Part 1

Unit -2
Introduction and theory of sound

Acoustics – definitions, terms related to acoustics.
Theory of sound : generation, propagation, transmission, reception of
sound, sound waves, frequency, intensity, wavelength, sound pressure,
measurement of sound, scales- decibel scale
,

• Characteristics of speech
• Music and hearing
• Distribution of energy in speech
• Music frequencies
• Intelligibility of speech, high fidelity reproduction of music
• Human ear characteristics- making of sound
• Binomial hearing g
• Behavior of sound in enclosed spaces.
STUDY OF SOUND ‐ ACOUSTICS
HANDLED BY G.YOGAPRIYA

INTRODUCTION
AND THEORY OF
SOUND


DEFINITIONS
• What is Acoustics ?

– Acoustics is defined as the
science that deals with the
production,
production control,
control
transmission, reception, and
effects of sound.

– The physical principles of
this science are utilized in
architecture to attain
distinct hearing conditions
in enclosed spaces.
p


• The study of acoustics helps
to

– To appreciate and analyse
the problems involved in
the propagation of sound
in these enclosures.

– To suggest ways and
means of producing
optimum conditions of
hearing

– To obtain both subjective
and objective assessments
j
of the results achieved.

DEFINITIONS
• What is resonance ?

• resounding or reechoing

• i
increasing the intensity of sounds by sympathetic
i th i t it f d b th ti
vibration

– Eg) Put two tuning forks of equal
frequencies side by side, but not
touching. Strike one tuning fork so that
g g
you can hear its tone, and then suddenly
silence it. You can still hear a faint tone.
This is because the second tuning fork
g
has started vibrating sympathetically.

DEFINITIONS
• What is Reverberation?
– It is the persistence of sound in a
particular space after the original sound
is removed.

– A reverberation, or reverb, is created
when a sound is produced in an
enclosed space causing a large number
of echoes to build up and then slowly
decay as the sound is absorbed by the
walls and air.

– This is most noticeable when the sound
source stops but the reflections
continue, decreasing in amplitude, until
they can no longer be heard.

DEFINITIONS
• What is Reverberation time?
– Standard reverberation time has been
defined as the time for the sound to die
away
a a to a le el 60 decibels belo
level below its
original level. The reverberation time can
be modeled to permit an approximate
calculation.


DEFINITIONS
• The reverberant sound in an auditorium dies away y
with time as the sound energy is absorbed by
multiple interactions with the surfaces of the room.
In a more reflective room, it will take longer for the
g
sound to die away and the room is said to be 'live'.

• In a very absorbent room, the sound will die away
room
quickly and the room will be described as acoustically
'dead'. But the time for reverberation to completely
die away will depend upon how loud the sound was
to begin with, and will also depend upon the acuity
of the hearing of the observer


• What is echo? DEFINITIONS
– In audio signal
processing and acoustics
acoustics,
an echo (plural echoes) is
a reflection of sound,
arriving at the listener
some time after the direct
sound.

• Typical examples are the
echo produced by the
bottom of a well by a
well,
building, or by the walls of
an enclosed room. A true
echo is a single reflection
g
of the sound source.

SOUND

1. A sound is a vibration in an
elastic medium, which may be y
a solid, liquid, or gas, which
can be registered by the ear.

2. It can be pleasant or
unpleasant, loud or soft, high
or low. Sound is made b
l S di d by
vibrations, something moving
back and forth. Stretch a
rubber band tightly between
your fingers, and pluck it. You
can see it quiver and hear the
sound it makes.

CHARACTERISTICS OF SOUND WAVES
Sounds are generally audible to
the human ear if their frequency
(number of vibrations per second)
lies b t
li between 20 and 20,000
d 20 000
vibrations per second, but the
range varies considerably with the
individual.
The range of audible sound is also differentiated into 3
main categories. Subsonic or low frequency sound is
defined in the range of 20Hz to about 500Hz.
Midrange frequencies inhabit the realm of 500Hz to
g q
6KHz (6000Hz)
With high frequency sound defined in the remaining
6KHz to 20KHz
20KHz.

Those with frequencies above the audible
range are called ultrasonic.
A sound wave is usually represented
graphically by a wavy, horizontal line; the
upper part of the wave (the crest) indicates a
f h (h ) i di
condensation and the lower part (the
trough) indicates a rarefaction.


GENERATION OF SOUND WAVES

GENERATION OF SOUND WAVES

Sound waves are generated by any
vibrating body. For example, when a
violin string vibrates upon b i
i li i ib being
bowed or plucked, its movement in
one direction pushes the molecules of
the air before it crowding them
it,
together in its path.

When it moves back again past its original position and on to the
other side, it leaves behind it a nearly empty space, i.e., a space with
relatively few molecules in it In the meantime however the molecules
it. meantime, however,
which were at first crowded together have transmitted some of their
energy of motion to other molecules still farther on and are returning
to fill again the space originally occupied and now left empty by the
g p g y p py y
retreating violin string.

