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[1] EE-340 DEVICES & ELECTRONICS PROJECT REPORT : NOISE CANCELLATION HEADPHONES M. HARIS USMANI [email protected] OVAIS BIN USMAN [email protected] UMAR MUSTAFA [email protected] SYED BILAL ALI [email protected]

Noise Cancellation Headphones

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The project report for our Noise Cancellation Headphones, implemented using discrete components.

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E E - 3 4 0 DE V I C E S & E L E C T R O N I C S P R O J E C T R E P O R T :

NOISE CANCELLATION HEADPHONES

M. HARIS USMANI [email protected]

OVAIS BIN USMAN [email protected]

UMAR MUSTAFA [email protected]

SYED BILAL ALI [email protected]

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ABSTRACT:

Suppose you are listening to your favorite music (or anything for that matter) and there’s a lot of noise around you. You try to boost up the volume to kill the noise, but that becomes painful to your ears. You wish you could enjoy your music at a lower volume.

Dr. Amar Bose, founder of ‘Bose’, experienced this scenario when he went on a flight to Europe. The noise from the engines didn’t allow him to listen to his music. There, he came up with the idea and designed the world’s first Active Noise Cancellation Headphone.

OBJECTIVE:

Noise Cancellation can be effectively used to prevent environmental noise from entering your ears, allowing you to listen to your music at lower volumes. It also enhances the clarity of music, especially in the lower end of frequencies.

We aim to implement active noise cancellation on simple headphones (which do not have noise cancellation).

We aim to enhance the user’s listening experience: by reducing environment noise, especially in the mid and lower frequency ranges.

We aim to reduce cost by minimizing the number of integrated circuits used- we focus on using discrete components such as RC Filters and BJTs.

We aim to make our headphones portable: we use two standard 9V Batteries to power our circuit.

We aim to make our circuit robust: after extensive testing of the designed circuit on a breadboard and software simulations, we preserved the circuit to a PCB.

CONCEPT:

In noisy environments (like building sites, public places or in heavy traffic) it is often impossible to concentrate without being distracted by the surrounding noise. Sound is a mechanical wave that is an oscillation of pressure transmitted through

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a solid, liquid, or gas and is composed of different frequencies. The audible range for humans is from 20-20,000 Hz, which is our frequency of interest.

Sound waves can interfere with each other to result in an attenuated wave whose amplitude depends on the respective phases of the interfering waves. The phenomenon of destructive interference occurs when the interfering waves are 180

degrees out-of-phase and results in a wave with zero amplitude.

This was the basic principle on which we designed the circuits to cancel undesired noise. The main steps involved: catching the surrounding noise and converting it to an electronic signal using microphones and filters, amplifying and inverting the signal, adding the inverted signal to the original signal (music etc.) and converting the signal back to sound through the headphones. The idea was that the inverted signal will destructively interfere with the surrounding undesired noise (shown in green) and cancel it. This was our anti-noise signal, as shown in the diagram below in blue.

REQUIREMENTS*:

1) Simple Headphone 2) Two Microphone Arrays 3) Amplifiers (made using BJTs) 4) Filters using RC Circuits

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5) Op-Amps as Voltage Follower and Adder 6) 3.5mm Audio Jacks

*excluding other lab equipment for testing and debugging.

PHYSICAL DESIGN:

We attached two microphone arrays to both sides of the headphone. The wires of both the mics and the speakers go in to a circuit which does the noise cancelation. The other input to this circuit is our audio signal. The output of this circuit is fed back to headphone speakers.

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BLOCK DIAGRAM (WITH ABSTRACTIONS)

This block diagram is just for one channel of the headphones. Other side would be a replica.

Input i.e. noise caught from the mics.

Human Ear

BJT Inverting Amplifier

A low pass RC Filter

Voltage Follower

Voltage Opamp Summer.

BJT unity gain inverting amplifiers

AUX IN

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DESIGN & IMPLEMENTATION

We designed the circuit twice. Once using Op-Amps for Analog Operations and then using simpler discrete components like BJTs.

Our final implementation was the one based on simple discrete components. Following are the steps involved in the implementation:

PICKING UP THE SURROUNDING NOISE:

We used an array of three small condenser microphones. These mics are commonly available and extensively used in toys and cheaper headsets. We could’ve used just one Mic for one Channel; instead we connected three mics in parallel in order to have a more favorable noise-to-signal ratio. This decreased our amplification requirements too. We biased our condenser mics at +9V thorough a 10kΩ Resister, while we place a high-filter to its signal output in order to get our frequencies of interest (~8-20,000 Hz) and avoid the 9V DC Bias.

