Download 15 SOUND

The Nature of Sound
What causes sound?
All sounds are created by something that vibrates.
Sound Waves
How does the sound made by a vibrating speaker get to your ears?
When an object like a radio speaker vibrates, it collides
with nearby molecules in the air, transferring some of its
energy to them. These molecules then collide with other
molecules in the air and pass the energy on to them. The
energy originally transferred by the vibrating object
continues to pass from one molecule to another. This
process of collisions and energy transfer forms a sound
wave. Eventually, the wave reaches your ears and you
hear a sound.
Sound Is a Compressional Wave
NYE WAVES.wmv
Sound Is a Compressional Wave
NYE WAVES.wmv
How do we make sounds?
Larynx or Voice Box
Moving Through Mediums
Most sounds you hear travel through air to reach your
ears. In fact, sound waves can travel through any type of
matter— solid, liquid, or gas. The matter that a wave
travels through is called a medium.
What would happen if no matter existed to form a
medium?
What would happen if no matter existed to form a
medium?
The Speed of Sound Through Different Mediums
The speed of a sound wave through a medium
depends on the substance the medium is made
of and whether it is solid, liquid, or gas.
In general, sound travels the slowest through
gases, faster through liquids, and even faster
through solids.
Sound travels faster in liquids and solids than in
gases because the individual molecules in a
liquid or solid are closer together than the
molecules in a gas.
Temperature and the Speed of Sound
The speed of sound waves also depends on the
temperature of a medium.
As the temperature of a substance increases, its
molecules move faster. This makes them more likely to
collide with each other. Then sound waves move faster as
the temperature increases.
Human Hearing
Human Hearing
The visible part of
your ear, the ear
canal, and the
eardrum make up the
outer ear. The outer
ear gathers the sound
waves.
The eardrum is a tough
membrane about 0.1mm
thick. When incoming sound
waves reach the eardrum,
they transfer their energy to
it and it vibrates.
The Middle Ear Amplifies
Sound Waves—When the
eardrum vibrates, it passes
the sound vibrations into the
middle ear, where three tiny
bones start to vibrate. These
bones are called the hammer,
the anvil, and the stirrup.
The bones amplify the sound
wave.
The Inner Ear
The inner ear contains
the cochlea (KOH klee
uh), which is a spiralshaped structure that
is filled with liquid and
contains tiny hair cells
It is the cochlea that
converts sound waves to
nerve impulses.
Hair Cells inside
the Cochlea
When these tiny hair cells in the cochlea begin to
vibrate, nerve impulses are sent through the auditory nerve
to the brain. It is the cochlea that converts sound waves to
nerve impulses.
Properties of Sound
Recall that the amount of energy a wave carries
corresponds to its amplitude. For a compressional wave,
amplitude is related to the density of the particles in the
compressions and rarefactions.
Intensity is the amount of energy that flows through a
certain area in a specific amount of time. When you turn
down the volume of your radio, you reduce the energy
carried by the sound waves, so you also reduce their
intensity.
How would the
intensity change
if the loop were
10 m away from
the radio?
When you hear different sounds, you do not need special
equipment to know which sounds have greater intensity.
Your ears and brain can tell the difference. Loudness is the
human perception of sound intensity.
Perception – understanding / interpreting what we are
sensing with our senses
Sound waves with high intensity carry more energy. When sound
waves of high intensity reach your ear, they cause your eardrum to
move back and forth a greater distance than sound waves of low
intensity do.
HOW LOUD IS LOUD?
It’s hard to say how loud too loud is. Two people are
unlikely to agree on what is too loud, because people
vary in their perception of loudness.
We need a scale to measure loudness. We measure
loudness with the decibel scale. Each unit on the scale for
sound intensity is called a decibel (DES uh bel),
abbreviated dB.
On this scale, the faintest sound that most people can hear is
0 dB. Sounds with intensity levels above 120 dB may cause
pain and permanent hearing loss.
Frequency and Pitch
Pitch
Frequency and Pitch
Frequency is a measure of how many wavelengths pass a
particular point each second. For a compressional wave,
such as sound, the frequency is the number of
compressions or the number of rarefactions that pass by
each second. Frequency is measured in hertz (Hz)—1 Hz
means that one wavelength passes by in 1 s.
Ultrasonic and Infrasonic Waves
Most people can’t hear sound frequencies above 20,000
Hz, which are called ultrasonic waves. Some animals
such as dogs and bats can hear ultrasonic.
