Transcript Week 3 - Truth Recordings
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 v
= frequency
in Hertz
(Hertz is cycles per second) = wavelength
(in metres)
= velocity of sound (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 2 nd harmonic 3 rd 4 th 5 th harmonic harmonic 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
Half wavelength Full wavelength 1.5x wavelength 2x wavelength 2.5x wavelength 3x wavelength 3.5x wavelength
Harmonic
1 st harmonic cancelled 2 nd harmonic reinforced 3 rd harmonic cancelled 4 th harmonic reinforced 5 th harmonic cancelled 6 th harmonic reinforced 7 th harmonic cancelled
Frequency
50Hz 100Hz 150Hz 200Hz 250Hz 300Hz 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