NOISE AND ITS EFFECT TO HUMAN

The sound is a pressure disturbance that it can be detected by the ear or mechanical compression or longitudinal waves that propagate through a medium, medium or intermediary agent can be a liquid, solid, gas. Most of the noise is a combination of various signals, but purely theoretically sound may be explained by the speed of oscillation or frequency measured in Hertz (Hz) and the amplitude or loudness of a sound measurements in decibels (dB). Humans hear when a sound wave, the vibration of air or other medium, to human ear druk. Limit the frequency of sound that can be heard by human ears roughly from 20 Hz to 20 kHz. The sound below at frequency 20 Hz called infrasonic, at other hand the sound over at the frequency 20 kHz called ultrasonic. There also the sound frequency have a general amplitude with variations in the curves of response.

Noise is sound or voice undesirable and detrimental to health and environmental comfort expressed in decibels (dB). Noise can also be defined as unwelcome noise, disturbing sound or sound annoying. Based on the Decree, the noise is unwanted sound that comes from the equipment, production process at a certain level can cause health problems and hearing.

Sound raises the noise caused by vibrating sound source. Vibration noise source is disturbing the balance of the surrounding air molecules so that the air molecules vibrating. Vibrations caused a wave source of mechanical energy propagation in the medium conditioned by propagation longitudinal pattern. Wave sound propagation in air is known as sound or noise while in the context of space and time so that it may cause interference comfort and health

The source of noise is the sound sources whose presence is considered intrusive hearing from both mobile and fixed sources. Generally, the source of the noise can be derived from industrial activities, trade, construction, power generation equipment, transporters and household activities. In industry, the source of the noise can be classified into three types, namely

1. machine, the noise generated by the activity of the machine.
2. Vibration,due to the noise generated by the vibration caused due to friction, impact or imbalances movement of the  machine. Occur on gears, flywheel, torque rod, piston, fan, bearings, and others.
3. The movement of air, gases and liquids. The noise is caused due to the movement of air, gases and liquids in the activities of the working process industry, for example in gas liquids pipelines, outlet pipe, exhaust, jet, flare boom, and others.

Noise is sound or noise that is disturbing. Noise can cause various disorders such as physiological disorders, psychological disorders, communication disorders and deafness. There is classification of the disturbances in the form of auditory disorders, such as disorders of the auditory and non-auditory disorders as communication disorders, safety hazards, the decline in performance of work, stress and fatigue. More detailed noise effect on the health of workers will be described as follows:

In general, high-pitched noise is very annoying, especially when intermittent or the arrival of a sudden. Disorders can include increased blood pressure, increased pulse, constricting peripheral blood vessels, especially in the hands and feet, and can cause pale and sensory issues. Noisy with high intensity can cause dizziness / headache. This is due to noisy situations can stimulate the vestibular receptors in the inner ear that will cause effect to dizziness / vertigo. Nausea, insomnia and shortness of breath because noise stimulation to the nervous system, balance organs, endocrine, blood pressure, digestive system and electrolyte balance. Psychological disorders may include discomfort, lack of concentration, insomnia, and irritable. If the noise is received in a long time, it can cause psychosomatic diseases such as gastritis, heart disease, stress, fatigue and others.

Communication disorders usually caused masking effect (sound cover less clear hearing) or disturbance sound clarity. Communication talks must be done by shouting. This disorder causes the disruption of work, to the possibility of error because they do not hear the signal or alarm. These disruption of communication are not directly endanger the safety of any person. Noisy very high can lead to the impression of walking in space or drift, which can cause physiological disorders such as dizziness (vertigo) or nausea.

The primary effect of noise on health is damage to the sense of hearing, which causes progressive deafness and this effect has been known and generally accepted from a bygone era. At first, the effect of noise on hearing is temporary and pemuliahan occur quickly after the work in a noisy area is stopped. But when work is constantly in a noisy area, there will be deaf settled and cannot be normal again, usually starts at a frequency of 4000 Hz and then widening to frequency vicinity and eventually the frequency normally used for conversation.

HOW TO CALCULATED OF THE SOURCES FREQUENCIES

The three sources of frequencies in machines are: generated frequencies, excited frequencies and frequencies caused by electronic phenomena. Generated frequencies sometimes called forcing frequencies are those frequencies actually generated by the machine. Some examples are imbalance, vane pass frequencies (number of vanes times speed), gearmesh frequency number of teeth times speed, various frequencies generated by antifriction bearings, ball passing frequency of the outer race, ball passing frequency of the inner race, ball spin frequency and fundamental train frequency. Generated frequencies are easiest to identify because they can be calculated if the internal geometry and speed of the machine are know.

