Noise Characteristics
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2.3  Noise Characteristics of Automotive Vehicles

The characteristics and levels of noise are the products of the exciting forces (engine, road and aerodynamic) and are also influenced by the elastic properties/dynamic characteristics of the vehicle structure and the interior body surfaces. Each of these surfaces also has a natural mode of vibration. In addition, there is the resonance of the air inside the enclosed car interior which can modify the acoustic field at specific locations and frequencies.

The noise inside a vehicle consists of both random background noise (which originates from mainly road surfaces and aerodynamic excitation) and discrete frequency components associated the engine. The structure borne noise is typically discrete whilst airborne noise is random in nature.

The random background noise is the main factor controlling the perception of loudness with regards to interior noise. Discrete frequency components control the peak amplitudes and thus the subjective ‘annoyance’ of the noise. A discrete frequency tone has been found to be more annoying when superimposed on a random background noise of varying intensity [3].

As a consequence, in controlling the noise level in the vehicle cavity an understanding of the preceding points is essential. If the background noise level can be reduced through suspension system refinement, the discrete frequency components remain. This then results in a subjectively ‘noisier’ vehicle.

It has been found [3] that for two identical vehicles, the subjectively quieter had a higher overall sound pressure level than the subjectively noisier vehicle. From this study it is clear that consideration of both the background and discrete noise levels is required to achieve a design that optimises the interaction of the two.

Investigations [3] have shown that the basic background noise component increases at a steady rate of about 5-6dB when the speed of a given vehicle is doubled.

Considering unweighted sound pressure level results from a vehicle interior, research [3] suggests that the noise consists of a significant level of low frequency sound in the frequency range 1-24Hz with a peak noise level of 120dB around 4Hz. Although the noise levels are very high in this frequency range, the larger proportion of the sound energy lies in the inaudible frequency range 0-20Hz and thus in terms of loudness this component of the noise has minimal effect.

Though the sound level in the audio range (above 20Hz) has a relatively lower amplitude, in terms of loudness it is significant (see figure 2.1).

As a car is accelerated, various resonances of the vehicle system occur and these are excited principally by the harmonics of the engine and wheels. The dominant overall harmonic in the vehicle system is the 2nd engine harmonic. Components higher than the 4th engine harmonic are generally considered insignificant.

The individual resonant peaks measured in a vehicle interior are broad and as a result a speed range of 5-7MPH can be categorised as leading to a given resonance peak in the vehicle. Such broad range of speeds indicates how the vehicle, when treated as a whole, is a very highly damped system.

A comparison of interior noise was made between both a self powered vehicle and one towed in gear with the clutch engaged [3]. It was determined that there was an average loss of 6dB for the towed vehicle. (Please note this was a vehicle from 1976 – equivalent data for contemporary vehicles will clearly be less)

Further research [6] suggests that typical car and truck bodies currently create on the average of 0.015 Pa of interior noise for 1 N excitation at attachment points (depending upon the nature of the attachment point). Though this is the case, similar-sized vehicles have been found to vary +/- 5dB in vibro-acoustic transfer for similar attachments.

With regards to sound power contribution, a study [37] has shown that at an engine speed of 3000rpm, the major contributors to interior noise are the lower dashboard and the floor panels. These were found to contribute at all frequencies. The windshield glass and the corner of the windshield instrument panel were the secondary major noise contributors. However, at high frequencies they were found to contribute to the vehicle interior noise but they absorbed energy at low frequencies. The vehicle doors, roof, rear seats, package tray and backlite glass were found to be energy sinks.

Through the continued improvement in the NVH characteristics of automotive systems there has been an overall reduction in the broadband noise in the passenger compartment. The current drive towards lighter and consequently fuel efficient vehicles has generally been achieved through the reduction in the material thickness of the vehicle body structure, unfortunately this leads to higher acoustic sensitivity. Then there is the fact that powertrains are becoming smaller with higher specific power outputs and increased rotational speeds. Couple with this the increasingly common use of balance shafts and thus there has been a reduction in excitation amplitudes of the primary firing orders, resulting in higher-frequency sounds becoming more significant. 




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