Sound Wave – What Is Sound Wave, How Is Sound Produced.
The fundamental elements of sound are the frequency of variation of sound waves and their intensity. The frequency is express in hertz (Hz), in vibrations per second (the human ear can distinguish noises with variable frequencies from 20 to 20,000 Hz, reaching the point of greatest sensitivity around 3,000 Hz). Regarding intensity, one of the most common measurement systems represents a special scale of the sound pressure level expressed in decibels (dB).
How is Sound Produced
Scientifically, it is proven that the manifestation of a feeling of discomfort in the human body coincides with an average value of 50 dB. When levels close to 90 dB are reach, there is a pain sensation; the physical pain threshold is 130-140 dB. For these data it is easy to deduce the importance of soundproofing in cars.
The main sources of noise in a car are:
– the engine (whose noise is transmit mainly through the intake filter and the exhaust manifold) and its accessories;
– change and transmission;
– the tires (noises related to rolling), and
– various and secondary sources (mainly body noise).
The engine’s operation generates vibratory phenomena of the whole assembly that cause, in addition to the sound emissions, a vibratory excitation of the body structure, which results in an unpleasant noise, especially intense at certain rotational speeds.
The distribution elements, tappets, rocker arms and valves produce noises within the high frequency range. In contrast, the intake and exhaust systems produce loud noises that cover the entire acoustic frequency range. The gearbox and transmission contribute to increased overall noise (typical of gearbox or differential gears).
The rolling noise of tires depends on the dynamic stresses to which they subjected while driving
Other noise, aerodynamic type, is cause by sliding the layers of air along the body. Logically this noise Is closely related to the vehicle’s speed and depends largely on studying each of the body elements more Than on the general shape of the same.
The main sources of intense noise are the side flares, the grille, the trim on the doors and windows (when their retention is poor). And the sheet’s roughness prevents the regular flow of wind layers. And also, high speeds, aerodynamic noises easily outweigh those of the engine and tread.
While the noise transmits to the outside of the vehicle propagates naturally through the liquid medium constitute by the air, the noise passenger compartment is transmits in different ways. And also, air and by solids. Part of the sound waves propagate through the air and hit the cabin walls, causing them to vibrate. And cause a secondary noise source or enters directly through holes in the sheets or defective gaskets (acoustic holes).
It another part is transmits through the elastic joint supports of the engine and the mechanical organs in general. Similarly transmission occurs through cables, pipes, pedals, steering bar, etc.
In all operating conditions, the study of noise in a vehicle always begins with an auditory check while driving on the road. It allows a first analysis of soundproofing problems to carry out. And also, Follow a series of systematic measurements to identify the type of noise.
These measurements include a magnetic
Tape recording of the sound levels in the front and rear seats as a function of vehicle speed and engine speed. And also, The determination was made using an analyzer and a microphone whose position is standardize to compare several vehicles. In this way, The evolution of sound levels is track by varying the motor’s rotational speed from 1,500 rpm to the maximum allow speed.
In addition to the preliminary road tests, the other tests generally carries out on the roller bench in an acoustically isolated environment.
The vibration signals are collects by piezoelectric type sensors. And analyzed with recording devices capable of reproducing everything on an oscillometer. When the study of the transmission of vibrations to the bodywork is carries out. The power unit with accessories and controls is first separated from the bodywork and mounted on rigid supports (which rest on the pavement of the test room). . Then the first cur
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