Chapter 13:
Sound
Camila Enríquez
Juliana Borrero
Objective:
Understand the production of sound
waves, compare the speed of sound in
different situations, and recognize the
Doppler effect.
• Relate frequency to pitch, identify
intensity, and decibel level.
• Approach the perception of loudness,
and resonance.
• Calculate harmonics and differentiate
between the harmonic series of closed
and open pipes.
•
Enduring understandings:
Sound waves are longitudinal. (sine curve)
• Frequency determines pitch
• Sound waves disperse in 3D
• Doppler Effect: relative motion creates a
change in
frequency.
• Intensity in decibels
• Length of vibrating string and vibrating air
column will affect harmonic series
frequencies.
•
The Doppler Effect
Australian
physicist Christian
Doppler
When the source that emits the
sound (this case the
ambulance) is approaching
you the wave you receive will
be in a higher frecuency
(shorter wave lenght). If the
source is leaving you, then you
will receive a lower frequency
wave (longer wavelenght.)
The ambulance is
moving toward the
woman, so the
frequency heard by
the woman is
greater than the
ambulance
frequency.
Remember: the speed of sound waves
DOES NOT change.
http://www.youtube.com/watch?v=a3RfUL
w7aAY&feature=related
Sound Intensity and Resonance

Intensity: the rate of energy flow through a unit area of
the plane wave.

In spherical waves, area = 4πr²

Intensity of sound wave decreases as distance from the
source increases.
◦ Same amount of energy spread over a larger area.
Harmonics

In the strings of stringed instruments, the ends are
not able to vibrate, but the center has a lot of
displacement.
◦ Ends are nodes, and the center is an antinode

The distance from one node to the next is always
half a wavelength.
◦ Wavelength is twice the string length.

The speed of waves is the frequency times the
wavelength.

Frequency is inversely proportional to wavelength.
◦ fundamental frequency is the lowest possible frequency of
a standing wave.

The fundamental frequency can be varied by
changing the string length.
◦ E.g. If the string is divided in half by another node, the
wavelength will be half as the one before and thus the
frequency will be doubled.

In a harmonic series, the fundamental frequency is
the first harmonic (f1), the next one is the second
harmonic (f2), and so on.
Standing waves in an air column

Standing Waves (stationary waves): On a tube/pipe some
waves travel down the tube, others are reflected back
upward. The combination of the waves traveling up and
down the tube are what we call standing waves.

If both ends of a pipe are open, all harmonics are present
◦ E.g. trumpet, saxophone, pipes of an organ.

Each end of the pipe is an antinode.

Fundamental frequency of a stringed instrument can vary
by changing the length of the vibrating air column.

If one end of a pipe is closed, only odd harmonics are
present
◦ One end is a node, the other is an antinode.
◦ The simplest standing wave is ¼ of a wavelength
◦ ¾ of a wavelength are still on the pipe.

The frequency of this harmonic is 3 times the fundamental
frequency.

Harmonics account for sound quality or timbre.
◦ Timbre (sound quality) is the characteristic sound of an
instrument, produced by a mixture of harmonics; it’s the
spectrum of the sound.

In chromatic musical scale: 12 notes, each with
different frequencies. The 13th note’s frecuency is
exactly twice of the 1st note’s (octaves, n).
Sound Barrier
The sound barrier refers to the point
in which an aircraft moves from a
transonic (a point in which air
surrounds the aircraft) to a
supersonic speed.
 Approximately velocity is 343 m/s.
 http://www.youtube.com/watch?v=wHrwg
RsX0BI


Bibliography

Holt Physics Textbook Chapter 13
Wikipedia,
http://en.wikipedia.org/wiki/Sound
 http://en.wikipedia.org/wiki/Sound_barrie
r
