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