Potential health risks of exposure to noise from personal music ...

Health risks from exposure to noise from personal music players3.3.2. Sound: Physical and technical backgroundSound or Sound waves comprise a wave phenomenon. Sound waves are ‘longitudinal’waves because sound waves consist of areas of higher and lower local pressure. Thepropagation of sound waves occurs in all media, i.e. in gases, liquids and solids as well asin more complex fluids like e.g. organisms and tissues.Fundamentally, sound waves are characterized by their spectrum. A spectrum is thesummation of individual frequencies (f) and amplitudes a certain signal has in thesurrounding medium. In daily acoustic settings sound is a complex summation of manydifferent sounds from different sources. Sound will not propagate through vacuum and itspropagation is influenced by material properties like density and compression / shearstrengths. Characteristic parameters of sound waves in a given situation derive from thefundamental wave equation which may be to challenging to evaluate for a given complexscenario.The exposure to sound in a typical setting is determined by many factors which are notalways easy to assess. For sound propagation, the geometry of the room, the surfacematerials and furnishings as well as its occupation, the materials and media surroundingthe source and the listener play a determining role. Like for any other wave, the soundwave at a specific location depends on interference from different sources which dependson the relative phase reaching the location from different sources or after travellingdifferent pathways. Thus, the distribution of energy and the energy absorption in soundexposure scenarios is not necessarily straightforward which leads to the many flavours ofacoustics as subfields of physics and engineering, medicine and architecture. Well-knownexamples are the different acoustical characteristics of a furnished and unfurnished room,the sound-design of commercial products like cars and the engineering of anti-soundreflectionsurfaces to be used in the prevention of sound propagation next to highways,railway lines, but also within sound-studios and in other architectural settings. Notablyalso details of the anatomy of the ear, the hair dress and clothing specific to one listenermay affect the sound distribution before the sound reaches the sound sensitive cells inthe inner ear of a specific observer.To assess the exposure from different sources in a specific point, it is common use toanalyse the different contributions by their frequency and to provide certainmeasurements related to sound (like power, amplitude etc) by their densities in thefrequency spectrum. Depending on whether sound waves are harmonic (‘tones’, ‘hum’)or relate to uncorrelated events. Sound with an equal energy distribution acrossfrequencies is called ‘white noise’, while most sources of sound exhibit dominatingfrequency bands originating from resonance phenomena. Typically, the above describedcomplex interaction of sound waves with the particular environment and media(absorption, refraction, reflection and interference) leads to a changing spectrum ofsound waves with progressing propagation or the modified position of an observer.3.3.3. Units of noise exposure3.3.3.1. Sound pressure level and dB SPLOne parameter of the acoustic (sound) wave which is generally used to assess soundexposure to humans is the sound pressure level expressed in µPa or Pa. Human ear’audible sound pressure levels range from 20 µPa (hearing threshold) till 20 Pa (painthreshold), resulting in the scale 1:10,000,000. Since using such a large scale is notpractical, a logarithmic scale in decibels (dB) was introduced which is also in agreementwith physiological and psychological hearing sensations.dB of sound pressure level (dB SPL) is defined as: 20 log 10 p1/p0 where p1 is actuallymeasured sound pressure level of a given sound, and p0 is a reference value of 20µPa,which corresponds to the lowest hearing threshold of the young, healthy ear. In thelogarithmic scale the range of human ear’s audible sounds is from 0 dB SPL (hearingthreshold) to 120-140 dB SPL (pain threshold) (see table 1 below).16