SOUNDPROOF TYRE FOR VEHICLE WHEELS

Abstract

A soundproof tyre and a process for the production thereof. The tyre includes particular polyolefin sound absorbent foams that provide damping for noise generated in a cavity of the tyre, together with resistance to hydrolysis, poor water absorption and an unexpected thermal and mechanical stability in use conditions. The sound absorbent material is applied on at least a portion of a radially inner surface of the tyre. The portion of the radially inner surface includes an impermeable elastomeric material layer. The sound absorbent material includes a foamed polyolefin material with closed macrocells having an average size of at least 1.5 mm according to ASTM D357 6.

Claims

1. Soundproof tyre for vehicle wheels comprising at least: a sound absorbent material applied at least on one portion of the radially inner surface of the tyre, preferably of the impermeable elastomeric material layer, wherein said sound absorbent material comprises a foamed polyolefin material with closed macrocells having an average size of at least 1.5 mm, more preferably of at least 3 mm, still more preferably of at least 4 mm according to ASTM D3576.

2. The tyre as claimed in claim 1, wherein said foamed polyolefin material with closed macrocells comprises at least one perforation, preferably at least 5, at least 10, at least 20, at least 30 perforations per 10 cm.sup.2 of at least one surface of the material itself.

3. The tyre as claimed in claim 1, wherein said foamed polyolefin material with closed cells, comprises a number of cells in 25 mm less than 30, preferably less than 20, more preferably less than 10.

4. The tyre as claimed in claim 2, wherein said foamed polyolefin material with closed cells is in sheet form with two opposite main surfaces and comprises in at least one of the two surfaces at least one perforation every 4 cm.sup.2, preferably at least one perforation every 2 cm.sup.2, still more preferably at least one perforation every 1 cm.sup.2.

5. The tyre as claimed in claim 1, wherein said foamed polyolefin material with closed cells is obtained through expansion of a polyolefin material selected from among homo- and copolymers of ethylene, of propylene, of C.sub.4-C.sub.20 alpha-olefin or mixtures thereof, preferably from among homo- and copolymers of ethylene and mixtures thereof.

6. The tyre as claimed in claim 5, wherein said polyolefin material is a low density polyethylene (LDPE), with a density equal to or less than 0.940 g/cm.sup.3, preferably with a density comprised between 0.910-0.940 g/cm.sup.3.

7. The tyre as claimed in claim 1, wherein the foamed polyolefin material has a density not greater than 40 Kg/m.sup.3, preferably not greater than 30 Kg/m.sup.3, more preferably not greater than 25 Kg/m.sup.3.

8. The tyre as claimed in claim 2, wherein the foamed polyolefin material comprises at least 10%, preferably at least 20%, more preferably at least 25% of cells open by the perforation.

9. The tyre as claimed in claim 2, wherein the foamed polyolefin material comprises at least one through perforation and at least one partial perforation.

10. The tyre as claimed in claim 2, wherein the perforations of the perforated foamed polyolefin material are uniformly distributed over the entire surface of the material.

11. The tyre as claimed in claim 2, wherein the perforations of the perforated foamed polyolefin material have an average width greater than 0.01 mm, preferably greater than 0.1 mm, preferably greater than 0.5 mm.

12. The tyre as claimed in claim 2, wherein the thickness of the foamed polyolefin material is greater than about 5 mm, comprised between about 5 and 50 mm, preferably between about 7 and 40 mm, more preferably between about 10 and 30 mm.

13. The tyre as claimed in claim 1, wherein said tyre is high performance (HP High Performance) or ultra high performance (UHP Ultra High Performance).

14. Process for producing a soundproof tyre for vehicle wheels which comprises: i) providing a vulcanised and moulded tyre; ii) optionally, cleaning at least one portion of the radially inner surface of the tyre, and iii) applying a sound absorbent material at least on the portion, optionally cleaned, of the radially inner surface of the tyre, wherein the sound absorbent material comprises a foamed polyolefin material with closed macrocells having an average cell size of at least 1.5 mm, more preferably of at least 3 mm, still more preferably of at least 4 mm according to ASTM D3576, preferably perforated.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0123] Further characteristics and advantages will be more evident from the detailed description of a preferred but not exclusive embodiment of a soundproof tyre, according to the present invention.