AMPLIFYING AND INVERTING THE NOISE SIGNAL:

Amplification using Op-Amps

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Once we have our environmental noise signal, we amplified the signal using a Negative Feedback Inverting Amplifier (using an Op-Amp), as shown in the schematic. The gain of this setup was adjusted by varying the ratio of the two 5kΩ resistors. Our desired gain was approximately -6.3 for the right channel and -6.8 for the left channel. This was experimentally determined by tuning the gain for the ‘least noise heard’ or ‘maximum noise cancellation’.

The actual approach we adapted was to use an inverting amplifier using a Bipolar Junction Transistor. We chose BJT over MOSFETS as they have a more linear response in our desired frequency range. The schematic for the BJT Circuit is shown on the next page. We opted for a Common-Emitter BJT Amplifier with an Emitter Resistor and a Bypass-Capacitor. This choice was made in order to get an ‘inverting’ amplifier, and one that is robust yet simple to implement. We designed the DC Biasing for 1mA current and a gain for ~-7. Placing a resistor (shown below as 1.48kΩ) after the by-pass capacitor allowed us the freedom to adjust the gain using a variable resistor without affecting our DC Biasing.

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Once our signal was inverted and amplified to our desired gain, we placed a voltage follower (using an op-amp) in order to use an Adder Circuit that follows. If we did not use a voltage follower, the input impedance to the Adder Circuit would vary, resulting in an unequal addition of the two signals.

ADDING THE INVERTED NOISE SIGNAL TO THE ORIGINAL SIGNAL:

After the voltage follower, we placed a Low Pass Filter. This was to cut the high pitched static in the anti-noise signal, in order to enhance the experience of the user. We filtered frequencies over ~5KHz, as our Noise Cancellation was working effectively for frequencies below this point. The Adder Circuit was implemented using an Op-Amp, that summed both our signals equally. Placing a voltage-follower before the adder guaranteed equal addition of both signals. Here, the music input from Auxiliary IN is summed with our Anti-Noise Signal.

Amplification using BJT

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Please note: the output of our adder circuit is inverting, hence we still need to invert the signal once more in order to get our desired output.

FEEDING THE RESULTING SIGNAL TO THE HEADPHONES:

Last in line was to invert the signal from the Adder with a unity gain. This was also first implemented using an Op-Amp inverter, but our final implementation was using a BJT Amplifier or similar characteristics to the once we used before. The only difference was the biasing point and gain. This time, our requirement was to have enough current to effectively drive the headphone speaker unit, hence we designed a DC Biasing for ~40mA. Our desired gain was -1, hence the resistor following the by-pass capacitor was kept larger than before. It is important to note that we successfully designed this amplifier to match the audio quality of an inverter implemented using an Op-Amp. We had to use

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larger capacitances in order to accommodate the low frequencies (further elaborated in difficulties).

IMPLEMENTING THE PCB:

Once the circuit was successfully implemented on the breadboard, it was time to transfer it to a PCB. We used ‘Labcenter Proteus’ to make our PCB layout design. After approximately 6-8 hours of designing, it took reasonable 8 hours from etching to complete component assembly. 3D output of circuit along with layout is given below.

3D RENDERING OF THE CIRCUIT

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PRINTED CIRCUITED BOARD LAYOUT

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DIFFICULTIES

MECHANICAL:

Making headphones with proper mic installation was a difficult task. We had to align the mics on both sides of headphones so that the noise caught from both sides was the same. Hence, precision and accuracy while building the headphones was of the key essence. For better noise catching, three mics were installed on both sides of the headphones. And also to avoid sound travel between the mics and the headphones body, the mics were installed on a soft foamy material placed between the mics and the headphones. This meant the system did not cut out any music mistaking it for environmental noise.

TECHNICAL:

A problem with our circuit was that for one side of headphones we were using the 741 IC (Op-Amp) as the amplifier while on the other side a BJT in Active mode was used as the amplifier, both were providing output to drive the speakers of the headphones. Both 741 and BJT had to be matched so no difference in sound occurs from both sides. Fine tuning of the circuit's filter was also a cumbersome task and so was the gain setting for the first BJT inverting amplifier.

High frequency noise cancellation was a bit difficult for us. High frequency noise cancellation requires both active and passive noise cancellation. Due to lack of time and resources we were not able to do perform effective passive noise cancellation. However that can be achieved by installing a certain material which does not allow high frequencies to enter ears from air, while the high frequencies caught by mics are filtered out using filters.

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THE MAIN SCHEMATICS (FOR LEFT OR RIGHT CHANNELS)

USING BJTS:

USING OP AMPS: