Noise Canceling Headphones for fMRI

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Noise Canceling Headphones for fMRI

• Wilmot Yeh
• 6/3/2005
• Music 220c

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Project Overview

• To aid studies in music and voice cognition
• Lack of existing solutions that provide good
  enough noise reduction
• Lack of existing solutions that provide
  high-quality sound

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Significant Obstacles

• No ferrous metals (minimum of any metal)
• No Dynamic (moving coil) transducers
• Very loud scan noise
• Dependent on scanning parameters
• 103dB from 1kHz to 7kHz
• RF Interference/Drain
• Scanning accomplished by RF energy at 64MHz and
  128MHz
• Drain and interference results in diminished
  resolution/accuracy

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Existing Approaches

• Sound delivery
• Pneumatic
• Piezoelectric
• Electrostatic
• Noise Reduction
• Passive
• Active (Chamber)
• Active (Headset)

Only electrostatic drivers are capable of
producing accurate, full-range sound Active
in-headset noise canceling achieves the greatest
and most consistent noise reduction.
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Electrostatic Transducer
Stators ( /- 350V-1000V)

• Charged, low-mass diaphragm suspended between
  stators carrying a differential audio signal
• As the charge on the stators change, force is
  exerted on the diaphragm

Spacers (dielectric)
Diaphragm (250V-600V static)
(Voltages are for headphones)
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Construction

• Materials
• Perforated aluminum sheeting (stators)
• Ideally coated/drilled circuit board
• 0.5 mil Mylar film (diaphragm)
• 20 mil Polycarbonate sheeting (spacers)
• Graphite, Foil, Glue, etc

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Cut and file

• One pair of stators per transducer
• As flat as possible
• No burrs/bumps

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Stretching the Mylar

• Mylar must be stretched flat
• Wide range of tension
• Depends on electronics

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Adding resistive film

• Can be done with variety of substances
• Graphite
• Soap
• Resistances of 15 MOhms
• Allow charge/prevent failure from arcing

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Cutting spacers/Assembly

• Spacers need to be cut so there is no chance of
  short-circuit
• Plastic-epoxy glue
• Aluminum foil lead from diaphragm

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Finished transducer

• Mounted (not yet) inside an industrial-application
  ear protector (29dB NRR)
• Enough room for noise-canceling pickup and
  compact RFI filter

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Electronics

• Requires a high-voltage bias and high voltage
  signal
• Bias at 250V-600V, signal from /- 300V 1000V
• Capable of outputting enough power to cancel
  103dB of broadband noise
• Capable of working with high resistance cabling
  and robust RFI filters/chokes

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Options
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Remaining Issues

• Noise canceling
• Closed-loop system
• Electret capsule from 220a (non-ferrous)
• Simple opamp-based circuitry (external)
• RF Interference/Drain
• RF chokes
• Bypass capacitor network