Noise Canceling Headphones Team Members Doan Thanh Khiet Tran Jasmine Khadem Kalina Guentcheva

Introduction In today’s noisy world it is hard to find a peaceful place. This new technology of noise cancellation can block out unwanted noise (signals) around us and achieve that ‘noise free environment’. By picking up ambient sound with a microphone and reproducing it out of phase, we can actively cancel or "null" out background noise.

How it works  The electronics behind this device consists of three op-amp circuits. Each circuit uses an op-amp in a different configuration. The first circuit is a non-inverting pre-amp. The second is a unity-gain phase-inverter. The third is an inverting headphone amplifier.

System Overview Power Plant Non- inverting pre-amp/ high pass filter Inverting amplifier (unity gain) Inverting amplifier Speaker Main PCB Switch Microphones

System Overview  Power Plant  used to power the system(± 9V DC).  Microphones  picks up the signal and converted into an electrical signal with a base-band bandwidth (~20 kHz).  Non-Inverting pre-amp/High-pass Filter  blocks the DC component and achieves a significant gain.  Switch  directs the signal between two different paths. It can go through a unity gain inverter and/or a controllable inverting amplifier.  Inverting Amplifier (Unity gain)  amplifies the signal.  Inverting Amplifier  cancels the noise.  Speaker  the output from the inverting amplifier goes to the speaker where the user can hear either the amplified signal or have a quiet zone.

Construction  There are two parts to this project: Building the electronics Modifying a pair of headphones

Building the electronics  Non-Inverting pre-amp/High-pass Filter The DC component and low frequency component of the audio signal (0~10 Hz) is blocked off.

Bode Plot of High-Pass Filter

Building the electronics  Inverting Amplifier (Unity gain) acts as an isolation barrier between the other stages in order to minimize noise.

Building the electronics  Inverting Amplifier At this stage, the microphone signal is inverted by 180 degrees and the background noise is canceled.

Overall circuit

Outputs Amplifying the signal Canceling the signal

Applications  Headphones that have adaptive noise cancellation are used for pilot’s radio headset in jet aircrafts.  Simply to block unwanted noise from the background.

Drawbacks/Improvements  Drawback: Consumption of power  the headphones require a battery that needs to be recharged. It also makes the headphones bulky. Works well only if the signal is continuous  if the signal is changed frequently then the noise will not be cancelled out efficiently. Only the hardware static filter was implemented (from 20 kHz to 30 Hz). The output is shifted by 45 degrees.  Improvement: Using a regulator that can generate –9V from 9V battery. Use a software control dynamic filter to achieve best performance at wide frequency range.

Conclusion  We have designed and successfully implemented a noise cancellation circuit.  Fast response time is a requirement for noise cancellation system.  Power consumption optimization is particularly important for mobile scenarios.

References  LM1575 Datasheet, October 26,  Head Wire: November 2,  Active Noise Cancellation: November 21,  Adaptive Noise Cancellation: November 15,  Adaptive Noise Cancellation: November 17,

Questions ? THANK YOU