Chapter 15 - Sound
Sound wave is a longitudinal wave
LT #1
Relate the properties of sound waves
to the way we perceive sound.
What is sound?
• Sound waves are longitudinal waves
• Sound waves is simply a pressure change that
is transmitted through matter.
• Produced by vibrations
• Transmitted through matter, they need a
medium.
• Sound is a mechanical wave
Interesting Fact….
• What is wrong with this video clip?
(Video clip: space balls)
• Sound CANNOT travel through a vacuum!
(Video clip: Bell jar)
How do we hear?
• Your eardrum is a pressure
detector.
• Eardrums vibrate when
there is a pressure change.
• These vibrations send
electrical impulses to the
brain that decipher it as
sound.
Parts
• Compression = high pressure
• rarefaction = low pressure
Wavelength = C to C or R to R
Amplitude = length of compression or rarefaction
Speed of Sound
• The speed of sound depends on the medium and
temperature
• Equation v sound in air = 331 + .6 T
– Sound in air at 20°C (72°F) = 343 m/s
– Sound in air at 0°C (32°F) = 331 m/s
• The speed of sound increases with increasing
temperature.
Speed of Sound
• Sound travels fastest in solids, then liquids,
slowest in air.
• Ex: 11 times faster in steel!!
• Sound travels fastest through materials with the
highest compressibility not the highest density.
• Waves (slinky, sound, light, etc.) slow down in
materials with higher densities.
Loudness
The loudness of sound
depends on the
amplitude of the sound
wave.
Small amplitude = soft
Large amplitude = loud
Loudness is measured in decibels (dB)
We perceive each increase of 10 dB as twice as loud
LOUDNESS
Pitch
The frequency of a sound
wave is the pitch of the
sound.
Higher Frequency = Higher Pitch video clip
Lower Frequency = Lower Pitch video clip
Longer Wavelength = Low Note (Pitch)
Shorter Wavelength = High Note (Pitch)
Interesting Fact
• Only a small range of frequencies are audible:
we can hear from about 20 Hz – 20,000 Hz
depending on age!
• We can’t hear:
– Ultrasonic Waves: above 20,000 Hz
– Infrasonic Waves: below 20 Hz
• LET’s take a test:
http://www.freemosquitoringtone.org/
Beats
• Beats occur when 2 frequencies (very nearly
identical) interfere and produce high and low
sound levels.
• The frequency of the beat = difference between
the 2 frequencies
LT #1 Quiz
Problem Solving
Solve problems relating to frequency, wavelength and velocity of
sound.
V=fλ
- Speed of sound in air depends on the temperature of air.
- Speed of sound in air at 20°C is 343m/s.
- Speed of sound increases at the rate of 0.6m/s for
every 1°C increase in temperature.
- Sound can travel through solids, liquids and
gases. It travels the fastest in solids.
Standing Waves
A wave appears to stand still
when the incident and
reflective wave interact.
Node – the point that
doesn’t move when two
pulses meet.
Created by destructive
interference
Standing Waves
A wave appears to stand still
when the incident and
reflective wave interact.
Anti-node- the largest
amplitude.
Created by constructive
interference.
Free and forced vibrations
• When an object is tapped, it will vibrate with
its own natural frequency - this are free
vibrations. (This is why different objects
sound different when tapped.)
• When there are vibrations around an object it
is forced to pick up that particular vibration –
these are forced vibrations.
Resonance
• When the forced vibrations match the natural
frequency of the object, the object easily picks
up the energy and starts to oscillate.
• This is called RESONANCE.
• Examples of resonance are swings, Tacoma
bridge collapse, breaking wine glasses.
Resonance or Standing waves in Strings
When waves are continuously sent (incident waves) and they
hit a rigid boundary (reflected wave) then they come back out of
phase.
These incident and reflected waves
interact to form standing waves
as shown.
The faster the vibrations, the more the nodes and antinodes.
Resonance or Standing Waves in Strings
Knowing the length, the actual wavelength of the
standing wave can be calculated.
1st Harmonic is also called the fundamental frequency. Other harmonics are
called overtones or octaves.
Resonance or Standing Waves in Strings
Over all, we can see that since the medium (string) is the
same, changing wavelength results in changing frequency!
Just like we saw in the slinky lab
If the wavelength halves, the frequency doubles.
Resonance or Standing Waves in Open Pipes
Open pipes are open at both ends.
You can produce notes in open pipes when
air is moved through pipe with a particular
frequency based on the pipes length.
These frequencies are capable of
producing standing waves.
Resonance or Standing Waves in Open Pipes
Resonance occurs when
anti-nodes are formed at
the open ends. Anti-nodes
occur at the open ends
because there is more
room for air to
Vibrate.
The wavelength can be calculated as shown.
Resonance or Standing Waves in Closed Pipes
Closed pipes are closed at one end.
Based on the length of the pipe,
certain frequencies are capable of
producing standing waves in closed pipes.
Resonance or Standing Waves in Closed Pipes
Resonance in closed pipes occur when there are
anti-nodes at the open end and nodes at the closed
ends.
Speed of Sound Lab
When the tube was at the right
length then the reflected wave
from the water met another
vibration coming down from the
tuning fork and they interfered
forming a standing wave.
Sound will amplify = Resonance
LT #3 Quiz
Doppler Effect
The change in wavelength caused by the
motion of the source or the observer.
Stationary Object
A stationary source would produce waves
that travel out equally on all sides. The
distance between the wave fronts are all the
same.
Source/Observer Moving Closer
The wave fronts are produced with the same frequency as
before. However, since the source is moving, the center of
each new wave front is now slightly displaced. As a result,
wave fronts begin to bunch up in front of the object
Source/Observer moving Away
The wave fronts again are produced with the same
frequency as before. This time, since the source and/or
observer are moving away, the wave fronts begin to spread
out.
Sound Barrier
Chuck Yeager was the first person to
break the sound barrier when he flew
faster than the speed of sound in the X-1
rocket-powered aircraft on October 14,
1947.
Determining Apparent Frequency
Here:
fo = apparent frequency as heard by the
observer.
fs = actual frequency of the source.
V = speed of sound (343m/s)
Vo = speed of the observer
Vs = speed of the source
Determining Apparent Frequency
Use +/- when the source and the observer
are getting closer.
Use -/+ when the source and the observer
are getting further away.
Source not moving
Example:
1. Find the apparent frequency heard by the
observer as the observer runs away from a
300 Hz siren, at the speed of 20m/s.
2. Find the apparent frequency heard by the
observer as the observer runs towards the
300 Hz siren, at the speed of 20m/s.
Observer not moving
Example:
1. Find the apparent frequency heard by an
observer as a cop car drives away with a
300Hz siren, at 20m/s.
2. Find the apparent frequency heard by an
observer as the cop car drives towards the
observer with a 300 Hz siren, at 20m/s.
LT #4 Quiz