Abstract
A moving vehicle has many sources of noise and vibration.
In an operating vehicle, contribution to sound and vibration comes from internal factors such as its powertrain and automotive components and external factors such as the road and wind. Some part of the sound is inherent in a vehicle during its operation and this includes the car audio or the active noise control systems.
For the driver, interaction with the vehicle is a fairly complex operation, with the audio and tactile feedbacks combining with several visible cues during the ever changing driving and boundary conditions. Industrial societies use passenger cars, with users developing precise expectations for the feel of their car. Moreover, these expectations, along with the cost and fuel consumption, drive individual purchase decisions.
Although vehicle manufacturers put in a lot of effort with specific attributes to align their products in a better way to customer expectations, noise and vibration play an important role in the overall harmony of the vehicle. Users expect their vehicles to perform all the intended functions while providing a comfortable and enjoyable environment. For instance, some noise and vibration elements affect comfort, such as wind, tire, boom noise, and gear whine, while others such as engine noise during acceleration, ride, and handling have a greater and more direct impact on overall appeal.
Manufacturers understand the role sound and vibration play in the way customers perceive their vehicles and use the parameters to establish and ensure commercial appeal. Most often, manufacturers integrate noise and vibration tightly with vehicle development and design for expressing a very strong brand identity.
Classifying sound and vibration quality components helps to translate these concepts into engineering metrics and targets. This also makes it easier to investigate sound and vibration quality concerns related to detectability issues, as they tend to be one-dimensional in the sound quality space. For instance, the single narrow-band frequency of axle whine is at first just audible, then becomes annoying as its level increases over the rest of the vehicle’s noise. Therefore, axle noise becomes detectable and objectionable beyond a specific level. On the other hand, the multi-dimensional acoustic image of the vehicle has multiple components that depend on time and frequency, interacting and combining to create an overall vehicle sound.
Sources of automotive sound and vibration
Automotive noise sources.
From the standpoint of sound quality, design challenges fall into two broad categories—interior noise and external noise. Again, both are dependent on the vehicle type, for instance, electric vehicles and those using the internal combustion engine for propulsion.
In vehicles using the internal combustion engine, the engine is a source of masking for all other sources. When the engine is not running, noise from all other sources become suddenly very noticeable, such as from fans, compressor, pumps, and other accessories and subsystems. The internal combustion engine provides a background noise that effectively masks the noise from other sources. Additionally, the internal combustion engine also drives most of the accessories, which therefore show an expected speed ratio and patterns of harmonics.
The motor of an electric vehicle also generates noise, but typically, this is in a much higher frequency range compared to that produced by an internal combustion engine and does not offer the same level of background masking. Therefore, in an electric vehicle, the noise from accessories and subsystems is more noticeable and annoying. To overcome this, some manufacturers of electric vehicles have adopted a strategy of injecting cool and pleasant powertrain sounds.
Kähler 1 discusses the contribution to automotive noise from the vehicle’s chassis and the suspension in the vehicle. According to Kähler, vehicles with a sportier configuration tend to have their chassis lowered and closer to the road—leading to higher road noise inside the cabin, especially when the road is rough the chassis stiff.
Kähler goes on to suggest that a vehicle with a softer suspension is better in suppressing the noise and vibrations from a rough road. Suspensions have different characteristics, and a softer suspension absorbs much of the road roughness. In contrast to the sportier cars, those with a softer suspension offer a smooth, soft, and a more enjoyable motoring experience.
Noise from an internal combustion engine
Internal combustion engine.
Although the internal combustion engine defines the character of the car using it, over the years, manufacturers have significantly improved the isolation of the passenger cabin from the engine. Therefore, more than the noisiness of the engine, the issue now is more about the quality of its sound and vehicle original equipment manufacturers (OEMs) have extensively mapped the sound and vibration quality attributes of the powertrain. There are two main sound quality criteria—maximum loudness and overall noise linearity.
Maximum loudness of the overall noise from the main engine: when running at idle and in slow and hard acceleration conditions. This is measured as an A-weighted sound pressure level and takes note of its firing frequency, along with the first few even, odd, and half-integer multiples.