[0124] Such description is set forth hereinbelow with reference to the enclosed drawings, provided as a non-limiting example, in which:

[0125] FIG. 1 schematically show, in radial half-section, a soundproof tyre for vehicle wheels according to the present invention;

[0126] FIG. 2 illustrates the damping performances for the cavity noise in the percussion test of a tyre according to the invention (samples 1-3), and of a comparison tyre C1, lacking sound absorbent material.

[0127] FIGS. 3 and 4 show the damping performances for the cavity noise in the noise measurement test in semi-anechoic chamber of a tyre according to the invention (sample 3), and of comparison tyres (C1), lacking sound absorbent material, and (C2), comprising a classic polyurethane foam with open microcells, at the speeds of 65 and 80 Km/h.

[0128] FIGS. 5a and 5b present the graphs relative to the inner noise measured in road tests at speeds comprised between 40 and 80 Km/h, at two points in the driver/passenger compartment of a car with tyres according to the invention, comprising foamed polyethylene material with perforated closed macrocells (sample 3), or comparative tyres comprising polyurethane foams with open microcells (C2, C3) or comprising foamed polyethylene material with smaller, non-perforated closed cells (C4 and C5).

[0129] FIG. 6 is the photograph of a sample of foamed polyolefin material with perforated closed macrocells preferably used in the soundproof tyre according to the present invention, next to a reference ruler with millimetre scale.

[0130] FIG. 7 reports the damping performances for the cavity noise in the noise measurement test in semi-anechoic chamber of a tyre according to the invention (sample 3), before and after the fatigue test.

DETAILED DESCRIPTION OF THE INVENTION

[0131] FIG. 1 schematically shows, in radial half-section, a soundproof tyre for vehicle wheels comprising foamed polyolefin material with closed macrocells according to the present invention.

[0132] In particular, in FIG. 1, “a” indicates an axial direction and “X” indicates a radial direction, in particular with X-X the line of the equatorial plane is indicated.

[0133] The tyre 100 for four-wheel vehicles comprises at least one carcass structure, comprising at least one carcass layer 101 having respectively opposite end flaps engaged with respective anchoring annular structures 102, termed bead cores, possibly associated with a bead filler 104. The zone of the tyre comprising the bead core 102 and the filler 104 forms a bead structure 103 intended for anchoring the tyre on a corresponding mounting rim, not shown.

[0134] The carcass structure is usually of radial type, i.e. the reinforcement elements of the at least one carcass layer 101 are situated on planes comprising the rotation axis of the tyre and substantially perpendicular to the equatorial plane of the tyre. Said reinforcement elements are generally constituted by textile cords, for example rayon, nylon, polyester (e.g. polyethylene naphthalate (PEN)). Each bead structure is associated with the carcass structure by means of backward folding of the opposite lateral edges of the at least one carcass layer 101 around the anchoring annular structure 102 in a manner so as to form the so-called turn-ups of the carcass 101a as illustrated in FIG. 1.

[0135] In one embodiment, the coupling between the carcass structure and the bead structure can be provided by means of a second carcass layer (not shown in FIG. 1) applied in an axially outer position with respect to the first carcass layer.

[0136] An anti-abrasive strip 105 made with elastomeric material is arranged in an outer position of each bead structure 103.

[0137] The carcass structure is associated with a belt structure 106 comprising one or more belt layers 106a, 106b situated in radial superimposition with respect to each other and with respect to the carcass layer, having typically textile and/or metallic reinforcement cords incorporated in a layer of elastomeric composition.

[0138] Such reinforcement cords can have cross orientation with respect to a circumferential extension direction of the tyre 100. By “circumferential” direction, it is intended a direction generically directed according to the rotation direction of the tyre.

[0139] In radially more external position with respect to the belt layers 106a,106b, at least one circumferential reinforcement layer 106c is applied, commonly known as “0° belt”, comprising at least one belt layer circumferential. The reinforcement layer (circumferential belt) can comprise a plurality of typically metallic and/or textile cords.

[0140] In radially outer position with respect to the belt structure 106, a tread band 109 is applied that is made of elastomeric material, like other semifinished products constituting the tyre 100.

[0141] Respective sidewalls 108 made of elastomeric material are also applied in axially outer position on the lateral surfaces of the carcass structure, each extended from one of the lateral edges 110 of the tread 109 up to the respective bead structure 103. The tyre portion comprised between the edges 110 identifies the crown C of the tyre. At such crown C, hence up to the edges 110 in radially inner position with respect to the tread, the belt structure 106 is preferably extended.

