Phys 1240: Sound and Music
LAST: wind instruments
TODAY: wind instruments (+ AM/FM)
NEXT: J. Merkel on “sound in the
studio”
Open tubes
overpressure
No new reading!
CAPA #9 due next Tuesday
Chris and David’s office hours
accordingly rescheduled:
Chris Tuesday (usual time, 2-4)
David Tuesday (usual time, 4-6)
Pressure waves “fit” in the
open tube
n (λ/2) = L
L
real tubes - end effect
overpressure
Since f λ = v,
fn= n (v/2L)
Usual harmonics same modes as a string!
L
Outer node is a bit outside tube
(about 0.3 * diameter)
1
CT 12.1.2
real tubes - shape
overpressure
If you double the diameter of a long
organ pipe, what happens to the
fundamental you hear?
A) Goes down by an octave
B) Goes down, but way less than an
octave
C) Goes up, by an octave
D) Goes up, but way less than an
octave
E) Something else
L
Higher modes reach
different distances out => not quite pure harmonics!
Closed tubes
(closed on one end)
Closed tubes
(closed on one end)
overpressure
overpressure
L
L
Closed end: antinode
Closed end: antinode
open end:node
open end:node
2
CT 12.1.3
CT 12.1.3
L
L
What is the wavelength of the
fundamental (shown above)
in a closed tube?
A) λ=L
C) λ=4L
E) λ=L/4
B) λ=2L
D) λ=L/2
Draw the next higher mode (zero
at right end, antinode at left, one
extra node in middle)
overpressure
What is the wavelength of the
fundamental (shown above)
in a closed tube?
A) λ=L
C) λ=4L
E) λ=L/4
B) λ=2L
D) λ=L/2
Draw the next higher mode (zero
at right end, antinode at left, one
extra node in middle)
overpressure
L
L
Closed end: antinode
Closed end: antinode
open end:node
open end:node
3
CT 12.1.4
Pressure waves “fit” in the
closed tube differently:
(odd n) (λ/4) = L
L
Since f λ = v,
What is the wavelength of the
standing wave (shown above)
in a closed tube?
fn= (odd n) (v/4L)
A) λ=L
B) λ=L/2
C) λ=3L/4
D) λ=4L/3
E) Something else
Lower fundamental
Missing harmonics
CT12.1.4b
CT 12.1.4
f1
6f1
C
f1
B
amplitude
A
f1
amplitude
A)The flute will sound lower
B)The clarinet will sound lower
C)They will have identical pitch
amplitude
If you have a clarinet and flute
of equal lengths, and play
the fundamental…
Which spectrum below best matches
that of a clarinet (of equal length)
playing its lowest note?
amplitude
A clarinet is basically sealed at
one end (you blow into a very
narrow hole), but open at the other
A flute is open at both ends.
A flute playing its lowest note is
shown in spectrum “A”.
6f1
6f1
D
f1
6f1
E) None of these looks right.
4
Remember the dust speck
example from last time:
overpressure
Displacement (not pressure) graphs.
Case 1: open tubes
displacement
L
N=1, the fundamental in the tube
Displacement (not pressure) graphs.
Case 1: open tubes
displacement
displacement graphs:
• Displacement is longitudinal
(despite the graph going “up”)
• Pressure nodes <=>
displacement antinodes
(and vice versa)
L
5
AM Amplitude Modulation
FMFrequency modulation
carrier
signal
CT 8.3.1
Under cover of night, a Girl scout
signals her friends on a distant hill
by alternately dimming and
brightening her flashlight. This
signal is most accurately described
as..
A: Frequency modulation.
B: Amplitude modulation.
CT 8.3.2
Musicians refer to tremolo when
the loudness fluctuates while the
pitch remains unchanged. Is this an
example of…
A: Frequency modulation.
B: Amplitude modulation.
6
CT 8.3.2b
CT 8.3.3
Musicians refer to vibrato when the
pitch is wiggled up and down as you
sing. Is this an example of…
A: Frequency modulation.
B: Amplitude modulation.
We say that "concert A" corresponds
to a frequency of 440 Hz.
Does that mean that a flute playing
concert A is emitting low frequency
electromagnetic radiation which we
"hear"?
Violinists can make “excursions”
of .2 semitones, 6 times/sec.
0.2 semitones-> strength of the
modulation
6 Hz -> “signal frequency”
440 Hz -> “carrier frequency”
A: Yes, sound is an electromagnetic
wave.
B: No, sound is not an
electromagnetic wave.
7