Tyres and pavement noise

Tyre / Pavement noise

Tyre Noise Source Mechanisms are described as follows:

Air resonance
- Pipe resonance is due to continuing waves, which action in the canal of the annoy tread. The beating complete will accept a amicableness of alert the aqueduct breadth if the aqueduct is accessible in either ends, or four times the aqueduct breadth if it is accessible in one end only.
- Helmholtz resonance - the aggregate of air in a atrium will act as a bounce resonating with the accumulation of air in the "throat" amid the atrium and the alien air [13].
- Air pumping in the tyre rolling process, air is trapped and aeroembolism in baby voids amid the footstep and pavement. Babble is generated back this aerial burden air is ejected to the atmosphere at the avenue of the acquaintance patch. Because of the aerial acceleration of the air jets created by this process, air pumping can be a above contributor to broadcast noise. The Helmholtz resonance ability amplify the noise.
Tread element vibration is produced by the blow amid the footstep blocks and alley or the acerbity of road. Getting in acquaintance with and abrogation the pavement accomplish footstep vibration, which is a above antecedent of tyre babble radiation.
Carcass vibration - as the tyre interacts with the alley macrotexture, accordance are generated in the tyre carcass. The footstep apparent and ancillary bank approach shapes of aerial frequencies comedy a important role in rolling babble generation. For ample admeasurement cross-bar bent barter tyre, cardboard [16] considers the beating of the ancillary bank the above babble source. The assignment [14] estimates for commuter car tyre that a approach appearance of atom agreement is generated at the position aloof afore the arch bend and appropriate afterwards the abaft bend back the tyre comes in acquaintance with the alley and rotates.
Circumferential distribution of the tread pattern - the first- and second-order apparatus of the footstep arrangement act as capital agitative forces. The first-order basic of the arrangement excites the ancillary bank accepting low activating stiffness, and makes the ancillary bank afford the complete apparent directly. The second-order basic of the arrangement excites the footstep apparent area, which vibrates aloof afore and afterwards the acquaintance patch.

A horn-shaped semi-closed space formed between tread surface and the road surface amplifies the sound pressure level.
Noise of rolling tyres is affected by the following factors:

Design factors - tyre construction, tread design;

Operational factors - speed, load, inflation pressure, pavement surface, steer angle.

Of these factors, the most important are tread design and pavement surfaces.
Rolling Speed influence on the noise level
Rolling tyres contribute to overall vehicle noise at cruising speed on smooth highways. Figure 1 presents the effect of speed for the four basic types of heavy truck tyres.

Figure 1 - Noise level vs. rolling speed for four basic types of heavy truck tyres
Figure 1 offers the following information:

for the same type of tread design and rolling speed, the noise level of radial tyre is lower than that of bias tyre;

for the same tyre construction and rolling speed, the lowest noise level is produced by a rib tread design;

increasing the rolling speed involves increase of noise level for all tyre types; but the increasing rate for a lug tread design is greater than that for a rib tread design (10 dB(A) from 50 km/h to 100 km/h against 5 dB(A) in the same speed range)

Pavement influence on the noise level
The typical rib design tyre may be as much as 8 dB(A) noisier on a brushed concrete surface (sound level of 78 dB(A)) than on smooth concrete (sound level of 70 dB(A)). This difference is not as large on a lug design [11].
Frequency analysis of the tyre rolling noise
The sound tonality is very important for people, because for two tyres which have exactly the same sound pressure level, one tyre may sound harsh and disagreeable and the other quite acceptable. The difference between the two sounds is their frequency spectrum. A tall peak in the frequency spectrum may originate from the repetitive nature of tread pattern. A hard work has been done for eliminating tonality problem through minimizing the repetitive features in the tread design.
Figure 2 shows a frequency spectrum from a radial rib truck tyre, measured in the semi-anechoic noise chamber [12]. Figure 3 presents a typical spectrum of noise from a cross-bar bias truck tyre, measured in an anechoic room, with the microphone positioned at a distance of 2 m from the tyre [16]. The cross-bar truck tyre has several tall peaks in spectrum at special frequencies which correspond to the number of lug-to-road contact points in the unit time and integer-multiplied [16]. The rib design truck tyre has a single tall peak in the noise spectrum, probably due to side wall resonance.

