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
Integrated Acoustics Laboratory
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
E R  20log
Ed
2
 Ec  2 E d Ec cosd  c 
Ed
2
• 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)
R  20log1  T2  
R  20logT2  
• More tolerant to amplitude error, phase accumulation
• Noise amplification out-of-band
• Stability & robustness
• Unity gain (0 dB) @ ±120°, -20 dB @ ±180°
• With 3 dB noise amplification, 0 dB @ ±138°, -10 dB @ ±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
s2  211s  12 s  
T s 
s2  22 2s   2 s  
2
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The 1970’s
• 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 90’s 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 Olson’s 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 antinoise 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
• 100’s of publications, patents, etc.
• Stay tuned for the next presentations!
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Fertig
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The Issues
E R  20log
2
30
Gain, dB
R  20logT2  
 Ec  2 E d Ec cosd  c 
Ed
2
40
• Feedback
R  20log1  T2  
Ed
20
Gain
10
0
-10
Phase
-20
10
100
1000
Frequency, Hz
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180
160
140
120
100
80
60
40
20
0
10000
Phase, deg
• Forward-acting
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”
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