[0142] In radially outer position, the tread band 109 has a rolling surface 109a intended to come into contact with the ground. Circumferential grooves, which are connected by transverse notches (not represented in FIG. 1) so as to define a plurality of blocks of various shape and size distributed on the rolling surface 109a, are generally obtained in this surface 109a, which is represented smooth in FIG. 1 for the sake of simplicity.

[0143] An underlayer 111 can be arranged between the belt structure 106 and the tread band 109.

[0144] An impermeable elastomeric material layer 112, generally known as “liner”, provides the necessary impermeability to the tyre inflation air, and is typically arranged in a radially inner position with respect to the carcass layer 101. The radially inner surface 113 of the impermeable elastomeric material layer 112 is adhered, for example by means of gluing, to a layer of sound absorbent material 301 comprising a foamed polyolefin material with closed macrocells preferably perforated.

[0145] The layer of sound absorbent material can be made to adhere to the radially inner surface 113 of the impermeable elastomeric material layer by means of gluing with suitable adhesives, such as an acrylic adhesive, or via fitting, or compression by making the sound absorbent layer of greater size than the inner diameter of the tyre.

[0146] With reference to FIG. 1, a tyre 100 is shown in radial section which bears a sound absorbent layer 301, made of foamed polyolefin material with closed macrocells preferably perforated. The sound absorbent layer 301 is made integral with the radially inner surface 113 of the impermeable elastomeric material layer 112 in the crown portion C by means of gluing, occupying in axial extension at least one part of said crown portion.

[0147] A process for producing a soundproof tyre for vehicle wheels comprises:

[0148] i) providing a vulcanised and moulded tyre

[0149] ii) optionally, cleaning at least one portion of the radially inner surface of the tyre, and

[0150] iii) applying a sound absorbent material at least on the portion, optionally cleaned, of the radially inner surface of the tyre,

[0151] wherein the sound absorbent material comprises a foamed polyolefin material with closed macrocells, characterised by

[0152] an average cell size of at least 1.5 mm, more preferably of at least 3 mm, still more preferably of at least 4 mm according to ASTM D3576, preferably perforated.

[0153] With regard to the sound absorbent material, the present process preferably provides for using the materials in the preferred application modes already indicated in relation to the soundproof tyre according to the invention.

[0154] The cleaning operation ii) of the present process is generally carried out if the tyre provided in step i) is contaminated on the radially inner surface of the impermeable elastomeric material layer by lubricants or oils or emulsions and anti-adhesive solutions applied there during the forming of the tyre, as occurs in the case of tyres prepared according to conventional processes.

[0155] The presence of these contaminants generally does not allow applying the sound absorbent material on the inner surface of the liner with an adhesion suitable for resisting subsequent stress during use, even employing highly adhesive, costly and hard-to-handle glues.

[0156] In such case, in order to remedy these problems, it is preferred to proceed with the cleaning operation, at least on the part of the radially inner surface of the liner affected by the application of the sound absorbent material.

[0157] The cleaning can be conducted according to any suitable method, both via mechanical removal with sponges, rags or brushes, and via dissolution of the contaminants with suitable solvents, or combinations thereof.

[0158] Advantageously, by instead providing a tyre in which such surface is substantially not contaminated, it is possible to proceed directly with the gluing of the sound absorbent material, minimizing or entirely avoiding the cleaning operation.

[0159] The operation of applying a sound absorbent material on at least one portion of the radially inner surface of the impermeable elastomeric material layer, optionally cleaned, is preferably conducted via gluing.

[0160] The gluing of the sound absorbent material is conducted by using adhesives or glues suitable for such purpose, preferably acrylic adhesives.

[0161] In order to apply the sound absorbent material via gluing, the adhesive can be applied at least on one portion of one of the two main surfaces of the sound absorbent material, on at least one portion of the radially inner surface of the impermeable elastomeric material layer, or on both, in corresponding or non-corresponding portions.

[0162] Preferably, since they are commercially available, sound absorbent materials in sheet or roll form are used, already arranged with an additional layer of adhesive material deposited on one of the main surfaces, adhesive layer suitably protected by a first removable film.

[0163] For the application, after having possibly cut the sound absorbent material to size, the first protective film is removed from the adhesive layer and it is applied on the desired surface portion of the liner under pressure, manually or with suitable automated systems.

