Transcript sound level. - Broadneck High School

(a). Before the bell is struck, the air around it is a region of
average pressure
(b). Once the bell is struck, however, the vibrating edge creates
regions of high and low pressure. The dark areas represent regions
of higher pressure; the light areas represent regions of lower
pressure
(a). The pressure of the air rises and falls as the sound wave propagates
through the atmosphere
(b). You can use a sine curve alone to model changes in pressure.
(c). Note that the positions of x, y, and z show that the wave, not matter,
moves forward. These points remain in the same horizontal position.
Sound travels more quickly
through more dense
materials – as the energy
passes more quickly from
molecule to molecule if the
molecules are closer!
The human ear is a complex sense organ that translates sound
vibrations into nerve impulses that are sent to the brain for
interpretation. The malleus, incus, and stapes are the three bones
of the middle ear that sometimes are referred to as the hammer,
anvil, and stirrup.
Perceiving Sound
Pitch: Marin Mersenne and Galileo
first determined that the pitch we
hear depends on the frequency of
vibration.
Pitch can be given a name on the
musical scale. For instance, the middle
C note has a frequency of 262 Hz.
The ear is not equally sensitive to all
frequencies. Most people cannot hear
sounds with frequencies below 20 Hz
or above 16,000 Hz.
Older people are less sensitive to
frequencies above 10,000 Hz than are
young people.
Loudness
Frequency and wavelength are
two physical characteristics of
sound waves. Another physical
characteristic of sound waves
is amplitude. Amplitude is the
measure of the variation in
pressure along a wave. In
humans, sound is detected by
the ear and interpreted by the
brain.
The loudness of a sound, as
perceived by our sense of
hearing, depends primarily on
the amplitude of the pressure
wave.
• The human ear is extremely sensitive to pressure variations in sound
waves, which is the amplitude of the wave. Recall from Chapter 13
that 1 atmosphere of pressure equals 1.01 x 105 Pa.
• The ear can detect pressure-wave amplitudes of less than onebillionth of an atmosphere, or 2 x10-5 Pa. At the other end of the
audible range, pressure variations of approximately 20 Pa or greater
cause pain and will damage hearing.
•
It is important to remember that the ear detects only pressure
variations at certain frequencies. Driving over a mountain pass
changes the pressure on your ears by thousands of pascals, but this
change does not take place at audible frequencies.
• Because humans can detect a wide range in pressure variations, these
amplitudes are measured on a logarithmic scale called the sound level.
• The unit of measurement for sound level is the decibel (dB). The sound
level depends on the ratio of the pressure variation of a given sound
wave to the pressure variation in the most faintly heard sound, 2 x 10-5
Pa. Such an amplitude has a sound level of 0 dB.
•
A sound with a pressure amplitude ten times larger (2 x 10-4 Pa) is 20
dB. A pressure amplitude ten times larger than this is 40 dB. Most
people perceive a 10-dB increase in sound level as about twice as loud
as the original level.
The Doppler Effect
Sound waves travel at approximately 340 m/s (or about 1000 ft/s).
If the source of the sound is stationary, the waves move away from
it in all directions at this velocity.
If the source of the sound is moving, then that velocity is ADDED to
the velocity of the sound waves if the source is moving TOWARD
you. If the source is moving AWAY from you, the velocity of the
source is SUBTRACTED from the sound wave velocity.
Waves moving faster cause more of them to hit your ear in a second.
This is a HIGHER frequency. Waves moving slower cause fewer of
them to hit your ear in a second. This is a LOWER frequency.
Demo 1
Demo 2
Demo 3
Sonic Boom
Sources of Sound
Sound is produced by a vibrating object. The vibrations of the object
create particle motions that cause pressure oscillations in the air. A loudspeaker has a cone that is made to vibrate by electrical currents. The
surface of the cone creates the sound waves that travel to your ear and
allow you to hear music. Musical instruments such as gongs, cymbals, and
drums are other examples of vibrating surfaces that are sources of
sound.
Click images for YouTube videos
Corn starch is a shear thickening non-Newtonian fluid meaning
that it becomes more viscous (“thicker”) when it is disturbed.
When it's hit repeatedly by something like a speaker cone it
forms weird tendrils. The video was shot at 30 fps and the
speaker cone was vibrating at 30 Hz which is why there is no blur.
This is the original video with the actual sound of the speaker.
-- Click Image for YouTube video --
Sound Resonance
Raising or lowering
the tube changes the length of the
air column. When the column
is in resonance with the tuning fork,
the sound is loudest. The shortest
column of air that can have an
antinode at the closed end and a
node at the open end is one-fourth
of a wavelength long
As the frequency is increased,
additional resonance lengths are
found at half-wavelength intervals.
Thus, columns of length .1/4, 3.1/4,
5.1/4, 7.1/4, and so on will all be in
resonance with a tuning fork.
A seashell acts as a closed-pipe resonator to amplify certain
frequencies from the background noise.
A string resonates with standing waves when its length is a whole
number of half wavelengths. Because strings are so small in crosssectional area, they move very little air when they vibrate. This
makes it necessary to attach them to a sounding board, which
transfers their vibrations to the air and produces a stronger
sound wave.
Sound Quality
A tuning fork produces a soft and uninteresting sound. This is
because its tines vibrate like simple harmonic oscillators and
produce the simple sine wave. Sounds made by the human voice
and musical instruments are much more complex, like the wave
shown. Both waves have the same frequency, or pitch, but they
sound very different. The complex wave is produced by using the
principle of superposition to add waves of many frequencies. The
shape of the wave depends on the relative amplitudes of these
frequencies. In musical terms, the difference between the two
waves is called timbre, tone color, or tone quality.
A violin, a clarinet, and a piano produce characteristic sound spectra.
Each spectrum is unique, as is the timbre of the instrument.
Consonance and dissonance
When sounds that have two different pitches are played at the same time, the
resulting sound can be either pleasant or jarring. In musical terms, several
pitches played together are called a chord. An unpleasant set of pitches is
called dissonance. If the combination is pleasant, the sounds are said to be in
consonance.
What makes a sound pleasant to listen to? Different cultures have different
definitions, but most Western cultures accept the definitions of Pythagoras, who
lived in ancient Greece. Pythagoras experimented by plucking two strings at
the same time. He noted that pleasing sounds resulted when the strings had
lengths in small, whole-number ratios, for example 1:2, 2:3, or 3:4. This means
that their pitches (frequencies) will also have small, whole-number ratios.
Beats
You have seen that consonance is defined in terms of the ratio of frequencies. When the ratio becomes nearly 1:1, the frequencies become
very close. Two frequencies that are nearly identical interfere to
produce high and low sound levels, as illustrated above.
Beat Frequencies
are caused by
two nearly
identical
frequencies
combining out of
“phase”
Applet Example
Go forth and make great music….