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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

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Investigating Electromagnetic and Acoustic Properties of Loudspeakers Using Phase Sensitive Equipment

Katie Butler

DePaul University

Advisor: Steve Errede


Why investigate loudspeakers
Why investigate loudspeakers? Loudspeakers Using Phase Sensitive Equipment

  • 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
How speakers work Loudspeakers Using Phase Sensitive Equipment

  • 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
Loudspeaker analogous circuit Loudspeakers Using Phase Sensitive Equipment

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
Low distortion power amplifier Loudspeakers Using Phase Sensitive Equipment

  • 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
Amplifier pictures Loudspeakers Using Phase Sensitive Equipment


Data acquisition technique for measuring electromagnetic properties of loudspeaker
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
Complex pressure properties of loudspeakerp and particle velocity u measurements

Acoustic measurements will be taken at the same time as electromagnetic measurements


Impedance and power

Electrical Impedance properties of loudspeaker

(Ohms, e)

Electrical Power (W)

Radiation Impedance

(Pa-s/m ac)

Acoustic Intensity (W/m2)

Impedance and Power


Phase sensitive equipment
Phase sensitive equipment properties of loudspeaker

  • SR830 Dual-Channel DSP lock-in amplifiers


The speaker

Italian Jensen C12N properties of loudspeaker

Ceramic magnet

12”, 8

50 watt rated power

Designed to emulate American made Jensens from the 1960s

The Speaker


Apparatus
Apparatus properties of loudspeaker

  • 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
Speaker Cabinet properties of loudspeaker

  • 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
Frequency Sweep Data properties of loudspeaker

Complex acoustic impedance; speaker in free air (blue), on baffle board (green), in cabinet (red)


Frequency sweep data1
Frequency Sweep Data properties of loudspeaker

Complex sound intensity; speaker in free air (blue), on baffle board (green), in cabinet (red)


Frequency sweep data2
Frequency Sweep Data properties of loudspeaker

Complex electrical impedance; speaker in free air (blue), on baffle board (green), in cabinet (red)


Frequency sweep data3
Frequency Sweep Data properties of loudspeaker

Complex electrical power; speaker in free air (blue), on baffle board (green), in cabinet (red)


Voltage versus particle velocity
Voltage versus particle velocity properties of loudspeaker

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 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 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
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
30 Hz – 20,000 Hz frequencies

Acoustic intensity (top)and EM power (bottom) versus frequency for speaker in enclosure at a height of 0.40 centimeters.


Sound intensity level s
Sound Intensity Level(s) frequencies

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
Acknowledgments frequencies

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.


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