Designing Direct Glazing for Improved Sound Attenuation

Tests with a new type of adhesive bond specifically designed for improved sound attenuation in automotive direct glazing applications produced a useful reduction in interior noise levels.

Particularly at low speeds and on poor road surfaces, the noise level inside a moving vehicle is determined to a significant degree by tyre noise generated in the low-frequency range (up to about 500 Hz). Vibrations transmitted from the tyres to the window glass via sound bridges produce changes in air volume, setting up pressure oscillations inside the passenger compartment which are perceived by the vehicle occupants as noise.

Finite element calculations, which are confirmed by test results, show that these resonance's can be reduced by the use of a windshield adhesive with a low modulus of elasticity and good damping characteristics.

The test results show that the most effective way of improving the acoustics inside the passenger compartment is to employ a two-phase windshield adhesive applied by the co-extrusion method. This acts as a barrier to structure-borne sound, thus reducing the transmission of resonance's from the body shell to the window glass. At the same time the superior damping characteristics produce a marked flattening of the resonance peaks.

The two-phase adhesive consists of an outer "shell" which provides the necessary stiffness, preferably a standard windshield adhesive; and a "core" of paste-like material with good damping characteristics. The acoustic performance of this adhesive system is critically influenced by the specific properties of the filler paste.

In terms of structural mechanics the use of a co-extrusion product to bond the rear window of a passenger vehicle reduces the first torsion by approximately 1 Hz, while the bending or flexing of the vehicle body remains virtually unchanged.

Experimental modal analysis shows that the use of a two-phase adhesive applied by the co-extrusion method and incorporating a low-viscosity core can dramatically lower the overall level of vibrations in the window glass by 30 ? 50 per cent, and in particular can bring about a flattening of the resonance peaks in the all-important roll vibration range between 100 and 500 Hz.

This results in a reduction in sound levels within this frequency range of between 1 dB and 2 dB + for rear-seat passengers.

The study also demonstrated the long-term durability of the adhesive bond and hence the retention of these positive acoustic characteristics over an extended period of time.

In short, this project has identified a solution for reducing interior noise levels with no weight penalty by means of a single low-cost measure, which must of course be considered in relation to the overall acoustic design of the vehicle. Further studies will show to what extent this procedure can be integrated into the production process, and what logistical or organizational changes are necessary in order to make this possible.

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The Authors

Mr. Stefan Böhm Senior Engineer LKT, RWTH Aachen Read more