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Transcript of Investigating Electromagnetic and Acoustic Properties of Loudspeakers Using Phase Sensitive...
Investigating Electromagnetic and Acoustic Properties of Loudspeakers Using Phase
Sensitive Equipment
Katie ButlerDePaul 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)
Electrical Power (W)
Radiation Impedance(Pa-s/m ac)
Acoustic Intensity (W/m2)
( )( )
( )em
V fZ f
I f
( , )
( , )( , )ac
p r fZ r f
u r f
*( ) ( ) ( )emP f V f I f *( , ) ( , ) ( , )acI 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 410 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.