Transcript Sound and Music copy

Unit: Waves
Topic(s): Sound and Music
Learning Goals:
•Describe Sound in terms of energy transfer
•Explain how amplitude of a sound wave relates to the energy of the
wave (via real world examples)
•Demonstrate the Doppler effect qualitatively or quantitatively (via
wave front diagram)
•Predict property of standing waves that are confined to a region
•Calculate wavelength and frequencies associated with possible
harmonics for musical instruments
Sound Facts
• Mechanical Wave
• Longitudinal Wave
• Propagate from source in 3-D
• Consists of alternating regions of compression (high
pressure) and rarefaction (low pressure)
• Speed of sound varies by medium (p329)
Sound Facts Part 2
• Frequency and volume (relative intensity dB) are the
deciding factors if you can hear something.
• Audible frequency range 20Hz-20kHz
• Age effects ability to hear frequency
• Frequency determines pitch (notes)
• Frequency Generator Demo
http://onlinetonegenerator.com
Energy of Sound
Sound energy is the result of energy transformation. (ex:
Speaker converts electrical energy to sound energy).
The power (or loudness) is related to how quickly the
energy is converted (P=energy/time)
Since sound waves are pressure waves, their amplitude is
related to their pressure and energy.
These pressure waves can cause other substances to
vibrate, and if the materials cannot handle the level of
vibration there may be damage.
Physics of Music
• Non-conventional instrument categories
• Tension and physical characteristics of strings matter
• “Air” instruments use valves and slides to change L.
Embouchure can change things too.
• Music scales use harmonics (aka overtones)
• Standing waves “in or on” an instrument correspond
to the harmonic series for that instrument (or object)
• Forced vibration (resonance)
• v=f
– Certain wavelengths are allowed based on length
of vibrating medium
– v is the speed of the wave in the medium (doesn’t
have to be speed of sound)
– f would be the pitch of the sound (note)
String Instruments
Standing waves have nodes on both ends (as
the ends are fixed)
Tension and string composition impact the
speed of the wave on the string which, in turn
impacts the pitch.
All the harmonics are possible
Open at both end
(air instruments)
Example: Flute
Woodwinds and many brass instruments do not
fit in this category, because once the musician
places their mouth over the end of the
instrument, the end becomes closed (fixed).
Standing wave has an antinodes at each end
All the harmonics are possible
Closed at one end
(air instruments)
Majority of woodwind and brass instruments
Closed at mouthpiece (node)
Open at end (antinode). Only odd harmonics
http://zonalandeducation.com/m
stm/physics/waves/standingWav
es/standingWaves.html
Frequency of wave on
string or in air
v
fn = n
2L
n is the harmonic (n=1 is first harmonic or
fundamental frequency)
v is the velocity of the wave in the medium
L is length of the medium
Doppler Effect
• Apparent change in frequency (pitch)of sound
due to a moving source or detector
• You can hear this in a passing ambulance.
• Radar makes use of Doppler Effect
• Astronomers use Doppler Effect with light to
determine if objects are moving towards/away
from us.
• http://www.walter-fendt.de/ph14e/dopplereff.htm