Improving the Environment of the Operator{7301} Caterpillar

Caterpillar Products: All

Introduction

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

• Power

Pressure 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.

There are some basic principles to how sound works. The list below explains those principles.

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Illustration 1	g01160067

1. 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.

2. 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.

3. 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.

4. 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 as annoying. Sometimes, reducing the frequency of a nearby sound can reduce the annoyance, even if the sound is as loud as ever.

5. When you are far from the source, low frequency noise is more annoying than high frequency noise. High frequency sound are more absorbed by the air over long distances than low frequencies. This is because absorption is dependent on the number of cycles, and there are more cycles in high frequency sound than in low frequency sound. It is also easier to shield a high frequency sound wave. If noise in the immediate area of the sound is not an issue,shifting the sound to a higher frequency may help. The sound could be absorbed by the time it reaches the hearer.

6. Sound travels long distances in a structure. Vibration that is inside a structure can travel a very long distance due to low internal damping of the structure. This is especially true of homogeneous structures like concrete and steel. The energy does not reduce, and as soon as a large surface is attached to the vibration, the large surface acts like a loudspeaker. This creates much noise. The best way to prevent this type of noise is to isolate the body from the vibration. Isolate as close to the source as possible.

7. Vibration that is inside of a structure needs a large area to convert into sound that travels through the air. When small objects vibrate, they do not usually produce much sound. This is because the small surface are of the object that is vibrating does not set many air particles in motion. However, if you connect a large panel to the source of vibration, more sound will be created, since more air particles will be set in motion. This is the same effect that was discussed in Step 6. For example, a tuning fork makes almost no noise until the fork is connected to a sounding board. Another example is a boiler furnace. The pump causes the pipework to vibrate, but when the vibration reaches a radiator, much more sound is emitted.

8. Small objects radiate less noise than large objects when they vibrate. With less surface area to excite the air molecules that surround the object, less noise is created. This principle follows the previous principle.

9. A structure produces less sound when damped. If vibration is induced in a panel, the vibration and the sound that is produced will diminish over time. The rate of sound reduction depends on the damping of the object. Higher damping results in the sound dying out more quickly. The damping also limits maximum vibration. A well damped panel will not vibrate as much when hit with the same force as a less damped panel. Sadly, most metals have low internal damping. To visualize this phenomenon, imagine ringing a bell. When you grip the rim of the bell with your hand, the sound dies out quickly. Your hand damped the vibration in the bell.

Measuring Sound

Sound is measured on the decibel scale. The sound is measured by sound meters that are designed to respond to sound close to the same way that a human ear responds to sound. There are many different types of devices that are used to measure sound. Despite their differences, there are three basic parts that all machines use. Each machine uses a microphone, a processing unit, and a read out or display.

Illustration 2	g01159764

Most sound measurement devices have the ability to measure sound using different weighting networks. The human ear responds differently to different frequencies. To a human, low frequency sounds seem quieter, even if they are the same decibel level as a higher frequency sound. Weighting networks are used to make the sound measuring machine display sound levels that are closer to how they are percieved. The most widely used is the A weighting. The A weighting is thought to most accurately model how the human ear responds to sound. Noise that causes hearing loss most closely correlates to the A weighting. The C weighting correlates well with the sound the human ear encounters when earplugs are worn. Illustration 2 shows how the weighting systems remove, or in some cases, add sound to the base reading from the meter.

Most sounds that you will want to measure fluctuate over time. If the sound varies rapidly, the needle on an analog gauge will fluctuate rapidly, making it difficult to get an accurate reading. In order to deal with sounds that fluctuate at different rates, two response times were standardized. You may select "F" for fast, or "S" for slow. The fast position will provide a time constant of 125 milliseconds. This provides a quick responding display that works well for slowly fluctuating sound. Switching to the slow position provides a time constant of 1 second. This will smooth out the reading when the sound fluctuates very rapidly. To measure an isolated impulse of sound, there is an I position. The 35 millisecond time constant is short enough to measure these impulses.

When you are taking sound measurements, make sure that your body is not acting as a shield or a deflector of sound. Experiments have shown that at 400 Hz, reflections from a person can cause errors as great as 6 dB if you are measuring less than one meter from your body.

How To Reduce Sound

There are two basic guidelines to follow in order to lower the sound levels in a given situation.

1. The first step is to try to eliminate the sound at the source. This can be done by applying acoustical materials to the surface of a machine, by damping the source of the sound, by reducing the frequency of the sound, or other means.

2. The second is to try and block the sound transmission path. This can be done by placing an enclosure or acoustic screens around the machine. Be sure to mount the enclosure on vibration isolators in order to prevent transmission of vibration and sound. Noise is further reduced by coating the walls, ceiling, and floor with absorbent materials. The materials will reduce sound reflection.

Potential Noise Sources

Static Checks

• CB radios

• AM/FM radios, tape or CD players

• Company radios

• Cab mounts that are worn or damaged

• Door latches that are worn or out of adjustment

• Door seals that are worn, damaged, or missing

• Window seals that are worn, damaged, or not sealing

• Holes, cracks, or openings in the cab

• Damaged or missing sound suppression in the cab

• Absence of a quality floormat with padding underneath. The mat should cover the entire floor.

• Floorplates that are not properly secured or have missing rubber seals

• Loose object in the cab (lunchbox, thermos, etc.)

• Panels and covers in the cab that do not fit or are not tight

• Objects that have been attached to the machine that are creating noise (antennas, wires, lights, brackets, fire extinguisher, sweeps)

• Engine enclosures or access doors that do not fit or that are not tight

• Missing or loose heat shields on the exhaust system

• Rain Cap

Operational Checks

• Check the exhaust system for leaks.

• Notice any unusual noises from the power train or the hydraulic system.

• Notice any unusual vibrations throughout the engine RPM range.

• Check for excessive wear in the components that move during operation.

Options for Reducing the Sound Levels for the Operator

• Add additional sound suppression between the engine compartment and the operator's compartment.

• Add sound suppression around the access door or panels to the different compartments.

• Install engine enclosures if the machine is not equipped with them.

• Install sound proofing on the inside of all of the cab walls or panels.

• Add sound absorption material under the cab.

• Equip the machine with an additional barrier floor mat. Seal the edges of the floor mat with duct tape.

• Seal all of the access cover seams in the cab with duct tape.

• Reduce the volume of all radios.

• Equip the machine with extensions for the exhaust. The extensions direct sound away from the cab.

• Provide the exposed person with appropriate hearing protection.

Reference: For more specific information please refer to Service Magazine, SEPD0559, "Material for Sound Supression in the Cab is Available"