AUD202
Audio and Acoustics Theory
Harmonics and Overtones
Waveforms / Wave Interaction
Phase Concepts / Comb Filtering
Beat Frequencies / Noise
Last Week >
Frequency and Dynamic Ranges of Hearing
Frequency / Wavelength / Period / Speed
Upcoming Assessments
Week 8 - NIHL Report
Week 11 - Sound Observations Report
Week 13 - Exam
Units of Measurement
= period of one cycle (in seconds)
f = frequency in Hertz
 = wavelength
v = velocity of sound
(Hertz is cycles per second)
(in metres)
(in metres per second)
HARMONICS & OVERTONES
Harmonics are integer
multiples of the fundamental
frequency.
Harmonics
For example, if the fundamental frequency
is 200Hz, the harmonics would be:
200Hz
400Hz
600Hz
800Hz
1000Hz
Fundamental frequency
2nd harmonic
3rd harmonic
4th harmonic
5th harmonic
An overtone is any frequency higher than
the fundamental frequency of a sound..
SIMPLE WAVEFORMS
Sine Wave
Fundamental frequency only, no harmonics
Sawtooth Wave
Odd and even harmonics at inverse amplitudes
Square Wave
Odd harmonics at inverse amplitudes
Triangle Wave
Odd harmonics at inverse squared amplitudes
Making a sawtooth from a sine wave
A sawtooth wave's sound is harsh and clear and
its spectrum contains both even and odd
harmonics of the fundamental frequency
Simple Harmonic Motion
Constant frequency without regard
to amplitude
Simple Harmonic Motion
PHASE
Phase Concepts
The word ‘Phase’ is used to describe a
specific location in a cycle of a periodic
(repeating) wave
“In phase”, “out of phase” and “completely
out of phase” are terms used to describe
relative positions of a wave cycle
Phase Concepts
Phase Angles
Wave Interaction
Two sine waves with the same amplitude
and frequency can add either
destructively or constructively
depending on their relative phase
Two sine waves travelling in opposite
directions can create a standing wave
Beat Frequencies
Beat Frequencies
When two sound waves of different (but close)
frequencies are played together, the alternating
constructive and destructive interference causes
an oscillation of the resulting waveform
frequency 1 – frequency 2 = Beat frequency
502 Hz - 500 Hz = 2 beats per second
COMB FILTERING
Comb filtering is caused by a wave
combining with a delayed version of
itself
Comb Filtering
A wave delayed by 180 degrees (half a
wavelength) results in the fundamental
frequency of cancellation.
Frequencies delayed by 1.5x, 2.5x, 3.5x the
wavelength will cancel, while whole number
multiples (1, 2, 3 etc) will reinforce.
Comb Filtering
A wave delayed by half a wavelength results in the
fundamental frequency of cancellation.
Wavelength
Harmonic
Frequency
Half wavelength
1st harmonic cancelled
50Hz
Full wavelength
2nd harmonic reinforced
100Hz
1.5x wavelength
3rd harmonic cancelled
150Hz
2x wavelength
4th harmonic reinforced
200Hz
2.5x wavelength
5th harmonic cancelled
250Hz
3x wavelength
6th harmonic reinforced
300Hz
3.5x wavelength
7th harmonic cancelled
350Hz
Single signal, no comb filter issues
1ms delayed version of the signal added
Comb Filtering
Common causes of comb filtering:
1. Two microphones at different distances from
the same source.
2. Reflective nearby surface causing a reflected
sound to arrive at the microphone slightly
after the direct sound.
3. Two identical sounds in a DAW with a 1 to
10ms delay
WHITE NOISE & PINK NOISE
Noise
White noise: Equal energy per frequency
Pink noise: Equal energy per octave
Pink noise is simply White noise with a
‘pinking filter’ added (-3dB/octave roll off)
Speed Writing Exercise
List anything that might cause NIHL
List occupations where NIHL might be a problem
List ways to avoid NIHL
Next Week >
Sound Envelopes
Instrument Acoustics