2.6.4. Infra sound

Infra sound is oscillation spreading through the air, liquid or solid body with frequency lower 16 Hz.

We can’t hear infrasound but we can feel it. High level infrasound violates vestibular system causing dizziness and headache. It reduces attention and work capacity, people may feel fear and total weakness.

All mechanisms working with rotational frequency lower 20 revolutions per second radiate infrasound. A car moving at the speed over 100 kilometers per hour also radiates infrasound. In machine engineering the infra sound sources are fans, compressors, combustion engines, diesel engines. Infrasound is impossible to reduce by modifying way it’s transmitted or by absorbing or isolation. Protective equipment is not effective also. It’s practicable to reduce infrasound exposure by control measures at the source. They include:

• increasing shaft rotation frequency to 20 revolutions per second and more;

• increasing rigidity of big oscillating constructions;

• elimination of low frequency vibration.

2.6.5. Ultra sound

Occupational technologies widely use ultra sound. Generators, purifying and degreasing equipment radiate ultra sound. Loading and unloading operations connects with contact ultra sound exposure. Ultra sound generators are used for plasma diffusion welding, cutting meals and coating metals. Intensive ultra sound exposure takes place during cleaning operations, chemical etching, and compressed air blasting of details and assembly operations.

Ultra sound impacts nervous system, causes headache, changes blood pressure and blood structure, loss of hearing, fatigue. It actions is transmitted through air, liquid or solid body.

To protect people from ultra sound isolation method is applied. Shields are installed to separate ultra sound source from people. Machines radiating ultra sound should be placed in specially equipped rooms. A properly designed control room can effectively reduce excessive level. This is achieved by applying sound absorbing material such as steel, duraluminium, plexiglas.

Total enclosure of ultra sound machines should have blocking network, which turns off the machine if enclosure is damaged.

2.6.6. Vibration exposure

Among all types of mechanical hazards vibration is the most dangerous. Vibration is mechanical oscillation (elastic wave) transferred through the solid body. Every elastic body or system driven out of balance has period and frequency of oscillation. Oscillation of that kind is called natural oscillations, which fade in time for motion energy transferring into friction and eventually heat.

Biological effect of vibration depends also on to which part of the body it’s applied. There are two major types of human exposure to vibration: vibration transmitted to the whole-body through a supporting surface, for example, the feet of a standing person or the buttock of a seated person; and vibration applied to a part of the body i.e. segmental vibration. When vibration is applied to the hand, it is termed "hand-arm" vibration.

Vibration affects physiological and functional condition of a person. Stable changes in physiological condition are called as vibration disease. Symptoms of vibration disease are headache, fingers numb, wrist and forearm pain, cramps, increased sensitivity to cold, sleeplessness. It also causes pathological changes in spinal cord, cardio-vascular system, bones and joints, capillary blood circulation. Each part of the human body has its own natural frequency of vibration, therefore the extent to which the human body is affected depends on the vibration frequency it is to which exposed. Vibration whose frequency is close to vital frequency of internal organs, most of which is within 6 - 30 Hz, is extremely dangerous. It causes its resonance what in turn results in shifting organs or even damages.

Resonance frequency of some organs:

• eyes – 22 - 27 Hz;

• throat – 6 - 12 Hz;

• chest – 2 - 12 Hz;

• head – 8 - 27 Hz;

• face – 4 - 27 Hz;

• stomach – 4 - 12 Hz;

• arms and feet – 2 - 8 Hz;

• spine – 4 - 14 Hz.

Local vibration may have impact to heart and nervous system. The most unfavorable for a person is simultaneous action of vibration, noise and low temperature.

Local vibration is classified by source:

• transport vibration produced by moving vehicles;

• transport and technological vibration produced by machines working;

• technological vibration transferred to operators of stationary machines or to workplaces which don’t have vibration themselves.

Vibration is characterized by three parameters: shift amplitude, oscillation velocity and oscillation acceleration. However only two of them are practicable: shift amplitude (m) and oscillation velocity (m/s). Oscillation velocity can be denoted in logarithmic scale. In this case it’ll be referred as oscillation velocity level, vibro-velocity level:

,

where  - oscillation velocity in specified point; _ref – referenced value, what is threshold oscillation velocity equal 510^-8 m/s.

Vibration assessment is carried out by following methods:

• frequency (spectral) analysis of specified parameter;

• integral analysis of frequency of specified parameter;

• vibration doze.