Transcript Audio Technology (Part 1)

SWE 423: Multimedia Systems
Chapter 3: Audio Technology (1)
Audio
• Audio is a wave resulting from air pressure
disturbance that reaches our eardrum generating
the sound we hear.
– Humans can hear frequencies in the range 20-20,000
Hz.
• ‘Acoustics’ is the branch of physics that studies
sound
Characteristics of Audio
• Audio has normal wave properties
– Reflection
– Refraction
– Diffraction
• A sound wave has several different
properties:
– Amplitude (loudness/intensity)
– Frequency (pitch)
– Envelope (waveform)
Audio Amplitude
• Audio amplitude is often expressed in decibels
(dB)
• Sound pressure levels (loudness or volume) are
measured in a logarithmic scale (deciBel, dB) used
to describe a ratio
– Suppose we have two loudspeakers, the first playing a
sound with power P1, and another playing a louder
version of the same sound with power P2, but
everything else (how far away, frequency) is kept the
same.
– The difference in decibels between the two is defined
to be
10 log10 (P2/P1) dB
Audio Amplitude
• In microphones, audio is captured as analog signals
(continuous amplitude and time) that respond
proportionally to the sound pressure, p.
• The power in a sound wave, all else equal, goes as the
square of the pressure.
– Expressed in dynes/cm2.
• The difference in sound pressure level between two
sounds with p1 and p2 is therefore 20 log10 (p2/p1) dB
• The “acoustic amplitude” of sound is measured in
reference to p1 = pref = 0.0002 dynes/cm2.
– The human ear is insensitive to sound pressure levels below
pref.
Audio Amplitude
Intensity
0 dB
20 dB
25 dB
40 dB
50 dB
60 - 70 dB
80 dB
90 dB
120 - 130 dB
140 dB
Typical Examples
Threshold of hearing
Rustling of paper
Recording studio (ambient level)
Resident (ambient level)
Office (ambient level)
Typical conversation
Heavy road traffic
Home audio listening level
Threshold of pain
Rock singer screaming into microphone
Audio Frequency
• Audio frequency is the number of high-to-low pressure
cycles that occurs per second.
– In music, frequency is referred to as pitch.
• Different living organisms have different abilities to
hear high frequency sounds
–
–
–
–
–
Dogs: up to 50KHz
Cats: up to 60 KHz
Bats: up to 120 KHz
Dolphins: up to 160KHz
Humans:
• Called the audible band.
• The exact audible band differs from one to another and deteriorates
with age.
Audio Frequency
• The frequency range of sounds can be divided into
–
–
–
–
Infra sound
Audible sound 20 Hz
Ultrasound
Hypersound
0 Hz – 20 Hz
– 20 KHz
20 KHz – 1 GHz
1 GHz – 10 GHz
• Sound waves propagate at a speed of around 344
m/s in humid air at room temperature (20 C)
– Hence, audio wave lengths typically vary from 17 m
(corresponding to 20Hz) to 1.7 cm (corresponding to
20KHz).
• Sound can be divided into periodic (e.g. whistling
wind, bird songs, sound from music) and
nonperiodic (e.g. speech, sneezes and rushing
water).
Audio Frequency
• Most sounds are combinations of different
frequencies and wave shapes. Hence, the
spectrum of a typical audio signal contains
one or more fundamental frequency, their
harmonics, and possibly a few crossmodulation products.
– Fundamental frequency
– Harmonics
• The harmonics and their amplitude determine the
tone quality or timbre.
Audio Envelope
• When sound is generated, it does not last
forever. The rise and fall of the intensity of
the sound is known as the envelope.
• A typical envelope consists of four sections:
attack, decay, sustain and release.
Audio Envelope
• Attack: The intensity of a note increases from silence to a
high level
• Decay: The intensity decreases to a middle level.
• Sustain: The middle level is sustained for a short period of
time
• Release: The intensity drops from the sustain level to zero.
Audio Envelope
• Different instruments have different
envelope shapes
– Violin notes have slower attacks but a longer
sustain period.
– Guitar notes have quick attacks and a slower
release