PULLEY 4N4782 - Caterpillar

Caterpillar Products: AllIntroduction
The purpose of this instruction is to explain some of the important concepts surrounding the issue of sound. Dealing with the dirt and dust that contribute to the total operator environment are also covered.What is Sound?
By definition, sound is a vibration that is transmitted. Sound travels in waves of compressed air molecules. Loud sounds are characterized by large, dense waves of compressed air molecules. Softer sounds are produced by smaller, less dense waves of air molecules. There are three measurable qualities of sound:
Pressure
Intensity
PowerPressure and intensity will both vary, depending on your position compared to the source of the sound. Power is independent of position. It is usually inferred through pressure measurements and the size of the measurement surface. The size, or amplitude of the pressure fluctuation is measured in decibels. This means that the decibel (dB) is a measure of pressure. The table below compares some common sounds with their decibel levels.
Table 1
Decibel Levels    
0 dB     Threshold of Hearing    
15 dB     Sound level in the woods    
35 dB     Library    
65 dB     Office    
90 dB     Heavy Truck    
105 dB     Jackhammer    
125 dB     Jet take-off    
140 dB     Threshold of Pain    There are some basic principles to how sound works. The list below explains those principles.
Illustration 1 g01160067
Slow repetitions produce low frequencies. Fast repetitions produce high frequencies. A noise that is repeated generates a frequency. The frequency that is produced depends on the time between the repetitions. For example, a large diesel engine that runs at low RPM will produce a low frequency. A small motorcycle engine that runs at high RPM will produce a high frequency.
Low frequency sound bends around obstacles and expands through openings. A low frequency sound radiates fairly equally in all directions. It diffuses around edges and through holes without losing intensity. It will then re-radiate from the edge or from the hole as if the sound were starting fresh. The sound spreads in all directions from the hole or edge. Because of this, screens and barriers must be very large in order to be effective against low frequency sounds.
High frequency sound is highly directional. High frequency sound is easy to reflect. High frequency sound is often produced by sources which radiate a high noise level in some directions but low levels in other directions. High frequency sound can be reflected from a hard surface in the same way that light is reflected from a mirror. High frequency sound will pass through a hole like a beam of light. It will not spread out from the hole like a low frequency sound will. High frequencies will not diffuse around edges, so barriers are effective against them.
When you are close to the source, high frequency noise is more annoying than low frequency noise. Since the human ear is more sensitive to high frequencies than low frequencies, a low frequency must be louder than a high frequency in order to be