Looking closer at vehicle noise

  • 09-Feb-2011 05:07 EST
Ahlmann_AS 900.jpeg

The Ahlmann AS 900 Booster swing-loader was recently the subject of a noise-reduction program, undertaken by Carcoustics International.

One example of how a good product can be enhanced by concentrating in one area is the Ahlmann AS 900 Booster swing-loader. Ahlmann cites advantages of the AS 900 being a spacious cab and a mono-boom that can swing through 180° as well as its high levels of efficiency. However, noise is an issue with the new model, according to Ahlmann Construction Machinery’s Marketing Manager Jan Lawerentz.

“For our customers, lowest possible noise levels were most important, both inside and outside,” said Lawerentz. “The demands made on cabs in construction and agricultural machinery have increased greatly and call for much high-precision work to achieve the expected standards of comfort inside the cab.”

The task of reducing the noise levels in the Ahlmann swing-loader was given to Carcoustics International, based near Cologne, Germany. First, the vehicle was assessed subjectively, together with its accessories, using a directional microphone. Normally a laboratory would be used for measurements, but this construction vehicle, weighing almost 6 t (6.6 ton), was beyond the floor’s load-bearing capacity.

Another consideration was the fact the noise level had to be measured under various conditions: fully laden, while idling, and in operation. Fortunately, with its own test track, Carcoustics could take outside measurements to get the required data.

Secondary acoustic measurements were also suggested. “These measures can be carried out on a complete product, ready to go into mass production,” said Carcoustics’ acoustics expert, Michael Hansen. Primary measures, by contrast, are deeply embedded in the design—for example, in an engine housing. In the case of the Ahlmann swing-loaders, it was a question of finding passive ways of attenuating noise, soundproofing, and muffling sound transmitters. Measurements were taken to see to what extent noise penetrated outside the engine compartment. Noise was traced escaping from the engine compartment into the wheelhouse, side panels, and seals.

The next inspection stage involved examining the acoustics of the engine compartment. Like Ross, Hansen and his colleagues made best use of the different materials available to achieve the best results, and the hood was fitted with noise-absorbing components such as insulating mats.

“The big challenge is to find those places where a noise-absorbing component will work best,” said Hansen. This can be measured using laser-scan vibrometry, whereby vibrations are illustrated visually. Rather like using a thermal-imaging camera, it is possible to display the places that generate particularly loud noise using an acoustic camera.

Engines invariably get hot and require air for cooling as well as combustion. In this case, the fresh-air-intake chamber was producing physical noise through both reverberation and static waves, generating air-reverberation noise. In one typical example, the solution involved covering the intake shaft with noise-absorbing parts and three sound-muffling components to reduce oscillation.

The driver’s cab was examined closely, following the measurements taken around the engine, and intensity measurements were taken. Tests were then carried out to establish to what extent surfaces were vibrating or noise penetrated within, through the ventilation system, both while idling and when at work.

Changes to the cab included fitting a noise-absorbent shield made of aluminum and glass-fiber matting to the heat screen between exhaust silencer and hydraulic tank. Noise-absorbing components were also attached to the frame, taillights, and side covers of the engine compartment, which, like the wheel arches, are plastic.

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