Name__________________________________Block_____Date____________________
Ch 26 Sound Notes
Mrs. Peck
Objectives:
1. Relate the pitch of a sound to its frequency 26.1
2. Describe the movement of sound through air 26.2
3. Compare the transmission of sound through air with that through solids, liquids, and a vacuum
4. Describe factors that affect the speed of sound 26.4
5. Describe loudness and sound intensity 26.5.
6. Give examples of forced vibration 26.6
7. Describe natural frequency 26.7
8. Describe resonance 26.7
9. Describe how sound waves interfere with one another 26.9
10. Describe beats 26.10
The Origin of Sound 26.1
sound is only thing that ear can hear
*source of sound is a vibrating object...
...............wh/ vibrates something larger...
...........................wh/ sends disturbance thru medium (usually air) in form of longitudinal waves
*frequency of vibrating source equals the frequency of sound waves produced
ex: vibrating vocal cords....vibrates air in throat & mouth of singer
pitch- how high or low frequency appears to be
high pitched sounds.....have high vibrational frequency
low pitched sounds.......have low vibrational frequency
Hearing Range: young person
older person
20 - 20,000 Hz
range shrinks especially at high-frequency end
infrasonic- sound with pitch too low to be heard by human ear
below 20Hz
<20 Hz
ultrasonic- normal upper limit of human hearing sounds w/ frequency above >20,000 Hz
Sound In Air 26.2
compression- a pulse of compressed air (or other matter); opposite of rarefaction
rarefaction- a disturbance in air (or matter) in wh the pressure is lowered; opp of compression
see fig. 26.3 , 26.4, 26.5
* sound- longitudinal wave particles move back & forth
in direction of motion of wave
for all wave motion: it is not the medium that travels across room, but a pulse that
travels
tuning fork: sets off series of periodic compressions & rarefactions (longitudinal waves),
higher in freq. & lower in amplitude
Media That Transmit Sound 26.3
2
sound needs a medium to travel thru....no media (vacuum) no sound
fig. 26.6
nothing to compress and expand
may still have vibrating obj. but no medium..no sound
speed of sound differs in diff. media
vsound faster in liquids than gases, and still faster in solids
vsound
solids > liquids > air
Speed Of Sound 26.4
sound is slower than speed of light
increase temperature....increase speed of sound
speed of sound depends on 1. medium
2. temperature
3. elasticity
speed of sound depends on substances elasticity:
elasticity-ability of mat to ∆ shape in response to force & then resume initial shape
once distorting force is removed
elastic mat. the atoms are close tog. and respond quickly to each oth motions, transmitting E w/ little
loss.
Loudness 26.5
can measure intensity of sound but not loudness
if increase amplitude of sound wave then increase intensity
intensity measured in decibels dB
loudness is a physiological sensation sensed in the brain
loudness differs for diff. people
Forced Vibration 26.6
fig. 26.8
forced vibration- vibration of obj made to vibrate by another vibrating obj that is nearby
many musical instruments have sounding boards which encounter forced vibration......
therefore amplifying sound thru force vibration
ex: vibration of guitar strings use wooden body as sounding board. wooden body
encounters forced vibration from guitar strings therefore amplifying sound
3
Natural Frequency 26.7
fig. 26.9
natural frequency- freq at wh/ an elastic obj. once energized will vibrate, a minimum amt of
E is needed to continue vibrating at that freq.
object’s natural frequency depends on its elasticity and shape
nat. freq is one at wh/ minimum energy is required to produce forced vibrations
nat. freq requires least amt of energy to continue vibration
Resonance 26.8
resonance- occurs when frequency of forced vibration matches object’s natural frequency
therefore increases amplitude
object must be elastic to resonate
in order for something to resonant, it needs a force to pull it back to its starting position and enough
energy to keep it vibrating
fig 26.11
When a tuning fork is struck, it sets another tuning fork into vibration. They have the same natural
frequency.......resonance occurs.
