Acoustics

Johnson K. Ch.1 and Acoustic Slides from LING 313

What are acoustic mediums

• movement that cause pressure fluctuations in the air

• Fluctuations that impinge upon the eardrum

What is an acoustic waveform

The record of sound producing fluctuations over time

What is a sound wave

• longitudinally travelling sound fluctuations

• regular pulses/pressure variations against air particles

what is a transverse wave?

• particles move up and down perpendicularly

• peaks and valleys are clearly visible

what is a longitudinal wave?

• particle movement and wave direction is the same

• particles move forward and backwards (like a domino effect)

• once recovered from the push, they push back

what is the compression zone?

• when particles are forced together

• this leads to high pressure

• peaks are high pressure zones

• measure pressure differences between compression and rare refraction zones

what are the characteristics of a simple sine wave?

• frequency: how many cycles per second (Hz)

• intensity: how strong the oscillations are measured (dB)

• phase: when the wave starts (degrees — relative to other waves)

Why do sound waves lose energy as they travel through air?

It takes energy to move

What are the main types of sounds?

• Simple periodic waves

• complex periodic waves

• component waves

what are simple periodic waves?

• sine waves with simple harmonic motion

• only children’s vocal fold vibrations come close to producing

what is frequency

• number of times the sinusoidal patterns repeats per unit time

• measured on horizontal axis

what is amplitude?

• distance between 0 line and peak or valley

• amplitude used for acoustic property (not loudness)

• amplitude in relation to time stays the same

• measured on the vertical axis

what is a phase?

• timepoint when the wave started

• to calculate the sine wave by taking amplitude values from a set of right triangles that fit in a circle

what is a wavelength?

distance between point A and B (on the x-axis)

what is a cycle?

each repetition of a pattern

what is a period?

• the duration of a cycle (point A-B)

• particle movement recorded as a function of time

  • one period measures a cycle of a specific wave

  • transverse waves measure time and height difference of a particle

what are complex periodic waves?

• composed of at least 2 sine waves (multiple)

• contains the fundamental frequency (f0)

what is the fundamental frequency?

• The rate the complex pattern repeats itself

• The  f₀ of a complex wave the greatest common denominator of component since waves

what is the fundamental frequency of a complex wave?

the greatest common denominator of the component sine waves

what are component waves?

• cannot see individual peaks corresponding to cycles

• complex periodic waves can be constructed by adding together sine waves

what is a power spectrum?

amplitude vs. frequency plot of the simple sine wave components of a complex wave

what is the fourier’s theorem

• any complex waveform can be decomposed into a set of sine waves

• set of particular frequencies, amplitudes and phase relations

what is the Fourier Analysis?

• take a complex wave (with arbitrary # of components)

• derive frequencies, amplitudes and phases of those components

• result is the power spectrum

what are aperiodic sounds?

• either a random waveform or a pattern than doesn’t repeat

• basically white noise

what is white noise?

a random pressure fluctuation (static type sound)

what are acoustic filters?

pass or blocks compoenents of sound of different frequencies

what is the low-pass filter?

allows low frequency components and blocks high-frequency components of a wave

what is a pass-band?

sound energy within a frequency range is passed by the filter

what is a reject-band?

sound energy in (gray shaded) region is blocked by the filter

what is the center frequency?

when the high and low cutoff frequencies of a band-pass filter equal each other

what is a bandwidth?

width in Hz of the filter’s peak (one-half of the acoustic energy in filter is within the stated width

what is the filter-slope?

the rate the filter’s attenuates frequency beyond cutoff (some particles blocked, though they would go through if they can find a “bigger hole”)

what is band-pass filter?

• acoustic filters do not have sharp boundaries

• typical for transition between pass band and reject band to extend over a range of frequencies

• important in modelling articulation and hearing

what happens when the high and low cut-off frequencies equal each other?

the center frequency and bandwidth equals the resulting filter

what are basic waves?

• waves travelling from one end to another

• each particles move independently to the wave