Active noise reduction hearing protectors: 50 years of development

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Active noise reduction hearing protectors 50
years of development

• Kenneth A. Cunefare
• Associate Professor
• Professor in Charge, Integrated Acoustics
  Laboratory
• George W. Woodruff School of Mechanical
  Engineering

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Outline

• How we got here
• Technology challenges of the past
• Technology challenges of the present
• Olson May 1953, 1956
• Meeker 1958, 1959
• A host of others

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Olson May, Electronic sound absorber, JASA
25(6), 1130-1136, 1953
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Precursor

• 48th ASA, Austin, 1955 (15 page program!)

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Precursor

• 51st ASA meeting, MIT, 1956 (49 pages)

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Olson, Electronic control of noise, vibration,
and reverberation, JASA 28(5), 966-972, 1956
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Meeker, Active ear defender systems component
considerations and theory, WADC TR 57-368, 1958

• Cast of characters
• Project directors
• Hawley Dolch through 1955
• Touger and Meeker 55-58
• Participants
• Simshauser
• Balakrishnan
• Olson Woll consultants
• Air Force Lt. David T. Blackstock Charles
  Nixon
• Target 25 dB from 50 to 1000 cps

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Meeker 1958

• Feedback, open-loop/feed-forward, and local
  control

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Meeker 1958

• Forward-acting system (System I)
• Cancellation
• Gain, phase critical
• Need for minimum phase network, compensation
• Recognition of limitations
• Non-flat component response
• Phase shift due to components, acoustics

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Meeker 1958

• Feedback system (System II)
• More tolerant to amplitude error, phase
  accumulation
• Noise amplification out-of-band
• Stability robustness
• Unity gain (0 dB) _at_ 120, -20 dB _at_ 180
• With 3 dB noise amplification, 0 dB _at_ 138, -10
  dB _at_ 180

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Meeker 1958

• Role of transducers and transfer paths
• . substantial phase shift above 500 cps
• correcting network/ compensation network
• Cut and try compensation!
• Implemented RC network filter for compensation at
  low and high frequency

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Meeker 1958

• Issues and opportunities
• Stability and gain in useful band
• Components of the day the limiting factor
• 20 dB reduction from 50 cps to 400 cps reasonable

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Meeker 1958

• Free-field sound absorber
• Concluded not viable due to constrained volume of
  effectiveness
• Still an area of research

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Meeker, Active ear defender systems development
of a laboratory model, WASC TR 57-368(II), 1959

• Feedback system prototype
• Addresses transducer development/limitations
• Microphones
• Earphones
• Amplifier phase shift limited performance
• Alternative embodiments
• Semi-insert
• Co-axial (close-coupled speaker and microphone)

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Meeker 1959 Feedback ANC experimental results
110 dB max noise field
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Meeker 1959

• Semi-insert concept

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The art as of 1959

• Feedback system preferred embodiment
• Need for
• Compensation for transducer dynamics
  (equalization, correcting networks,
  compensation networks)
• Minimum phase designs, from all components
• System design considering all elements
• Higher power handling components

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Patent U.S. Patent US 2,972,018. Filed Nov. 30,
1953, Patented Feb. 14, 1961
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The 1970s

• Dorsey et al., An active noise reduction system
  for use with ear defenders, 8th International
  Aerospace Symposium, Cranfield, 1975.
• Addresses many of the same issues as Meeker et
  al.
• Surprised by transducer affect
• Equilisation philosophy - compensation

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The 1970s

• Dorsey et al.
• Showed 10 dB reduction 150-700Hz, 20 dB 300-600
  Hz
• Up to 6 dB amplification 1-3 kHz
• Low-frequency performance limited by earphone
  actuator phase shift, oscillation
• 124 dB noise field target

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The Bose Era

• Patents and publications galore
• A. G. Bose, and J. Carter, Headphoning, U.S.
  Patent No. 4455675, June 19, 1984

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The 90s and beyond digital control

• Pan et al., Application of feed-forward active
  noise control to a circumaural hearing
  protector, Active 95.
• Off-line error path estimation

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Pan et al. Active 95

• Band-limited noise 40-750 Hz, tones, chirp

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Casali Robertson, Narrow-Band Digital Active
Noise Reduction in a Siren-Cancelling Headset
Real-Ear and Acoustical Manikin Insertion Loss,
NCEJ 42(3), 1994

• Adaptive digital version of Olsons sound absorber

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The Consumer Side

• NoiseBuster headphones use a microphone inside
  and outside the earcup (1) to listen to the noise
  coming into the ear (2). Using electronics (3),
  the system takes that information and uses it to
  create a noise wave that is identical to, but
  directly opposite of, the one coming into the ear
  (4). The "anti-noise" wave is output through a
  speaker, also located in the earcup. When the two
  waves (the noise wave and the anti-noise wave)
  meet, the noise is significantly reduced (5).
  69
• Active cancellation 40-1,200 Hz, maximum 18 dB
  between 100-200 Hz
• Amazon.com

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The Consumer Side

• Acoustic Noise Cancelling Headset Technology
• The story of this breakthrough technology begins
  on a long-distance flight that Dr. Amar Bose took
  in 1978. When he tried on the airline's
  conventional headphones, the experience was
  disappointing-engine roar and other cabin noises
  interfered with any enjoyment of music. Turning
  up the volume did little to mask the noise and
  further distorted the music. Would it be
  possible, he wondered, to achieve active noise
  reduction in a pair of headphones to enhance the
  enjoyment of music?
• Dr. Bose's experience on that flight prompted the
  creation of the Bose Noise Reduction Technology
  Group. They, in turn, developed Acoustic Noise
  Cancelling headset technology to dramatically
  reduce unwanted noise and allow the enjoyment of
  music, films or even silence, in places where
  noise is a problem.
• 299
• Bose.com

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The Remaining Issues

• Transducer dynamics
• Compensation
• Acoustic path limit
• Low-frequency performance

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Where we stand

• Push to higher noise level systems (140 dB)
• Push to lower higher frequencies
• Adaptive/feedforward/feedback
• 100s of publications, patents, etc.
• Stay tuned for the next presentations!

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Fertig
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The Issues

• Forward-acting
• Feedback

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Carter, Active noise reduction,
AFAMRL-TR-84-008, 1984

• Classical feedback w/compensation
• 10 dB reduction 50-500 Hz
• 20 dB reduction 80-300 Hz

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Carter 1984
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Carter 1984

• Adaptive feed-forward concept
• Considered too expensive