Linearity of the overall engine noise against RPM—the requirement is the noise must grow linearly with the RPM and not show significant valleys and peaks. Therefore, a loud vehicle signature is more acceptable to the driver when it grows linearly with engine RPM, rather than a quieter engine that booms when accelerating or at steady state. Sometimes, the boom, a sudden deviation from the mean noise, may not be from the engine alone, but could be triggered by a low-frequency mode by the motion of an accessory or from the tires.
Noise from diesel engines
Diesel engine.
Passenger vehicles powered with diesel engines produce noise acceptable at high-speed cruise conditions, but not at low speeds and when idling. Their sound signature characteristically differs from those belonging to the internal combustion engine because of sharpness, tonality, impulsiveness, and irregularities. The latter are commonly termed as diesel knock and clatter respectively.
Kähler 1 claims that the electric engines are quieter than internal combustion and diesel engines are. According to Kähler, although initially, the diesel engines were very loud, their modern versions are far quieter. As diesel engines work at lower revolutions when driving on highways at high speeds as compared to a petrol engine, they are substantially quieter. However, as the speed drops, the diesel engine sounds louder than petrol engines. However, this is influenced by the nature and extent of soundproofing in the cabin of the car.
According to Car Audio Help (CAH), 2 noise from the alternator contributes greatly to the sound from car stereo—a high-pitched whine rising and falling with the engine speed. However, properly grounding the equipment usually solves this problem. It requires the electronic system to be grounded to a bare part of the metal chassis, preferably with a bolt and nut. Other sources of electrical noise are imperfect connectors between the battery and other components, including the factory ground strap. CAH suggests the use of twisted pair audio cables for less noise pickup.
Another suggestion from Pomerantz 3 for reducing alternator noise is to introduce a filter on the power line. When installed between the battery and the alternator, the filter can minimize the noise. Pomerantz also suggests a noise filter on the power lead of the radio receiver to cut down on signal pollution.
Tuning the automotive engine
Engine tuning.
If the automotive engine is already at its overall quietness and linearity, achieved by addressing its powertrain targets, the interior sound may be further tuned to match the desired acoustic image. Manufacturers typically tune the engine by manipulating the performance of the intake and exhaust systems. They balance the requirements of the engine sound and power performance for achieving the best possible compromise. Both intake and exhaust noise issues need review and addressing.
Engineers achieve tuning in both intake and exhaust by completely passive means, active means, or a hybrid mix of both. For instance, European OEMs use valves in the exhaust of their high-performing vehicles, which allows the vehicles to comply with the pass-by tests, while generating sporty sounds at higher engine RPM.
Intake tuning typically increases the balance of the harmonic content in the mid-frequency range, increasing not only the integer engine orders but also the half orders. This changes the quality of the engine sound creating sporty, aggressive connotations. It introduces a roughness to the sound depending on the level difference between the integer orders and the half orders.
Sometimes tuning alone may not help reducing the noise, especially for old cars. According to Pomerantz, 3 the ignition system may be the source of the noise—varying in speed when accelerating. As a remedy, Pomerantz suggests using resistor-type spark plugs and shielded carbon-core wires for spark plugs, coil, and distributor cap.
Gear whine issue
Gear whine.
While OEMs carefully design for engine sound quality, gear mesh frequencies appearing as typical driveline sound quality issues are heard as pure tones over background noise. It is possible to detect and measure the tone over masking by comparing slices of A-weighted gear-mesh orders to the overall noise.
The perception of tonal components that differential and drive shafts, transmission, and transfer cases generate or radiate depends on whether it is a loud tone or has a level varying with time and/or RPM. Often, people do not complain of a loud noise simply because it is always present. However, a gear whine that is heard say only between speeds of 70 and 80 km/h is very noticeable. That makes it imperative to express the maximum allowed level for the tone as a function of the engine RPM.
Tire and road noise
Tire and road noise.
Successful and ongoing reduction of powertrain and driveline noise in vehicles has led to the increasing importance of tire and road noise for overall sound quality perception. In general, road noise starts to be noticeable at vehicle speeds above 50 km/h. However, its contribution to overall interior noise of the vehicle maximizes between 60 and 100 km/h, decreasing at higher speeds as aerodynamic noise starts to predominate.
