1. Integrated Acoustics Laboratory 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

2. Integrated Acoustics Laboratory 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 Outline

3. Integrated Acoustics Laboratory Olson & May, Electronic sound absorber, JASA 25(6), 1130-1136, 1953

4. Integrated Acoustics Laboratory Precursor 48th ASA, Austin, 1955 (15 page program!)

5. Integrated Acoustics Laboratory Precursor 51st ASA meeting, MIT, 1956 (49 pages)

6. Integrated Acoustics Laboratory Olson, Electronic control of noise, vibration, and reverberation, JASA 28(5), 966-972, 1956

7. Integrated Acoustics Laboratory 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

8. Integrated Acoustics Laboratory Meeker 1958 Feedback, open-loop/feed-forward, and local control

9. Integrated Acoustics Laboratory 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

10. Integrated Acoustics Laboratory Meeker 1958 Feedback system (System II) 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°

11. Integrated Acoustics Laboratory 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

12. Integrated Acoustics Laboratory 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

13. Integrated Acoustics Laboratory Meeker 1958 Free-field sound absorber Concluded not viable due to constrained volume of effectiveness Still an area of research

14. Integrated Acoustics Laboratory 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)

15. Integrated Acoustics Laboratory Meeker 1959 Feedback ANC experimental results 110 dB max noise field

16. Integrated Acoustics Laboratory Meeker 1959 Semi-insert concept

17. Integrated Acoustics Laboratory 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

18. Integrated Acoustics Laboratory Patent: U.S. Patent US 2,972,018. Filed: Nov. 30, 1953, Patented: Feb. 14, 1961

19. Integrated Acoustics Laboratory 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

20. Integrated Acoustics Laboratory 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

21. Integrated Acoustics Laboratory The Bose Era Patents and publications galore A. G. Bose, and J. Carter, Headphoning, U.S. Patent No. 4455675, June 19, 1984

22. Integrated Acoustics Laboratory 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

23. Integrated Acoustics Laboratory Pan et al. Active 95 Band-limited noise 40-750 Hz, tones, chirp

24. Integrated Acoustics Laboratory 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

25. Integrated Acoustics Laboratory 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

26. Integrated Acoustics Laboratory 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

27. Integrated Acoustics Laboratory The “Remaining” Issues Transducer dynamics Compensation Acoustic path limit Low-frequency performance

28. Integrated Acoustics Laboratory 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!

29. Integrated Acoustics Laboratory Fertig

30. Integrated Acoustics Laboratory The Issues Forward-acting Feedback

31. Integrated Acoustics Laboratory 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

32. Integrated Acoustics Laboratory Carter 1984

33. Integrated Acoustics Laboratory Carter 1984 Adaptive feed-forward concept Considered “too expensive”