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SOUND WAVES
Sound Waves
Speed of Sound • Depends on the material of the vibrating medium
• Sound can vibrate water, wood (speaker enclosures,
pianos), metal, plastic, etc.
• Sound speed in dry air is 330 meters/second at 0o C
• Faster in warm air, slower in cold
• Water 4 times faster, steel 15 times faster
Speed of Sound
• Medium velocity m/sec
air (20 C) 343
air (0 C) 331
water (25 C) 1493
sea water 1533
diamond 12000
iron 5130
copper 3560
glass 5640
II. PROPERTIES OF SOUND
Pitch - description of how high or low the
sound seems to a person - The property of sound that varies with
variation in the frequency of vibration
Loudness - how loud or soft a sound is perceived to be.
- how much energy a sound wave carries
High frequency means more vibrations
hitting the ear.
Pitch is how high or how low a sound
seems to be.
Healthy humans can hear from 20 Hz to
20,000 Hz
We are most sensitive from 440 Hz to
7,000 Hz.
Frequency and pitch
Sound Waves
Molecules in the air vibrate about some average position creating the compressions and rarefactions. We call the frequency of
sound the pitch.
Loudness of Sound in Decibels
SOUND LOUDNESS (dbs) HEARING DAMAGE
Average Home 40-50
Loud Music 90-100 After long exposure
Rock Concert 115-120 Progressive
Jet Engine 120-170 Pain
Speed of sound
1) 332 m/s in air at 0 C.
2) Changes by 0.6 m/s for every
Celsius degree from 0 C.
3) Subsonic – slower
4) Supersonic – faster than sound
(Mach 1 = speed of sound)
5) Sonic boom (pressure cone)
The ratio of the speed of a moving
body to the speed of sound
INFRASOUND - sounds with frequencies below the
normal human range of hearing. Sounds in the 20-200 Hz range
they are felt rather than heard
(earthquakes, heavy machinery).
ULTRASOUND - sound waves with frequencies above the
normal human range of hearing. Sounds in the range from 20-100kHz
Ultrasonic sound has a frequency
greater than 20,000 Hz.
a) Dogs (up to 35,000 Hz)
b) Bats (over 100,000 Hz)
c) Medical diagnosis
Supersonic Flight SUPERSONIC
(of speed) greater than the speed of sound in a given medium (especially air)
Having frequencies above those of audible sound
An explosive sound caused by the shock wave of an
aeroplane travelling faster than the speed of sound
SONIC BOOM
Interference • the result of two or more sound
• waves overlapping
Different sounds that you hear include (A) noise, (B) pure tones, and (C) musical notes.
Standing waves in these open tubes have an antinode
at the open end, where air is free to vibrate.
Doppler Effect
- is the apparent change in the frequency of a sound caused by the motion of either the listener or the
source of the sound.
Sounds from Moving Sources
• A moving source of sound or a moving observer experiences an apparent shift of frequency called the Doppler Effect.
• If the source is moving as fast or faster than the speed of sound, the sound waves pile up into a shock wave called a sonic boom.
• A sonic boom sounds very much like the pressure wave from an explosion
Doppler Effect 4 cases
• Source moving toward receiver
• Source moving away from receiver
• Receiver (observer) moving towards source
• Receiver (observer) moving away from source.
Wave Reflection
• When a sound wave reflects from a
surface we generate an echo
• Wave reflection from surfaces depends on
the characteristics of the surface
• Smooth hard surfaces reflect best
• Rough soft surfaces reflect poorly
• Energy not reflected is absorbed or
transmitted through the material
Wave Reflection
• Think of arrows pointing in the direction of the
wave motion
• We can trace the path of these arrows
Angles Equal
Wave Reflection
Acoustics of room
design is very
interesting. Need some
reflections to “liven” the
room. Too many
reflections and the sound
gets mushy. Look in a
concert hall or
auditorium to see the
different sound
treatments
Wave Refraction • If there is a change in the characteristics of a
medium, waves are bent
• This occurs because different parts of the wave
front travel at different speeds
• Think of a marching around a curved track
• The inside people have to move more slowly
than the outside people to keep the lines
straight
PROPERTIES OF SOUND WAVE
Reflection
ECHO – reflected sound wave
REVERBERATION – Multiple echo
Wave Refraction
Wave Reflection & Refraction
• The combination of reflection and
refraction enables imaging
• Ultrasonic medical imaging
• Naval SONAR for detecting submarines
• Bats catch mosquitoes
Diffraction of Sound
the bending of waves around small*
obstacles and the spreading out of
waves beyond small* openings.
* small compared to the wavelength
Natural Frequencies
• Objects have “natural” frequencies based
on their size and structure
• Guitar strings are an example
• Timpani heads
• Air columns
Forced Vibrations
• Can externally impose a vibration on an object
• Guitars and violins and pianos
• Set the wood into motion at the frequency
of the string
• This provides a larger surface to interact with the air
• Harp vs. Piano
Resonance • When the forced vibration matches a natural
frequency we get a “resonance” condition
• Think about a swing on a playground
• You go high when you pump the swing at its
natural vibration frequency
• Sympathetic vibrations in tuning forks
• Famous Tacoma Narrows bridge collapse
Resonance - the ability of an
object to vibrate by absorbing
energy at its natural frequency.
5. Uses of sound
a. Acoustics – the study of sound.
Soft materials dampen sound; hard
materials reflect it (echoes and
reverberations).
b. SONAR – Sound Navigation and
Ranging (echolocation).
c. Ultrasound imaging
d. Kidney stones & gallstones.