Tire noise is mainly present between the frequency ranges of 500–1300 Hz and contains both broadband and narrow-band components. From the tires, the noise reaches the interior occupants of the vehicle primarily through holes and leakages, and partly through insufficient transmission losses of the vehicle’s windows, doors, and floor.
According to Kähler, 1 it is extremely important to choose tires properly. The level of road noise can change dramatically simply by changing the existing tires on a car. The road noise decreases if the tires are narrow and small in diameter. Kähler explains the reason as thicker tires have more rubber in contact with the road, creating more noise.
Manufacturers have various models of tires with differing contents of rubber. According to Kähler, harder tires produce considerably more noise than softer tires. In addition, road noise also depends on the tire pattern—some patterns are quieter.
Aerodynamic noise
Aerodynamic noise.
For a vehicle traveling at a speed above 100 km/h, aerodynamic or wind noise is the predominant component of its interior noise. The aerodynamic coefficient of the vehicle is responsible for the noise and is a function of the shape and cross-sectional area of the vehicle. Factors affecting the aerodynamic noise generated are turbulence and externally varying wind conditions such as crosswind.
Aerodynamic noise from turbulence occurs through holes in the sealing around doors, windows, windshields, hood, and so on. Very low frequency beating noise can occur with the wind rushing past a sunroof or a partially open window as the vehicle cabin resonates like a Helmholtz resonator being excited by the airflow along the boundary of the sunroof or window opening.
Sound proofing the car
Sound proofing.
Another way of reducing cabin noise is to sound proof the interior of the car. This is achieved best by applying different sound dampening/deadening materials throughout the car. Several companies produce excellent solutions for this.
Kähler 1 suggests install-ing such material by cutting patches to size and sticking them to places that transfer the most noise. According to Kähler, these places would be the roof, doors, floor, and the firewall—the wall between the engine and the cabin. Although the floor and the boot may be handled easily, fitting sound proofing materials within door panels and the firewall may require the services of an expert.
Vibration dampers, sound barriers, sound absorbers & gasketing materials.
According to Cascade Audio Engineering (CAE), 4 it is necessary to identify the noise first before choosing the correct product for soundproofing. According to CAE, using a combination of materials is necessary to reduce the noise effectively. For this, CAE suggests a sound control system consisting of gasket products, sound absorbers, sound barriers, and vibration dampers. These are most effective when installed in stages.
According to CAE, while gasket materials are best for sealing speakers, they are effective also in eliminating buzzes, rattles, and squeaks. CAE suggests the use of sound absorbers primarily for absorbing the mid and high frequencies from airborne sounds. However, sound absorbers do not damp vibrations as these are made typically from very lightweight material.
For eliminating or reducing structural vibration and resonance, CAE suggests the use of vibration dampers. However, vibration dampers will not block sound from entering the cabin. They suggest use of sound barriers for blocking sounds. Sound barriers are effective for reducing road noise from tires, drivetrain, airflow, and exhaust.
CAH 2 suggests the use of viscoelastic sheets and spray-on-materials as these are commonly available in the automotive aftermarket. According to CAH, these sheets are relatively easy to apply and they work effectively to dampen road noise and rattles. Interior door panels do better with a liquid spray, but these are messier to apply. Note that most sound deadeners tend to be heavy and may affect the mileage of the vehicle. The use of viscoelastic sheets and spray-on-materials is therefore a better alternative.
Wojdyla 5 also suggests using sound-deadening pads to act like sound-absorbing stickers to absorb noise-making vibrations that enter the cabin. These are easy to apply as one can simply cut them into patches and stick them on to the sheet metal at key spots to reduce the sheet vibrations.
Conclusion
Vehicle manufacturers take great care to reduce cabin noise and allow a pleasant driving experience. Additionally, several methods and techniques are available for reducing the various types of automotive noise a moving vehicle typically generates. Some are do-it-yourself types, while others need help from an experienced technician. As the perception of noise and vibration is subjective and varies from person to person, it is ultimately a personal choice for deciding what works best.