Some of the calculated frequencies may be present in most machines without indicating a vibration problem. These frequencies at acceptable levels without sidebands, include but not limited to imbalance vane pass frequencies blade pass frequencies and gearmesh frequencies. Other calculated frequencies should not be present in any form at prescribed calibration levels. These frequencies include but not limited to ball pass frequencies of the outer and inner races ball spin frequency, fundamental train frequency. Calculated frequencies should not be modulated with any degree of significance by other frequencies. If any of  these frequencies are generated, a vibration problem exists.

When a rotating unit has a mass balance, it will generate a sine wave that has very little distortion. This signal can be observed in the time domain. The frequency domain spectrum will have a spectral line at one times speed of the unit. For example a 1776 RPM fan that is out of balance will have one spectral line at 29,6 Hz. Most pumps and fans can generated vane or blade pass frequency, which is the number of vanes or blades times the speed of the unit. A high vibration at this frequency could be the result of buildup on the vanes or blades, the vanes or blades hitting something or looseness associated with the rotating unit.  

Gearmesh frequency is normally seen in data taken from a gearbox or gear train. The frequency is the number of teeth on agear times the speed of that gear. For two gears in mesh, the gearmesh frequency will be the same for each gear, thr ratio of the number of teeth to gear speed ia constant. In a gear train, all gears will have the same gearmesh frequency. This vibration caused by teeth rotatingagainst each other. Multiplies and submultiples of gearmesh frequency are sometimes observable in the frequency spectrum and will be discussed later. For illustration consider a gear with 67 teeth is in mesh with a 22-tooth pinion gear. The gear is rotating at 6,4 Hz. Calculated a)what is the gearmesh frequency (GF) b) what is the speed of the pinion gear. The answer are:  

GF = number of teeth x gear speed

a) GF = 67 x 6,4 Hz  = 428,8 Hz

b) Speed of pinion = gear speed / number of teeth

speed of pinion = 428,8 Hz / 22 = 19,5 Hz

Excited frequencies, natural frequencies are the property of the system. Amplified vibration called resonance occurs when a generated frequency is tuned to a natural frequency. Natural frequencies are often referred to as a single frequency. Vibrations are amplified in a band of frequencies around the natural frequencies. The amplitude of the vibration in this band depends on the damping. When refer to natural frequency, often means the center of frequency. Natural frequencies can be excited by harmonic motion if the harmonic motion is within the half power points of the center frequency and contains enough energy.

The half-power points are down 3 dB on their side of the center frequency. The frequency range between these half-power points called the bandwidth of the natural frequency. The half-power point is 0,707 times peak at the center frequency. It is a general rule to stay a least 10% away from each side of the center frequency. If some frequency is within the bandwidth the center of frequency and his frequency contains enough energy to excite the natural frequency, the natural frequency will be present.

HOW TO MEASURING THE VIBRATION AMPLITUDE

Amplitude of the vibration level can be measure with four ways. The value of amplitude can be measure with peak to peak, zero to peak, RMS and average. Peak to peak is the distance from the top of the positive peak to the bottom of the negative peak. This method of measurement is most often used when referring tp displacement amplitude.

The value of zero to peak is the measurement from to zero line to the top of the positive peak or the bottom of the negative peak. This type of the measurement is most often used to describe the vibration level from a velocity transducer or accelerometer.

The Root Mean Square (RMS) is the true measurement of the power under the curve. The RMS value is 0.707 x peak only applies to pure sine wave. The true RMS value can be calculated by the square root of the sum of the squares of a given number of points under the curve.  The formula for true RMS they are:

RMS = square {(P₁² + P₂² + P₃² _+… P_n²)/n}

For measurement the value of the true RMS, the measurement for signals that it contain pulses must consider the value of the crest factor and duty cycle. The crest factor (CF) is the ratio of the peak value to the RMS value with the DC component removed. These formulas can be write:

CF = (P-DC)/RMS

For accurate measurement of pulse, the value of the crest factor is 7 normally can be used. The duty cycle is the ratio of the pulse width (PW) to pulse recurrence frequency (PRF). The formula for duty cycle they are:

duty cycle = PW/PRF

There are also several forms of pseudo are used in some equipments. This value of the pseudo RMS can be formula with

RMS = 0.707 x peak

RMS = square (AC² + DC² )

Analog meters is the equipments that can be use as to measure average amplitude. There are various the value of constants are the used to calculate peak, peak to peak, zero to peak, or RMS. Most measurements that are not true RMS measurements are either overstated or understated. When describing the vibration level of a machine, the RMS value should be used if possible. However some cases require peak to peak measurements, for example when measuring mils of displacement. Other case require zero to peak displacement measurement such as high places on the roll.

