Transcript SPH3U - K-Moncrief

SPH3U: Waves & Sound
Musical Instruments 1
Music
** There is a scientific difference between
music and noise. **
Music

Music: sound that originates from a
source with one or more constant
frequencies

Noise: an irregular mixture of
frequencies
Music

Music consists of notes with constant
frequencies; Noise constantly changes
frequency.

If the frequency of a sound increases,
the pitch of the note increases
Music
Sound traces on an
oscilloscope.
(a) Pure sound from
a tuning fork
(b) Random noise
(c) “oooooo” sound
Music

In music, a pure tone is a sound where
only one frequency is heard. Much of
the time what we hear in music is a
combination of frequencies rather than
pure tones.
Music

A musical interval of an octave is when
one sound has a frequency double of the
other. For example a 200Hz sound is
one octave above a 100Hz sound.
Musical Instruments

Most musical instruments consist of a
vibrating source and something to
enhance the sound through resonance.
Musical Instruments

Recall from our discussion on standing
waves and resonance that musical
instruments have certain natural
frequencies that will cause resonance
within the instrument and standing
waves. It is the ability of musical
instruments to create strong standing
waves that allow them to play loud clear
pure tones of sound.
Musical Instruments

In general, large instruments produce
low frequencies and small instruments
produce high frequencies.

Musical Instruments are grouped
depending on how the vibrations are
produced.
String Instruments
guitar
piano
zither
violin
String Instruments
String instruments are made of a
vibrating string and a resonating box
 Plucking, bowing, or striking a string
causes a vibration
 The string itself does not make a loud (or
nice?) sound.

String Instruments

The frequency or pitch produced by a
vibrating string depends on 4 things:
The tension of the string
 The diameter of the string
 What the string is made out of
 The length of the string

String Instruments
Equations for these relationships can be found on pg. 281 in your text.
frequency1 length2

frequency2 length1
frequency1 diameter2

frequency2 diameter1
Tension1
frequency1

frequency 2
Tension 2
frequency1

frequency 2
Density 2
Density 1
Practice

A piano string with a pitch of A (440Hz)
is under tension of 140N. What tension
would be required to produce a high C
(523 Hz)?
Percussion Instruments
drums
Percussion instruments involve hitting
one object against the other.
Vibration occurs when the object is hit
Percussion Instruments
Some percussion instruments have “single
pitch” and only create one frequency (e.g.
the triangle)
Others might have components that will
create different pitches (e.g. the
xylophone) or have the ability to adjust to
make different pitches. Different
frequencies and pitches depend on the
natural vibrational frequency of the
material.
The Human Voice

In the human voice, air from the lungs
passes over the vocal chords. This
causes the vocal chords to vibrate. The
vocal chords are two bands of skin.
They act like a double reed.
The Human Voice

The pitch of a human voice is
determined by the muscular tension as
well as the size of the vocal chords. The
sound resonates in the pharynx, mouth,
and nasal cavity (and over the tongue
and lips).
The Human Voice

A large resonating cavity, and larger
vibrating object will cause a lower sound.
This is why men tend to have lower
voices than women, and why children
have higher voices still.
The Musical Notes and their
Fequencies
Today’s Activity
1) Reflect on today’s lesson – what pieces of information are useful to you as you
begin to design your own instrument?
2) With your group, research more about musical instruments by reading Chapter
10.2 and by visiting the following websites:

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http://www.phys.unsw.edu.au/music/
http://www.phy.mtu.edu/~suits/notefreqs.html
http://kellerphysics.com/acoustics/Lapp.pdf
http://cnx.org/content/m12413/latest/
http://www.physicsclassroom.com/class/sound/U11L5a.cfm
3) Practice:
a) A 1.0m string has a frequency of 220 Hz. If the string is shortened to 0.8m, what
will its frequency become?
b) A string under a tension of 150N has a frequency of 256 Hz. What will its
frequency become if the tension is increased to 300N?
c) A steel string of diameter 1.0mm produces a frequency of 880Hz. What is the
frequency of a steel string with the same length and tension if the diameter is 2.0mm?