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NOISE CANCELLATION IN HEADPHONES

M. Tech. Credit Seminar Presentation. NOISE CANCELLATION IN HEADPHONES. Vikash Sethia (Roll no. 02307023) Supervisor: Prof. P. C. Pandey 18 th November 2002. Presentation overview. Introduction Passive attenuation Active reduction Adaptive algorithms used in ANC headphones

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NOISE CANCELLATION IN HEADPHONES

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  1. M. Tech. Credit Seminar Presentation NOISE CANCELLATION IN HEADPHONES Vikash Sethia (Roll no. 02307023) Supervisor:Prof. P. C. Pandey 18th November 2002

  2. Presentation overview • Introduction • Passive attenuation • Active reduction • Adaptive algorithms used in ANC headphones • Practical applications • Conclusion Topic: Noise Cancellation In Headphones - Vikash Sethia

  3. Introduction • Acoustic noise • Solutions • Passive reduction techniques • Sound absorption and isolation - earmuffs, earplugs, audio-limiters • Good attenuation - about 15 dB and more at middle and high frequencies • Acoustic wavelength dependence of its size and mass • Hence bulky and heavy at low frequencies Cont … Topic: Noise Cancellation In Headphones - Vikash Sethia

  4. Active attenuation • Known as active noise control (ANC) - sound field modification, particularly sound field cancellation, by electro-acoustical means • Principle of destructive interference – cancel noise with “anti”-noise signal of the same magnitude but of opposite phase at some specific points • Adaptation to noise Topic: Noise Cancellation In Headphones - Vikash Sethia

  5. Filter Compressor and Amplifier Passive attenuation Headphone Error microphone - + Audio signal Fig. 1 Passive headphone attenuation with noise filtering • Filters – usually bandpass filters and audio-limiters • Poor attenuation inside the passband i.e. frequency band of interest Cont.. Topic: Noise Cancellation In Headphones - Vikash Sethia

  6. Fig. 2 Typical passive attenuation • Good attenuation at middle and higher frequencies Topic: Noise Cancellation In Headphones - Vikash Sethia

  7. Analog control Active ReductionAnalog feedback control Headphone Error microphone _ + Audio signal Fig. 3 Active headphone attenuation with feedback control • Error signal at the error microphone used as an error correction to the noise • Control elements – lead, lag and notch filters Cont… Topic: Noise Cancellation In Headphones - Vikash Sethia

  8. Merits – Good performance against broadband noise at low frequencies • Demerits • Possibility of excessive phase-shift leading to instability • Poor performance against narrowband or tonal noise Fig. 4 Typical active attenuation Cont… Topic: Noise Cancellation In Headphones - Vikash Sethia

  9. Active digital control Headphone Error microphone Noise reference - microphone + Audio signal Adaptive filter Fig. 5 Active headphones with feedforward control Cont.. Topic: Noise Cancellation In Headphones - Vikash Sethia

  10. Requires two microphones • Noise reference microphone – picks the disturbance or noise • Error microphone – picks the error signal and the noise that is not sensed by the reference microphone • Adaptive filters - use adaptive algorithms on DSP devices • Good performance against narrowband or tonal noise Topic: Noise Cancellation In Headphones - Vikash Sethia

  11. Digital Controller ∑ Loudspeaker ∑ Analog Controller Error Microphone Combined analog and digital control Audio d(n) signal + y(n) + x(n) - - To ear e(n) Fig. 6 Block diagram of combined analog and digital control Topic: Noise Cancellation In Headphones - Vikash Sethia

  12. Adaptive algorithms used in ANC headphones • Least mean square (LMS) algorithm For feedforward structure Loudspeaker output Error signal Weight update where Topic: Noise Cancellation In Headphones - Vikash Sethia

  13. Filtered-x LMS algorithm For feedback structure Error signal Estimated reference noise Loudspeaker output Weight update • Faster rate of convergence than LMS algorithm • Complicated because of the accuracy of the loudspeaker model Topic: Noise Cancellation In Headphones - Vikash Sethia

  14. Fig. 7 Spectrum of cockpit noise in a jet aircraft (upper curve), the noise at the ear of the pilot when wearing a conventional headset (middle curve) , and at the pilot’s ear when using a headset with active noise control. (Elliott, 1993) Topic: Noise Cancellation In Headphones - Vikash Sethia

  15. Practical applications • Audiometry – to diagnose hearing defects and speech discrimination • In industry – particularly in noisy areas • For military uses • Commercial applications – listening to music Topic: Noise Cancellation In Headphones - Vikash Sethia

  16. Conclusion • Good passive attenuation at middle and high frequencies, and better active control at low frequencies • Good performance of analog control at broadband frequencies, and digital control at narrowband frequencies for ANC headphones • Possible combination of analog and digital model for use at wide range of frequencies Topic: Noise Cancellation In Headphones - Vikash Sethia

  17. Thank you Topic: Noise Cancellation In Headphones - Vikash Sethia

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