Download Ch. 15 Sound

Ch. 15
Sound
Milbank High School
Sec. 15.1
Properties of Sound
• Objectives
– Demonstrate knowledge of the nature of
sound waves and the properties sound
shares with other waves.
– Solve problems relating the frequency,
wavelength, and velocity of sound.
– Relate the physical properties of sound
waves to the way we perceive sound.
– Define the Doppler shift and identify
some of its applications.
Sound Waves
• Are pressure variations that are
transmitted through matter
• Air molecules colliding
• Longitudinal waves
• Speed depends on air temperature
– Warmer=faster
Sound Waves
• Can’t exist in a vacuum
• Echoes
– Reflected sound waves
– Sonar
Loudness
• Depends on amplitude
• We hear based upon pressure
differences
– Larger amplitudes create a higher
pressure differences
• Sound level
– Decibels (dB)
– 0 dB (most faintly heard), 2 x 10-5 Pa
– Tenfold increase, 2 x 10-4 Pa is 20 dB
– 110 is painful (most concerts)
Pitch
• Difference in frequency
– Middle C: 262 Hz
– E: 327 Hz
• Human hearing
– 20-16000 Hz
– SIM – Pitch/Beat Freq
– SIM – Pitch/Beating Freq
Doppler Shift
• Police siren as it drives by
SIM
• More sound waves reach you when
the vehicle is moving towards you
• Vehicle + waves = higher frequency
• Waves – vehicle = lower frequency
• Bats:
– Insects flying away: lower frequency
– Insects flying towards: higher frequency
Sec. 15.2
The Physics of Music
• Objectives
– Describe the origin of sound.
– Demonstrate an understanding of
resonance, especially as applied to air
columns.
– Explain why there is a variation among
instruments and among voices using
terms timbre, resonance, fundamental,
and harmonic.
– Determine why beats occur.
Sources of Sound
• Produced by vibrating objects
– Vocal cords
– Brass instruments
– Reed instruments
– Stringed Instruments
– Others
Resonance
• When one object vibrating at the
same natural frequency of a second
object forces that second object into
vibrational motion.
• Sometimes occurs in a column or
tube (trombone)
• Length of air column determines the
frequency
Closed pipe resonance
• One closed end (marimba, pipe
organ, sea shell)
• First sound heard at ¼ wavelengths
• Additional resonance lengths found
at half-wavelength intervals
Open-end Resonance
• Woodwinds, brass
• Sounds loudest at half-wavelengths
• Wavelengths half as long as in a closed
ended tube
• Leads to twice the frequency
Sound Quality
• Fundamental
– Lowest frequency
• Harmonics
– Odd-numbered multiples of the
fundamental frequencies
• Dissonance
– Unpleasant set of pitches
• Consonance
– Pleasant set of pitches
Musical intervals
• Octave
– Frequencies in a 1:2 ratio
– Ex: 1st note-440 Hz, octave higher
would be 880 Hz
– Also corresponds to harmonics
Beat Notes
• Beat
– Oscillation of wave amplitude
– Beat frequency is the difference
between the two frequencies