What is Sound?
Before getting to know how noise canceling works, it is important to understand what the sound is and how people perceive the sound. A sound is a form of vibration by air molecules that travels through the air. When the air molecules move in a pattern, a sound wave is generated. The waves generate due to the movement of the air molecules by the applied force, for example, when a person claps, the air molecules around the hand get pushed by clapping and thus generate sound waves.
The fundamental sound wave can be shown in this graph.
When a sound wave is generated, the interval between one compression and the next compression is defined as the wavelength. The vibrations can squeeze the air molecules tightly or gently, and how intense the squeeze is called amplitude. The amplitude of the sound can describe the volume of the sound. The higher the amplitude, the louder the sound becomes
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This fundamental graph is directly related to the movement of the air molecules below.
As the air molecules compress with each other, it represents the compression on the graph. Viceversa, the molecules being further from each other represent the rarefaction part on the graph. The air molecules do not move on the way with the sound wave, but they oscillate back and forth along the wave’s direction.
How do people perceive sound?
Basically, the sound wave travels through the ear to the brain to make people recognize it as the sound. To start with, the sound wave enters through the narrow ear canal. Then, when the sound hits the eardrum, the eardrum gets vibrated, which vibrates three tiny bones called the malleus, stapes, and incus. These bones increase the vibrations and transfer them to the cochlea. Since the cochlea is filled with fluid, the transfer of vibrations creates waves in the cochlea. The hair cells inside the cochlea are influenced by the waves, and its bundles (top of the hair cells) have movements. When hair bundles get movement, ion flows to the top of the hair cells, causing the release of the chemicals at the bottom of the hair cells. These chemicals bind to the auditory nerve cell and generate an electrical signal that travels along the auditory nerve to the brain. The auditory nerve carries the electrical signals to the brain, and makes us recognize what is called “sound”.
Active Noise Cancellation
Now, let’s go to the main topic, the mechanism of active noise cancellation. Firstly, active noise cancellation is the system of reducing background noise by using microphones and speakers. To cancel the background noise, the headphone detects the surrounding noise and sends the anti-noise to the surrounding noise.
The question would be how and what parts of the headphone works to activate the noise canceling.
There are complicated engineering involved in noise cancellation, and the mic is definitely one of them in noise cancellation technology. The mic inside the headphone detects the surrounding noise frequency and then calculates an anti-sound wave that is equal but opposite to the background noise. For every compression (high pressure of the air molecules) of the background noise, there would be rarefaction (low pressure of the air molecules) created or vice versa. This anti-sound wave is then added to the normal audio that is sent from a smartphone. When the normal audio wave, the anti-noise wave, and the background noise wave are mixed, the anti-noise and the background noise cancel out, leading people to listen to the normal audio only.
In reality, there is no such thing as the perfect noise cancellation. The background noise is not constant and has a variety of frequencies, however, it is certain that this active noise cancellation definitely plays a huge role in the reduction of noise, and many people desire this noise cancellation.
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