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Tutorial 2 - decibels.docx

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McMaster University
Paul Faure

September 18 , 2013 Psych 3A03: Audition Tutorial 2: Decibels ISO Standard Based on Human Hearing - , where  P x absolute pressure of incident sound  P ref ISO reference = 2-μPa - When P refis 20μPa, we refer to this measure as dB SPL (SPL – sound pressure level; 20μPa implied) - The ISO standard is the average threshold of human hearing, and is the pressure created in air by a tone of 1-4kHz (where we are most sensitive, and where the most dominant form of speech is located) with an intensity of 10 - 12W/m 2 - For a 1kHz sinusoid at the threshold of human hearing (0dB SPL), air particle displacement is ca. 7.68 x 10 m (only very slightly being displaced), also implies that you have 20μPa as an incidence pressure - 1 Angstrom (Å) = the diameter of a Hydrogen atom, .:at threshold of hearing air particles are displaced ~ca 768Å Usefulness of the Decibel Scale - The ratio of sound pressure that causes permanent damage from short exposure to the limit (threshold) that undamaged human ears can detect is more than one million (>10 ) 6 - Because power is proportional to the square of the pressure, the ratio of the maximum power that causes permanent hearing damage to the minimum 12 power that is just audible is more than one trillion (>10 ) - To deal with such a range, the logarithmic decibel scale is quite useful - E.g. lo10of a trillion = 12, so a ratio of 1
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