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Noise Canceling Headphones for fMRI

Noise Canceling Headphones for fMRI. Wilmot Yeh 6/3/2005 Music 220c. 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. Significant Obstacles.

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Noise Canceling Headphones for fMRI

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  1. Noise Canceling Headphones for fMRI Wilmot Yeh 6/3/2005 Music 220c

  2. 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

  3. 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

  4. 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.

  5. 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)

  6. 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

  7. One pair of stators per transducer As flat as possible No burrs/bumps Cut and file

  8. Stretching the Mylar • Mylar must be stretched flat • Wide range of tension • Depends on electronics

  9. Adding resistive film • Can be done with variety of substances • Graphite • Soap • Resistances of 15+ MOhms • Allow charge/prevent failure from arcing

  10. Cutting spacers/Assembly • Spacers need to be cut so there is no chance of short-circuit • Plastic-epoxy glue • Aluminum foil lead from diaphragm

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

  12. 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

  13. Options

  14. 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

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