dr. john varriano dr. ted clarke jonathan fili department of physics christian brothers university...
TRANSCRIPT
Dr. John VarrianoDr. Ted ClarkeJonathan Fili
Department of PhysicsChristian Brothers University
Electricity & Magnetism: Music to Our Ears
TAAPT Annual MeetingMarch 21, 2015
Austin-Peay State UniversityClarksville, TN
1. Electric “Guitar”
electromagnetic induction ac circuits standing waves musical scale
Physics
small coil with lots of turns (e.g. coil from a relay) small magnet (neodymium) steel wire wood, screws hook-up wire, connectors, soldering iron, solder amplifier, speaker, audio cables
Materials
Coil
relays
resistance 200-400
Example: NTE ELECTRONICS R46-5D12-12 POWER RELAYSPDT, PC BOARD Coil Voltage 12VDCContact Voltage 120VACContact current 12A
www.newark.com $3.89
Characterizing Coil
Measure resistance with meter.
R = 202
f = 500 Hz
Irms = 3.00 mA
Vrms = 4.00 V
Z = = 1333 Vrms
Irms
L 0.42 H
typical guitar pick-up coils R = 5-10 k, L = 1-9 H, C=1 nF
Determine inductance using impedance method.
222
22
)()1(
)(
RCLC
LRZ
VR
Lcoil
AIrms
Vrmsf C
22 )( LRZ
12-tone equal temperament
f12 = 2f0f0 f1 f2
octave
01 nff
)12/1(00
1212
2
2
n
ffnf
0)12/(2 ff m
m (1)
bridgenutL0
Lmdm
mm L
vf
Lvv
f
2
2 000
(2)
(3)
Combining (1), (2), (3) gives
)12/(0
)12/(0
2
11
2
mm
mm
Ld
LL
Fret Bars?
02
12 fnnff
0fnf mm
2. Theremin (Ætherphone)
capacitance ac circuits (RLC circuit resonance) frequency mixing (heterodyne principle)
Physics
Thereminwww.musiciansfriend.com
• Moog (already built ) $389-$499• Moog (kit) $339
amplifier, speaker, audio cables• Moog 15-watt amplifier/speaker ($99)
Materials
Theremin
http://en.wikipedia.org/wiki/Theremin
frequency “antenna”
volume “antenna”
Patented in 1928 by Lev Sergeyevich Termen (a.k.a. Leon Theremin in America)
played without physical contact hand and antenna act as plates of a capacitor distance between hand and antenna determines capacitance
http://www.thereminworld.com
Pitch Control uses 2 oscillators
one has fixed frequency other frequency determined by capacitance of antenna-hand distance difference of these two frequencies is audible frequency (heterodyne)
Why use 2 oscillators and heterodyne? Why not just 1 oscillator?
Capacitance is on order of picofarads so resonant frequency is in radio spectrum. Must mix two radio frequencies to get audible frequency.
Volume Control capacitance detunes oscillator which then attenuates signal
The theremin was used in movie soundtracks such as Miklós Rózsa's for Spellbound and The Lost Weekend and Bernard Herrmann's for The Day the Earth Stood Still, and as the theme tune for the TV show Midsomer Murders.
http://en.wikipedia.org/wiki/ThereminInteresting Trivia
http://www.northwestchicagofilmsociety.org/
http://www.youtube.com/watch?v=3ULhiVqeF5U