Sound Waves
Three aspects of sound
§  Sound source
Sound is produced by a vibrating object
§  Medium
Transfer of energy through a medium via longitudinal
waves
§  Detection
...by an ear or instrument (microphone, etc)
Medium is required
Sound waves=longitudinal
Sound waves require a medium
to transfer the energy.
“In space no one can hear you
scream.”
Keep in mind that although we sometimes draw sound waves
as transverse, they’re really longitudinal. Air molecules that
have been pushed closer to other are in the compression zone,
while the air molecules that are more widely spaced are in
the rarefaction zone. http://illuminations.nctm.org/ActivityDetail.aspx?id=37
Speed of sound
Pitch? Volume?
...depends on the mechanical characteristics of the medium.
Frequency of sound waves is often called pitch.
Amplitude of sound waves is often called volume.
The Frequency Response of the human ear is ~20-20,000 Hz.
In air:
v ≈ (331.4 + 0.60T ) m/s
>20,000 Hz (20kHz) = ultrasonic frequencies
<20 Hz = infrasonic frequencies (eq, thunder, volcanoes, heavy
machinery)
Dogs can hear up to 50,000 Hz
Bats can hear up to 100,000 Hz
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Calculating sound intensities
“Loudness” of a sound wave is simply a measure of its
intensity = how much power the wave has per unit area.
Not done
Intensity... loudness?
E /t P
= = I (W/m2 )
A
A
The human ear can generally detect sounds as quiet as 10-12
W/m2, and as loud as 1 W/m2. What we perceive as loudness,
however, does not vary directly with intensity. Sounds that we
€ as being approximately twice as loud are actually 10
perceive
times as intense.
Sound intensities
0 dB
10 dB
20 dB
40 dB
50-60 dB
70 dB
90 dB
Threshhold of human hearing
Leaves rustling
Whisper
Quiet radio
Normal conversation
Vacuum cleaner, street traffic
Limit of intensity w/no long-term
damage to hearing
120 dB
Siren; loud concert
130-140 dB
Gunshot
150 dB
Jet airplane
Standing Waves
When a wave is reflected from
a boundary and interferes with
itself, a standing wave is
produced.
Standing waves have nodes and
antinodes. By carefully examining
the placement of these features,
we can determine various
qualities of the wave.
Resonance
Resonance
Objects composed of materials
that have even a little elasticity
will vibrate with a natural
frequency if that material is
disturbed. Bells, tuning forks,
guitar strings, diving boards... all
vibrate with their own
characteristic, resonant,
frequencies. When the frequency of a forced
vibration (due to an external force)
matches an object’s natural
frequency, an increase in amplitude
occurs.
What are some common examples of
resonance?
•  Pushing a kid on a swing
•  Rubbing finger on a wine glass
•  Blowing air across the top of a
bottle
•  Plucking a guitar string
•  Rubbing a bow across a violin string
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