Chapter 2 Flashcards
Motions of Vibrating Air Molecules
Sound can be defined as the propagation of a pressure wave–sound wave–in space and time
Sound must be propagated through a medium
The medium is composed of molecules that are compressible (degree of compressibility depends on the medium)
Force displaces molecules of the medium from “rest positions” and moves them closer to nearby molecules and results in a “bunching up” of molecules
All sound-conducting media have
ll sound-conducting media have elasticity & mass
Elastic objects oppose displacement
Massive objects oppose being accelerated and decelerated
Generally, the medium of interest for speech is air
Molecules at rest are still in motion–Brownian Motion
Average spacing between molecules is roughly equivalent
When a molecule is subjected to a force
When a molecule is subjected to a force, it moves away from its rest position and in the direction of the force
As it moves farther from its rest position, it opposes increasing displacement by generating an increasing recoil force which is exerted in the direction of the rest position
This displacement continues as molecules bump into neighboring molecules and this is how the sound wave is propagated
Pressure can be defined
Pressure can be defined as the force exerted over a unit area (P=F/A)
Proportional to the density of air
When air molecules are more densely packed in some unit volume
When air molecules are more densely packed in some unit volume, they collide with each other more frequently and generate more force and pressure within the volume.
Conversely, when air molecules are less densely packed, the collisions are less frequent and the pressures are relatively lower
The density of air at rest is the same at any spatial location–atmospheric pressure
Atmospheric pressure serves as a reference for positive & negative pressure
compression or condensation
High-density and high-pressure areas are called areas of compression or condensation
rarefaction
Low-density and low-pressure areas are called areas of rarefaction
Pressure Waves Propagate Through Space
Individual molecules don’t propagate through space
Molecules move around a rest position, movement results from inherent elastic and inertial forces
The pressure wave moves across space as a sequence of compressions and rarefaction areas
The air molecules move back and forth which alternately bunches them up and spreads the apart
Sound is a longitudinal wave
When a pressure wave moves away from the source of the sound waves, the alternating regions of high and low pressures the same direction as the force
Measuring Variation of a Pressure Wave in Time & Space
There are specific measures of the temporal (time) and spatial (space) variation of pressure waves
A waveform shows a magnitude (e.g., displacement, pressure, etc.) as a continuous function of time
A waveform shows one complete cycle of motion
Period (T) is the time taken to complete one full cycle of motion
Frequency (f) is the number of cycles of vibrations completed in one second
Hertz (Hz)–cycles per second
T=1/f
Period and Frequency have a nonlinear relationship
Wavelength
Wavelength–distance covered by a high-pressure region and its succeeding low-pressure region (or vice versa)
It is the measurement of spatial variation of a wave
Symbolized by greek letter lambda λ
Spatial Measures
Wavelength–distance covered by a high-pressure region and its succeeding low-pressure region (or vice versa)
It is the measurement of spatial variation of a wave
Symbolized by greek letter lambda λ
Has an inverse relationship to frequency
The higher the frequency, the shorter the wavelength
When sound waves encounter an object along their path
When sound waves encounter an object along their path, straight-line propagation may change
Frequencies with very long wavelengths generally may bend around objects in their path
Frequencies with very short wavelengths may strike the object and cause pressure variation to reflect off the object
The pressure reflections interact with the original pressure wave as well as with the multiple reflections
The relationship between wavelength and objects within the path of a pressure wave is related to their respective magnitudes
The motions of air molecules can produce pressure waves
The motions of air molecules can produce pressure waves, which are the basis of sound
Pressure waves vary across space and time
Can be described and measured in mathematical terms
The simple motions and resulting pressure waves discussed previously were sinusoidal motions and waves
Simplest form of vibration
Complex vibrations can be broken down into component individual sinusoids
Sinusoidal Motion
Simple Harmonic Motion
Linear projection of uniform circular speed (UCS)
Frequency:
number of full cycles occurring in a one second interval (f)
Amplitude:
: displacement of an air molecule from the rest position (A)
Phase:
position of the sinusoidal motion relative to some arbitrary reference position (φ)
Sinusoidal Motion Summary
A sine wave is a waveform that results from the linear projection of UCS
A sine wave is periodic
A sine wave involves only a single frequency
Simplest type of acoustic event
Described by a simple formula including the parameters of period, amplitude, and phase
Complex Acoustic Events
Acoustic event contains more than one frequency
Two types of complex events
Periodic: waveform pattern repeats over time
Aperiodic: no repeating pattern can be identified
Periodic
: waveform pattern repeats over time