 The average values are measured by analog meters. The value average is the converted to peak by multiplying a constant of 1.57. These calculated values are accurate only when measuring pure sinusoids. The following constants may be helpful. However they apply to pure sine waves only. The more of the signal deviates from a true sine wave, the more a error is introduced.

Average = 0,637 x peak

Average = 0,90 x RMS

Peak to peak = 2 x peak

Peak = 1,414 x RMS

Peak = 1,57 x average

RMS = 0,707 x peak

RMS = 1,11 x average

PRINCIPLES OF NOISE CONTROL

Vibrations are the physical movement or motion of a rotating machine. Vibrations are always has inherent characteristically with frequency and amplitude. Since the vibration frequency and amplitude cannot be measured by sight or touch, this characteristic must be measure and convert into a usable product that can be analyzed. The apparatus that can convert the mechanical vibration into an electrical signal is called a transducer. The transducer output is proportionate to how fast the machine is moving (frequency) and how much the machine is moving (amplitude). The frequency describes what is wrong with the machine and the amplitude describe relative severity of the problem. The motion can be harmonic, periodic and random. All harmonic motion is periodic. However, all periodic motion not harmonic. Random motion means the machine moving in unpredictable manner.

Harmonic Motion

Harmonic motion is characteristically a sinusoid or some distorted version, depending upon the harmonic content. All harmonic motion is periodic, meaning it repeats at some point in time. In linier system imbalance in rotating equipment could generate harmonic motion. However, with many variables such a gear problems, looseness, bearing defects, misalignment such sinusoids are not often found. It is important to understand that a sine wave simply a plot of circle against time. All harmonic motion repeatable and just is one form of periodic motion.

HARMONIC MOTION

Periodic Motion

All motion that repeats periodically are periodic motion. This includes harmonic motion, pulse, etc. Periodic motion is any motion that repeats itself in equal time periods. For example, a misalignment motor coupling that is loose could have a bump once per revolution of the shaft. Although this motion is not harmonic it is periodic. The signal will have one pulse every t seconds as indicated by picture.

PERIODIC MOTION

Random Motion

Every motion are occurs in a erratic manner and contains all frequencies in a particular frequency band called random motion. Random motion is any motion that is not repeatable. Rain hitting a roof and bowling pins being knocked over are examples. Random motion is also called noise. When random motion generated by a machine, a recording of the noise played backed ten times faster than it was recorded can sound like a TV set after the station has signed off the air. A time signal of random noise will contain all frequencies in a given range. The frequency spectra from such time signals will be up off the baseline as indicated by picture. Often, random motion in a machine is caused by severe looseness.

RANDOM MOTION

In any noise control system, there are three basic elements they are

1. The sources of noise

2. The path through which the sound travel

3. The receiver of the sound

There are several sources of sound in many situations, various path for the sound and more than one receiver. The basic principle noise control would be the same as for the simple case. The object of most noise control programs is to reduce the noise at the receiver. This may be can do by making modifications the sources,  the path or the receiver.

The sources of noise or undesirable sound is a vibrating surface, such panel in an item of machinery. The path for the sound may be the air between the sources and the receiver as is the case for machinery noise transmitted directly to operator’s ear. The path may also be indirect, such as sound being reflected by the wall to person in the room.

Noise Control at The Source

The best solution for a noise control problem is the modification at the sources of sound. Components of a machine may be modified to effect a significant change in noise emissions. Noise at the sources may indicate others problems such as need for maintenance. For example, excessive noise from roller bearing in machine may indicate wear failure in one of the rollers in the bearing. Replacement of the defective bearing may also solve the noise problems, in addition to preventing further mechanical damage to the machine.

The noise generate by large vibrating panel can be reduced by applying damping material to the panel surface or by uncoupling the panel from the vibrating force. Making the panel stiffer by the increasing the panel thickness or reducing the dimensions of panel may reduce amplitude of vibration.

Noise Control in The Transmission Path

Modification the path through which the noise propagated is often use when modification of the noise source is not possible. For noise sources located outdoors, one simple method for noise control would be move the sound source farther away from receiver.

For noise source located indoors, the transmission path may be modified placing a wall or barrier between the source and receiver. Reducing of traffic noise from vehicles on freeways passing nears residential areas and hospitals has been achieved by installation of acoustics barrier along the roadway. The more expensive method, the noise control procedure is to enclosed the sound source in an acoustic enclosure or enclose the receiver in the personnel booth. The exhaust noise from engines, fans, and turbines is often control by using mufflers or silencers in the exhaust line for the devices.

Noise Contorl in The Receiver

One possible approach to limit the noise exposure of a worker to industrial noise is to limit the time during which person is exposed to high noise levels. The hearing protector (earplugs or acoustics muffs) can be effective in preventing noise-induced hearing loss in an industrial environment.