Infrasonic, or subsonic, waves have frequencies below 20
Hz—too low for most people to hear.
Wind, earthquakes, and elephants make infrasonic
sounds.
The Doppler Effect
The change in pitch or wave frequency due to a moving
wave source is called the Doppler effect.
The Doppler Effect
The change in pitch or wave frequency due to a moving
wave source is called the Doppler effect.
The Doppler effect happens any time the source of a
sound is changing position compared with the observer. It
occurs no matter whether it is the sound source or the
observer that is moving.
Resonance in Air Columns
If you have ever used just the mouthpiece of a brass or
reed instrument, you know that the vibration of your lips
or the reed alone does not make a sound with any
particular pitch. The long tube that makes up the
instrument must be attached if music is to result.
When the instrument is played, the air within this tube
vibrates at the same frequency, or in resonance, with a
particular vibration of the lips or reed.
A resonating tube with one end closed is called a closedpipe resonator.
An open-pipe resonator is a resonating tube with both
ends open that also will resonate with a sound source. In
this case, the sound wave does not reflect off a closed
end, but rather off an open end.
HOW LONG MUST A PIPE BE TO RESONATE
AT A CERTAIN FREQUENCY ???
CLOSED PIPE - The
shortest column of
air that can have an
antinode at the
closed end and a
node at the open
end is one-fourth
wavelength long
L =
/ 4
F1 is the
FUNDAMENTAL
FREQUENCY
As the frequency is
increased, additional
resonance lengths are
found at halfwavelength
intervals.
Thus, columns of
length /4, 3 /4,
5 /4, and so on will
all be in resonance
with a tuning fork.
Resonance frequencies in an open
pipe
The shortest
column of
air that can have
nodes at both
ends is one-half
wavelength long
L=
/2
As the frequency is
increased, additional
resonance lengths
are found at halfwavelength
intervals. Thus
columns of
Length
/2,
,
3 /2, and so on will
be in resonance with
a tuning fork.
Sound Quality
A tuning fork produces a soft and uninteresting sound.
That’s because its tines vibrate like simple harmonic
oscillators, producing the simple sine.
Sounds made by the human voice and musical instruments
are much more complex, like the wave in Figure 15–15b.
Both waves have the same frequency or pitch, but they
sound very different. In musical terms, the difference
between the two waves is called timbre, tone color, or tone
quality.
CLARINET SOUND WAVE
The sound spectrum: fundamental and harmonics The
complex sound wave in Figure 15–15b was made by a
clarinet. The air column in a clarinet acts as a closed pipe.
Thus, columns of length /4, 3 /4 , 5 /4, and so
on will all be in resonance with a tuning fork.
f1 =
/4
3 * f1
5 * f1
is the FUNDAMENTAL FREQUENCY
is the 1st HARMONIC
is the 2nd HARMONIC
FOR AN OPEN PIPE RESONATOR, THE HARMONICS ARE
Thus, columns of length /2,
,
3 /2, and so
on will all be in resonance with a tuning fork.
f1 =
/2
2 * f1
3 * f1
4 * f1
is the
is the
is the
is the
FUNDAMENTAL FREQUENCY
1st HARMONIC
2nd HARMONIC
3RD HARMONIC
HARMONICS OF INSTRUMENTS
Musical intervals Two notes with frequencies related by
the ratio 1:2 are said to differ by an octave.
For example, if a note has a frequency of 440 Hz, a note
an octave higher has a frequency of 880 Hz, the next
octave higher is 1760 or (2n * 440). A note one octave
lower has a frequency of 220 Hz.
For example, if one note has a frequency of 400 Hz, the note an octave above it is at 800
Hz, and the note an octave below is at 200 Hz. The ratio of frequencies of two notes an
octave apart is therefore 2:1. Further octaves of a note occur at 2n times the frequency of
that note (where n is an integer), such as 2, 4, 8, 16, etc. and the reciprocal of that series.
For example, 50 Hz and 400 Hz are one and two octaves away from 100 Hz because they
are ½ (or 2 −1) and 4 (or 22) times the frequency, respectively. However, 300 Hz is not a
whole number octave above 100 Hz, despite being a harmonic of 100 Hz.
Beat Notes
Two frequencies that are nearly identical interfere to
produce high and low sound levels, as illustrated in
Figure 15–18. This oscillation of wave amplitude is called
a beat. The frequency of a beat is the magnitude
of difference between the frequencies of the two waves,
fbeat = |fA - fB|.