•In other words, the vibratory motion set up by
the i li t i
th violin string causes alternately i a given
lt t l in i
space a crowding together of the molecules of
air (a condensation) and a thinning out of the
molecules (a rarefaction).


•Taken together a condensation and a
rarefaction make up a sound wave; such a
wave is called longitudinal, or
compressional, because the vibratory
compressional
motion is forward and backward along the
direction that the wave is following.

•Because such a wave travels by disturbing
the particles of a material medium, sound
waves cannot travel through a vacuum.


THE PROPAGATION OF SOUND

1. Sound is propagated in air,
much like blowing up a large
balloon,
balloon which expands equally
in all directions. For sound to
be generated and heard it must
g
have a source, a medium
through which to pass and a
receiver.

2.
2 Propagation means "movement
movement
through“ Sound will propagate
through air and water.

All media have three properties which affect the
behavior of sound propagation:

A relationship between density and pressure.
This relationship, affected by temperature,
determines the speed of sound within the
i f i i
medium.

The motion of the medium itself, e.g., wind.
Independent of the motion of sound through the
medium,
medium if the medium is moving, the sound is
moving
further transported.

The viscosity of the medium. This determines
Th i i f h di Thi d i
the rate at which sound is attenuated. For many
media, such as air or water, attenuation due to
viscosity is negligible.

TRANSMISSION OF SOUND
TRANSMISSION OF SOUND

There are three requirements for
sound to "occur" in an environment:

(1) A vibrating source to initiate
sound,
sound
(2) A medium to transmit sound
vibrations throughout the
environment and
(3) A receiver to hear or record sound
vibrations.
Sound is initiated in an environment
by a vibrating source.


Water is a more efficient transmitter of sound compared to
air as sound travels faster and further in water.

The movement of the gas or liquid medium is identical to
surface waves found on any large body of water
water.

The wavelengths of speech are of the size of ordinary
objects, unlike light, whose wavelengths are extremely small
.Because of this, sound does not ordinarily cast "acoustic
shadows
shadows" but, because its wavelengths are so large, can be
transmitted around ordinary objects


 For example, if a light is shining on a person, and a
book is placed directly between them, the person will
them
no longer be able to see the light (a shadow is cast by
the book on the eyes of the observer).

However, if one person is speaking to another, then
p
placing a book between them will hardly affect the
g y
sounds heard at all; the sound waves are able to go
around the book to the observer's ears.

On the other hand, placing a high wall between a
highway and houses can greatly decrease the sounds of
the traffic noises if the dimensions of the wall (height
h ffi i h di i f h ll (h i h
and length) are large compared with the wavelength of
the traffic sounds.


RECEPTION OF SOUND – HEARING MECHANISMS - HUMAN EAR
CHARACTERISTICS- MAKING OF SOUND

 Th ear consists of three basic
The i t f th b i
parts
 The outer ear serves to collect and
channel sound to the middle ear.
 The middle ear serves to
transform the energy of a sound
wave into the internal vibrations
of the bone structure of the
middle ear and ultimately
transform these vibrations into a
compression wave in the inner
ear.
 The inner ear serves to transform
the energy of a compression wave
within the inner ear fluid into
nerve impulses that can be
transmitted to the brain.


Sound waves enter your outer ear and
travel through your ear canal to the
middle ear.
The ear canal channels the waves to
y
your eardrum, a thin, sensitive membrane
stretched tightly over the entrance to your
middle ear. The waves cause your eardrum
to vibrate.
 It passes these vibrations on to the
hammer, one of three tiny bones in your  Inside the cochlea,
ear. The hammer vibrating causes the there are hundreds of
anvil,
anvil the small bone touching the special cells attached to
hammer, to vibrate. nerve fibers, which can
The anvil passes these vibrations to the transmit information to
stirrup,
stirrup another small bone which touches the brain.
The brain processes
the anvil. From the stirrup, the vibrations
the information from the
pass into the inner ear. ear and lets us
The stirrup touches a liquid filled sack
p q distinguish between
and the vibrations travel into the cochlea, different types of
which is shaped like a shell. sounds.

Monaural and Binaural hearing
• Monaural‐ Hearing with one ear
• Binaural‐Hearing with both the ears.

In open air as well as in enclosures, the hearing is direct and
binaural (i.e.with both the ears). This helps to localize sound and
obtain a correct idea of the sound perspective.

Binaural hearing could be easily compared to binocular vision. Just
as an observer gets an impression of direction and distance or depth
with two eyes, similarly his two ears enable him to appreciate the
direction of sound and to a certain extent, the distance of sound as
well.

It is quite easy to imagine how the two ears enable him to see the
direction of sound which is primarily due to the difference in the
intensity of the two sounds reaching the two ears It is the depth or
ears.
the perspective of sound.

It may be observed that the reflected sound varies much weakened
in intensity as the recede from the speaker Consequently the ratio of
speaker.
direct to the reflected sound varies considerably.

