4. In this lesson, you are to explain how sound waves are produced; relate
frequency to pitch; compare the speed of sound in various media; relate plane
waves to spherical waves; recognize the Doppler Effect, and determine the
direction of a frequency shift when there is relative motion between a source
and an observer.
(a) The sound from the tuning fork is produced by
(b) the vibrations of each of its prongs.
(c)When a prong swings to the right there is a
region of high density and pressure.
(d)Once a prog would swing back to the left, a
connected part of lower density and pressure
occurs.
5. PRODUCTION OF SOUND WAVES
The motion from one place to another and back again is called vibration. Sound waves
are made by vibratory object in a medium.
7. Once a piano player hits a piano key, the hammer inside the piano hits the wire causing
it to vibrate as shown in Figure 11.7.The wire's vibrations transferred to the piano's
board. As the soundboard vibrates, it exerts a force on air molecules around it, resulting
in air molecules in motion. Accordingly, as the soundboard vibrates back and forth its
kinetic energy is then converted into sound waves. Of this, the vibration of the
soundboard gradually dies out.
Figure11.7 Once the piano wire vibrates, energy is transferred to the piano's
soundboard, in response, the energy is then transferred into the air in the form of
sound.
8. As mentioned in lesson 1, sound waves traveling in the air are classified as longitudinal
waves. When sound waves travel from the source, energy is transferred from one point
(air molecule) to another. Moreover, the rate of energy that is transferred through a unit
area of the plane wave is termed the intensity of the wave. Since Power, P, is defined as
the rate of energy transfer, we can describe intensity in terms of power.
Sound Intensity
The SI unit for power is the watt. So, the unit for intensity is watts per square meter ( W/m2). Consider a spherical wave;
energy propagates in an equal amount in all directions. In this case, the power emitted by the source (P) is distributed
over the spherical surface (A=4πr2). given that there is no absorption in the medium.
9. Intensity= Power / 4π
( distance from the source)2
I- intensity
P- power
A- area ( 4πr2)
10. The inverse square law (sound) is relevant to small sources of sound that produce sound equally
in all directions. Supposed that the medium in which the sound travels is the same and does not
reflect or absorb the surface or the air. Given this condition, sound travels as a spherical wave. It
extends out uniformly in alldirections and its wave fronts form the surface of a sphere.
From the intensity of a spherical wave equation:
Rearranging this formula would help you to determine
the power of the source.
P = 4πr2 I
11. Figure 11.9 exhibits the general presentation of inverse Square Law. The illustration
presents a point source that extends uniformly infuse in all directions without a limit to its
range. Moreover, the intensity of the influence at any given radius(r) is the source strength
divided by the area of phenomena. The inverse square law applies also to other phenomena
(such as force, light, and radiation)
12. The graph presents at low frequencies (below 50 Hz) or high frequencies (above 12
000 Hz) must be relatively intense to be heard, whereas sounds in the middle of
the spectrum are audible at lower intensities. The threshold of hearing describes
to have the softest sounds that can be detected by the average human ear. It has a
frequency of 1000Hz and an intensity of 1.0x10*-12 W/m*2. On the other hand,the
threshold of pain is the loudest that the human ear can tolerate have an intensity
of about 1.0 W/m*2.
