It’s very enlightening to drive a car dating from before WW1. You’ll find that vibrations and noise assail your senses from all directions. The exhaust barks, the gearbox and rear axle whine, the gearlever vibrates, and the wooden body and wheels creak. The engine feels rough, unless you’re in one of the rare cars with six cylinders. Vibration shakes the mudguards and other hang-on parts, especially when the engine is idling or running at small throttle openings. If the throttle is opened further, the vibration dies down, as if by magic, only to appear again when the revs get close to the low maximum the engine is capable of. If the car hits a bump in the road, it not only leaps into the air, but also twists and vibrates in a way that makes you finally understand why modern automotive sales brochures emphasise torsional stiffness…
Now get back into your modern car, and the totally different experience must prompt you to wonder what happened to all the noise and vibration. The amazing thing is that the inputs, ie the causes of the vibrations, are still there, but the resultant responses by the various components have been totally transformed.
How did it happen? How did a source of vibration like an engine, or a rough road, get tamed? Before we can answer this question, it is worthwhile to look at the nature of vibration.
The nature of vibration
Many objects respond to a disturbing force by vibrating, and often this action sets up a vibration in the air around them. When these waves reach our eardrums, we experience it as a noise. At other times, the nature of the vibration is such that we feel it directly, mostly because we’re in contact with something that touches the vibrating object.
Vibration can best be described in terms of four words that most readers will already be familiar with. Any regularly repeated motion has:
o an amplitude, which is the amount the disturbance deviates from an equilibrium position
o a frequency, which is the number of cycles completed per second, measured in hertz (one Hz = one cycle per second)
o a period, which is the time taken per cycle, measured in seconds
o a wavelength, which is the distance from one crest (or trough) to the next crest (or trough).
Note that the period and the frequency are inverses of each other, ie if you invert cycles per second you get seconds per cycle.
Vibration occurs in a number of different forms, or modes, depending on the nature of the disturbance as well as the nature of the system being disturbed.
o A single once-only disturbing force will give rise to the so-called natural or free vibration of a body. This is what happens when a temple bell is struck; the bell vibrates at its natural frequency, which depends on the shape, dimensions and material of the bell. The sound reaches our ears because the bell’s vibration has been communicated to the air, and the airwaves have set our eardrums in motion. However, the sound doesn’t last long, because the vibration dies away due to friction between the particles in the interior of the body, and can only be revived by striking the bell again.
o Complicated, or built-up, shapes tend to vibrate in more than one mode, and hence will have more than one natural frequency. Many automotive vibrations are of this kind. For example, a crankshaft and flywheel combination will vibrate in a number of different linear modes. Furthermore, it’s also prone to torsional vibration, which is an alternate twisting and untwisting of the shaft. An experienced racing mechanic can tell whether a crankshaft is cracked by suspending it from a support and then striking it to make it ring like bell. If the tone is dull and dies quickly, it’s likely to be cracked.
o Forced vibration occurs when an external vibrating disturbing force acts on a vibrating system. The system will initially respond by vibrating at its natural frequency, but will also try to adapt to the new vibration. If there is some form of damping, the natural frequency will die out, leaving only the forced vibration at the same frequency as the disturbance. This explains why a crankshaft’s vibrational pattern is so complicated. The repeated shock loads constantly interfere with the natural frequency, so that the movement never gets a chance to die out.
o Damping can take many forms, and is always present to some extent. Some forms, such as the friction of the air surrounding an object, and internal damping by the particles of the substance, are usually very weak, so that the vibration takes a long time to die down. Other forms, for example fluid friction, dry friction and magnetic damping, are usually strong enough to kill the vibration quickly. Damping can be classified as overdamped, when the vibration cannot occur, or underdamped, when too much vibration occurs. Critical damping refers to a borderline vibration between the two extremes.
o A very destructive vibration called resonance occurs when the disturbing vibration has the same frequency as the natural vibration of a system. In this case, the amplitude of the vibration will increase dramatically, and could result in destruction of the object. A very famous example is the collapse of the Tacoma Narrows bridge in the USA in November 1940. Gusts of wind amplified the natural vibration frequency of the structure, and it was seen to be vibrating with increasing amplitude until the bridge eventually collapsed, taking some cars and people with it.
o Any rotating shaft goes through critical speeds, where the rotational speed coincides with one of the natural vibrating frequencies of the shaft. Propshafts tend to vibrate in this way. The usual cure is to change the natural frequency by shortening the shaft, and fitting a support bearing in the centre. This is the reason why most long wheelbase saloons and light commercial vehicles have divided propshafts.
