Session 18
The physics of sound
and
the manipulation of digital sounds
Pictures vs. Sounds
• Get in groups of 2 or 3 students.
• Create a list (to be turned in) of all the
modifications that we made to pictures.
• Which of these modifications do you think
have similar techniques with sound?
How does Hearing Work?
• The outer ear “catches” sounds
• The eardrum vibrates
• The inner ear translates the vibrations to
nerve impulses for the brain to interpret
Acoustics, the physics of sound
• Sounds are waves of air
pressure
– Sound comes in cycles
– The frequency of a wave is
the number of cycles per
second (cps), or Hertz
• (Complex sounds have
more than one frequency
in them.)
– The amplitude is the
maximum height of the
wave
Volume and Pitch
• We perceive volume as changes in amplitude
– If the amplitude doubles, it’s about a 3 decibel (dB)
change.
– As an absolute measure, it’s in comparison to
threshold of audibility
• 0 dB can’t be heard.
• Normal speech is 60 dB.
• A shout is about 80 dB
• We perceive pitch as changes in frequency
– Higher frequencies are perceived as higher pitches
– We can hear between 5 Hz and 20,000 Hz (20 kHz)
Logarithmic Scales
• Human hearing works with ratios not
differences:
– The A above middle C is 440 Hz
– The A above that has twice the frequency
(880 Hz)
– The A above that has twice the frequency of
that (1760 Hz)
Digitizing Sound
• We can estimate a curve
by creating rectangles
• We’ll do the same to
estimate the sound curve
– Analog-to-digital
conversion (ADC) will give
us the amplitude at an
instant as a number: a
sample
– How many samples do we
need?
Nyquist Theorem
• We need twice as many samples as the
maximum frequency in order to represent (and
recreate, later) the original sound.
• The number of samples recorded per second is
the sampling rate
– If we capture 8000 samples per second, the highest
frequency we can capture is 4000 Hz (where human
voices max out)
• That’s how phones work
– If we capture more than 44,000 samples per second,
we capture everything that we can hear (max 22,000
Hz)
• CD quality is 44,100 samples per second
Encoding a Sound
1-D Array of samples
Each sample = 2 bytes or 16-bits
0
1
2
3
4
getLength(sound)-1
. . .
16-bits allows for 2 16 = 65,536 combinations
For signed integers, split roughly in half:
- 215 = -32,768
+215 -1 = +32,767 (zero takes away one)
Given this information
• How much memory is necessary to store 1
minute of CD quality stereo sound?
Sound Basics
makeSound(fileName)
– Will create a new Sound object from the data in the
file with the passed file name
play(soundObj)
– Will start the sound playing. Let’s you repeat
immediately
blockingPlay(soundObj)
– Will play the complete sound before continuing
openSoundTool(soundObj)
– Will open a sound tool on the object (can do this from
the menu too)
The Sound Tool
• Not all of the sound is
shown when you
explore a sound
– Skips values to fit in
the window
• You can zoom in
– To see all sample
values
• You can zoom out
– To fit the sound in the
window again
Increase volume
def increaseVolume(sound):
for sample in getSamples(sound):
value = getSampleValue(sample)
setSampleValue(sample,value*2)
More General Code
def changeVolume(sound, factor):
""" Changes the sound volume by a given factor with
factor < 1 decreasing and factor > 1 increasing
the volume """
for sampleNum in range(0, getLength(sound)):
sample = getSampleObjectAt(sound, sampleNum)
value = getSampleValue(sample)
setSampleValue(sample, value * factor)
Main Program
""" Select a sound and repeatedly adjust its volume. """
def main():
print "Select the Media Folder"
setMediaFolder()
print "Select the sound (.wav) file to play repeatedly"
fileName = pickAFile()
sound = makeSound(fileName)
volumeAdjustment = requestNumber("Enter factor to increase (>1) “ + \
“or decrease (<1) the volume.")
for counter in range(10):
blockingPlay(sound)
changeVolume(sound, volumeAdjustment)
print "Open Sound Tool to 'view' sound"
openSoundTool(sound)
But…
• What happens if we increaseVolume() too
many times?
– Clipping