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.