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Harmonics and OvertonesWaveforms / Wave Interaction
Phase Concepts / Comb FilteringBeat Frequencies / Noise
AUD202Audio and Acoustics Theory
Units of Measurement
= velocity of sound (in metres per second)
f
v = wavelength (in metres)
= frequency in Hertz (Hertz is cycles per second)
= period of one cycle (in seconds)
Harmonics
For example, if the fundamental frequency is 200Hz, the harmonics would be:
200Hz400Hz 600Hz 800Hz
1000Hz
Fundamental frequency2nd harmonic3rd harmonic4th harmonic5th harmonic
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
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
Wave Interaction
Two sine waves with the same amplitude and frequency can add
either destructively or constructively depending on their relative phase
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
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.
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
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
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