Sound Attenuation声衰减

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Sound Attenuation
INTRODUCTION
In the broadest sense,a sound wave is any disturbance that is propagated in an elastic medium,
which may be a gas,a liquid,or a solid.Noise is defined as any unwanted sound perceived by
the hearing sense of a human being.Excessive noise can impair hearing,and may also put stress
on the heart,the circulatory system,and other parts of the body.Worker exposure to excessive
noise over an extended period may result in a permanent loss of hearing.The introduction of a
noise source into a given environment can be potentially hazardous,as well as objectionable to
nearby tenants and residents–depending on its sound level.Numerous laws have been enacted
at both national and local government levels to limit excessive noise.Such regulations are
typically grouped together based upon the land use characteristics and the proximity to
residential or other sensitive areas.
Every situation in noise control involves a system composed of three basic elements:source,
path,and receiver.Before a solution to a complex noise problem can be designed,the dominant
source of the noise must be known,the characteristics of the significant transmission path must
be understood,and a criterion for the level of noise considered permissible or desirable in this
situation must be available.
25.1 BASICS OF SOUND
25.1.1 Concept of sound.A person perceives sound as any vibration of the eardrum in the
audible frequency range that results from an incremental variation in air pressure at the ear.A
variation in pressure above and below the atmospheric pressure is called sound pressure and is
measured in units of Pascal(Pa).
The number of pressure variations per second is called the frequency of sound,which is
measured in cycles per second,called Hertz(Hz).A young person with normal hearing can
perceive sound in the frequency range of roughly 20?20,000 Hz,defined as the normal audible
frequency range.A sound that has only one frequency is known as a pure tone.Pure tones are