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THE GENERATION OF SOUND
Periodic and aperiodic sounds No sound can be produced without a supply of force or energy. It is the breathing
mechanism that constitutes the energy supply, the expiratory phase. In order that
sound will be generated, the steady flow of energy in one direction has to be
converted into oscillations. For speech, the larynx performs this function, with the
vocal cords. The alternate opening and closing of the vocal cords results in theemission of successive puffs of air into the space above the larynx and this stream of
pulses constitutes the basis of the sound generated by the larynx.
Sound waves are air pressure
waves. At a given instant of time a
sinusoidal air pressure wave will be
distributed in space. It will have a
peak pressure at one point, zero
pressure at a distance of one-
quarter wavelength from the
maximum, a peak pressure at one
wavelength from the first peak, etc.A wave can be described as a
disturbance that travels
through a medium transporting
energy from one location to
another location.
Sound-> longitudinal wave and
a mechanical wave.
Longitudinal wave is a wave
which particles of medium
move in a direction parallels tothe direction in which the wave
moves. Mechanical wave requires a medium in order to transport their energy.
Theyre not capable of transmitting invacuum (vaco). Wave length is the length of one
complete circle of a wave.
Compression (region of high air pressure)/Rarefaction (region of low air pressure)
A wave is an energy transport and the amount of energy carried by a wave is related to
the amplitude a measure of force. A high energy wave is characterized by a high
amplitude and a low energy wave by a low amplitude. Amplitude is an amount of
maximum displacement of a particle on the medium from its rest position.
The greater of the ampl
Intensity: The amount of energy which is transported past a given area of the mediumper unit of time is known as the intensity of the sound wave. The greater the
amplitude of vibrations of the particles of the medium, the greater the rate at which
energy is transported through it, and the more intense that the sound wave is.
Loudness is subjective depending on the person/intensity is objective.
Frequency of vibrations refers to how often the particles of the medium vibrate when
a wave passes through the medium, the unit of frequency is Herz (Hz) or cycle per
second.
Intensity -> decibels (dB)
Frequency-> rate of vibration of the vocal folds.
Increasing length and mass (ex. Double-bass/violin): Increasing tension -> higher
frequency and vibration
Each standing wave pattern is referred to as a mode of vibration, so the string is
vibrating simultaneously in a number of different modes, that is over its whole length.
Imagine Fundamental Frequency (Fo)= 100 Hz which
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This mode of vibration involving the whole length of the string is called the
fundamental mode and the frequency to which it gives rise is the fundamental
frequency. The frequency produced by other modes (B, C, D) are harmonics. The
prequency of harmonics are multiples of the fundamental frequency.
The period of a wave is the time for a particle on a medium to make one complete
vibrational cycle. Frequency and period are different, yet related quantities. Frequency
is the cycles per second and period second per a cycle.P=L/f second/cycle F=L/P
Amplitude is a measure of force. It is linked with the sensation of loudness, and it
refers to the maximum displacement in a cycle of movement.
The time taken by one cycle of opening and closing of the cords depends upon the
balance between the subglottal
pressure and the resistance offered by
the vocal folds. The physical factors
which regulate the frequency of
vibration are the mass, length and
tension of the vibrating structures. The
vocal folds have many modes of
vibration in addition to the
fundamental mode. Frequencies
generated by these other modes of
vibration are multiples of this
fundamental frequency.
The glottal wave is periodic, that is it consists of repeated cycles of motion, and
therefore it must be made up of a fundamental frequency plus a range of harmonics.
The fundamental frequency has the greatest amplitude, and the amplitude of each
succeeding harmonic would be falling off progresively as the frequency increases.
In speech the fundamental frequency is
changing all the time but the components of
the larynx tone are always harmonics of the
fundamental and the effect of the
resonances of the vocal tract is to produce a
peak in the spectrum of the output at the
harmonics which are the closest to the true
resonance
The spectrum of the resulting sound always
has the same general outline or envelope
although the fundamental frequency is
continually changing. This means a
certain sameness of quality is heard in a
range of sounds with different
fundamentals. If this were not the case,
speech sounds could not fulfill the
linguistic function that they in fact have.
The term used for a resonance of the
system in this context is a formant.
Differences in formant structure are the
result of differences in articulationwhich affect the shape and hence the
dimensions of the vocal tract. The most
important modifications are due to
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different places in the tract, the location corresponding to what is referred to
phonetically as the point of articulation of the sound.
What about voiced consonants such as /z/?
CORRELATOS ARTICULATORIOS, ACSTICOS Y AUDITIVOS
The spoken message undergoes a series of modifications during speechcommunication.
It is convenient for us to grasp the dynamic nature of speech and the non-monotonic
relations between articulatory, acoustic and auditory parameters of speech analysis.
The speech chain:Transmission Phase
- String of language units
- Chain of nerve impulses
- Sequence of muscle movements
- Train of sound waves
Reception Phase
-Train of sound waves
-Sequence of muscle movements
-Chain of nerve impulses
-String of language units
Non-monotonic relationsThe changes in the acoustic parameters as the articulatory parameters vary
through ranges of values are often non-monotonic. The same type of relation is
also often observed between certain acoustic parameters and some aspects of the
auditory response to the different acoustic patterns.
ACOUSTIC-ARTICULATORY RELATIONSHIPSAccording to the source-filter theory of speech production,(Fant, 1960), the
production of speech sounds can be described as a process involving: The generation
of sound sources, and The filtering of these sound sources by the airway above the
glottis.
In the formation of vowel sounds the action of the glottis produces the basic source
of sound; this sound is then transmitted through the pharynx and oral tract to the
outside air. We can think of the tract as a filter that emphasizes some of the
components of the source sound, namely, those at or near the resonant frequencies
of the tract. Estimates of the acoustic consequences of changes in vocaltract shapes
lead to the following relation between tongue-body position and the first formant
frequency: vowel height is ____ correlated with F1 frequency.
ACOUSTIC-AUDITORY RELATIONSHIPSIt seems that not all items of acoustic information are equally important from
the point of view of speech reception. No doubt that the perception of speech is
necessarily structured by the inherent constraints of the auditory system.
Results from psychoacoustic studies have shown that the listeners sensitivity
to frequency changes is not the same across the frequency range. The auditory
system is more sensitive to frequency changes at lower than higher frequencies.
In addition, subjective auditory impressions of loudness differences do not
match sound pressure differences. For soft sounds , large changes in perceived
loudness result from relatively small changes in sound pressure, while for loud
sounds, relatively large pressure changes produce only small changes in perceived
loudness
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