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Paul Faure (56)

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Paul Faure

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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