None of us wants to hear that annoying humming sound of engine while travelling or unwanted noise while you are listening to your favorite songs or speaking with someone over the phone. May it be the headphones /earphones technology is equipped to remove out noise and give us a pleasant hearing experience. Noise Cancellation is one such efficient technique.
What is NOISE?
Any unwanted information /signal /data which can interfere with the communication affecting the overall quality of the sound transmitted or received.
Need for NOISE CANCELLATION:
Too much exposure for low frequency signals over long period can result in fatigue and other health disorders.
To maintain the actual quality of the sound /voice which is transmitted through the headphones/earphones.
Volume need not be kept high while hearing which will reduce chances of damage to ears.
Finds applications in automobiles, which will remove the unwanted sound caused by running of automobile.
What is Noise Cancellation?
Noise cancellation is a technique which removes unwanted ambient sounds leaving behind rich quality of original sound. This makes it feasible to hear clearly even at low volumes.
Noise cancellation though is currently not advanced to defeat high frequency ambient noise, but can effectively overcome low frequency noise with the help of electronics.
In Further sections we will look into different types of noise cancellation techniques and their applications in real world.
Types of Noise Cancellation:
Passive Noise Cancellation
Active Noise Cancellation
Passive Noise Cancellation:
This is a form of cancellation involving zero electronics and devices with passive noise cancellation are commonly found in large number.
Passive noise cancellation combats unwanted noise and ambient sounds by means of design. In the sense the way the headphones are built play a major role in this type of cancellation.
This technique reduces most of mid and high frequency unwanted signals.
They are less sophisticated by design as no electronics is involved.
Cheaper to manufacture compared to active noise cancellation devices.
In summary passive noise reduction involves blocking of external sound completely merely with the aid of the design without assistance of electronics.
Active Noise Cancellation (ANC):
Active Noise Cancellation involves noise removal by addition of an exact sound specifically designed to cancel the first. ANC is best suited to overcome low frequency noise signals with the aid of electronics.
Principle of Working:
ANC involves the concept of Destructive Interference. The underlying principle is to listen the noise from the external world using microphones which cannot be nullified passively .This is further inverted or phase shifted and added with original noise signal, the output signal erases the noise without affecting the overall quality of the sound in normal audio.
Fig. 1: Image Explaining Concept of Active Noise Cancellation
ANC Working Explained:
The most common mechanism is feed-forward topology. Consisting of following components:
A microphone to sample the unwanted sound or disturbance
An electronic control system to process the reference signal and generate the control signal
A loud speaker which produces the signal to cancel disturbance driven by control signal.
Fig. 2: Simple Block Diagram Explaining Working of Active Noise Cancellation
The input noise signal is referenced from external world with the aid of microphones. The number of microphones for this purposes varies accordingly. The received signal is phase shifted by 180°. This inverted signal is further interfered with original signal to remove out only the noise without changing the characteristics of actual sound.
There are many different methods to achieve Active Noise Cancellation, using Adaptive Filters or Digital Signal Processing.
What is an Adaptive Filter?
Adaptive Filter has a characteristic of adapting the response of the filter to the input signal characteristics which enhances the performance and stability of the system.
Usage of Adaptive Signal Processing in Active Noise Cancellation Systems:
Adaptive Filter finds application when the parameters are not fixed or specifications are unknown.
Fig. 3: Diagram showing Adaptive Filter in Active Noise Cancellation System
Adaptive Noise Canceller
There are several algorithms which can be used to configure adaptive filter, each of these are used to compute the filter
coefficient which will help to minimize the error signal in shorter interval of time few of them are as mentioned below:
It consists of Digital Signal Processor to perform complex computations and algorithm processing
Least Mean Square (LMS) – Most Frequently Used as it is simple to realize and efficient.
Normalized Least Mean Square (NLMS).
Application of Noise Cancellation in Automobiles:
Need for Noise Cancellation?
One of vital field of application for noise cancellation would be automobiles. In general any automobile consists of an engine and other moving parts which cause vibrations to build up and are transmitted across the body of the vehicle. One such disturbance is due to noise. The reason why automobiles are getting costlier is due to such enhancements which will give a pleasant driving experience.
Types of Cancellation:
Structure Based (Passive Noise Cancellation)
The issue of noise is considered right from the point of vehicle design. Most of the cabin and passenger compartment noise are eliminated by the means of sound damping materials which passively reduce the noise inside the automobile. But this method has a drawback associated with it. Since this approach increases vehicle weight and invites more side-effects. Active noise cancellation is more advantageous.
Active Cabin Noise Cancellation Technique:
Here the leverage of electronics is brought into account. . This is similar to functioning of headphones. One inherent drawback is here the
system has to combat with frequencies of higher range, hence it becomes difficult to cancel noise throughout the cabin uniformly after few
hertz of order in hundreds.
The cabin of the vehicles are equipped with microphones which are located to capture noise inputs from different parts of the car.
The captured signal is sent to one of the control unit inside the automobile, the controller might be a dedicated one or the functionality might be embedded in multimedia ECU.
Based upon the characteristics of the input signal, the controller produces a control signal of same amplitude but of inverted phase which is fed as input to speakers located at the various corners of the automobile.
The outcome is subtraction of noise with the anti-noise signal producing a silencing effect inside the cabin.
Active noise cancellation not only finds an application in Cabin noise suppression, but also in mitigating engine combustion and exhaust system noise.
In case of exhaust system noise cancellation, the shape of the exhaust pipe plays a vital role in suppressing the noise produced by combustion of fuel.
As a part of noise cancellation, actuators in the exhaust stream are electronically controlled through solenoid valves, etc. These actuators control the opening of the exhaust stream channel which will damp the pressure waves and their by the disturbance
Applications of Active Noise Cancellation:
Noise reduction in HVAC systems especially air conditioners.
Automobile Engine noise vibration elimination
Consumer goods – Refrigerators, Washing Machines, lawn movers, vacuum cleaners etc.
Public Phone Booths.
Trucks and mining equipment.
Naval ships and Pleasure Boats
Aircraft and Cabin noise control
Potential Drawbacks of Active Noise Cancellation
The complete system of noise cancellation includes DSP, microphones and loudspeakers.
Integrating noise cancelling hardware into existing hardware.
Excessive cost of installation.
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