The first fork is struck (not in image) & sends out a sound wave (longitudinal wave...a series of
compressions & rarefactions)
a.
b.
c.
d.
e.
a compression meets the fork and gives it a tiny & momentary push
the fork bends and then returns to initial position
(returns to initial position) just at the time a rarefaction arrives
the fork keeps moving back and overshoots in the opposite direction (bcs of rarefaction)
just when it returns back to its initial position, the next compression arrives to repeat the cycle
now the tuning fork bends farther because it is already moving
Since the frequency of these pushes corresponds to the natural frequency of the fork, the pushes
successively increase the amplitude of vibration. This is because the pushes occur at the right time
and are repeatedly in the same direction as the instantaneous motion of the fork.
eg. 1. when you adjust the tuner on a radio set, you are adjusting the natural frequency of the
electronics in the set to match one of the many incoming signals...the set resonates to one
station at a time, instead of playing all of the stations resonance occurs whenever
successive impulses are applied to a vibrating object in rhythm with its natural frequency
2. the loose front end of a car that vibrates at only certain speeds
3. a crystal glass that shatters by a singer’s voice
4. when troops are marching on a bridge and the marching is the same frequency as the natural
frequency of a bridge, steadily the bridge will sway back and forth increasing in amplitude
Interference 26.9
4
sound waves (like any waves) can interfere with each other.......constructively or destructively
fig. 26.13 Comparison of interference of transverse waves and longitudinal waves
crests of transverse waves correspond with compressions of sound wave
troughs of transverse waves correspond with rarefactions of sound wave
***interference effects intensity...”loudness” of sounds
Fig 26.14 interference of sound waves
constructive interference:
increases loudness
if sit equidistant between two speakers emitting simultaneous identical sound waves
of constant frequency.. then the waves are in phase and have constructive interference
(compressions and rarefactions arrive in phase or in step)...so sound is louder bcs the waves add
up.....increase amplitude...increase in intensity or “loudness”
destructive interference:
decreases loudness
if sit to side (not equidistant from speakers) so that the paths from the speakers differ by
a half-wavelength....then the waves are out of phase & have destructive interference
(compressions and rarefactions arrive out of phase or out of step....compression from one speaker arrives
at same time as a rarefaction of other speaker......similar to a crest and trough at same time)...so sound is
softer bcs the waves cancel each other...decrease amplitude....decrease intensity
or “loudness”
only decrease in loudness not complete silence for there is always some reflection
of sound to fill in canceled spots
in some gyms or auditoriums get some “dead zones” where get interference from
sound from source and reflected waves off of walls....just move head to one side
to get out of destructive interference pattern area
COOL APPLICATION OF SOUND DESTRUCTIVE INTERFERENCE
Antinoise technology: uses destructive interference to decrease loudness of noisy
devices such as jackhammers, planes, engines ect. Antinoise devices have a
microphone that captures incoming sound...creates a mirror image (that is out of
phase with incoming sound)...then this mirror image signal is sent to headphones
that are worn by operator. The compressions of actual sound are canceled by the
rarefactions of mirror image sound fed into headphones.....decreasing loudness of
machine’s sound.....cool
Beats 26.10
5
interference of 2 slightly different frequencies causes beats
beats: fluctuation in loudness of combined sounds....loud...soft....loud....soft....loud...
beats: periodic variation in loudness of sound
beats- a throbbing variation in the loudness of sound caused by interference when two
tones of slightly different frequencies are sounded together
fig. 26.16 creating of beats
the 2 sound waves have slightly different frequencies
-periodically the compressions of both will be in phase (increase in loudness)
-periodically the rarefaction of one will interfere with the compression of the
other..out of phase....so (decrease in loudness
fig. 26.17 sine curves from an oscilloscope illustrating beats
first 2 sine curves are the 2 sound waves of slightly different frequencies 10Hz & 12Hz
last sine curve is the composite sine curve of the 2 waves above superimposed....
which illustrates beats being formed
COOL APPLICATION OF BEATS:
musicians use beats to tune instruments.....use a tuning fork or oboe to produce a
sound of a certain frequency...then instrument plays that same frequency too. If the
instrument is slightly out of tune (slightly different frequency) you will hear beats
produced. Keep tuning instrument until hear single tone & no beats.
determining the beat frequency & tone heard during interference of 2 slightly different frequencies
to determine the beat frequency:
subtract the smaller frequency from the larger frequency....this is the frequency
of beats that will be heard (number of beats per second)
eg. 262 Hz and 264 Hz
264 Hz - 262 Hz = 2 Hz (2 beats per sec)
to determine the tone of the beats heard:
the tone will be the frequency half way between the 2 frequencies
find the avg. frequency of the 2 frequencies
eg. 262Hz and 266 Hz
(262 Hz + 266 Hz) divided by 2 = 264Hz
beats occur with any kind of wave and are a practical way to compare frequencies