A laboratory investigation of noise reduction by riblike structures on the ground

Abstract

This paper investigates the use of periodically spaced edges or wells for the control of road traffic noise. The wells have uniform height and width and are placed on the ground. Physical scale modeling is used to assess the acoustic performance of these structures under laboratory conditions. It is shown that, in certain situations where the use of conventional barriers would not be appropriate, strategically designed riblike structures can provide insertion losses of typically 10-15 dB. The findings are explained in terms of acoustic scattering with the approaches used in the study of resonators and diffraction/interference gratings.

Using natural means to reduce surface transport noise during propagation outdoors

“…More extensive laboratory measurements have shown that excess-attenuation spectra are influenced by the number and spacing of roughness elements and by their profile or shape [98]. These factors are not investigated in Bougdah et al [97] since only identical rectangular, thin acoustically-hard rib-like elements (vertical strips) were considered.…”

“…Bougdah et al [97] reported laboratory measurements over arrays of up to 17 thin walls with (equal) heights and spacing between 8 and 25 cm. They measured a maximum overall insertion loss of 10.3 dB for a 3.25-m wide 14-wall array with height and spacing of 0.25 m with the wall nearest the source located at the specular reflection point halfway between source and receiver which were at 0.4 m height and separated by 10 m. They discussed three physical effects other than surface-wave creation and the effective ground impedance that may be involved.…”

Exploiting ground effects for surface transport noise abatement

“…As long as the height of the walls is 0.3 m, which is approximately the wavelength of sound in air at 1 kHz, or less, they can be considered as a form of artificial ground roughness. The potential usefulness of regularly-spaced low parallel walls for road traffic noise reduction was suggested and demonstrated by outdoor experiments in 1982 [37]. An array of sixteen 0.21 m high parallel brick walls with edge-toedge spacings of about 20 cm placed on compacted grassland was found to give a broadband (between 100 Hz and 12,500 Hz) insertion loss (IL) of slightly more than 4 dB(A) including insertion losses of up to 20 dB(A) in the 1/3 octave bands between 400 and 1000 Hz.…”

Meteorological effects on the noise reducing performance of a low parallel wall structure

“…Sound energy is trapped in a zone close to the surface [8], and the decay of sound intensity with distance becomes less pronounced [8]. Conditions for surface wave generation are met when, upon grazing incidence, the imaginary part of the equivalent surface impedance by which such a LPW could be represented, exceeds its real part [1] [2][10] [8]. However, the surface waves can be mitigated by making the walls (partly) absorbing or by (partially) filling the space in between the walls with a porous medium such as gravel [3] [9].…”

“…Bougdah et al [2] discussed possible phenomena when sound waves interact with a LPW structure. The cavities formed by the parallel walls could act as quarter-wave length resonators; sound waves passing over the tops of the walls are partly cancelled at specific sound frequencies by reflections coming from the bottoms of the cavities.…”