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Psychology (5,217)
PSYCH 3A03 (56)
Paul Faure (56)
Lecture

# Week2 outline

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School
Department
Psychology
Course
PSYCH 3A03
Professor
Paul Faure
Semester
Fall

Description
This outline summarizes major points covered in lecture. It is not intended to replace your own lecture notes. Instantaneous pressure p(t)  Proportional to velocity of system; sound pressure is proportional to air particle velocity  p(t) = (m  v) / (t  area), where m=mass, v=velocity, t=time Sound pressure and energy  Energy—ability to do work, flows through medium  Power—rate of energy transfer through medium, rate at which work is accomplished  Power = Work/Time  Sound Energy—energy unit = Joule (J) = 1 N·m  Sound Power (W)—energy unit = Watt (W) = 1W = 1 J/s = 1 N·m/s  Note: sound energy and sound power are proportional but are not equivalent to each other 2 2 2  Sound Intensity ()—energy unit = Watt/m (W/m ) = 1 J/s·m  Sound intensity—amount of energy that is transmitted per second through an area of one square meter  Energy flowing through a volume  Acoustical energy from point source in expanding spherical wave front Absolute vs relative measures  Power, intensity, pressure are absolute measures  Relative measures used as a reference for stating relative magnitude  Relative acoustic measures employ levels  Levels = ratio of absolute sound energy  Level expressed as a ratio, e.g. Power incidentower reference  Need to know the reference or standard in order for a level measure to be useful bel  Number of bels calculated as: N(bels) = log (l10el); e.g. N(bels) = log (10 x ref  Bels: a relative measure without units  Advantage: provides a smaller range of values due to compressed scale with log system  Disadvantage: fractions commonly arise when dealing with smaller range  Solution: use decibels, calculated as: N(dB) = 10·N(bels); e.g. dB = 10·log ( / ) 10 x ref Intensity vs Pressure  Sound pressure (P)2is a scalar quantity; sound intensity () is a vector quantity  Equation:  = [ P rms/(ρ·c) ], where ρ = density of medium and c = speed of sound in medium  Equation: Z =cρ·c = characteristic impedance of medium Decibels and pressure 2  Standard dB equation cannot be used with sound pressure because  P rms  dB = 10·log ( / )  used for calculating decibels using absolute intensity or power 10 x ref  dB = 20·log (10/Px) refsed for calculating decibels pressure  ISO standard for P ref= 20 µPa  When using the acronym dB SPL (SPL = sound pressure level) this implies P ref= 20 µPa Usefulness of dB scale  large dynamic range from threshold of hearing to threshold of pain or even hearing damage  dynamic range can be as large 10 12 or 1014  possible to have negative dBs; it just means that the numerator is smaller than the denominator  Remember: once calculated, a dB is a dB is a dB Standard dB reference  dB SPL—sound pressure level, re 20 µPa -12 2  dB IL—intensity level, re 10 W/m  dB SL—sensation level, re some other measure, e.g., individual threshold  dB HL—hearing level, re level above average threshold of hearing at that frequency Psych 3A03 14 September 2012 Week
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