kroger et al., 2004; Ryszard, 2001). The stick-slip mechanism is considered to be significant in the situations where great tangential forces are applied to tires, such as acceleration, braking or cornering. During free-rotation or when the driven wheel is rolling at a constant speed, the stick-slip mechanism is considered to be much less important (Sandberg and Ejsmont, 2002). Studies found that increased microtexture decreases the stick-slip vibrations within the contact patch by producing larger friction coefficient which can only be reached at rather high sliding speeds (Kroger et al., 2004). However, in such cases, strains induced by the shearing forces would not be relieved and, as a result, tangential vibrations at the trailing edge of contact patch may increase and become prominent (Ejsmont, 1990).
Stick-Snap Mechanism
Stick-snap is a phenomenon resulted from adhesion at tire-pavement interface.
When tire tread surface becomes “sticky” and pavement surface is very smooth, the strength of adhesive bond increases. This bond is stretched and then released at the trailing edge of contact patch. The tread element then gets back to its "rest position",
49 causing radial and tangential tire tread vibrations. It is difficult to separate stick-snap noise from slip-stick noise. It is also possible that stick-snap mechanism enhances the excitation of “inverse impact” effect when a tread element leaves the contact patch.
Although clearly presented from laboratory experiments, adhesion stick-snap mechanism does not contribute much to field tire/road noise since in-service pavement surfaces are usually covered by dirt that reduces adhesive bonds (Sandberg and Ejsmont, 2002). The stick-snap phenomenon is found mostly occur at the trailing edge of the contact patch (Richard et al., 1998; Kuijpers and van Blokland, 2001), and result in higher noise levels for frequencies above 1000 Hz (Nilsson et al., 1980).
The stick-snap noise can be reduced with higher microtexture on pavement surface, where the attraction force between tire tread and pavement surface is decreased (Kroger et al., 2004). It is less significant on wet pavements as well.
2.2.2.3 Air Pumping
Air pumping occurs when a tire rolls as air is compressed and gets pushed out at the front of the contact patch while another portion of air is expanded and sucked in at the rear. It generates vibrations in the surrounding air and constitutes a source of sound. This effect can be more significant if cavities exist on tire tread or pavement surface where air can be enclosed. A common analogy of air pumping mechanism is the sound made by clapping (Nilsson et al., 1980), where hands compress the air between them and force it out and vibrating to create sound.
Air pumping is one of the major sources of tire/road noise (Plotkin et al., 1980; Sandberg and Ejsmont, 2002). It is important at both the leading and trailing edges of contact patch (Richard et al., 1998). Air pumping noise evidently occurs at a 1-3 kHz frequency range (Ejsmont, 1992; Donavan and Lodico, 2009). Both tread pattern and pavement textures can affect air pumping noise. To reduce the noise generated by air pumping, it is necessary to connect all the cavities efficiently to the open air so that pressure gradients are lowered. This could be achieved by increasing
50
pavement porosity or macrotexture to allow air to escape when it is compressed by a rolling tire (Descornet and Sandberg, 1980; Nilsson, 1982; Fujikawa et al., 2006).
2.2.2.4 Air Turbulence
There is another potential mechanism that may contribute to rolling tire noise, i.e. air turbulence created by a tire rolling in the still air and displacing and dragging the air around the tire. There is a general consensus that this effect is a minor contributor to tire/road noise compared to vibration mechanisms and is relatively insignificant at highway speeds (Donavan and Oswald, 1980; Nilsson et al., 1980;
Eberhardt, 1984). From extensive experimental studies, Sandberg and Ejsmont (2002) concluded that air turbulence noise is negligible in most situations and it is unlikely to affect the overall sound level.
2.2.2.5 Horn Effect
It is noticed that most noise generation mechanisms (such as impact-induced vibration, friction-induced vibration and air pumping) exist close to the leading and trailing edges of the tire-pavement contact patch, where a horn-shape geometry is formed by tire tread and pavement surface. This acoustical horn forces a multi- reflection of the sound wave and hence significantly amplifies its magnitude. It provides a better match between the impedance of sound source at the contact patch and sound field around the tire (Schaaf and Ronneberger, 1982). Horn effect is believed to be the most important amplification mechanism for tire/road noise. It enlarges the overall noise level by 10-20 dB in the frequency range up to 2000-3000 Hz (Schaaf and Ronneberger, 1982; Kropp et al., 2000; Graf et al., 2002). The noise amplification is more significant when the excitation is closer to the contact patch.
Measurements also found that the horn effect leads greater directivity of sound towards the front and rear of a tire (Ronneberger, 1985).
