Sound is created by pressure disturbances traveling through an elastic medium like air. These pressure disturbances propagate as waves, which can be periodic or aperiodic. Periodic waves have regular, repeating patterns of vibration and are associated with the perception of pitch. They can be analyzed into combinations of sinusoidal components called harmonics. In contrast, aperiodic waves do not have a regular repeating pattern and are generally not associated with a clear pitch. Both periodic and aperiodic waves are important in speech communication.

2.  Sound is created by a disturbance travelling in
an elastic medium like air and water. For
instance, when an excess pressure is produced
on some region of the air, that region tends to
expand towards the neighbouring zones. This,
in turn, compresses those zones, creating a
new excess pressure which will tend to expand
next, and, again, a new excess pressure is
further created. The pressure disturbance will
thus propagate through the air, and eventually
it will reach some receiver (for instance a
microphone or an ear). Excess pressure is
called sound pressure(see Ladefoged,1996:3f).

3.  This kind of movement in which it is not
the medium itself but some disturbance
what is travelling, is called a wave. There
are many other types of waves, such as
radio waves, light, heat radiation, the
ripples on the surface of a lake,
earthquakes, etc. When the wave takes
place in a liquid or gaseous medium
(except surface waves), the wave is called
an acoustic wave. When a wave is audible,
it is called a sound wave.

4.  most waves are the result of many
successive disturbances of the medium,
instead of only one. When those
disturbances are generated at regular
intervals and are all the same shape we are
in the presence of a periodic wave, and the
number of disturbances per unit time is
called the frequency of the wave. It is
expressed in a unit called Hertz (Hz),
meaning cycles per second (a cycle is all
that happens in between a disturbance). In
the case of sound waves, frequency is
between 20 Hz and 20,000 Hz.

5.  Acoustic waves of frequency smaller than
20 Hz are called infrasounds, and those of
frequency greater than 20,000 Hz are
called ultrasounds. Neither of them can
ordinarily been heard by humans. Several
animals (such as the dog, for instance) can
hear very low frequency sounds, such as
those created by ground waves during an
earthquake.

6.  Even if there are many sounds which are nearly
periodic, such as those sounds produced by pitched
musical instruments, the vast majority of sounds in
Nature are aperiodic, that is, successive disturbances
are not equally spaced in time, and are not of constant
shape either. This is what in a technical sense is called
noise. Aperiodic waves usually cannot convey the
sensation of pitch. Some examples are the consonants of
speech, urban noise, the noise of the wind and the sea,
and the sound of many percussive instruments such as
drums, charlestons, etc.


7.  PERIOD (n.) A term derived from the study of
the physics of sound, and used in acoustic
phonetics, referring to the time it takes for a
cycle of pressure variation in a sound wave to
repeat itself regularly over and over. The
shorter the period, the more cycles there will
be in a given unit of time, and thus the higher
the frequency. Waveforms which show a
repeating pattern of vibration are periodic
waves; those which do not are aperiodic.
Speech makes use of both types of waveform:
vowel sounds have periodic waveforms;
fricatives, for example, involve aperiodic
waveforms. Crystal(2008:357)

9.  Speech communication involves both
periodic and aperiodic sounds which are
characterized, respectively, by the
presence and the absence of the periodic
acoustical excitation produced by the
vibration of the vocal folds in the human
larynx. Depending on the vocal fold
function, the degree of periodicity may
vary even within speech sounds belonging
to the same phonemic category.

10.  Many speech sounds, such as vowels, that
typically occur in periodic forms can also be
produced without periodic vibration of the
vocal folds (e.g., in whispered speech). The
degree of periodicity also varies in vowels of
natural speech due to aspiration noise and the
irregular oscillation of the vocal folds that
takes place, for example, in the production of
creaky voices. In addition, the degree of sound
periodicity allows linguistic differentiation
between certain consonant sounds (such as
distinguishing the unvoiced /s/ from the voiced
/z/). Thus, periodicity is an inherent feature in
speech communication and it affects both the
phonemic identity and the quality of speech
sounds.

11.  The description of the sine waves which
compose a given sound is called the spectrum
of the sound (see Ashby,2011:58). The
spectrum of sound is important since it allows
a description of sound waves which is closely
related to the effect of different devices and
physical modifiers of sound.
 O’Connor (1973:74) states that periodic
sounds give rise to a clear sensation of pitch
whose height is related to the frequency of
vibration- the higher the frequency, the higher
the pitch. But not all periodic sounds have the
simple and elegant shape (sinusoidal shape) of
the vibrations.

13.  These are not sinusoidal in shape and yet the
remarkable thing about it is that it can be
analyzed into a combination of two shapes
which are sinusoidal. This is done by
measuring the separate amplitudes at equal
interval of time along the horizontal axis
adding the amplitude values together
whenever both are on the same side. The
more complex the periodic shapes , the more
sinusoidal components will be needed to built
it up. These sinusoidal components are known
as the harmonies of that sound. The higher
harmonies are always simple multiples of the
lowest harmonic which is known as the
fundamental frequency or simply fundamental.

14.  Two quite distinct sounds may obviously
have the same fundamental frequency but
differ in quality, which in turn related to
the harmonic structure of the sounds. We
can therefore specify periodic sounds by
stating the frequencies and amplitudes of
the fundamental and whatever higher
harmonics are present.
 No speech sounds are absolutely periodic,
that is, perfectly from one cycle to the
next, but some are so nearly periodic (e.g.
vowel sounds) . The wave forms of spoken
vowels are very complex.

16.  An aperiodic sound is the one whose
pattern does not repeat itself as do those
of the periodic sounds. Aperiodic sounds
such as /t/ can show that it is no longer a
case of a tidy harmonic structure, with
each harmonic being a simple multiple of
the fundamental.

17.  for aperiodic sounds there is no fundamental ,
no harmonics; on the contrary , noise is going
on at every frequency , which is why we do
not perceive any clear pitch for such sounds as
for these periodic ones. The spectra of
aperiodic sounds cannot therefore be a series
of vertical lines representing the frequencies
and amplitudes of the separate harmonics; it
must be a continuous line with these
differences in the amplitude profile over the
frequency range which enables us to
distinguish one aperiodic sound from another.
Also , the same aperiodic sound can be of
different spectra (see O’Connor ,1973:78f).