Transcript Chapter 2 Impulsive Sounds, Alone and in Sequence

Chapter 2
Impulsive Sounds, Alone and in
Sequence
WAVE DESCRIPTION
Picture of a Transverse Wave
l
Crest
l
Wavelength
A
Amplitude
Trough
Transverse and Longitudinal
Waves
Wave Motion
Click on the phrase above and then
select either transverse or longitudinal
waves
Sound is a longitudinal wave disturbance
Wavelength is the distance between compression.
Sound is a Longitudinal Wave
The tuning fork moves air molecules back and
forth in the direction the sound wave travels
Wavelength (l)
• Distance between adjacent crests in a
transverse wave
• Distance between compressions in a
longitudinal wave
• Distance a wave travels during one
vibration
• Units - meters or feet
Period (T)
•
Time required to make one vibration.
• Time required to generate one wave.
• Time required for the wave to travel one
wavelength.
Frequency (f)
The number of vibrations per unit of time
made by the vibrating source.
Units - cycles per second
1/s
Hertz (Hz)
Examples of Frequency
• What is the frequency of the second hand of
a clock?
Frequency = 1cycle/60 sec
•
Period = 60 sec
What is the frequency of US Presidential
elections?
Frequency = 1 election/4 yrs
Period = 4 yrs
In symbolic form
or
T
T 1T
Tff  f T
T
f f ff T
NATURAL FREQUENCY
• Demo - Drop Different Sounding Objects
• Rap on Table
• Finger Snap
• Tuning Fork
• Objects have natural frequencies at which they vibrate.
• The natural frequency depends on elasticity and shape.
A Selection of Natural Frequencies
RESONANCE
• Resonance occurs when successive
impulses are applied to a vibrating object at
the object’s natural frequency.
• Result - increased amplitude
• Examples:
Swinging
Marching on a bridge
Tacoma Narrows Bridge
FORCED VIBRATIONS
• Demo - Tuning Fork Touching a Table
• Sound is intensified because of the larger
surface area that can vibrate the air.
• The surface is forced to vibrate at the
frequency of the tuning fork. (It is not a
resonance phenomenon.)
• Examples: Musical sounding boards
Equally Tempered Scale
In most acoustics texts
the octaves are
numbered from the left
side of the piano
keyboard. The note
labeled C4 is middle C.
Octaves
• If the note has the same pitch name, then
the frequency is related by a power of 2.
 C4 = 256 Hz
 C5 = 512 Hz
A4 = 440 Hz
A5 = 880 Hz
Repetition Rates
• Consider a drummer playing with his left hand (o) and his
right hand (x). He might play
-------o-------o-------o-------o-------o-------o-------o-------o
• Next he plays a flam
-------o-x-----o-x-----o-x-----o-x-----o-x-----o-x-----o-x----Or,
-------o---x----o---x----o---x----o---x----o---x----o---x----o
Repetition Rates
• We would have no trouble identifying this
as the same frequency as the original
• But put the beat of the other hand at the
exact midpoint and the listener hears twice
the repetition rate.
Electronic Experiment
Shaper
1
Frequency
Generator
Sum
Delay
D
Shaper
2
Audio
Ampl.