Generation mechanism and affecting parameters
There are mainly, three major sources of noise in a vehicle i.e., wind noise, engine-powertrain and tyre/road noise. In general, these sources contribute to 10-15%, 70-80% and 10-15% respectively, but tyre noise alone can account for 75-80% of noise at higher cruising speed like on highway driving situations. Tyre noise, typically lies in the frequency range of 500-2500Hz.
Tyre noise improvements in last few years has taken a critical role in reducing the sound signature of a vehicle. Decrease in noise from internal combustion engines and low noise levels of electric vehicles demand more development in tyre industry to reduce noise. Tyre manufacturers spend a lot of time and money for R&D and perform a lot of tests in order to meet lower noise level without compromising on the performance (safety, braking, ride comfort etc). Road noise is a major concern in urban areas mainly near hospitals, schools, libraries etc.
Below Picture summarizes the tyre noise and its causes and measuring methods.
In this blog we will be understanding Part -I of the above picture i.e. tyre noise with focus on generation mechanism and affecting parameters. In part-2 of the blog we will be discussing about test methodology and quantifying parameters.
Mainly there are three types of mechanisms by which tyre generates noise, namely,
- Impact Mechanism,
- Adhesion Mechanism and
- Air-Displacement mechanism.
Impact Mechanism-When tyre rolls on a road, tread of tyre hammers on the non-compressible road surface, which causes deflection of the tread along the circumference of tyre. Due to this hammering and deflection, Forced and Free vibrations are induced in the tyre. This causes Vibroacoustic noise mainly in the frequency range of 300 to 1500 Hz. This noise is generally called as whining noise.
This frequency range depends on tread length and vehicle speed as defined in below equation.
F= Frequency [Hz]
V= Velocity of vehicle[m/sec]
L= Tread length[m]
For example, velocity of vehicle is 30km/hr and tread length is 40mm, Frequency will be 208Hz.
Adhesion mechanism- This mechanism comes into action during sudden acceleration, cornering and braking. Adequate amount of traction between tyre and road is important for safe driving. In the absence of enough traction, abnormality in tyre noise is observed. This type of noise, generally lies above a frequency range of 1-2 kHz. This noise is known as sizzling.
Air Pumping- It has been seen that at the leading edge of the tyre’s contact patch, air gets compressed and penetrates into road surface and at trailing edge, compressed air gets released from road surface and flows backwards. This phenomenon is cyclic in nature, as tyre rotates continuously which causes noise of frequency above 1 KHz and depends on RPM of the wheel. This kind of noise is well known as hiss.
Air Turbulence- Air between trailing edge of tyre, vehicle body and wheel arch get whirling effect due to which aerodynamic noise is generated. This aerodynamic phenomenon is generally not significant enough at lower speed, but it amplifies as vehicle’s speed increases. Frequency of this kind of noise is generally in the range of 300 to 400 Hz.
Parameters affecting tyre noise
Read part 2 of the blog: Test Methodology and quantifying Parameters