51 Iwai et al. (1994) and Kropp et al. (2000) found that horn effect can be reduced when porous surfaces are used. Further studies have indicated that changing the porous layer thickness or adopting a double-layer porous system can optimize the amplification characteristics of horn effect (Lui and Li, 2004). Another horn effect reduction measure is to narrow tire width so that the reflecting area of the "horn"
constituted by tire tread and road surface is reduced.
2.2.2.6 Cavity Resonance
The acoustic cavity inside the tire carcass also contributes to tire/road noise amplification (Bschorr, 1985; Scavuzzo et al., 1994). The noise amplified by the cavity resonance is less important for exterior-vehicle noise emission than for interior-vehicle noise. The cavity resonance commonly does not affect tire vibration characteristics (Bolton et al., 1998), unless in some special cases where a structural resonance may couple with the cavity resonance resulting in further amplification.
The frequency of the cavity resonance is determined by tire and rim geometry.
Typical resonance frequencies for passenger car tires are 220-280 Hz while that for heavy truck tires is around 150 Hz (Sandberg and Ejsmont, 2002).
2.2.2.7 Helmholtz Resonance
Helmholtz resonance occurs as an action in a simple mass-spring vibration system. Applied to tire/road noise, the volume of tread cavity leaving contact with the road surface acts as a spring, and the air present between tread and pavement acts as a mass. This mechanism is observed when a trapped volume of air is connected to the outside through a narrow channel (Kinsler et al., 2000). As a cavity moves out of the contact patch, a mass-spring system is suddenly created at the moment when the cavity opens to the surrounding air. Along with the cavity moving further away from the contact patch, the volume and mass of the air immediately outside the cavity will increase. The resonance frequency is determined by the mass, spring and damping
52
constants, thus there is a “tone burst” associated with each cavity leaving the contact patch, starting at a high amplitude at a medium frequency and fading-off at a higher frequency. Nilsson et al. (1979) found that the Helmholtz resonance has most of its energy concentrated in the frequency range of 1000-2500 Hz.
Helmholtz resonance could be suppressed by good “ventilation” of the cavity formed by tire tread and road surface. The ventilation may be provided by a porous pavement or a ventilated tread pattern. The resonance frequencies may be shifted by changing the tread cavity volume or altering the tire diameter and thus contact patch length (Sandberg and Ejsmont, 2002). It is indicated that the Helmholtz resonance is closely related to the air pumping mechanism and these two mechanisms should be studied simultaneously (Kim et al., 2006).
2.2.2.8 Pipe Resonance
Another resonance mechanism existing in tire/road noise emission is the pipe resonance occurring within the tread grooves. In a pipe of length L and open at both ends, an air displacement may create a standing wave with a fundamental wavelength of 2L. If a pipe is closed at one end and open at the other, the fundamental resonance will be at a quarter of a wavelength. All tread grooves (with different shapes and dimensions) serve as pipe resonators to some extent when they are in contact with a smooth road surface. Pipe resonance is considered a major noise amplification for grooved tires on smooth pavement (Koiki et al., 1999). Saemaan and Schmidt (2002) found that a tire with longitudinal grooves is about 3.5 dB(A) noisier when compared to the same slick tire. The frequency of pipe resonance generally depends on the groove geometry (Oswald and Arambages, 1983), and is believed to concentrate in high frequency range.
Pipe resonance reduction may be realized in two ways. One is to avoid the tread pattern having a resonance frequency close the impact frequency. Another is to efficiently ventilate the grooves, which can be achieved by porous pavement surface
53 (Koiki et al., 1999). Similar as the Helmholtz resonance effect, pipe resonance cannot be completely separated from air pumping mechanism either (Kim et al., 2006).
These phenomena have to be considered simultaneously in the investigations of tire/road noise generation and amplification.
This section discussed the critical mechanisms in tire/road noise generation and amplification. It is important to note that all the mechanisms coexist in tire/road noise emission and interact with each other. It is impossible to extract each individual mechanism explicitly in practice. The relative importance of a specific mechanism varies with tire-pavement combination and operation condition. Nevertheless, some general consensuses can still be obtained from this literature review. Tire vibrations and air pumping are the most important noise generation mechanisms, while air turbulence noise is negligible to the overall noise level at highway speeds. Horn effect has a significant influence on tire/road noise amplification, while acoustic resonances are less important, especially for smooth tires. Some of the mechanisms can be reduced by applying porous pavement surfaces, such as the air pumping, horn effect, Helmholtz resonance and pipe resonance.