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

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Health risks from exposure to noise from personal music playersWhile sound pressure measurements should give a reading of the sound pressure indB SPL, in the context of human hearing it is more practical to provide also a value whichcorresponds more closely to the hearing sensation or loudness in phones. The A, B, andC filters used currently in sound-level meters were aimed at mimicking isoloudnesscurves over frequency under different conditions of sound intensities, i.e. for sounds oflow, medium, and high loudness levels, respectively (IEC 651 1979). The “A” networkmodifies the frequency response to follow approximately the equal loudness curve of 40phons, while the “C” network approximately follows the equal loudness curve of 100phons. A “B” network is also mentioned in some texts but it is no longer used in noiseevaluations. The popularity of the A network has grown in the course of time. In currentpractice, the A- weighting curve filter is used to weight sound pressure levels as afunction of frequency, approximately in accordance with the frequency responsecharacteristics of the human auditory system for pure tones. This means that energy atlow and high frequencies is de-emphasized in relation to energy in the mid-frequencyrange.Correlation between noise effect hearing loss and sound exposure levels measured in A,B, or C weightings would not be very different. B (or even C) weightings provide a bettercorrespondence between loudness and moderate (or high) acoustic levels, however Aweighting differs only from B and C as underweighting frequencies below about 500 Hz.Since the human ear is much more resistant to noise-induced hearing loss (NIHL) at andby low frequencies A weighting is more in correspondence with NIHL risk.It should be noted that the A-filter has been adopted so generally that sound pressurelevels frequently quoted in audiology literature simply in dB are in fact A-weighted levels.Many older general purpose sound level meters are restricted solely to A-weighted soundpressure level measurements.3.3.3.3. Decibel measures in audiometryDifferent decibel measures are used in audiometry (evaluation of hearing sensitivity)than in sound pressure measurement. They depend on the reference value.Pure-tone audiometric thresholds are expressed in dB HL (hearing level) and are referredto hearing thresholds of normal hearing young individuals. The differences betweendB HL and dB SPL arise from isophonic curves. Their corresponding values are given inthe table below.Frequency [Hz] dB SPL dB HL250 12 0500 5 01000 2 02000 -2 04000 -5 08000 13 0Table 2: Audiometric hearing thresholds of normal ears: conversion of dB SPL intodB HL (extracted from ISO, 2003)Similarly to dB HL, the dB nHL (normal hearing level) values are referred to hearingthresholds of normal hearing individuals but they regard non-tonal sound stimuli (e.g.clicks).18
Health risks from exposure to noise from personal music players3.3.4. Methodology of noise measurementSounds are usually identified by their frequency spectrum, which is also relevant tohuman perception because the ear analyses sounds in the cochlea by a spectral analysis.The elemental component of a frequency spectrum is a sine wave or sinusoid with aspecific frequency. All sound waves can be described as a linear superposition ofsinusoids. Each sinusoid can be characterized by its frequency, its amplitude and thephase in relation to the zero-time mark. Sinusoids with the same frequency andamplitude superimpose either constructively by adding up to a sinusoid with doubleamplitude if the phase difference is zero and destructively by cancelling out if the phasedifference is 180 degrees (or antiphase) resulting in no sound of that characteristicfrequency at a given point.Sound originating from speech and music can similarly be described by their spectrum.In general terms signals can be divided in signals with a tonal character and with a noisycharacter.• Signals with a tonal character exhibit a spectrum made up of a basic frequencycomponent (f0) with harmonics (components that have a frequency which is aninteger multiple of the basis frequency (n*f0) and a related phase.• Signals with a noisy character exhibit a spectrum which is more complex than alinear superposition of basic frequencies and their harmonics.Sound measurements are done by determining the amplitude of the spectral componentsor by detecting the sound pressure through a physical device, e.g. a microphone. Thetotal sound level of a signal is a root-sums-of-squares of the amplitude of all the spectralcomponents.Signal levels, including noisy signals and music, are measured by placing a calibratedsound meter (SPL meter) at the centre head location of a potential listener. This methodis generally used to determine the risk for hearing loss in working conditions.The method distinguishes between various possible measures:• The averaged level, which is the average level of all frequency components over acertain time periodoThe level measurement can be recorded by filtering according to the A, Bor C filter; dB (A)• The peak level indicating the highest level recorded either of the total (weighted)signal or of specific components• The 8-hour equivalent level (L equ, 8h ) which is a measure for the risk on hearingdamage based on certain criteriaThe method can also be used to determine the level of music in the open field. Due tothe dependence of sound waves on the exact listening situation, as detailed in 3.2, it isclear that this type of measurement is not suitable to head phone use where only a smallspace between the head phones and the inner ear is exposed to sound waves.Sound levels of signals presented through headphones are usually measured by artificialears. Most common are two types, the occluded ear simulator (OES) and the 2 cccoupler. In audiometry and hearing aid specifications all measurements are measuredusing one of these two couplers. The design of the couplers is based on the resonanceproperties of the ear canal and the impedance of the tympanic membrane.In the link of sound transfer from the open field to the ear, there is another transfercharacteristic to be included and that is the baffle effect of head and torso. The headeffects are usually determined by using a manikin, or as they are also called HATS, headand torso simulator. It consists of a torso and head in which artificial ears are included.The sound pressure is measure at the eardrum. If compared with the free field, this givesthe head-related transfer function (HRTF).19
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