[0164] The sound absorbent material is typically sold with a further (second) removable protective film arranged on the other main surface, that not covered by the adhesive. This second film, which mainly accomplishes protection functions for the foamed sound absorbent material, is generally constituted by a thermoplastic film.

[0165] In the soundproof tyre according to the present invention, this second film can be left adhered, or is preferably removed, after the application of the sound absorbent material.

[0166] The Applicant has observed that the acoustic performances of the tyre according to the invention are generally improved if this second film of the sound absorbent material is removed.

[0167] The Applicant has found that these particular sound absorbent materials, used for soundproofing building and equipment or creating anti-noise road barriers, hence for static applications and/or in controlled temperature conditions, are surprising suitable and effective in damping the resonance noise, even when positioned inside the cavity of a tyre—i.e. in conditions of considerable mechanical and thermal stress—even if they are composed of polyolefins, well-known to have lower melting points with respect to the polyurethanes typically used for this application.

[0168] Advantageously, these materials have hydrolytic stability and do not absorb water. In addition, due to the morphological characteristic of the macrocells, they are particularly light, contributing to the overall reduction of the weight of the finished tyre.

[0169] The following examples are now provided for illustrative and non-limiting purposes.

Examples

[0170] Phonometric Tests

[0171] In order to evaluate the performances in the attenuation of the cavity noise of sound absorbent materials of the present invention and of the comparative materials, sample tyres were prepared by applying strips of the pre-selected materials on the inner surface of the liner according to the modes described in detail below.

[0172] The acoustic performances were then measured by means of phonometric tests conducted both on the tyre mounted on the rim (percussion test or Hammer test) and on the tyre mounted on a car, with evaluations in a closed setting (test in a semi-anechoic chamber) and on the road (measurements of the noise of the driver/passenger compartment and opinion of the tester).

Preparation of the samples

[0173] The inner surface of the liner made of bromobutyl material of a tyre Pirelli 275/45R20 at the crown portion has been cleaned with a soft abrasive sponge in order to remove all contamination of the anti-adhesive solution applied during the vulcanisation step.

[0174] Subsequently, the selected sound absorbent material has been applied as a single strip on such surface, by means of an acrylic adhesive layer interposed between the foam and the liner. The other surface of the foam, non-adhesive, was covered by a removable protective film.

[0175] The sound absorbent material has been applied by covering inner surface of the liner for the entire circumference, symmetrically with respect to the equatorial plane.

[0176] The performances of the tyre comprising the polyethylene foam with perforated, closed macrocells A according to the invention (samples 1-3) were compared with those of a base tyre lacking sound absorbent foams (sample C1) and with those of comparative tyres comprising the polyurethane foam with open microcells B in two different thicknesses (20 e 10 mmm) (samples C2-C3) and polyethylene foams with non-perforated closed microcells D1 and D2 (samples C4, C5)

[0177] Foam A was a polyethylene foam with closed macrocells, with double perforation; density 25 Kg/m.sup.3 measured according to ASTM D3575-08 Suffix W; Cells/25 mm<10 according to BS 4443/1 Met.4; thickness 10 or 20 mm, sold by Sogimi with the commercial name Stratocell Whisper® (FIG. 6). This foam had a first surface, covered with adhesive and with a first protective film, and a second surface, directly covered by a second protective film. In samples 1 and 2, the second protective film was maintained; this was instead removed in sample 3.

[0178] Foam B was a polyurethane foam with open microcells PL38LWF (Tekspan Automotive), density 35-41 Kg/m.sup.3 (ISO 1855), number of cells/25 mm>40, thickness 10 or 20 mm.

[0179] Foam D1 was a cross-linked polyolefin foam with closed microcells, number of cells/25 mm>40); density 33±3.5 Kg/m.sup.3 measured according to ISO 845-88; sold by Tekspan Automotive with the commercial name K630.

[0180] Foam D2 was a cross-linked polyethylene foam with closed microcells (number of cells/25 mm>40); density 30±5 Kg/m.sup.3 measured according to ISO 845-88; sold by Tekspan Automotive with the commercial name of X105 SM.

