After reading this article you will learn about Ultrasound:- 1. Meaning of Ultrasound 2. Low-Frequency Ultrasound 3. Biological Effects 4. Permissible Levels.
Meaning of Ultrasound:
Ultrasound is a mechanical vibration of an elastic medium in which it is propagated in the form of alternating compressions and expansions. Ultrasonic vibrations cover frequency ranges beyond about 16-20 kHz, which are not audible to the human ear.
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The higher the frequency of ultrasonic vibration the more it is absorbed by the surrounding medium, and the less deeply it penetrates into the human tissue. Absorption of ultrasound is accompanied by a rise in temperature of the medium. When ultrasound passes through a liquid, it causes cavitation.
Ultrasound finds wide applications in many fields of engineering and science for the purpose of analysis and testing (e.g., detection of flows, analysis of matter, etc.). Ultrasound also finds applications in medicine for therapeutic, surgical and diagnostic purposes. In all these applications, high ultrasound frequencies from 500 kHz to 5 MHz are applied at low intensities (0.1 – 2.0 W/cm2).
High-frequency ultrasound does not propagate in air, and its action on workers is possible only by direct contact between the ultrasound source and the skin. Moreover, such contact may only arise from defective ultrasound equipment.
Low-Frequency Ultrasound:
Low-frequency ultrasound (18-30 kHz) of high intensity (up to 6-7 W/cm2) is widely employed in many industrial processes (e.g., cleaning and degreasing of parts, machining of hard and brittle materials, welding, brazing, soldering, electrolytic coating and acceleration of chemical reactions).
Low frequency ultrasound may also arise from aerodynamic phenomena of industrial processes, and it is a component of the noise produced by jet engines, gas turbines, and powerful pneumatic motors.
Low-frequency ultrasonic equipment (welding machines, drilling machines, parts-cleaning tanks, etc.) generally produce a complex vibration consisting of audible noise and low-frequency ultrasound.
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The latter is easily transmitted by air together with high-frequency audible noise; but unlike normal noise, the low- frequency ultrasound is considerably attenuated by the distance from the source, and has no uniform intensity in the air.
The acoustic pressure at the workplace has a very wide spectrum which lies between 80 and 120 dB (according to the type of ultrasound equipment) at full operating energy. The highest noise levels in the audible range are measured at the frequencies close to resonance (i.e., the working frequency), and at frequencies of 10-11 kHz.
Biological Effects of Ultrasound:
Low-frequency ultrasound produces local effects when parts or materials being processed by ultrasound are touched. It also produces general effects in the body. This happens because the hands are often introduced into the zone where ultrasound acts most strongly.
Exposure to powerful equipment sources (6-7 W/cm2) of ultrasound represents a serious danger. Such high-intensity ultrasound may damage peripheral nervous and vascular structures at the point of contact. In particular, ultrasound exposure by contact takes place during loading and unloading of parts in ultrasonic cleaning tanks.
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A 3-minute exposure of the fingers in a cleaning bath with a 1.5 kW generator causes a sensation of pricking, or sometimes of itching. Five minutes after the cessation of such exposure to ultrasound, a sensation of cold or numbness of the fingers is experienced.
Repeated short contacts of 20-30 seconds or more with a fluid through which ultrasound is passed may give rise to the development of neurovascular syndrome of the fingers, hands and forearms.
If the workers employed on low-frequency ultrasonic equipment are systematically exposed to the noise levels exceeding the recommended standards, and to ultrasound intensities of more than 100-110 dB, they may develop functional disorders of the central and peripheral nervous system, the cardiovascular system, hearing and vestibular analysers, and endocrinic and humoral changes from the normal.
All these changes have much in common with symptoms of exposure to high-frequency noise observed in other industrial activities.
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The workers exposed to low frequency ultrasonic noise complain about headache and excessive fatigue (increasing towards the end of the work-day), a feeling of pressure in the ears, hesitant gait, dizziness, and disturbed sleep.
These complaints are nearly always associated with signs of moderate neurovascular syndrome of the hands (much less often, also of the feet) with varying degrees (numbness, thermal asymmetry, impaired sensitivity such as “glove” sensation, etc.).
Vestibular disorders, increased body and skin temperature, fall in blood sugar level and eosinophilia are sometimes observed in worker chronically exposed to ultrasound. When intense ultra sound is accompanied by high noise levels, remarkable hearing loss and more pronounced vestibular impairment can be observed.
Airborne ultrasonic vibration produces effects on the central nervous system and the functions of other systems and organs, not only through the ear but also via extra-auditory routes, because even deaf mute persons exposed to ultrasound develop the same disorders.
Clinical examinations of workers exposed to ultrasound have revealed that the percentage of cases with pronounced signs of ultrasound pathology is considerably higher among persons who are intensely exposed to low frequency ultrasound with levels up to 120-130 db.
No such signs at all are observed in workers exposed to ultrasound of 90-105 db. Workers who are not only exposed to airborne ultrasound but also to contact with vibrating surfaces develop more pronounced symptoms of ill health, and in particular the signs of vegetative polyneuritis.
The degree of the pathological changes depends on the intensity of ultrasound and duration of exposure. Contact with the vibrating medium and the presence of noise in the exposure spectrum aggravate the state of ill health.
Ultrasound has a much weaker effect on the hearing function than high- frequency noise, but ultrasound causes more pronounced anomalies in the vestibular function, sense of pain and body temperature regulation. Direct skin contact with intense high-frequency ultrasound leads as a rule to the same disorders as the low-frequency ultrasound.
Permissible Levels of Ultrasound:
Many industrialised countries have recommended the maximum permissible levels of sound and ultrasonic pressure for workplaces and ultrasonic equipment. Such standards for Russia are shown in Table 1. These maximum permissible levels are standardised in third-octave bands with the mean geometric frequencies of 12.5, 16,20 and 40 kHz.
These criteria have been established for ultrasound exposure during and 8- hour work-day. If the total duration of exposure to high-frequency noise and ultrasound is less than 4 hours per shift, the noise levels shown in Table 1 may be increased by 6 dB for lengths of exposure from 1 to 4 hours, and by 12 dB for exposures from 15 minutes to 1 hour.
The sound pressure levels in the octave bands with mean geometrical frequencies from 63 Hz to 8 kHz, and the noise levels at industrial workplaces must not exceed the standards established for noise.