1 - Noise generation overview
This Section provides a brief overview of the main factors involved in the generation of road traffic noise. Road traffic noise is a typically a combination of propulsion noise and tyre/road interaction noise. Propulsion noise, predominantly from the engines and exhausts, is dominant at low speeds and is not discussed in this study. Tyre/road noise is the focus of this study and the noise generation mechanisms are well documented existing literature. A more detailed discussion on the mechanisms of tyre/road noise generation can be found in other documents, including: Part 2 Section 4 of TRL’s Published Project Report PPR443 “A review of current research on road surface noise reduction techniques”, Transport Research Laboratory, P G Abbott, P A Morgan and B McKell, January 2010, and Section 2 of “Sub Task 3 – Noise Evaluation”, Collaborative Research Programme, Highways England, Mineral Product Association and Eurobitume UK, November 2017.
1.1. Tyre/road noise generation mechanisms
There are three main noise generation mechanisms:
- Aerodynamics (air pumping);
- Impacts / shocks, and
- Adhesion effects.
These noise generation mechanisms are discussed further in the following subsections.
1.1.1. Aerodynamics (air pumping)
This refers to the movement of air between tyre and road surface and within the cavities formed by the tyre tread pattern. The most commonly referred effect is that of air pumping, where air trapped in the tread is suddenly released. Significant noise levels can be produced via this mechanism especially were the road surface is non-porous and relatively smooth, as the air has less opportunity to escape when trapped. Noise generated by these aerodynamic mechanisms are generally in the frequency range of 1000 to 2000Hz.
1.1.2. Impacts / shocks
Impacts and shocks are caused by the interaction of the tyre and road surface, whereby the tyre tread blocks will be displaced and then snap back in to place as they enter and leave contact with the road surface, causing vibrations in the tyre. This vibrational energy in the tyre is then radiated into the air as sound. Noise generated by these mechanisms tends to occur at lower frequencies below 1000Hz.
1.1.3. Adhesion effects
This is the stick/slip process between the tyre tread, gripping the micro texture of the surface and then slipping out of contact. This results in vibration excitation of the tyre which is then radiated into the air as sound.
1.2. Road surface properties influencing tyre/road noise
1.2.1. Texture and Porosity
The surface texture amplitudes and wavelengths (variations in surface peak and troughs) of a surface are an important factor in noise generation. Texture wavelengths of 0.5 to 10mm reduces air pumping. However, texture wavelengths of between 10 and 500mm increases low-frequency noise due to higher tyre vibrations.
Surface texture can also be positive or negative (see image right). Positive texture also generates more tyre vibrations increasing low frequency noise compared to a negative surface texture.
Surface porosity is the amount of air voids in the surface and increased porosity reduces air pumping and generally increases sound absorption, reducing noise propagation. However, increased porosity has a negative trade-off with durability as porous surfaces are susceptible to degradation.