PSYC 212 Lecture Notes - Lecture 14: Sound Pressure, Phon, Logarithmic Scale

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28 Feb 2019
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Decibels(and(loudness
The$main$determinant$is$the$amplitude$of$waveform
loudness$is$generally$measured$in$decibels
Formula:$dB$=$20log(p/p0)
They$are$always$relative$to$the$smallest$pressure$perceivable
dB$are
The$reason$why$we$need$to$square$the$pressure$when$calculating$intensity:$sound$waves$propagate$in$
3d.$Intensity$corresponds$to$the$energy$of$the$sound$wave$when$it$hits$a$2D$surface,$like$the$eardrum
MOST$IMPORTANT$TO$REMEMBER:$dB$scale$is$relative$to$smallest$pressure$perceivable$and$it$is$a$
logarithmic$scale.
Frequency(and(pitch
PITCH=$the$psychological$aspect$of$sound$related$mainly$to$the$fundamental$frequency$
The$frequency$(Hz)$=$number$of$cycles$per$second
E.g.$is$there$are$2$cycles$in$a$second,$the$frequency$is$2Hz
Higher$frequencies$will$be$associated$with$high$pitches,$and$vice$versa.
Sounds$that$only$have$one$frequency$are$called$pure$tones.
Audibility
Audibility$of$a$sound$is$due$to$a$mix$of$frequency$and$amplitude
AUDIBILITY=$capacity$to$hear$a$sound
It's$more$difficult$to$hear$a$sound$that$has$a$low$frequency
Music$is$in$the$middle$of$our$audible$range;$you$won't$play$music$that$can't$be$heard$by$someone
If$sounds$are$too$loud$[amplitude$is$too$high]$the$sound$will$become$unpleasant$and$can$even$become$
painful.
EQUAL-LOUDNESS$CURVE=$a$graph$plotting$sound$pressure$level$[dB$SPL]$against$the$frequency$for$
which$a$listener$perceives$constant$loudness.
Note:$the$unit$used$is$the$'phon'$which$corresponds$to.$The.$dB$value$of$the$curve$at$1k$Hz
On$the$graph,$decibels$are$calculated.$Based$on$sound$pressure:$dB$=$20log(p/p0)$
For$instance,$a$sound$of$40dB$is$equal$to$40$phons$at$1kHz$and$2kHz,$but$to$10$phons$at$0.1kHz
…$or,$sounds$of$70$dB$at$0.2kHz$and$60dB$at$1kHz$will$sound$equally$lead$[60$phons]
§
Lecture$14
Thursday, February 28, 2019
1:10 PM
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Frequency
Complex$sounds$can$be$understood$as$a$combination$of$several$frequencies.$In$nature,$every$sound$is$a$
complex$sound.$
Complex$sounds$can$be$described$by$their$spectrum.$
In$the$complex$waveform,$there$is$a$lot$of$power$at$the$lower$frequency$and$a$lot$of$power$at$the$
other$higher$frequency.$If$you$combine$all$the$waveforms$to$the$point$where$you$have$a$square$
wave:
§
The$height$of$the$bar$=$amplitude$of$the$frequency
Location$of$bar$on$graph$=$the$frequency$we$are$characterizing
The$first$waveform$is$a$pure$tone;$the$second$one$is$also$a$waveform;$the$third$is$adding$
waveforms$A$and$B,$one$bar$corresponds$to$the$first$1k$Hz$and$the$second$represents$the$other$
frequency.$$So$you$can$tell$that$there's$a$lot$of$one$frequency$and$little$of$the$other.$The$fourth$
waveform:$if$you$imagine$that$you$add$a$higher$and$higher$frequency$with$a$lower$amplitude,$it's$
going$to$lead$to$that.$The$more$we$add$a$frequency$with$a$high$intensity,$the$more$the$waveform$
resembles$a$square:
§
Many$sounds$have$a$harmonic$spectrum.
HARMONIC$SPECTRUM=$the$spectrum$of$a$complex$sound$in$which$energy$is$at$integer$multiples$of$the$
fundamental$frequency$
Typically$caused$by$a$simple$vibrating$source$[e.g.$a$string$of$a$guitar,$or$reed$of$a$saxophone]
FUNDAMENTAL$FREQUENCY$=$the$lowest-frequency$component$of$a$complex$periodic$sound
TIMBRE=$the$psychological$sensation$by$which$a$listener$can$judge$that$two$sounds$with$the$
same$loudness$and$pitch$are$dissimilar.
§
Timbre$quality$is$conveyed$by$the$profile$of$the$harmonics.
For$the$four$sounds$here,$the$fundamental$frequency$is$the$same$but$the$harmonics$[overtone]$
are$what$change:$
§
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