PH 105 Dr. Cecilia Vogel Lecture 9

OUTLINE  Finish ear/hearing  Source power level  Sound loudness  sound intensity level  sound pressure level  dB and logs  distance dependence  loudness level  dB meters  multiple sources

Ossicle Physics  Pressure =  Pressure can be increased by  decreasing the area (knife edge)  round window of inner ear has 20X _______ area than eardrum  increasing the force (leverage)  ossicles act as a small force large lever arm large force

Inner Ear  Vibration in middle ear  creates pressure wave in cochlear fluid in  which vibrates the basilar membrane  stiff part resonates at  helps us determine stiffer less stiff basilar membrane  vibrations in basilar membrane bend  which send nerve signals to

Limits of Hearing  The ear is sensitive to large range of pressure amplitudes  pressure variations as low as about ________ N/m 2 can be heard  compare to ______ N/m 2 atmospheric pressure  pressure variations as high as about _____ N/m 2 can be heard without pain  1/1000 atmospheric pressure  The ear is sensitive to large range of frequencies  from about ___ Hz to about ___ __Hz  more sensitive in

Critical Bands  Basilar membrane vibration helps us determine  If two sounds have similar basilar membrane response  they are said to lie within a critical band  Size of critical band is about 4 semitones for range of freq’s used in music  the size of the critical bands is larger at   Just noticeable difference

Limits of Hearing  The ear is sensitive to large range of frequencies  from about  most sensitive near  The ear is sensitive to large range of pressure amplitudes  pressure variations as low as about ________ can be heard  compare to  pressure variations as high as about _________ can be heard without pain 

Logarithmic Sensations  The loudest sound we hear  does  Every time we _____ the pressure or intensity of the sound,  we  2XIntensity, add  4XIntentity, add  8Xintensity, add  … 1,000,000XIntensity,  Thus the loudness level is

Logarithmic Math  Important log math:  log(2x)-log(x) = log(2)  log(AB) = log(A)+log(B)  ex: log(20) = log2 + log10 =  log(A/B) = log(A)-log(B)  ex: log(2/3) = log2 – log3 =  logA n = nlog(A)  ex: log 4 = log2 2 = 2log2 =

Source Power Level  Consider the  The sound power level of a source:  W in numerator is  in

Source Power Level  The sound power level, L W, is used to  such as  It does  any more than the wattage of a light bulb tells  Also depends how  Intensity =

Sound Intensity Level  How loud a sound is more closely related to the  The sound intensity level:  I in numerator is

Sound Pressure Level  Ears, microphones, sound level meters respond to  Since intensity is proportional to  The sound pressure level is:  p is  Under normal circumstances

Fading into the Distance  A source sounds  because the sound gets  for example  sphere: source  hemisphere: source wall

Free Field  For a spherical or “free field” wave  The sound intensity level, L I, at a ______ ______ from a source is  L I =  where L w is the  As you go further from the source the L I drops  each time the distance doubled,  demo

Hemispherical Field  For a hemispherical wave  The sound intensity level, L I, at a ____________ a source is  L I =  where L w is the  As you go further from the source the L I drops  each time the distance doubled,

Example  A dentist drill has a power level of 70 dB  What is the sound intensity level at the hygienist, who is 1m away from the drill, which is held in midair?   What is the sound intensity level at the patient, who is 25 cm away?  

Example Continued  Find pressure variation on the patient's eardrum, assuming L p = L I = 71 dB?

Multiple Sources  Multiple sources of sound  which are    If you have n equal sources   The sound level in dB  L I =  If 1 washing machine has a sound intensity level of 30dB, _ washing machines will have a level of ____, _ washing machines ____, etc  demo?

Subjective Loudness Level  The subjective loudness level of a sound  depends  depends on other properties, too,   how well can you hear at different freq’s?  Graph from text:  each curve shows sounds of equal

Sound Level Meter Weighting  Our ears are not as sensitive to sounds at the  so very low and very high sounds have to have  to have the same  The _________ on sound level meter mimics this:  meant to give _______ to sounds that sound  even if the

Summary  Sound Power Level, L W  power of source  Sound intensity level, L I  intensity of sound  Sound pressure level, L p  same as L I under normal circumstances  what instruments respond to  Loudness level  subjective sense of loudness  Logarithmic  multiplication —> addition  Homework Chapter 6: