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Sound When a sound wave meets an
obstacle, some of the sound is reflected back and some
passes into the obstacle material. Of the sound that
passes into the obstacle, some is absorbed by the
material and some is transmitted through the material.
Reflection and absorption are dependent on the
wavelength of the sound. The percentage of the sound
transmitted through an obstacle depends on how much
sound is reflected and how much is absorbed.
Questions you may have about these
effects are:
Reflection
When a sound wave reaches the interface
between different materials, some of the sound is
reflected off the interface, while the rest of it goes
into the other material. The percentage of sound that is
reflected is a function of the material, the quality of
its surface, and the wavelength of the sound.
If the surface or interface is
relatively smooth, more sound will be reflected than if
the surface is rough. For example, more sound will be
reflected from a smooth wall made of mud than a pile of
dirt. The reason is that the rough or porous surface
allows for many internal relfections, resulting in more
absorption and less reflection.
When sound reflects off a smooth flat
surface, an echo or reproduction of the sound can be
heard. Echoes are more noticeable if the surface is far
enough away to allow for a time-lag between when the
sound is made and when it is hear.
Fishermen use the principle of echoes to
find fish underwater. They use a device that measures
the time it takes for a sound to reflect off a fish.
This is called sonar, and it is very similar to radar.
Some sound is also reflected at an
internal interface. For example, some of the sound in
air that hits a wall will be reflected by the air to
wall surface. But of the sound that is transmitted
through the wall, some will also be reflected internally
at the wall to air surface. Often that reflection is not
considered, but it is a factor.
Absorption
As sound travels though any medium,
there is some loss due to absorption. Since sound is a
regular vibration of the atoms or molecules in a
material, some of the kinetic energy of the waveform is
lost due to molecular collisions. That sound energy is
turned into random heat energy.
The volume of sound traveling though air
not only becomes less as it spreads out, but some of its
energy is lost due to absorption. The rate of loss is a
function of the frequency or pitch of the sound.
For example, if you yelled as loud as
you could, someone 1 mile or 1500km away probably could
not hear you, even under the best of conditions. On the
other hand, an Elephant can make a low pitched rumble
that can be heard by other elephants about 5 miles or
6000km away.
Some materials absorb sound more than
others. Drapes and ceiling tiles are used to absorb
unwanted sound and eliminate echoes. Music recording
studios use sound absorbing materials on their walls to
eliminate any undesired or outside sounds, when
recording a song.
Transmission
Sound waves that are not absorbed in a
material are then transmitted through that material.
Since sound is a regular vibration of matter, it is
transmitted through almost all materials. Sound travels
through air, water, and even walls.
If you know how much sound is reflected
at the surface of the obstacle and how much is absorbed
within the obstacle, you can determine how much is
transmitted through the obstacle.
For example, suppose someone is playing
music in the next room, and the volume is at 100%. The
wall will reflect all frequencies almost the same. Let's
say 30% is reflected. That means that sound goes into
the wall at 70% the original volume.
Now suppose the material in the wall
absorbs 80% of the middle and high frequencies. That
means that 20% of the volume passes through the wall the
middle and high frequencies. Thus you would hear a
volume of 14%. That is 70% going into the wall times 20%
of that coming out = 0.7 X 0.2 = 0.14 = 14%.
But suppose only 30% of the volume of
low frequencies is absorbed. That means 70% passes
though, resulting in a volume of 49% of the original
sound for the low frequencies (0.7 X 0.7 = 0.49 = 49%).
Before the sound leaves the wall, some
of it is reflected at the wall to air surface or
interface. Typically, the internal reflection is less
than the reflection from the air to the wall. Let's say
the internal reflection is 10% for all wavelengths.
Thus for middle and high frequencies,
12.6% of the sound passes through the wall (0.9 X 0.14)
and 42.1% of the sound at the low frequencies passes
through (0.9 X 0.49).
That is why you barely hear the higher
notes of the music, but you can hear the boom-boom-boom
of the bass sounds.
Sound is externally reflected, absorbed,
transmitted, and internally reflected when the sound
wave encounters an obstacle. Reflection and absorption
are dependent on the wavelength of the sound. The
percentage of the sound transmitted through an obstacle
depends on how much sound is reflected and how much is
absorbed. |
