![]() Exposure to sounds at or above 100 dB can cause serious damage to human hearing ( Fig. 8.17). Thus, the acoustic energy expressed by the decibel sound scale ranges from very weak below about 40 dB but climbs rapidly. Furthermore, a sound produced at 80 dB is twice as loud as the 70 dB sound, and a 100 dB sound is 8 times as loud as a 70 dB sound. If we compare the sound level associated with 70 dB with a sound of 60 dB, we find that the 60 dB sound is only half as loud. It is important to keep in mind at the decibel scale is logarithmic. The difference in reference levels used, combined with the difference between the density of air and water (the latter being denser), means that the volume in dB of a sound transmitted through the air cannot be directly compared with its volume in water ( Stafford, 2013). As such, dB levels are always reported as “dB re 1 μPa” for a sound in water or “dB re 20 μPa” for a sound in the air. In water, a different reference is used: 1 μPa. For instance, in air, measurements are referenced to 20 micropascals (μPa). Decibel levels indicate how loud a sound is relative to some reference (represented in units of amplitude called micropascals, or μPa). The loudness of a sound is measured in decibels (dB). Substituting the value of a micropascal for the rms pressure in the plane wave intensity expression, we find that a plane wave pressure of 1 μ Pa corresponds to an intensity of 0.67 × 10 −22 W/cm 2 (i.e., 0 dB re 1 μ Pa). In seawater, ρ c is 1.5 × 10 5 g cm −2 s −1 so that a plane wave of rms pressure 1 dyne/cm 2 has an intensity of 0.67 × 10 −12 W/cm 2. The average intensity, I, of a plane wave with rms pressure p in a medium of density ρ and sound speed c is I = p 2/ ρ c. Pressure ( p) ratios are expressed in dB re 1 μ Pa by taking 20 log p 1/ p 2 where it is understood that the reference originates from the intensity of a plane wave of pressure equal to 1 μ Pa. Therefore, taking 1 μ Pa as I 2, a sound wave having an intensity, of, say, one million times that of a plane wave of rms pressure 1 μ Pa has a level of 10 log(10 6/1) ≡ 60 dB re 1 μ Pa. The presently accepted reference intensity is based on a reference pressure of one micropascal (μ Pa): the intensity of a plane wave having an rms pressurex equal to 10 −5 dynes per square centimeter. Absolute intensities can therefore be expressed by using a reference intensity. ![]() Two intensities, I 1 and I 2, have a ratio, I 1/ I 2, in decibels of 10 log I 1/ I 2 dB. The decibel (dB) is the dominant unit in underwater acoustics and denotes a ratio of intensities (not pressures) expressed in terms of a logarithmic (base 10) scale. Kuperman, in Encyclopedia of Physical Science and Technology (Third Edition), 2003 VIII Appendix: Units Research needs and policy decisions, based on what is currently known, are considered.William A. On the other hand, it is also possible that low-frequency noise provides some protection against the effects of simultaneous higher frequency noise on hearing. Although the effects of lower intensities of low-frequency noise are difficult to establish for methodological reasons, evidence suggests that a number of adverse effects of noise in general arise from exposure to low-frequency noise: Loudness judgments and annoyance reactions are sometimes reported to be greater for low-frequency noise than other noises for equal sound-pressure level annoyance is exacerbated by rattle or vibration induced by low-frequency noise speech intelligibility may be reduced more by low-frequency noise than other noises except those in the frequency range of speech itself, because of the upward spread of masking. Intense low-frequency noise appears to produce clear symptoms including respiratory impairment and aural pain. The effects of low-frequency noise are of particular concern because of its pervasiveness due to numerous sources, efficient propagation, and reduced efficacy of many structures (dwellings, walls, and hearing protection) in attenuating low-frequency noise compared with other noise. ![]() Low-frequency noise is common as background noise in urban environments, and as an emission from many artificial sources: road vehicles, aircraft, industrial machinery, artillery and mining explosions, and air movement machinery including wind turbines, compressors, and ventilation or air-conditioning units. The sources of human exposure to low-frequency noise and its effects are reviewed.
0 Comments
Leave a Reply. |