The reason for NVH control
The automotive market is very competitive, and when people shop for a car the noise levels often play a role in the final choice. It is also one of the criteria CAR uses when evaluating a test vehicle. Furthermore, noise and vibration reduction are not only necessary to reduce the irritation factor and prevent parts from breaking, but also to help make the use of cellphones and in-car entertainment more practical and enjoyable, to say nothing of the latest fad – voice control of various functions. This is the driving force behind the huge amounts of money and time that has been spent on the quest to reduce NVH.
The major automotive manufacturers have tackled NVH reduction by creating special departments, staffed by trained engineers and scientists. It’s interesting to note that General Motors, for one, considers three sources: engine noise and vibration, road noise and vibration, and wind noise. As Elizabeth Pilibosian, Director of Noise and Vibration at General Motors (Detroit), says: “The challenge is to balance all three sources, which are generally in three different frequency ranges, to make a quality-sounding vehicle. Our goal is to have a good balance among the three, not just reduce one or two of the sources. It’s got to be like an orchestra – nobody can be out of tune.”
The problem is made even more difficult due to noise measurement equipment being unable to provide a readout of what is irritating and what is not, because sound volume is not a good guide. For example, it has been found that sounds as low as 40 dB(A), are sometimes linked to increased driver fatigue under certain conditions.
Nevertheless, from 1980 to 2000 the average noise level in cars has been reduced by 0,3 dB(A) per year. This implies a total reduction of 6 dB, which might not sound like much, but if one keeps in mind that this is more than the usual difference in sound levels between travelling fast and slowly in the same car, it becomes meaningful.
As a car becomes quieter, the objective becomes one of tuning the sound to change its nature, rather than eliminating it, because other sources of sound become significant. For example, a noisy exhaust may be masking a potentially irritating road noise.
The scientific study of automotive sound levels has resulted in some definitions. The following are interesting, but have not been universally accepted. Unwanted sound is called Noise, but Harshness refers to sudden events of short duration at higher frequencies. Vibration is usually of low frequency, so that it’s rather felt than heard. Many other words are being used to describe the different kinds of disturbance, and one of the aims in the industry is to get universal agreement on precise definitions.
Some NVH control examples
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 or noise transmitters. The latter either transmit a vibration, or magnify the sound or harshness.
A. Sources of vibration
Every time combustion takes place, the relevant piston delivers a blow to the crankshaft that results in a sudden angular acceleration. This is immediately followed by a deceleration due to another cylinder going into a compression stroke, causing the fluctuation in engine speed that triggers torsional vibration.
For example, in the case of a four-cylinder diesel, it may mean that within ONE revolution the instantaneous idling speed changes TWICE; from the average of 800 r/min to 860 and then back to 800 r/min, before dipping to 740. This effect is less severe at higher engine speeds, but it certainly explains why many engines tend to induce annoying vibration while idling.
Historically, the annoying effect of this vibration was initially countered by increasing the number of cylinders, whereas later engines employed more sophisticated crankshaft balancing techniques. Many modern engines employ balancer shafts that run at twice engine speed to counter vibrational frequencies that only occur at twice engine speed.
The last ten years have seen the majority of engine designers specify 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, while 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 ’30s, 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, have just been announced by Honda for two 2005 models. The company is using this rather drastic solution to curb vibrations that will arise in a 3,0-litre V6 engine when the driver employs cylinder de-activation to save fuel. In this mode the engine runs on only three cylinders, and the resulting vibrations could not be tamed any other way. This system is bound to become more popular in the future.
Tyres are often a major source of noise, especially if the rest of the car is quiet. This is because a 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.
B. Noise transmission
The structure of a car is, unfortunately, a very good noise transmitter, and this is made worse by the modern technique of utilising unit construction. In earlier days, when the body panels were mounted onto a separate chassis with insulating material in between, it was a lot easier to isolate the various noises. In addition, the passenger cavity acts like a soundbox, often resulting in a booming noise at certain speeds. This is especially true of single-cab LCVs, which are notoriously difficult to quieten.
The sounds that the occupants hear are made up of contributions from the powertrain, 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 powertrain 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.
This approach, which is actively being followed by a number of manufacturers, resulted from the realisation that the interior drivetrain noise could be separated into airborne noise and structure-borne noise. The powertrain surfaces, intake and exhaust systems, and the ancillaries directly transfer airborne noise to the interior, and these sources can be identified and tuned. The structure-born noise results from vibrations that are transmitted directly by the structure and radiated into the cabin.
The future
Active noise cancellation is an exciting prospect that’s just around the corner. Typically, such systems use piezo-electric drivers to generate sound of the same frequency but a waveform that is a mirror-image of the soundwave, so that the waves cancel and the noise quietens down.
