Any person in your daily life is confronted with manifestations of noise in the city streets, stadiums, public premises and industry. Is no exception and a conventional car where you have to spend a lot of time and feel the whole range of extraneous factors. For understanding the physical essence of this phenomenon and its impact on a living organism and the organization of the system of protection from him, try to understand the concept and nature of noise.
The nature of noise and its types
The noise is nothing like the combination of sounds of different intensity and frequency, creating additional emotional burden and harmful for humans. Thus, in the undesirability and harmfulness to the impact of the object lies the main difference from the ordinary sound. In the car, under the condition of high noise, the driver and passengers get tired faster and this can lead to emergency situation. In this regard, there is a need of struggle against this undesirable phenomenon. And in order to know our enemy, let's see what happens to the noise.
In everyday parlance it is most often characterized by the words "strong" and "weak". But what lies behind them, not many know. They are based on two parameters that determine the degree of perception and influence of the mind - is the sound level (intensity) and sound pressure level. What we change, adjusting the sound volume of the speaker system. The generally accepted scale for measuring these indicators is a logarithmic scale with a unit of measure in decibels (dBA). The critical value of the sound level is 130 dB, in which it causes the effect of the pain. The location of the maximum sound pressure level in the frequency spectrum of the noise can be divided into:
- low-frequency (LF) - up to 300 Hz;
- medium frequency (MF) from 300 to 800 Hz;
- high-frequency (HF) - more than 800 Hz.
Taking into account the amplitude fluctuations of a sound wave during the time intervals the noise is divided into continuous, intermittent and pulse. The constant noise is characterized by amplitude variations within 5дБа for 8 hours. To constant noise can be attributed to structural noise transmitted from the engine, transmission, shock-absorbers, wheels. Intermittent has a discrete pattern of change in pulse shape jump lasting about 1sec. Intermittent can be attributed to the buzz included the heating system. To the pulse are creaking plastic parts. Human hearing adapts to the "background" level and accustomed to it. Do not notice sounds that are quieter than this level, but sensitive to "sharp" noises exceeding the background amplitude. Than the "sharper" sound, so it is more noticeable to the human ear.
Ways of formation of noise fields in a car
Sound vibrations from the primary sources penetrate into the inner space of the vehicle through the loose door and window openings, ducts and process vents to the body. In the process of restriction from the actions of such acoustic oscillations have two main components. Is the suppression and restriction of the appearance of background noise in the sources, that is, through improvement of the powertrain, design, chassis, exhaust system. Of great importance in this matter is the height and texture of the tire tread. On the other hand the thickness of the glass, trim and panels directly depends on the degree of noise isolation of the car.
Structural noise is spread from the source of noise and vibrations on the machine frame by means of associated fastening elements, and mechanisms of transmission of rotation, such as suspension, transmission, steering. The resulting mechanical vibrations result in movement of other body parts, which create a secondary background noise. In the formation of the latter the important role played by reflected sounds. Arising in the process of the acoustic wave tend to like to go around the obstacle and be reflected from various surfaces, which creates an additional burden on the environment. For example, sound vibrations are reflected from the roadway and returned toward the bottom of the car. Additional vibration of the floor also cause energy accompanying the exhaust gases. All this together is connected to the General cacophony of sounds by our hearing the noise.
The rigidity of the system and why it is needed
As already mentioned, the vibrations make a significant contribution to the overall structure of the noise impact. It should be noted that the constant motion of the particles of solid bodies causes not just the sound, but can lead to more meaningful results, coinciding with other waves in a certain part of the space. For harmonic vibrations, the stiffness of the vibrating system determines the possibility of resonance in a certain frequency range. While increasing stiffness reduces its probability at high frequencies with the same amplitude of the input. In addition, for resonance at low frequencies it is necessary to increase the input.
To comment on this as an example. The man shakes the tree with a diameter of 10 cm do it easily "finds" resonance and is not even one. If he's going to shake the tree with a diameter of 20 cm, the "search" points of coincidence of the oscillations takes him more strength. And these forces will not suffice for the sway of the tree 40 cm this shows that when the same amplitude of the input impact of increase in the rigidity of the system allows to reduce the probability of occurrence of the system in resonance. In relation to the car's high rigidity allows you to "not pay attention" to small values of structural and airborne noise, in view of the fact that they will not be sufficient for the emergence of this phenomenon in different parts of the body of the car.
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