Defining Sound & Time Domain Representation

cypriotcamelUrban and Civil

Nov 29, 2013 (3 years and 10 months ago)

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Acoustics


Acoustics is the science concerned with the
production, control, transmission,
reception and effects of sound.

A Definition of Sound


‘The sensation produced through the ear, what is
or may be heard; vibrations causing this
sensation;
musical

~ (produced by continuous &
regular vibrations, opp. to
noise
)…’













Oxford English Dictionary

Musical Sound


Source
-

creates sound


Medium
-

transfers sound


Receptor
-

receives sound

Source


Excitation Mechanism
-

energy supply


Vibrating Element
-

tone characteristics


Resonator
-

amplifies / modifies sound

Class

Example

Excitation
Mech

Vibrating
Element

Resonator

Stringed

violin,
guitar

bow, pluck

string

sound
box/board

Woodwind

clarinet, sax

blowing,
reed

air column

pipe & bell

Brass

trumpet

blowing,
lips

air column

pipe & bell

Percussion

drum,
piano

stick,
hammer

membrane,
string

cavity/solid
material/tu
be

Vocal

singer

larynx
muscle

air
column/tiss
ues

tissues &
mouth

Electronic

synth,
sampler

electric
current

oscillator,
waveform

fx, amp.,
speaker

Medium


Medium Proper
-

sound propagation (e.g.
air, water, wood, steel)


Boundaries
-

reflection, absorption,
reverberation (e.g. walls, ceiling)


Receptor


Eardrum
-

conversion into mechanical
oscillations


Inner Ear
-

primary frequency sorting,
conversion into nerve impulses


Nervous System
-

processing, display
identification, storage and transfer to other
brain centres

Ear Anatomy

Mechanism of a Wave


Vibrations must be converted into pressure
oscillations before they can be heard


Sound is transmitted through compressed air

Mechanism of a Wave


The molecules that make up air can be
compressed together


Sound pressure waves are longitudinal waves
of compressed air


piston

air

compressed air

direction of piston

air that is now
compressed

air that used to be
compressed

direction of disturbance

It’s not wind!

It’s a compression easing along.

Like a line of dominoes falling down.

None of the dominoes move very much.

They just hit the one in front.

A Musical Sound Wave is a Periodic
Disturbance

A tuning fork is attached to the end of the piston. This
creates a periodic (regular) disturbance.

Just as
compressed
molecules are crowded and so hit
those around them more than normal,
rarefied
molecules have lots of space and so don’t hit the
molecules around them as much as air at normal
pressure.

Tuning Fork Pushes
& Pulls
Piston

Air molecules tend to move into a low pressure region,
creating a new low pressure region further down the
tube.

Tuning Fork Pushes
& Pulls
Piston

Periodic pushing & pulling creates regular alternate
waves of high and low pressure.

Tuning Fork Pushes
& Pulls
Piston

pressure

cm

Compression = peak


Rarefaction = trough

Time Domain Representation

Distance Can Be Seen As Time Here


Tube = 100cm


Say sound travels 100cm/sec


A compression will reach the end of the tube
after 1 second


It will get halfway along the tube in half a
second, etc.

100cm

pressure

seconds

It is possible to an measure waves in terms of time as
opposed to distance.

Amplitude

amplitude

amplitude

seconds

pressure

The greater the pressure variations above and below
atmospheric pressure, the greater the amplitude of the
wave.

Wavelength

cm

pressure





The distance travelled by one cycle of a waveform.

Period

seconds

pressure





The length of time it takes for one cycle of a waveform to
pass through a given point.

Frequency


The number of cycles that move through a
given point per second






f = 1 /


Frequency Cont...



If




= ½
s



Then

f = 1/0.5s = 2Hz

Frequency Cont...


If




= 80m
s (1ms = 1/1000
th

of a sec)



Then

f = 1/0.08s = 12.5Hz

Frequency Cont...


If it takes 50ms for a wave to repeat (


=
50ms or 1/20
th

of a second)


Then f = 1/50ms = 1/0.05secs = 20Hz


The higher the fundamental waveform
frequency the higher the pitch

Phase

A sine wave is 90 º out of phase with a cosine
wave .

0

1


-

1


sine wave

0

1


-

1


cosine wave