It is for this reason that as we go farther from the speaker the direct
sound is weakened while the reflected or reverberant sound
becomes more and more noticeable. It is this factor which permits
binaural hearing to appreciate distance or depth.

 It is not difficult to appreciate how monaural hearing causes loss of
direction. It is common experience to observe that the two ears
enable the person consciously to suppress sounds coming from one
direction and to concentrate on desired sound from a given direction.

Single ear is unable to do this and consequently the noises and the
reverberation present in the room or apparently increased.


Behavior of sound in enclosed places


1. Sound is absorbed in the 5. Sound is emitted by the
y
air also appreciable to resonance of the wall in
reflected sound. both directions.
2. Sound in reflected at the
2 S di fl t d t th 6. Sound is inter‐reflected
6 S di i t fl t d
wall surface. between bounding surface
3. Sound is absorbed from setting at reverberation.
the wall surface or its 7. Resonance of the enclosed
surface finished. volume of air by direct cross
4. Sound is conducted by the
4 S di d db h reflection.
reflection
wall to other part of the
surface.


CHARACTERISTICS OF SPEECH

• Any audible sound has three important
characteristics
1. Frequency(Pitch)
2.
2 Loudness
3. Tonal quality


DEFINITIONS
• What is Frequency?
q y
• The number of cycles per unit of time is called the frequency. For
convenience, frequency is most often measured in cycles per
second ( ) or th i t h
d (cps) the interchangeable H t (H ) (60 cps = 60 H )
bl Hertz (Hz) Hz),
named after the 19th physicist.


Characteristics of speech
Frequency(Pitch)
 It is independent of intensity of loudness of sound.
 Pitch is a psychological phenomenon determined by
frequency of a sound wave whereas frequency is a
physical quantity and can be measured .
 Sound is called a pure tone when it consists a single
frequency and when two or more frequencies are
present it is called a complex tone.

Pure tone complex tone

Loudness:
– Loudness is a physiological effect of sensation produced
through the ear and depends on the intensity of sound
or amount of energy present in sound waves while
or amount of energy present in sound waves while
entering it.

– In oat , Loudness falls down as distance increases
“Loudness is inversely proportional the the square of the
distance from source “(Not applicable for semi closed
enclosures)


Tonal quality or timbre

Timbre is a general term for the distinguishable
characteristics of a tone. Timbre is mainly determined
by the harmonic content of a sound.

 timbre is what makes a particular musical sound
different from another, even when they have the
same pitch and loudness.


DEFINITIONS
• What is wavelength?
g
• The wavelength of sound is the
distance between analogous
p
points of two successive waves.

λ = c / f
where
λ = wavelength (m)
c = speed of sound (m/s)
f = frequency


• Sound Intensity.
– The sound intensity in a specified direction is the
DEFINITIONS
amount of sound energy flowing through a unit
area normal to that direction. The sound intensity
is normally measured in watt per square metre
is normally measured in watt per square metre
(W/m2).

– The scale for measuring intensity is the decibel
scale.


DEFINITIONS
• Sound pressure
p
Sound pressure or acoustic pressure is
the local pressure deviation from the ambient
(average, or equilibrium) atmospheric pressure
(average or equilibrium) atmospheric pressure
caused by a sound wave.
Sound pressure can be measured using
Sound pressure can be measured using
a microphone in air and a hydrophone in water.
The SI unit for sound pressure p is the Pascal.


HIGH FIDELITY REPRODUCTION OF MUSIC

High fidelity—or hi‐fi—reproduction is a term
used by home stereo listeners and h
db h li d home
audio enthusiasts (audiophiles) to refer to high‐
quality reproduction of sound [ or i
li d i f d images, to
distinguish it from the poorer quality sound
produced by inexpensive audio equipment.
d db i i di i

Ideally, high‐fidelity equipment has minimal
Id ll hi h fid li i h i i l
amounts of noise and distortion and an
accurate f
t frequency response.

Frequency Ranges
Frequency Ranges


THRESHOLD OF AUDIBILITY
The threshold of audibility at any specified frequency is
the minimum value of sound pressure of a pure tone of
that frequency which is just audible The term is used for
complex waves such as speech and music . It is
measures in dynes per sq cm ( dynes – cgs‐force)
cm.

THRESHOLD OF FEELING
Sound pressure that can cause discomfort and pain. It is
situated around 120 dB above the threshold of hearing. It is
measures in dynes per sq cm and also in watts per sq cm.


PHON
A unit of apparent loudness equal in number to the
loudness,
intensity in decibels of a 1,000‐hertz tone perceived to
be as loud as the sound being measured.

EFFECT OF REVEBRATION ON HEARING
The hang over effect of one syllable on the next
g y
distorts the speech by blending the end of the first
with the beginning of the second so as to mask the
second syllable completely. Th
d ll bl l t l The reverberation
b ti
characteristics may give an effect of frequency
distortion because of reverberation time being
different for various frequencies. The effect of both will
depend upon position of the observer .


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