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Chapter 2
Impulsive Sounds, Alone and in Sequence
Picture of a Transverse Wavel
WavelengthWavelength
AmplitudeAmplitude
AA
CrestCrest
TroughTrough
WAVE DESCRIPTION
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
- meters or feet
• Distance between adjacent crests in a transverse wave
• Distance between compressions in a longitudinal wave
• Distance a wave travels during one vibration
• Units
Wavelength ()
• Time required to make one vibration.
• Time required to generate one wave.
• Time required for the wave to travel one wavelength.
Period ()
The number of vibrations per unit of time made by the vibrating source.
Units - cycles per second
1/s
Hertz (Hz)
Frequency (f)
• What is the frequency of the second hand of a clock?
Frequency = 1cycle/60 secFrequency = 1cycle/60 sec Period = 60 secPeriod = 60 sec
• What is the frequency of US Presidential elections?
Frequency = 1 election/4 yrsFrequency = 1 election/4 yrs Period = 4 yrsPeriod = 4 yrs
Examples of Frequency
T 1T
f
T
f
T
fT
f Tf fTf
In symbolic form
oror
f
• Demo - Drop Different Sounding Objects
• Rap on Table
• Finger Snap
• Tuning Fork
NATURAL FREQUENCY
• Objects have natural frequencies at which they vibrate.
• The natural frequency depends on elasticity and shape.
A Selection of Natural Frequencies
• 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
RESONANCE
Tacoma Narrows Bridge
• 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
FORCED VIBRATIONS
Equally Tempered ScaleIn 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 A4 = 440 Hz
C5 = 512 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
FrequencyGenerator
DelayD
Shaper2
Shaper1
SumAudioAmpl.