9.2 The Impact of Acoustic Factors


Sound is the mechanical oscillations of an elastic medium with the frequency(f) within the hearing range (16 Hz – 20 kHz).

If (f) of the oscillations is <16 then it is called infrasound, while if the (f)>20 kHz it is called ultrasound.

 It is noise, relative to periodic sounds (e.g. music) that has adverse effects in a man.
Noise is an undesirable sound of a non-periodic nature, containing oscillations of various frequencies and intensities.

  • The measure of how potentially harmful noise is is given by noise levels and classes stated in the health and safety regulations.
  • The detrimental effect of noise depends on: frequency; intensity; continuous or irregular; duration; type of activity performed by the exposed person.
  • For instance, the jobs which present a high level of hazard – hazard which can result in hearing impairment – are: turbo-compressors testing; bridge building; working with pneumatic tools; lumberjack; etc.
  • In addition, long term exposure to excessive noise can damage the CNS, and adversely affect: the cardiovascular system(arrhythmia can develop); digestive system(ulcers); neuropsychic sphere(causing insomnia, fatigue, etc.).
  • Urban protection systems include walls out of sound-absorbing materials which isolate, for instance, highways.

Ultrasound covers a wide range of mechanical oscillations – from 20 kHz up to several hundreds GHz-

Ultrasound may be produced mechanically in special pipes, however, magnetostrictive (magnetostrictive generators produce low-frequency ultrasound(20 kHz to 100 kHz)) or piezoelectric sources (piezoelectric generators produce high-frequency ultrasounds) ultra are preferred.

  • High-frequency ultrasound is used in both diagnosis(1 – 3 MHz) and diagnosis (2-40 MHz).
  • Hypersound(1 – 200 MHz) is used in acoustic microscopy.
  • The interaction between ultrasound and a tissue depends on the intensity of the ultrasound. At higher intensities it is said that is has a biological effect, because it results in a change of the biological environment.
  • If the ultrasound has no effect, i.e. it is passive, then the biological system will change the course of acting of the ultrasound. This type of interaction forms the basis of ultrasonic diagnosis methods.

Ultrasound of low and medium intensities does not cause ionisation, but it rather affects the biological system directly:

  • by heating it. This is caused by the absorption of acoustic energy.
  • by mechanically changing it. This is due to forced due to high frequency of ultrasound.
  • by producing cavitation. This is the formation of gas bubbles (which have the potential to collapse and cause mechanical damage to surrounding biological structures). Cavitation is also a source of free radicals – these can damage the chemical environment of the biological media.

Depending on the nature of the oscillations, cavitation can be:

  • transient (collapse)
  • permanent (resonance)

The study of ultrasound is important to understand the possible hazards of ultrasonic applications in:

  • diagnosis
  • therapy
  • stimulation
  • inhibition

Notes compiled by Andrei Cociug.

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