a range of compression wave frequencies to which the human ear is sensitive
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a range of a range of compressioncompression wave wavefrequencies to which thefrequencies to which thehuman earhuman ear is sensitive is sensitive
Sounds are produced bySounds are produced byvibrating mattervibrating matter
1. reedsreeds
2. stringsstrings
3. membranesmembranes
4. air columnsair columns
Sound is a Sound is a mechanical wavemechanical wave (longitudinal). (longitudinal). It will It will notnot travel through a vacuum. travel through a vacuum.
• PITCH = The impression about the frequency of Sound
• high pitched – high frequency (ex: piccolo)• low pitched – low frequency (ex: fog horn)
• The frequency range for normal human hearing is . between 20 Hz and 20,000 Hz . (for younger people, older people lose the higher frequencies)
• Infrasonic or subsonic = Sounds below 20 Hz • Ultrasonic = frequencies above 20,000 Hz
Range of Some Common SoundsRange of Some Common Sounds
Intensity Range for Some Common SoundsIntensity Range for Some Common Sounds
Compression waves travel through air or along springs.
These waves travel with
areas of compression and rarefaction.
The medium does not travel from one place to another, but the pulse that travels.
Any matter will transmit sound, whether it is a solid, liquid or a gas.
However; sound cannot travel through a vacuum.
Sound wave requires medium
DING
No Sound
VACUUM
The velocity of sound in air The velocity of sound in air dependsdepends
on the air temperature. The speed on the air temperature. The speed ofof
sound in dry air is sound in dry air is 331.5 m/s331.5 m/s at at 0 0 ººCC.. This speedThis speed
increasesincreaseswith with
temperature: temperature: about about 0.6 m/s0.6 m/sfor every 1 for every 1 ººC C increase in increase in
temperature.temperature.
Speed of Sound • The speed of sound in dry air at 00 C is about
330 m/sec. (1200 km / hr) [or ~ .000,001 x the speed of light of 300,000 km / sec]
• So air at room temperature (~20oC) is ~340 m/sec.
• QUESTION : . How far away was the strike if there is a 3 second delay between the lightning flash and the sound of the thunder?
• 340 m/sec x 3 sec = 1020 m, over 1 km (~2/3 mile) away
Sound generally travels Sound generally travels fastestfastest
in solids and slowest in in solids and slowest in gases,gases,
but there are some but there are some exceptions.exceptions.
Medium Velocity (m/s) Medium Velocity (m/s)Medium Velocity (m/s) Medium Velocity (m/s)
Air 330 Carbon dioxide 260Air 330 Carbon dioxide 260
Helium 930 Hydrogen 1270Helium 930 Hydrogen 1270
Oxygen 320 Water 1460Oxygen 320 Water 1460
Sea water 1520 Mercury 1450Sea water 1520 Mercury 1450
Glass 5500 Granite 5950Glass 5500 Granite 5950
Lead 1230 Pine wood 3320Lead 1230 Pine wood 3320
Copper 3800 Aluminium 5100Copper 3800 Aluminium 5100
• The speed of sound in a material does NOT depend on its density (mass per unit volume [g/cm3 ]).
• The speed of sound in a material DOES depend on the elasticity of a material.
• Elasticity = the ability of a material to change shape in response to an applied force, then resume its original shape when the force is removed.
• Steel is elastic, putty is inelastic.
• Sound travels 15 times faster in steel than in air and about 4 times faster in water than in air.
Speed of Sound
LOUDNESS • The intensity of a sound is proportional to the
square of the amplitude of the sound wave. (i = ka2)
• Loudness is measured in decibels (dB)
• 1 10 100 1000
• The decibel scale is logarithmic, increasing by factors of 10
VIBRATINGLOUDSPEAKER
AMPLITUDE
MICROPHONE
OSCILLOSCOPE
LOUDNESS • TABLE of Loudness Levels• SOURCE OF SOUND LEVEL (dB)• Jet Engine (from 30 m) 140 • Threshold of pain 120 • Loud rock music 115 • Old subway train 100 • Average factory 90 • Busy street traffic 70 • Normal speech 60 • (Shshshshhh!) A library 40 • Close Whisper 20 • Normal breathing 10 • Hearing threshold 0
Forced Vibration• Sounding boards are used to augment (increase) the volume
(amplitude) of a vibrating object (like a string).
STRINGS
SOUNDING BOARD
Natural Frequency
• Everything vibrates, from planets and stars to atoms and almost everything in between.
• A NATURAL FREQUENCY is one at which minimum energy is required to produce forced vibrations
• and also requires the least amount of energy to continue this vibration
Resonance
Resonance – when the frequency of a forced vibration on an object matches the object’s natural frequency, a dramatic increase in amplitude of the vibrations occurs.
• For example, a swing, or the hollow box parts of musical instruments are designed to work best with resonance.
• In order to resonate, an object must be elastic enough to return to its original position and have enough force applied to keep it moving (vibrating)
A resonant air column isA resonant air column issimply a standing simply a standing
longitudinallongitudinalwave system, much likewave system, much like
standing waves on a standing waves on a string.string. closed-pipe resonatorclosed-pipe resonator tube in which one end is tube in which one end is
openopenand the other end is closedand the other end is closed
open-pipe resonatoropen-pipe resonatortube in which both endstube in which both ends
are openare open
A A closed pipeclosed pipe resonates when resonates when the the length length
of the air columnof the air column is approximately is approximatelyan an odd numberodd number of of quarterquarter
wavelengths long.wavelengths long.
l = {(1,3,5,7,…)/4} *
With a slight correction for tube diameter,With a slight correction for tube diameter,we find that the resonant wavelength of awe find that the resonant wavelength of a
closed pipe is given by the formula:closed pipe is given by the formula:
= 4 (l + 0.4d),= 4 (l + 0.4d),
where where is the wavelength of sound, is the wavelength of sound,l is the length of the closed pipe,l is the length of the closed pipe,and d is the diameter of the pipe.and d is the diameter of the pipe.
An An open pipeopen pipe resonates when resonates when the the lengthlength
of the air columnof the air column is approximately is approximatelyan an even numbereven number of of quarterquarter
wavelengths long.wavelengths long.
l = {(2,4,6,8,…)/4} *
With a slight correction for tube diameter,With a slight correction for tube diameter,we find that the resonant wavelength of anwe find that the resonant wavelength of an
open pipe is given by the formula:open pipe is given by the formula:
= 2 (l + 0.8d),= 2 (l + 0.8d),
where where is the wavelength of sound, is the wavelength of sound,l is the length of the closed pipe,l is the length of the closed pipe,and d is the diameter of the pipe.and d is the diameter of the pipe.
Interference • Sound waves interfere with each other in the same
way as all waves.• Constructive interference - augmentation
• Destructive interference - cancellation
Beats • BEATS - A periodic variation in the loudness of
sound. . (faint then loud, faint then loud and so on … )
• What is the frequency when a 262 Hz and a 266 Hz tuning fork are sounded together ?
• The 262 Hz and 266 Hz forks will produce 4 beats per sec. and the tone heard will be half- way between at 264 Hz as the ear averages the frequencies.
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