Modern cars are exceptionally comfortable and quiet. In fact, technology has advanced so far that one is inclined to take its benefits for granted. To be reminded of this, all you need do is take a ride in an old-school 4×4!
Automotive engineering has come a long way in the last 60 years. Just how far is easily demonstrated by a drive in an old beam-axled, leaf-sprung Land Rover from the 1960s. Noise and vibration assail your senses from all directions. The gearbox and rear axle whine, the gear lever vibrates and the engine feels very rough.
If the vehicle hits a bump, it not only leaps into the air but twists and vibrates in a way that makes you totally understand why modern automotive sales brochures emphasise torsional stiffness. When Land Rover finally introduced coil springs in its vehicles, the ride changed dramatically, but the engine had to wait a few more years to become civilised.
When you drive one of the latest luxury Land Rovers, the experience prompts you to wonder: what happened to all the rough stuff? The amazing thing is that the causes of the noise and vibrations of old are still there, but the responses by the various components to them have been transformed.
CONTROLLING NOISE LEVELS
Automotive engineers lump all forms of noise and vibration together in the phrase “noise, vibration and harshness (NVH)”, and the industry spends a great deal of time and money to make vehicles quieter and curb vibration.
When people shop around for a car, the noise levels often play a role in the final choice, with the result that the major manufacturers have tackled NVH reduction by creating special departments, staffed by trained engineers and scientists.
Finding a solution to a particular problem is often difficult because noise measurement equipment is unable to provide a read-out of what is irritating to the human ear. Sound volume is not a good guide, because one man’s noise is another man’s music.
As cars get quieter, the objective often becomes one of tuning the sound to change its nature rather than eliminating it, because other sources of sound become more significant. For example, a noisy exhaust may be masking potentially irritating road noises.
In general, a vibration can either be reduced at the source, damped at the source, or damped where it becomes problematic. For this purpose, the parts that make up a car can be divided into sources of vibration and noise transmitters.
Sources of vibration
Every time combustion takes place, the relevant piston delivers a blow to the crankshaft. This results in a sudden angular acceleration that is immediately followed by a deceleration due to another cylinder going into a compression stroke, causing a fluctuation in engine speed that is a source of regular vibration.
Historically, this vibration was countered by increasing the number of cylinders, whereas later engines employed more sophisticated crankshaft balancing techniques. Many modem engines employ balancer shafts that run at twice engine speed to counter the vibration frequencies that only occur at twice the engine speed.
In the last ten years, the majority of engine designers have specified dual-mass flywheels to reduce the harmful effects of torsional vibration. This construction divides the traditional flywheel into two parts. One is attached to the crankshaft and the other is part of the clutch assembly. The two parts are connected by a radial damping spring such as one would normally find in the centre of a clutch-driven disc. The complete dual-mass unit dampens the worst of the torsional vibrations.
Rubber engine mountings came into general use in the early 1930s, and their ability to dampen engine vibration has lately been advanced by going for sophisticated liquid-filled mountings, because the traditional rubber mountings are not effective under all vibration conditions.
Electrically excited engine mountings, which calculate the engine vibration patterns from measured pulse intervals in crank rotation and then vibrate electrically in exactly the opposite way, are coming into fashion.
Tyres are often a major source of noise, especially if the rest of the car is quiet. This is because the tread contains blocks that flail against the road surface, causing the underlying reinforcement to vibrate and send sound waves outwards. Different portions of the tyre vibrate at different speeds, thus causing noise of a higher or lower frequency.
Cutting down on the number of blocks generally tends to reduce a tyre’s wet road grip, whereas making the blocks shallower tends to reduce tyre life.
The basic structure of a car is, unfortunately, a very good noise transmitter. This applies especially to the modem technique of designing the chassis and body structure as one unit. In addition, the passenger cavity acts like a sound box, often resulting in a booming noise at certain speeds. This is especially true of single cab bakkies.
The sounds that the occupants hear are made up of contributions from the power train, from road noise and from wind noise. The latter two can only be reduced, not changed, by aerodynamic improvement and tyre research, because the sound waves arising from these sources are jumbled up in an irregular pattern.
However, the waves arriving from the power train are regular, and even predictable, so that they can be modified by tuning the shape and material of the interior. The aim is to produce a characteristic sound that will be appealing to buyers.