Noise Control

Mitigating unwanted sound through the use of noise control techniques can be a requirement of lifestyle, legal issues, or environmental constraints. Unwanted noise can refer to a loud kitchen appliance, a heater, a fan or a noisy computer. Typically, an undesired noise source can be categorized as aerodynamic, (for example, pneumatic drills) or mechanical (unwanted vibration noise) in their origins, and there are several methods which can be employed at the source, or during the transit of the sound waves, to actively reduce the undesired noise.

Around the home and office, mechanical vibration is the most common form of unwanted noise. The first step toward eradicating the noise is to determine the source and evaluate what is causing the vibration. It can often be as simple as ‘dampening’ the source, rather than actively trying to ‘insulate’ the surrounding environments from the noise which can be time consuming and certainly more costly. A loud computer on a desk or wooden floor is a common source of unwanted vibration noise,

and in most cases simply placing the hard drive on a solid rubber mat will sufficiently remove most of the vibration. This is known as damping, and it is a very effective form of noise control because it absorbs the vibration (I.E. the source of the noise) and converts it to heat. Springs, counter weights and viscous fluids can also act as vibration dampers.

At times, it is undesirable or inefficient to dampen an object producing unwanted noise at its source, in which case one would target the transmission phase of the mechanical wave, rather than the emission phase. This would involve soundproofing an area surrounding the source and using a porous material like high density foam or fibre glass to absorb the noise. Introducing a ‘mass barrier’ constructed from a dense material like concrete or steel, to ‘insulate’ the outside environment is also an effective method of transmission noise control.

Both of these methods employ the use of passive noise reduction techniques. Active techniques eliminate the need for materials to insulate or dampen the sound, because they focus on the phenomenon of ‘interference’ to cancel out the unwanted noise. This is achieved by ‘activating’ a sound wave which emits sound at the same frequency as the unwanted source, but with the opposite polarity. The result being the two waves are ‘out of phase’ with each other and cancel each other out (Similar to the behavior of two colliding ocean waves).