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Understanding  SoundCSC121,  Introduction  to  Computer  Programming

What  is  Sound?

• “Sound  takes  place  when  bodies  strike  the  air,…  by  its  being  moved  in  a  corresponding  manner;  the  air  being  contracted  and  expanded  and  overtaken,  and  again  struck  by  the  impulses  of  the  breath  and  the  strings,  for  when  air  falls  upon  and  strikes  the  air  which  is  next  to  it,  the  air  is  carried  forward  with  an  impetus,  and  that  which  is  contiguous  to  the  first  is  carried  onward;  so  that  the  same  voice  spreads  every  way  as  far  as  the  motion  of  air  takes  place.”

What  is  Sound?

• Aristotle described  the  generation  and  propagation  of  sound  in  De  Anima (c.  350  BCE)

Sound  Waves

• a  typical  sound  has  an  measurable  average  ambient  air  pressure• but  sound  pressure  is  not  continuously  unchanging• as  Aristotle  noted,  there  are  fluctuations  in  pressure  that  we  identify  as  condensations (increases)  and  rarefactions(decreases)• these  changes  form  a  wave pattern•much  like  that  of  dropping  a  pebble  into  a  pond

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Sound:  push  and  pushback

a)  a  piston  pulse  initiates  push  on  the  air  cells

b)  the  pressure  deforms  the  air  cells  that  pass  the  energy  to  their  neighbors

c)  darker  cells  indicate  higher  pressure,  lighter  cells  lighter  pressure

d)  the  energy  is  reflected  on  the  rigid  surface  of  the  wall

e)  reflection  and  diffraction  is  evident  in  the  sound  propagation

Sound  Waves

• Here  is  an  idealized  sinusoidal  wave  with  an  amplitude A  and  a  wavelength λ

Sound  Waves

• the  frequency of  the  wave,  measured  as  the  number  of  repeats  or  cycles  per  second

• typically  expressed  in  Hertz or  Hz

Sound  Wave  Properties

• speed  &  propagation  falloff• intensity• superposition  &  interference• reflection  &  diffraction

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Sound  Waves

• the  speed  of  propagation  of  a  sound  wave  does  not  depend  on  the  wavelength  or  frequency  of  the  sound

Propagation  Falloff

• The  intensity or  amplitude  of  a  sound  does  decrease  over  distance

Superposition

• the  principle  of  superposition:  sound  waves  pass  through  one  another  with  little  disturbance

Superposition  and  interference

• If  the  sound  waves  are  precisely  opposing  signals,  then  they  can  cancel  each  other  out

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Example:  Noise  Cancellation  Technology

Active  noise  cancellation  is  available  for  audio  headsets.  It  seeks  to  apply  the  principle  of  superposition.

External  microphones  on  the  headset  capture  the  sound  on  its  way  to  the  ear.  Fast  acting  electronics  attempt  to  apply  an  opposing  pressure  to  cancel  the  noise.

In  practice,  they  reduce  the  noise  by  a  factor  of  100  or  20  dB.

Reflection

• the  law  of  reflection holds  true  for  all  radiating  energies:• the  angle  of  incidence  equals  the  angle  of  reflection

Refraction

•when  light,  for  example,  falls  on  a  matte-­‐painted  wall,  it  reflects  diffusively• a  similar  phenomenon  occurs  when  sound  waves  encounter  surfaces  that  are  rough

Intensity

• the  intensity  or  power  of  a  sound  wave  can  be  measured• sound  intensity  is  normally  expressed  in  log form• specifically,  the  base  10  logarithm• logarithmmeasures  the  exponent  that  yields  the  number• the  base of  the  logarithm  is  the  number  we  are  raising  to  a  power• and  the  power is  the  logarithm  of  the  number• example:  log10(26)  =  1.41497…• i.e.,  101.41497… =  26

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decibelsDecibels Pressure (Pascals) Sound

0 0.00002 threshold  of  human  hearing20 0.0002 rustling  of  leaves40 0.002 residential  area  at  night80 0.2 vacuum  cleaner100 2 pneumatic  hammer  at  2  meters120 20 rock  concert130 63.2 threshold  of  pain150 632 jet  engine  at  30  meters  (hearing severely  

damaged)180 20,000 rocket  engine  at  30  meters  (instant death)