Download Investigating Electromagnetic and Acoustic Properties of

Investigating Electromagnetic and Acoustic
Properties of Loudspeakers Using Phase
Sensitive Equipment
Katie Butler
DePaul University
Advisor: Steve Errede
Why investigate loudspeakers?
•Most important link in the
audio chain
•Last piece of equipment
audio signal passes through
•Many variables in
loudspeakers; permanent
magnet, size and weight,
material of cone, size and
type of enclosure, etc.
TheToneChamber.com
How speakers work
• Voice coil (electromagnet) is positioned in constant
magnetic field from permanent magnet
• Current across voice coil constantly changes, changing
the magnetic field polarity and strength causing the
voice coil and diaphragm to move
Cross-section of typical loudspeaker
Loudspeaker analogous circuit
Using electrical components to model the mechanical
components of the loudspeaker, further work must be
done to accurately calculate these using data collected
Low distortion power amplifier
• No audio amplifiers readily
available in lab
• Need to amplify signal from
function generator to power
loudspeaker
• Building amp using widely
available LM3875 chip amp
• Constant voltage source,
typical for powering
speakers
Component going into amplifier
Amplifier pictures
Data acquisition technique for measuring
electromagnetic properties of loudspeaker
Based on UIUC Physics 498POM PC-Based Pickup Impedance Measuring System
Complex pressure p and particle velocity u
measurements
Acoustic measurements will be taken at the same time as electromagnetic
measurements
Impedance and Power
Electrical Impedance
(Ohms, e)
Radiation Impedance
(Pa-s/m  ac)
V( f )
Zem ( f ) 
I( f )
p(r , f )
Z ac (r , f ) 
u (r , f )
Electrical Power (W)
Acoustic Intensity (W/m2)
Pem ( f )  V ( f )  I * ( f )
I ac (r , f )  p(r , f )  u * (r , f )
Phase sensitive equipment
• SR830 Dual-Channel DSP lock-in amplifiers
The Speaker
•
•
•
•
•
Italian Jensen C12N
Ceramic magnet
12”, 8
50 watt rated power
Designed to emulate
American made Jensens
from the 1960s
Apparatus
• Took measurements 3
ways: in free air, on baffle
board, in speaker cabinet
• Microphones are on
movable arms controlled by
computer program
• Current and voltage cables
attached underneath
• Foam sound absorbers
used under speaker to
prevent reflections
Setup with speaker on baffle board
Speaker Cabinet
•Designed and built by Steve
Errede, based on Marshall
1965B 410 straight speaker
cabinet
•Sound absorptive material
placed in cabinet behind
speaker
Frequency Sweep Data
Complex acoustic impedance; speaker in free air (blue), on
baffle board (green), in cabinet (red)
Frequency Sweep Data
Complex sound intensity; speaker in free air (blue), on baffle
board (green), in cabinet (red)
Frequency Sweep Data
Complex electrical impedance; speaker in free air (blue), on
baffle board (green), in cabinet (red)
Frequency Sweep Data
Complex electrical power; speaker in free air (blue), on baffle
board (green), in cabinet (red)
Voltage versus particle velocity
Magnitude of voltage (left) and magnitude of particle
velocity (right), the electromagnetic resonance (120.5 Hz)
appears as a resonance in particle velocity at 0.40
centimeters above the speaker
Acoustic pressure of speaker in free air versus
mounted on baffle board
Acoustic pressure across surface at 0.40 centimeters above
speaker in free air (left) and speaker mounted on 24” square baffle
board (right), driven at 120 Hz
Particle velocity of speaker in free air versus
mounted on baffle board
Particle velocity across surface at 0.40 centimeters above
speaker in free air (left) and speaker mounted on 24”
square baffle board (right), driven at 120 Hz
Sound intensity across surface of
speaker driven at various frequencies
Magnitude of
sound intensity
across surface of
speaker in
enclosure
Driven at 130.5 Hz
(left), 3485.0 Hz
(center), and
10,000 Hz (right)
30 Hz – 20,000 Hz
Acoustic intensity (top) and EM power (bottom) versus frequency
for speaker in enclosure at a height of 0.40 centimeters.
Sound Intensity Level(s)
RHS plot: Sound Intensity Level SIL= 10log10(I/Io) (blue),
Sound Pressure Level SPL=20log10(p/po) (green), Sound Particle
Velocity Level SUL = 20log10(u/uo) (red) versus frequency for
speaker in enclosure at a height of 0.40 centimeters.
LHS plot: The differences dSLip = SPL-SIL (blue), dSLiu = SIL-SIU
(green) and dSLpu = SPL-SUL (red) versus frequency for speaker in
enclosure at a height of 0.40 centimeters.
Acknowledgments
Special thanks to Professor Steve Errede for his
commitment to our projects. Also thank you to
Gregoire Tronel for sharing the lab space and
equipment.
Thank you to the REU program for this research
opportunity, which is supported by the National
Science Foundation Grant PHY-0647885.