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Transcript
AUD202
Audio and Acoustics Theory
Sound
Ranges of Hearing
Frequency / Wavelength / Period / Speed
What You Need to Learn Today
What sound is and how it travels
The frequency range (Hz) and dynamic
range (dB) of human hearing
How to calculate frequency,
wavelength, period and speed of sound
Acoustics:
the physics of sound
Psychoacoustics:
the human perception of sound
Sound is Vibrations
When an object vibrates it causes nearby
molecules to vibrate causing a chain
reaction
Sound is Vibrations
Sound travels as a series of compressions and
rarefactions through a medium
Sound is a Mechanical Wave
Sound travels through air as a longitudinal wave
However it can also travel as a transverse wave
(such as on a guitar string or a wave in water)
One complete cycle is one compression
and one rarefaction
1 cycle per second = 1cps = 1 Hertz = 1Hz
Frequency range of hearing
The human ear is capable of hearing frequencies
between 20 and 20,000 cycles per second
(20Hz to 20kHz)
Frequency range of hearing
20Hz
10kHz
40Hz
15kHz
80Hz
16kHz
100Hz
17kHz
150Hz
18kHz
200Hz
19kHz
Dynamic Range of Hearing
The typically accepted amplitude range of
human hearing is 120dBSPL
Sound amplitude is referenced to the
Pascal (Pa) (the unit for pressure)
0dBSPL = 0.00002 Pascals (20
µPa)
120dBSPL = 20 Pascals
Dynamic Range of Hearing
60 decibels of amplitude
-60dBFS to 0dBFS
Sine Wave
The sine wave is a pure tone
with no harmonics. It is a
‘simple waveform’
1kHz sine wave
Other Simple Waveforms
Sine
Square
Triangle
Sawtooth
HIGHER FREQUENCY
shorter wavelength
LOWER FREQUENCY
longer wavelength
Characteristics of Waveforms
Frequency
(number of cycles per second)
Wavelength
(length of one cycle in metres)
Amplitude
(strength or power of the wave)
Period
(time for one cycle in seconds)
Units of Measurement
= period of one cycle (in seconds)
f
= frequency in Hertz (Hertz is cycles per second)
 = wavelength
v = velocity of sound
(in metres)
(in metres per second)
Velocity of Sound Formula
v = 331.4 + (0.6x°C)
The velocity of sound through air
at 20 degrees Celsius is 344m/s
What is the wavelength
of 20Hz?
l=v f
Wavelength (m) = speed of sound / frequency (Hz)


= 344 / 20
= 17.2 metres
What frequency has a wavelength
of 1.72cm?
f v 
Frequency (Hz) = speed of sound / wavelength (m)
f = 344 / 0.0172
f = 20,000Hz
What frequency has a period
of 2 milliseconds?
f 1 T
frequency in Hz = 1 / period (in seconds)
f = 1/.002
ms = millisecds.
There are
in 1 second
f =1000ms
500Hz
What period has a frequency of 200Hz?
Answer in milliseconds.
T 1 f
period in seconds = 1 / frequency (Hz)
T = 1/200
T = 0.005 seconds
T = 5 milliseconds
What is the speed of sound at
1,263,719°C?
v = 331.4 + (0.6x°C)
v = 331 + (0.6 x 1,263,719)
v = 758,562.8 m/s
The Sound Barrier
The Sound Barrier
Velocity of Sound in Various Mediums
Air > 344 m/s
Hydrogen > 1284 m/s
Water > 1482 m/s
Human Brain > 1540 m/s
Gold > 2000 m/s
Steel > 5200 m/s
Diamond > 12000 m/s
Speed of Sound Through Air
Calculate the speed of sound at:
0°C
5°C
10°C
20°C
30°C
v = 331.4 + (0.6xC)
Answers
0°C = 331.4 m/s
5°C = 334.4 m/s
10°C = 337.4 m/s
20°C = 343.4 m/s
30°C = 349.4 m/s
What are the frequencies of the
following wavelengths?
3.4m
5m
50cm
80cm
Answers
3.4m = 101 Hz
5m = 68.8 Hz
50cm = 688 Hz
80cm = 430 Hz
Calculate the Wavelength of:
140Hz in air (at 344m/s)
400Hz in steel (at 5000m/s)
500Hz for an electrical signal in wire (at
300,000km/s)
Range of Hearing
The ‘accepted’ range of hearing of a young
undamaged ear is 20Hz to 20kHz
The typical amplitude range is 120dBSPL
The Decibel
The Bel is named after Alexander Graham
Bell 1847 –1922.
The decibel (dB) is a tenth of a Bel
In audio, amplitude is measured in decibels
Loudness is Subjective
Amplitude is Objective
Loudness
Loudness is a
subjective term
describing the
strength of our
perception of
sound.
Loudness is measured in Phons
Threshold of Pain 120dB
Threshold of hearing is
To Recap
Sound is audible pressure variations
which travel as a series of compressions
and rarefaction through a medium.
Sound is a mechanical wave which is
transmitted as longitudinal and
transverse waves.
Audio Engineering Society
www.aes.org
Username: jmcacademy
Password: student1
Papers related to NIHL on AES website
Hearing Loss from Noise and Music
Sound Pressure Levels in Symphony Orchestras
and Hearing
How the Ear Works and Why Loud Sounds Cause
Hearing Loss
Preventing Hearing Loss
Investigation of the Loud Music Exposure Hearing
Loss
Moodle Key to enrol in Trimester 1 classes
amplitude
Next Week
Wave Interaction
Beat Frequencies
Phase Concepts
Comb Filtering
Waveforms
Noise