[0181] In the following Table 2, the samples of the tyres thus prepared are reported along with their structural characteristics:

TABLE-US-00002 TABLE 2 Sample No. 1 2 3 C1 C2 C3 C4 C5 Type of A A A no B B D1 D2 foam Foam  25 .sup.1  25 .sup.1  25 .sup.1 —  38 .sup.2  38 .sup.2  33 .sup.3  30 .sup.3 density Kg/m.sup.3 Number of <10 .sup.4 <10 .sup.4 <10 .sup.4 — >40 >40 >40 >40 Cells/25 mm Average >1.5 mm   >1.5 mm   >1.5 Mm.sup.  — <1.5 mm <1.5 mm <1.5 mm <1.5 Mm.sup.  cell size Width 90 mm 90 mm 180 mm  —  180 mm  180 mm  180 mm 180 mm  Thickness 10 mm 20 mm 20 mm   20 mm   10 mm   10 mm 10 mm of the Foam Second film Si Si No Key: A: polyethylene with closed macrocells and double perforation; B: polyurethane with open non-perforated microcells; D1: cross-linked polyolefin with closed microcells; D2: cross-linked polyethylene with closed microcells. Test density: .sup.1 (ASTM D3575-08 Suffix W), .sup.2 (ISO 1855), .sup.3 (ISO 845-88); Number of cells: .sup.4 (test: BS 4443/1 Met.4), Tyre 275/45R20

[0182] Percussion Test (Hammer Test)

[0183] This internal test, with substantially qualitative character, is employed for a preliminary selection of the materials based on their effectiveness in damping the cavity noise.

[0184] The tyres of the samples 1-3 according to the invention (with polyethylene foam A) and the comparative tyre C1 (without foam) were mounted on rim 9JX20 E.T.R.T.O. and inflated to the pressure of 2.6 bar.

[0185] Each tyre, without load, was hit with dynamometric hammer and the amplitudes of the sounds produced at the various frequencies by the percussion were registered along the axis X and reported in the diagram of FIG. 2.

[0186] As can be observed from the graphs, the phenomenon of the cavity resonance is shown with a series of peaks approximately between 170 and 200 Hz.

[0187] The intensity of the resonance peak of the base tyre (C1), lacking sound absorbent foam, present at about 190 HZ, resulted damped for all the samples 1-3, proportionally to the thickness of the foam. A considerable increase of the sound absorbent activity was also observed for the sample lacking both protective films (sample 3), which at the thickness of 20 mm showed the best damping activity from among the tested samples.

[0188] Measurement of the noise in the driver/passenger compartment in a semi-anechoic chamber

[0189] With this test, the acoustic damping performances of tyres according to the invention (sample 3) were compared with those of comparison tyres, lacking sound absorbent foams (C1), or comprising conventional polyurethane foams (C2), in a semi-anechoic chamber.

[0190] The tyres under evaluation have been mounted on 9JX20 E.T.R.T.O. rims, inflated to the pressure of 2.6 bar and mounted on a car.

[0191] For each set of tyres, the intensity of the noise was measured inside the driver/passenger compartment with the increase of the speed, between 20 and 150 Km/h. The official tests of the automobile manufacturers evaluate the damping performances for the cavity noise of the tyres at speeds generally comprised between 40 and 80 Km/h, since at speeds lower or greater than this range there are other noise generation phenomena which render the measurements of little significance.

[0192] Reported in FIGS. 3 and 4 are the intensity curves of the sound measured in the driver/passenger compartment of the vehicle for the different tyres under examination with respect to the frequencies, respectively at the speed of 65 and 80 Km/h.

[0193] As can be observed, at the frequency of the cavity resonance peak (about 190 Hz), the tyre sample 3 shows a damping effectiveness for the noise comparable to that of the tyre comprising a conventional polyurethane foam (C2). Therefore, from this test, it is inferred that the soundproof tyres of the present invention, advantageous for the hydrolytic stability and the non-hygroscopicity of the sound absorbent foams, are at least comparable in terms of acoustic performances to known soundproof tyres, comprising conventional polyurethane foams.

[0194] Measurement of the Noise of the Driver/Passenger Compartment on the Road

[0195] With this test, the acoustic damping performances on the road of the tyre sample 3 according to the invention were compared with those of comparison tyres, lacking sound absorbent foam (C1), or comprising conventional polyurethane foams (C2, C3), or polyethylene foams with non-perforated, closed microcells (C4, C5).

[0196] The tyres under examination were mounted on 9.0Jx20 rims, inflated to the pressure of 2.3-2.5 bar, and mounted on a VW Tuareg 3.0 TD car.

