A noise-attenuating device The present invention relates to a device for attenuating noise, which comprises an upwardly extending screen wall with resonator members arranged at the upper edge of the screen wall in order to improve the noise reduction capacity of the screen wall, the resonator members delimiting a plurality of elongate cavities lying adjacently in the horizontal direction and having an opening at one end of the resonator members. The invention relates preferably but not exclusively to those devices intended for attenuating noise from road traffic.

A plurality of different screen constructions have previously been proposed for so-called secondary noise control of traffic noise, that is to say screens which are positioned next to the road in order to screen and attenuate the noise between the noise source (motor vehicles, trains, etc.) and those living in the surrounding area. Such noise screens have in this respect been provided with different configurations at their crests or screen tops, for example with a Y-shape or a T-shape, in order to improve the noise-attenuating capacity of the screen, that is to say in order to increase the acoustic impedance encountered by incident sound waves at the crest of the screen, without the height of the screen having to be increased.

Furthermore, EP 057 497 and EP 742 542 describe noise-attenuating screens which at the top bear a form of resonator body, which bodies delimit a number of adjacent ducts, through which sound waves from a noise source can pass, that is to say the ducts are in this case open at both ends.

One object of the present invention is to propose a new type of screen top configuration which, in a passive manner, provides improved and more effective noise attenuation than the previously known noise screens with noise-attenuating elements on the screen crest.

For this purpose, the device according to the invention referred to in the introduction is characterized in that the resonator members are closed at their other end so as to limit the depth of the respective cavities. It has been shown that screen crests provided with such resonators can provide a generally more distinct noise attenuation, irrespective of whether the resonator bodies are oriented essentially vertically with the openings at the top or essentially horizontally with the openings facing towards the noise source, by virtue of the fact that the resonators create a secondary sound field with a destructive effect on the original sound field.

Other features of the device according to the invention are indicated in the dependent claims below.

The invention is described in greater detail below with reference to the appended drawing, in which: Fig. 1 shows diagrammatically in perspective a noise screen with resonators according to a first embodiment of the present invention arranged on its crest; Fig. 2 shows in perspective a noise screen with resonators according to a second embodiment of the invention arranged on its crest; Fig. 3 shows an embodiment of the device according to the invention with an L-configuration, and Fig. 4 shows a variant of the L-configuration in Fig. 3.

Fig. 1 shows a first embodiment of a device 10 according to the invention for changing the acoustic impedance encountered by incident sound waves at the crest of a noise screen wall 12, that is to say for improving the noise-attenuating capacity of the screen wall 12.

The device 10 comprises a plurality of adjacent resonators 14 arranged vertically at the upper end or the crest of the screen wall 12 and on that side of the latter which faces towards the noise source. The resonators 14 have the shape of an elongate cavity,

formed by longitudinal and transverse wall elements 16 and 18 respectively, and have an opening at at least their upper ends. In the embodiment according to Fig.

1, the resonators 14 have the same length 1 for application in the case of harmonic sound spectra from the noise source, for example noise from a transformer.

The length 1 and the cross-sectional area of the resonators 14 can in this respect be adapted to the relevant dominant sound frequency occurring in the noise source in order to optimize the acoustic impedance. In a practical example, the screen wall 12 can have a height h of approximately 3 metres, the length 1 of the resonators 14 can be approximately 1 metre and the cross-sectional area can be approximately 100 cm2. Thanks to the resonator bodies 14, a secondary sound field is generated which is added, with destructive effect, to the original sound field, originating from the noise source, above the screen.

The acoustic impedance, and consequently the strength of the secondary sound field also, are thus dependent on the frequency of the primary sound pressure. The length 1 of the resonators 14 is, like organ pipes, of central importance. If it is assumed that each resonator 14 in Fig. 1 has the shape of a straight pipe with a closed bottom, it applies that, when the length 1 coincides with uneven multiples of quarter wavelengths of the sound above the screen 12, the abovementioned destructive effect occurs. This effect makes it possible to tune the resonator bodies to the frequencies at which the highest noise levels lie.

For attenuating a more broadband noise spectrum, such as road traffic, the length of the resonators can be varied and tuned so as to achieve a broader noise reduction in terms of frequency. Such an example is shown in Fig. 2, where the length 1 of the resonators 14'may vary between approximately 0.3 and 1 metre. Although the resonators 14,14'may be open at both their ends, they are preferably closed at their

lower ends. The upper opening may also face towards the noise source.

In the embodiments in Figs 3 and 4, the resonators 14a and 14b respectively extend essentially horizontally from the crest of the screen wall 12 in the direction of the noise source to form an essentially L-shaped noise screen. Those ends of the resonators 14a, 14b which face the noise source are then open, while the rear ends are closed. In Fig. 3, the resonators 14a are the same length in order, like the embodiment in Fig. 1, to attenuate noise of more harmonic sound spectra, while the resonators 14b in Fig. 4 are of varying length (approximately 0.3-1 metre) for noise reduction which is broader in terms of frequency.

Although the resonators in the exemplary embodiments shown have a square cross section, it is possible within the scope of the invention to design the resonators with other cross-sectional shapes, such as round, oval or polygonal cross sections.