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Lecture 27
Electricity & Applications
to
Music
Probe the Class OP
Last time we looked at some
electrical circuits
We identified charge.
 Current is the charge per unit time that passes
through a point of a “circuit”.
 The current increases with the voltage across a
“resistor” such as a light bulb.

– The brightness of the bulb depends on the current
through it.

We used OHM’S LAW
– Voltage (volts) = Current (Amps) x Resistance (ohms)
Mr. Ohm’s Legislation
V  IR
V
I
R
V
R
I
200 Watts
60 Watts
POWER
Let’s move a charge q
from one side to the
other in a time “t”.
 V=work/charge
 Work we do = qV
 Power = qV/t
 i=q/t
 Power = iV

V
i
Resistance = R
More on Power through a Resistor
Work
Power 
 IV
Time
2
P  IV  I  ( IR )  I R
also
V
V 
P  IV   V 
R
R
2
R is fixed, so
more power = more
current = more light!
The USA at night.
LOTS OF BULBS!
Which bulb in (b) is the brightest?
Will the bulb in (a) be brighter, dimmer, or
the same brightness as any one bulb in (b)?
What would happen if one of
the bulbs in (b) burned out?
A Magnet
S
N
+Q
OP Survey
Magnets - Summary
 Magnets
Do NOT attract chages.
 Magnets have N and S poles
 Like poles repel
 Unlike poles attract
Other Observations
A magnet moving into a coil produces an
electric current (and voltage!).
 There is a “magnetic field” around a wire.
 A loop of wire acts like a small magnet.

A
wire moving near a magnet will
have a current generated in it.
The Dynamic Microphone
V
The VOLTAGE is passed on to the next part of the circuit
Dynamic Microphones
The Voltage is the audio signal.
 Dynamic microphones are extremely rugged and
may be the most widely used microphones in
television production.
 They are relatively inexpensive (for
professional-quality microphones) and usually
have good frequency response.
 They tend to be somewhat less sensitive to
high-frequency sounds than are condenser
microphones

They Come in all Colors too!
Condenser Microphones
Add a Resistor to get the Voltage
Electrets
Both plates are electrically charged, and sound
hitting the faceplate causes a change in voltage.
 Electret condensers are a popular type of
condenser microphone.
 Electrets are manufactured with a permanent
electric charge in the capacitor and therefore
require the use of only a very small battery as a
power source to boost the output signal of the
microphone to a usable level.
 As a result, electret condensers tend to be
significantly smaller than other condenser
microphones.

Different strokes …
Bigger is Lower!
Frequency Response
Directionality
“Stereo” Microphone
The Process We Are Looking At
Sound Production
Microphone
conversion
to electrical signal
Amplify the Voltage
Loud Speaker
of some type
LISTENER
Amplifier
SLIGHT TURN
The Simple Transistor Amplifier
(Bipolar)
The Voltages
NOTE SCALE DIFFERENCES
Don’t push an amplifier too hard
The Speakers … all kinds
Exploded View
Comments
A speakers maximum response is at
resonance.
 The heavier the structure, the lower the
resonance frequency.

– In order to cover the entire audio range we
need multiple speakers.
 Woofer/Sub-Woofer for low frequency
 “Tweeter” for high frequency
 Mid range speakers as well.
Frequency Ranges
for different types of speakers.
Another Music Application
Multiple Sources of Sound
The Strings
The “Package”
A Combination
Amplifier
and
Speaker
The Instrument
Next Step (If We Get To It)
The “new Electronics
 The Digital World
 Compression
 The CD to the IPOD