[0197] The car was brought to the velocity of about 80 Km/h, on a rough asphalt track, at the temperature of 9-13° C., after which the motor was turned off and the noise in the driver/passenger compartment was measured and evaluated by the tester, until the vehicle stopped.

[0198] The measurement of the noise in the driver/passenger compartment was carried out by arranging the microphones at the centre of the car (right channel) and window side (left channel), at car speeds comprised between 40 and 80 Km/h and at the frequencies from 0 to 22000 Hz.

[0199] As is inferred from the FIGS. 5a and 5b, the noise measured in the two different positions inside the driver/passenger compartment (upper graph FIG. 5a, car centre, lower graph FIG. 5b, window side), increases with the increase of the speed.

[0200] From the graphs, it is observed that the tyre 3, according to the invention, has at least the same if not greater effectiveness in reducing the noise in the driver/passenger compartment with respect to soundproof tyres comprising polyurethane foams with conventional open microcells (C2, C3), and polyethylene foams with closed non-perforated microcells (C4, C5).

[0201] Reported in the following tables 3 and 4 is the intensity of the sound measured at the peak frequency of about 190 Hz and at the speeds of 60 and 80 Km/h, respectively, in the two positions inside the driver/passenger compartment of the car for the different tyres under examination:

TABLE-US-00003 TABLE 3 60 Km/h Velocity Car centre Sample (dB) Window side (dB) 3 61.0 62.7 C1 61.4 63.6 C2 60.8 62.9 C3 61.1 63.0 C4 61.2 63.3 C5 61.0 62.9

TABLE-US-00004 TABLE 4 80 Km/h Velocity Car centre Window side Sample (dB) (dB) 3 63.4 65.1 C1 64.0 66.0 C2 63.8 65.6 C3 63.6 65.5 C4 63.8 65.5 C5 63.6 65.3

[0202] From the above data, it can be appreciated that the tyres according to the present invention (sample 3) generally have a damping effectiveness for the noise at least equal to if not greater than that shown by the classic polyurethane foams (C1 e C2).

[0203] More particularly, the data relative to the measurements at the speeds of 60 and 80 Km/h show that the polyethylene foams with perforated macrocells, employed in these tests, have a capacity of reducing the cavity noise that is even better than that of the classic polyurethane foams and polyethylene foams with non-perforated microcells.

[0204] Evaluation of the Tester of the Noise on the Road

[0205] The car tester, in the above-described driving conditions, expressed the following opinion with regard to the noise perceived in the driver/passenger compartment:

TABLE-US-00005 TABLE 5 Opinion Sample regarding Tyre noise Notes 3 + Generally similar to or slightly better than C2 C1 +++ The noise was annoying, discontinuous and penetrating C2 ++ Passing from 80 Km/h to the lowest speeds, the cavity noise increases, still remaining acceptable Noise: +++ high; ++ average; + average-low;

[0206] Also from the tester's opinion, it can be concluded that the tyre according to the invention shows sound absorption performances comparable to if not greater than those of soundproof tyres comprising conventional polyurethane foams.

[0207] Evaluation of the Duration of the Sound Absorbent Foams

[0208] A tyre 275/45 R20 110W according to the invention (sample 3), inflated to the pressure of 3.0 bar, was subjected to a fatigue test in a closed setting which consisted of making it rotate at a constant velocity of 80 Km/h, at the temperature of 25° C., at a constant load of 1380 Kg, on a street tyre with diameter 2.0 m for 400 hours, verifying the integrity of the sound absorbent layer at intervals of 80 hours, upon stopping and dismounting of the tyre. The tyre according to the invention did not show signs of intermediate deterioration, and it exceeded the predetermined 400 hours without detachments or damage of the sound absorbent layer.

[0209] Evaluation of the Acoustic Performances of the Sound Absorbent Foams after the Fatigue Test

[0210] The tyre according to the invention (sample 3) was subjected, before and after the fatigue test in an above-described semi-anechoic chamber, to the measurement of the acoustic performances.

[0211] FIG. 7 reports the graphs of the noise—measured in Pa according to the weighing curve “A” more similar to the human ear—produced by the tyre according to the invention at the frequencies of 192 and 208 Hz, and in the range of velocities from 80 to 60 Km/h, before and after the fatigue test. As is seen in the overlapped curves, the sound absorbent material surprisingly maintained the same sound absorption activity after 400 hours of rolling.