So far we have described the disturbing effects that are produced by noise within the frequency range of human hearing from about 20 to 16 000 Hz. But noise above and below this frequency range can also disturb people. Very low frequency noise or infrasound (usually considered to be at frequencies below 20 Hz) may be very intense although “inaudible” in the traditional sense of hearing; but nevertheless it may cause a sensation of pressure or presence in the ears. Low‐frequency noise (LFN) between about 20 and 100 Hz, which is within the normally audible range, is usually more disturbing than infrasound.
LFN in the region of 20–50 Hz seems to be the worst problem and more disturbing than infrasound. Infrasound below 20 Hz can excite some of the body organs into resonance as can low‐frequency sound in the range 20–75 Hz. Kryter [7] has described some qualitative results obtained by the U.S. Air Force and NASA when people were exposed to intense (above 130 dB) low‐frequency (below 100 Hz) pure tones and noise. Some effects observed included chest wall, abdominal and nasal cavity vibration. Additional effects included nausea, giddiness, and coughing. However, these effects only appeared when the levels were above about 130 dB. Such low‐frequency levels are only experienced in a very few environments. Such physiological effects can be related to the resonance of organs in the body. This will be discussed further in this chapter. Sources of infrasound and LFN noise include (i) air‐conditioning systems in buildings with variable air volume controls or with large‐diameter slow‐speed fans and blowers and long ductwork in which low‐frequency standing waves can be excited, (ii) oil and gas burners, (iii) boilers, and (iv) diesel locomotives and truck and automobile cabins.
Ultrasound covering the range 16000 to 40000 Hz is generated in many industrial and commercial devices and processes. Examples of ultrasonic sources include dental drills, ultrasonic cleaners, humidifiers, ultrasonic welders, air jets, sonic weapons, and jet and rocket engines. Although ultrasound is inaudible, it can produce effects on people. Probably the tinnitus, dizziness, nausea, and headaches sometimes attributed to ultrasound [7] are really caused by parts of the noise which are in the audible range below 20 kHz. However, other studies [7] have suggested that ultrasound becomes audible because subharmonics can be generated in the middle ear by ultrasound. The inner ear will then respond to the subharmonics.
Currently, our best information is that infra‐ and ultrasound can be tolerated at high sound pressure levels up to 140 dB for very short exposure times, while at levels between 110 and 130 dB, infra‐ and ultrasound can be tolerated for periods as long as 24 hours without apparent permanent physiological or psychological effects. There is no current evidence that levels of infrasound, LF, or ultrasound lower than 90–110 dB lead to permanent physiological or psychological damage or other side effects. However, a report recently published in the UK by the Health Protection Agency (HPA) has recommended an exposure limit for the general public to airborne ultrasound sound pressure levels of 70 dB (at 20000 Hz), and 100 dB (at 25000 Hz and above) [18]. For more in‐depth discussion on the known effects of infrasound, LFN, and ultrasound, the reader may consult Ref. [19].
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