Chapter 2 Impulsive Sounds, Alone and in Sequence.
Transcript of Chapter 2 Impulsive Sounds, Alone and in Sequence.
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Chapter 2
Impulsive Sounds, Alone and in Sequence
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Picture of a Transverse Wavel
WavelengthWavelength
AmplitudeAmplitude
AA
CrestCrest
TroughTrough
WAVE DESCRIPTION
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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.
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Sound is a Longitudinal Wave
The tuning fork moves air molecules back and forth in the direction the sound wave travels
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- 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 ()
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• Time required to make one vibration.
• Time required to generate one wave.
• Time required for the wave to travel one wavelength.
Period ()
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The number of vibrations per unit of time made by the vibrating source.
Units - cycles per second
1/s
Hertz (Hz)
Frequency (f)
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• 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
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T 1T
f
T
f
T
fT
f Tf fTf
In symbolic form
oror
f
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• 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.
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A Selection of Natural Frequencies
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• 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
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• 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
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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.
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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
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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
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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.
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Electronic Experiment
FrequencyGenerator
DelayD
Shaper2
Shaper1
SumAudioAmpl.