The present invention concerns a road surfacing with reduced rolling noise emission and a method for its manu¬ facture.

A considerable environmental problem in today's society is noise emission from roads and streets. At lower speeds and high acceleration, noise from the power unit dominates at the receiver point in most cases compared to the rolling noise..

At speeds exceeding 50 km/h, however, rolling noise gene- rated by the tire/road contact will dominate. This means that a considerable part of the noise emission, along streets and roads even in built up areas, is caused by the tire/road noise. In such areas it would not help much to further reduce noise emission from the power units. The fact that tire/road rolling noise dominates the total noise already at 50 km/h contributes to a certain understandable disinclination of car manufacturers to further reduce noise emission from the power units. A reductionof the tire/road rolling noise, thus, would contribute to solve acute noise problems at streets and highways where speeds generally exceed 50 km/h and -also to a general reduction of traffic noise even at lower speeds, since reduced tire noise levels will increase the motivation to also reduce the noise emission of the power unit.

Tire/road noise is generated by the contact between the tire and the road surface. The major part of the sound radia¬ tion occurs from the tire close to the contact patch.. For tires with more powerful tread patterns this takes place from the trailing portion of the contact patch. At lower frequen¬ cies (below about 800 Hz) tire/road noise is dominated by direct radiation of tire carcass vibrations. At higher fre¬ quencies air-resonances between tire and road surface will probably substantially influence noise radiation. This means that a considerable noise reduction could be achieved if such air-resonances are not excited to a greater extent. To achieve this it is very important that air pressure neu¬ tralization between tread pattern cavities can occur.

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Known methods up to now for tire/road noise reduction are:

1. Measures on tread pattern. Length of tread blocks in di rection of rotation are distributed so that tonal component are spread around the mean frequency. Thereby tonal peaks are reduced.

2. Changing the rubber compound so that higher compliance is obtained.

3. Influencing the road surface texture, so that an optimu texture depth in regard of tire noise is obtained.

Further developping these constructional principles cou give 2-4 dB(A) in additional noise reduction. Laboratory studies have revealed that the most probable cause of the high frequency noise production from tires (most important for the percieved noise impression) is that tire tread bloc oscillations would cause air movements. If the positive and negative pressure, respectively, that thereby is caused in tire grooves could be equalized, the noise would be decreas One way of obtaining such equalization is, according to the present invention, to make the road surface porous. This can be done e.g. by binding a granulate of homogenous gra size by a suitable amount of binder to achieve the porosity. Porosity could also be produced by creating air-permeable channels in other ways, e.g. by drilling or otherwise makin holes at production of a road surfacing. An additional posi tive effect of the porosity is that such road surfaces will obtain sound absorption abilities, that are considerably better in the high frequency range than for an ordinary non porous road surface. This is of vital interest since the sound generation mainly occurs extremely close to the road surface. As a great portion of the tire/road noise then wil propagate close to the road surface a great effect of noise absorption of the paving will result.

At a wet road surface the porosity will also add draina of water film which would reduce the noise level as well as lower the risk of hydro planing.

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As mentioned, tire vibrations start air-resonant oscil¬ lations in the contact region a d produce the major part of the high frequency noise. By reducing these vibrations in the tire the radiated noise is also reduced. One way of re- - ducing tire vibrations would be to construct the road sur¬ face so that a substantial increase in its compliance is ob¬ tained. If, however, the road surface is given only increased compliance, the noise will increase due to a greater compres¬ sion pressure being built up when the tire contacts the road surface. If, on the other hand, according to the invention, the higher compliance is combined with porosity of the road surface the air pressure differences will be neutralized and a significantly reduced noise level in the far field will be obtained compared to on one hand hard and porous surface and on the other hand soft and dense road surface. An additional beneficial consequence of making tire road surface simultaneous¬ ly soft and porous is that the tire to some extent sinks down into the road surface. This means that the tire tread release angles will be smaller which is in favour, with re- spect to noise radiation.

The obtained "down sinking" is furthermore an advantage with respect to road holding (e.g- panic brakings). It will further cause breaking up of ice layers on the surface.

The soft road surface furthemaore causes less tire vibra- tions to be transferred to the vehicle cabin and, thus, greater driving comfort.

