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Art 321
Sound, Audio, Acoustics
Dr. J. Parker
Sound
What we hear as sound is caused by rapid
changes in air pressure!
It is thought of as a wave, but that can be
confusing since we are immersed in the
medium. A wave seems to be on the
surface.
Sound is 3D
Sound sources is at the centre of a sphere.
Pressure changes (higher, lower, higher …) move
outwards from the source.
Pressure and Amplitude
Loudness is a matter of how big the
pressure change is.
If we let normal ambient air pressure be 0,
then sound amplitude is +x to –x for some
way to measure pressure. Air pumps at
gas stations here use Pascals, but we can
use whatever we like.
PHYSICS ALERT !!
1 Pascal = 1 Newton per square metre
Standardized normal air pressure is:
P0 = 2 X 10-5 N/m2 = 0.00002 N/m2
= 20μ Pa.
Why do we care? Because a decibel, the
standard measure of sound level, is:
10 log10
where p is the rms pressure
of the sound.
Who cares?
All sound folks use decibels, or db. You get
to know (feel) how loud things are after a
while.
Threshold of Hearing (TOH)
0 dB
Rustling Leaves
10 dB
Busy Street Traffic
70 dB
Front Rows of Rock Concert
110 dB
Military Jet Takeoff
140 dB
Instant Perforation of Eardrum 160 dB
How loud is it?
The sound of leaves is 10 db.
The sound of a pencil dropping is 20 db.
How much louder is that ?
10 times!
Decibels are based on powers of 10.
If one sound is 10x times more intense than
another sound, then it has a sound level which is
10*x more decibels than the less intense sound.
Frequency
If you stand in one place, the pressure
waves will pass you. The number of peaks
that pass per second is the frequency.
Measured in Hertz (Hz), formerly cycles per
second.
60 Hz = low hum
440 Hz = ‘A’ on piano
4000 Hz = limit of telephone (voices)
Frequency
If we draw a curve that represents pressure
VS distance we can see that sound does
look like a wave.
Frequency
Since the speed of sound in air is a constant
(pretty much) then the number of peaks
that pass is a second is related to the
distance between them is a fixed way.
Also, the horizontal axis could be distance
just as easily as time.
Frequency
V = l/f where v is speed of
sound, f is frequency.
Amplitude
Frequency
Frequency is a precise way to specify pitch.
A 440
E above C
Summed
Oboe A
Oboe E
Phantom
B 3960
Frequency
Interesting sidebar
The speed of sound is different in different
media (air and water). When a sound
moves from one medium to another, part
of the sound is transmitted, part is
reflected back!
Sound can reflect
Multiple sounds
What we hear with multiple sounds is the
sum of all of them.
Sound Systems/Tech
A sound system is a collection of electronic
components designed to record and
display sound. Details depend on
applications:
Theatre: large output, many speakers
Studio: large input, many microphones
Microphone
Converts air pressure differences into
electrical voltage.
Microphone
Speakers
For display of audio.
Amplifier
Changes volume (voltage/current/power)
levels of a signal. Needed to get good
sound levels from a speaker.
Mixer
Control volumes of multiple inputs into
(multiple) outputs.
Complete Sound System
Recording
How can sound be recorded?
How can it be stored on a computer?
What tools do we use for this?
Where do the wires go?
Why am I doing this?
It is still sound if it is on a hard drive?
How much disk space do sounds need?
Recording
To record sound on a computer is very simple.
Recall that microphones change sound (pressure)
into voltage (electricity).
A computer ‘sound card’ can change voltages into
numbers (just as we do: 12 volts, for instance)
which can be stored in a computer memory.
A sound is therefore a sequence of numbers that
represents voltages (that represent pressure).
Digital Sound
How BIG the numbers can be dictate
accuracy of voltage samples ->volume or
amplitude.
8 Bits (binary digits) can store numbers
between 0 and 255 (256 different
frequencies)
16 bits has 32768 different frequencies.
Called depth or quantization.
Digital recording
How fast you play the samples back dictates
frequency.
Obviously one plays them back as fast as
they were recorded. However, more
samples per second means higher
frequencies can be recorded.
Sampling
Sampling
Quantization Distortion
Quantization is the process of selecting whole
numbers to represent the voltage level of each
sample. The A/D converter must select a whole
number that is closest to the signal level at the
instant it’s sampled. This produces small
rounding errors that cause distortion.
Quantization distortion increases at lower levels
because the signal is using a smaller portion of
the available dynamic range, so any errors are a
greater percentage of the signal.
Clipping
File Sizes
Sampling Rate x Resolution x Number of
Channels x Time in Seconds / 8 =
File Size (in Bytes)
44100 x 16 x 2 x 60/8=10,584,000 /minute
Sizes
Bytes per minute, uncompressed.
Sampling Rate Resolution Number of Channels
44,100
16
2
44,100
16
1
22.050
16
1
11.025
16
1
11.025
8
1
File Size
10,584,000
5,292,000
2,646,000
1,323,000
616,000
Recording sound – MAX OS X
http://audacity.sourceforge.net/download/
Recording Sound – Windows XP
Sound recorder
•To open Sound Recorder, click Start, point to All
Programs, point to Accessories, point to
Entertainment, and then click Sound
Recorder.
Audacity
Sound Forge
Sound Effects
http://www.pacdv.com/sounds/index.html
http://freesoundfiles.tintagel.net/Audio/
http://www.a1freesoundeffects.com/
http://www.pachd.com/sounds.html
http://www.soundhunter.com/
http://simplythebest.net/sounds/WAV/sound_effects_WA
V/index.html
http://www.acoustica.com/mp3-audio-mixer/sounds.htm
http://ljudo.com/default.asp?lang=tEnglish&do=it