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Sound

Sound is aural sensation caused by pressure variations in air and is sensed by the ear. The pressure variations can be due to the vibration of a solid object or a turbulence in a fluid. Sound in a fluid can be defined as a time-dependent disturbances of the particles in the fluid. Sound can be voice, speech of expression, or music with organized tone and rhythm. But when the sound is unwanted, it is noise.

Noise

Today, more sound sources are introduced to environment by the increasing of using modern technologies. They can be transportation noise from cars, machine noise from industries, appliance or equipment noises from personal and leisure activities. Noise or excessive sound in environment have adverse effects on hearing, verbal communication, and human behavior. Therefore, noise becomes a major concern in product design and product competitiveness. Besides, excessive sound can also affect the normal system operations by introduce fatigue damage on mechanical systems, such as the aircraft structure.

As the increasing demand for improved acoustic comfort, noise control and abatement becomes tremendously important in many aspects of engineering and product design

Noise Source & Transmission

Sound travels from source to receiver through direct transmission when they are very close together. But in general, sound transmission is usually through interaction among sound source, medium path and sound receiver.

Sound source induces an oscillatory movement to the ambient medium through the source media interface. The movement is transmitted through the medium path by the propagation mechanism. And sound is then detected by the receiver through the receiver media interface.

The two dominant noise sources are the structure-borne source caused by a vibrating surface and air borne source caused by gas fluctuation.  can be a speaker, vibrating surface, unsteady flow combustion

The medium path for sound transmission can be air, fluid, solid, or acoustic material for absorbing, damping or barrier

The receiver can be ear, or detecting microphone.

Noise Control

The objective of noise control is the making use of available noise controlling technology to provide a economic and practical solution for achieving the reduction of noise, or noise level attenuation and obtaining an acceptable noise environment according to the required standard or specification at the reception area or receivers.

The three elements in sound transmission are also the three key elements for noise control.

Noise control at the source is the most effective method but the involvement of source design also makes this approach the most difficult one.

Noise control at the receiver is also not a good solution because each receiver should be considered individually.

Noise control at the transmission path may be the most practical one by preventing noise propagation from the source to the receiver.

However, the coupling among these three elements is sometime not neglectable, and they should be treated simultaneously in order to achieve a global noise control. A practical way is to measure the actual noise level of the noise system, set the targeting acceptable level or the attenuation level, design a most appropriate solution for the noise system, verify the design by measuring the actual noise level again after the installation of the noise control system.