By Jim Kerr

Automobile manufacturers do a lot of research into reducing cabin noise in new vehicles. Drive a modern vehicle and you quickly become aware of the quiet atmosphere that allows normal voice conversations or listening to audiophile sound systems. However, noises that we didn’t hear before, such as the wind blowing past a rearview mirror, now seem to be much louder – but only because everything else has become much quieter. Perhaps that is why when a noise does occur while driving, it becomes so annoying.

Noises can be mechanical in nature, such as the rattle of a loose panel, or the squeak of two panels rubbing together. Noises can come from the tires, as they roll across different road surfaces; or noises can come from the air passing around our vehicle. Not all noises are objectionable: some drivers like a throaty exhaust note during full throttle acceleration or the mechanical busy-ness of a double overhead cam engine as it rises in r.p.m. Others prefer quiet isolation. Most vehicles fall in between.

There are three ways of reducing noise. Stop it at its source, block its path into the interior, or change the level or frequency so it doesn’t annoy anyone. Stopping noise at its source may seem to be the preferred method, but it is not always possible. Some tire noise is always present, and rough road surfaces make it even worse. Different tread designs do make a huge difference in noise generation, so you may experience more or less noise with different tires. Most tire dealers can provide noise level recommendations.

Other noises come from powertrain components. These are easiest to change when designing new assemblies. Additional ribs strengthen components to reduce vibration. After all, noise is simply a vibration we don’t like.

Damping noise transmission is done with several methods. Fluid-filled motor mounts block engine vibrations to the passenger compartment. Rubber isolation between suspension mounts and the body can block noise. Sandwiched firewalls, where a layer of plastic is bonded between two layers of metal, does a remarkable job of blocking noise transmission from the engine compartment into the interior. Much of the damping material used on vehicles remains unseen. Foam-filled pillars add structural strength and prevent vibrations in the material.

Body panels can be damped by sticking a mastic or pliable sheet to the inside of the panel. I watched Ford demonstrate how they optimize the placement and size of the sound damping material. They generate vibrations in the body panels with large audio speakers operating over many different frequencies. Body vibrations are then monitored by projecting a laser beam on the panel and measuring the areas where the body vibrates the most. Sound damping materials are then placed in these areas and the body is retested. Adding additional damping material may seem to be better, but this adds weight to the vehicle, which reduces fuel economy. It is better to add small amounts in key places than large amounts everywhere.

Changing the frequency of a noise is another method of building quiet passenger compartments. Human hearing doesn’t pick up very low or high frequencies, so any vibrations in that spectrum are not objectionable to us. By changing the stiffness or mass of a component, the frequency that it vibrates at will be different.

With the mechanical noise reduced, wind noise becomes more apparent. A wind “rush” is the sound of air passing the windshield pillars, wiper blades and mirrors. By changing angles, location or smoothing edges, wind rush can be reduced but not eliminated. You can hear the wind rush even in a smooth-bodied glider.

Wind whistles are usually caused by air leaking from inside the body to the outside. A wind whistle will disappear when a window is partially opened. Poor fitting weatherstripping or misadjusted doors and windows are the most common causes of this type of whistle.
Finally, wind whistles can come from wind passing over openings in the body, such as door gaps, suspension access holes or window mouldings. To reduce these noises, body panels are adjusted so the front panel is always even or higher than the panel behind it. This reduces air entry into the openings. Diagnosing this type of wind noise can involve several body adjustments and a roll of masking tape. If you tape over an opening and the noise disappears, then you have found the source of the problem.

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