An indoor road-wheel facility at the Technical University of Gdańsk was used to study the noise emission from a variety of tires with different tread patterns. The tires were run both on a smooth steel drum and a drum covered by a replica road surface. All tread patterns were hand-cut to generate several families of simple treads with regular pitch for a systematic study of how groove design influences noise. Most of the observed, tread influenced phenomena could be explained by generation mechanisms such as radial vibrations induced by tread block impact, pocket air pumping and pipe resonances in the grooves. For instance, it was observed that, when speed increases, sooner or later the tread block impact frequency will coincide with the pipe resonance frequency and then generate excessive noise at that speed.

Over the past 20 years, studies on the subject of tire/road noise generation have been published by several authors. It is likely that a reader would find the empirical evidence and the resulting conclusions published in them rather inconclusive and muddling. After the International Conference on Tire Noise in 1979 it was summarized that "The disagreement on which generating mechanisms are most important appeared to be almost as great as before". The controversy is still a fact and may sometimes lead to rather "hot" discussions.

This report has the ambition of trying to bridge the gaps between the differing views and to emphasize on discussing the major results of research regarding the road surface influence on tire/road noise generation appearing in the latest decade. It also includes supplementary data from the author's research where this is appropriate.

The importance of tire/road noise as part of total vehicle noise has recently received growing attention. It has been recognized as a major component of passenger car noise, even at moderate speeds, and as a significant component of truck noise at high speeds. The continuing development of more silent vehicle power units will even more emphasize tire/road noise. During the last decade, in many countries, more deeply textured anti-skid road surfacings were implemented which appeared to be noisier and drew public awareness on what could be called "pave¬ment noise". As a matter of fact, the surface characteristics of roads are now known as the main factors influencing car tire/road noise. Then, in order to harmonize safety and environmental require¬ments and also to define a reference surface for measurement and regulation purposes, a better knowledge of the noise-relevant surface characteristics is required.

Unacceptable errors in the prediction of traffic noise occur in some cases when the road surface is largely different from that on which the prediction model is based. The reason is that tyre/road noise has appeared to be the dominating component of the noise from free-flowing traffic and that this noise is to a substantial extent dependent on the road surface.

The mechanisms for tyre/road noise generation and its relation to road characteristics are described. Relevant road surface characterization methods are suggested. The major method is the measurement of the road texture profile and subsequent spectral analysis of the profile curve. Supplementary methods concern the measurement of acoustical and mechanical impedances. It is concluded that the road surface effect on traffic noise is extremely complicated and that it is very difficult to generalize any simple relations.

For free-flowing traffic it is shown that the tested road surface types and conditions may influence the traffic noise by up to 11 dB(A). This calls for a correction term for the road surface in the prediction models. Despite the complicated relations, it appears feasible—within stringent limitations—to use a table where the correction term is a variable of vehicle type, vehicle speed as well as road surface type and condition.