When driving cars with studded tires on a road surface having substantially increased compliance in combination with porosity a considerable lower wear, compared to usual asphalt surfaces, will be obtained. Due to the fact that the studs sink into the rubber and flex, parts of the surface will not be torn away when the studs contact the road surface. This is also confirmed in laboratory tests where a studded ^" tire was run on a rubber surface of the kind stated for about two hours, without any measurable wear being noticed. A corresponding test on an ordinary asphalt surface cause a

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remarkable wear (in the range of 1^2 mm) . Furthermore, it has been noticed that the increase of noise that is created when changing to studded tires on an ordinary road surface- does not appear at driving on a porous rubber surface. The laboratory tests performed have revealed that the compliant/porous surface gives about 10 dB units lower nois in the high frequency region compared to a standard asphalt surface of type AEB12T (the Swedish notation) . A considerab decrease also in the tonal components of the tire noise spectra was found.

An important question is how a porous road surface shou be manufactured in an economical manner. A number of altern tive methods are possible.

For instance granulated waste rubber (e.g. from scrap tires) could be screened to a uniform grain size and bound by polyurethan rubber. Other possible binders are latex, emulsified solutionsof synthetic rubber etc. It would also be possible to granulate unvulcanized rubber and to heat th rubber to vulcanization temperature in connection with pres sion, whereby a complete granulate product will be achieved without binder.

In connection with such pressing process any kind of pattern could also be applied in the surface layer for impr ved road holding capability. The invention will be further described with reference to the annexed drawings, wherein Fig. 1 shows a cross sec¬ tion through a road structure. Fig. la shows at an enlarged scale the encircled portion <_ of Fig. 1, Fig. 2 is a diagra showing a reduction spectrum (difference spectrum) for a porous and soft rubber surfacing according to the invention relative to a conventional asphalt surfacing at different frequencies, and Fig. 3 is a diagram showing the sound pres sure levels for asphalt and rubber, respectively, at diffe¬ rent frequencies. In the embodiment of Figs 1 and la a road 1 is shown comprising a substratum 2 of concrete or the like and a roa

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surfacing 3 consisting of a multiplicity of vulcanized or otherwise interconnected balls or chips 4 of rubber, plas¬ tic or other polymeric material between which exist communi¬ cating spaces, which together form air-permeable channels or pores. Of special importance is that such communication exists that air pressure equalization can take place between the different grooves of the tread surface. The road sur¬ face according to the present invention shall have a Youngs modulus of maximum 7 MPa as measured on a solid non-porous test body. For a normal asphalt surface Youngs Modulus is considerable. The relaxation time for the material used in the road surface of the present invention shall be about ten times shorter than the corresponding relaxation time for an asphalt surfacing. The specific flow resistance

₃ shall be less than 200 • 10 MRS Rayls/m. The typical value

3 for the flow resistance is around 10 • 10 MKS Rayls/m

4 (Ns/m ) . An additional advantage of the road surfacing of the invention is that it is possible to make a road sur¬ facing having approximately the same material characteristics as those of the tire rubber. Hereby a mechanical impedance counted from the road surface and downwards is obtained that is approximately equal to the impedance from the boun¬ dary of the road surface and upwards toward the tire. This equality in mechanical impedance results in a great power transfer between the tire and the road surface. This will give the following advantages:

1. A great mechanical power is transferred from the wheel to the road surface. This results in improved road holding characteristics. 2. A part of the tire vibrations are transferred to the road surface where the radiation damping is greater. Vibrations in the road surfacing, thus, give rise to lower sound pres¬ sure levels in the far field than corresponding vibrations in the tire structure.

The asphalt forming an ingredient of the asphalt road

surfacing AEB12T mentioned is of a type notified as A 120 according to the building code of the National Swedish Road Administration and having a penetration of 200-250 measured according to ASTM D 5/73. In the diagram of Fig. 2 there is shown a difference spectrum at different frequencies for a surfacing according to the present invention in relation to an asphalt surfa¬ cing of the above mentioned type.

In the diagram of Fig. 3 there is shown the sound pres- sure levels at different frequencies for a surfacing accor¬ ding to the invention and a conventional asphalt surfacing respectively. As will appear, there is a difference in dB(A level between 1000 and 3200 Hz of 8.0 dB and between 200 an 1000 Hz of 5.2 dB. This latter, lower difference is probab- ly dependent of influence from background sound levels.

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