Figure 2 - Frequency spectrum of radial truck tyre rolling noise [12]

Figure 3 - Frequency spectrum of cross-bar bias truck tyre [16]
Sound intensity level on the contour map of the tyre
Noise generating portions of a passenger car tyre rolling on a drum, without driving torque were measured. Four noise generating portions are identified from the contour map of sound intensity level shown in figure 4 [14].

Figure 4 - Contour map of sound intensity level [14]
The noise generation portions are:

Side wall area right above the trailing edge, near the contact area of the tyre; this vibration is synchronized with the first-order component of the tread pattern (400 Hz to 600 Hz for passenger car tyre);

Leading edge area which radiates the sound in a wide frequency range from 800 Hz to 2 kHz; the sound frequency spectrum includes the second-order component of the tread pattern;

Trailing edge area which radiates sound having the same frequency range as that of the leading edge area, but having a slightly higher sound pressure level than that of the leading edge area;

Upper area of the wheel housing which radiates sound of frequency from 500 Hz to 600 Hz; it seems that a sound resonance system formed between the wheel housing and the road surface is excited by the main sound sources located in the three areas presented above.

Figure 5 - Acoustic intensity distribution of a cross-bar bias truck tyre [16]
When the tyre rolling speed is modified, the peak frequencies of the spectrum in figure 3 also are changed. If a peak frequency reaches 450 Hz or its vicinity, the peak magnitude increases drastically [16]. Figure 5 shows the acoustic intensity distribution of peak frequency of 450Hz, at rolling speed of 50 km/h, for a cross-bar truck tyre. From Figure 5 one can determine that this frequency component is generated from the side wall and its vicinity, just above the contact patch.
Tread Pattern influence on tyre/road noise
The influences of tread parameters of a passenger car tyre on the A - weighted sound levels and noise spectra are presented in table 1 [13].
Table 1 - Influences of tread pattern of a passenger car tyre on the rolling noise [13]

Tread patterns have an unequal pitch arrangement in the tyre circumferential direction in view to dispersing the pattern noise synchronized to the interval between neighboring patterns. The dispersing regards the modification of pure tone sound, which is offensive to the ear, into sound of a wide frequency range, which is less offensive to the ear. Changing the interval between the neighboring patterns, as shown in figure 6, does this [14].

Figure 6 - Frequency modulation due to pattern pitch [15]
The basic factors of the cross-bar type tread pattern of a truck tyre are: groove width (W), groove depth (P), groove length (L), and groove angle (a), figure 7. The smaller these factors become, the lower the noise level.

Figure 7 - Basic factors of the cross-bar type tread pattern [15]
The factor values have to be determined after due consideration of the balance with other characteristics, such as the traction and wear resistance.
For the tread pattern of three cross-bar truck tyre, whose design is presented in figure 8, the influence of the groove angle on the noise level is shown in figure 9. The groove angle diminishes starting with ULX, UL2 and then downs to UL5 (where the angle is 0^o [16]. Considerable noise reduction is obtained merely by improving the tread pattern.

Figure 8 - Tread pattern of three cross-bar truck tyre

Figure 9 - Noise level vs. highway rolling speed for the tread patterns in figure 8 [16]

Methods for reducing the tyre rolling noise

Optimization of tyre construction - increasing the stiffness of the tyre carcass (for instance, increasing the number of the ply sheets, adding reinforcement rubber, and using steel ply materials).

The tread radial vibration

Tread design modifications (changes of tread shape, arrangement, and size); for instance, utilizing the circumferential-oriented grooves;
Reducing the hardness of tread rubber;

The pipe resonance

Controlling the frequency spectrum by changing the groove length(s);
Controlling the sound amplitude by changing the angle and width of the grooves, and intersecting them with another ones;
Tread pattern randomization for decreasing the noise level which may occurs when pipe resonance and tread impact frequencies coincide at certain speeds;

The air pumping