聲學(xué)基礎(chǔ)教學(xué)課件fundamentalsofacoustics071

1、Basic KnowledgeIn following pages, we will learn: Introductions of this course. What is sound. What sound is caused by. What a sound wave is. What a cycle is.Prerequisites: An understanding of basic mathematics. The fundamental principles of mechanics. Electricity and access to a scientific calculat

2、or.Objective:Throughout the course, emphasis is placed on the fundamental of acoustics, with discussions and problems extending into some phases and applications of acousticsThe Generation, Transmission and Reception of Acoustic WavesOn completion of this course, students will be able to understand:

3、 the terms used to describe sound waves, the generation, transmission, and reception of acoustic waves and their application to numerous fields. the relationship between sound pressure, sound intensity and sound power. the terms used to describe sound fields.What is sound?Sound is mechanical wave of

4、 energy that changes the pressure of its medium (air or water) as it moves. These changes of pressure are detected by our sense of hearing and transmitted to our brains for interpretation. Sound waves are described by their wavelength, amplitude and frequency and intensity (in decibels).Any sound, w

5、hatever it might be, is caused by something vibrating. Without vibration there can be no sound. The vibrating body causes the air particles next to it to vibrate. Those air particles, in turn, cause the particles next to them to vibrate. In this way a disturbance of the air moves out from the source

6、 of the sound and may eventually reach the ears of a listener. The Transmitting MediumWhen we hear a sound, air vibrates against our eardrums causing them to vibrate also. These vibrations are detected and analyzed by our brains. Although it is usually the air that acts as the transmitting medium, s

7、ound can be transmitted by other media, e.g. water, building structures, etc.Sound WaveSound does not consist of air moving towards us in bulk. It travels through the air as a sound wave. A sound wave consists of a disturbance moving out from a source to surrounding places with the result that energ

8、y is transferred from one place to another. Compressions and RarefactionsAs the wave passes, the disturbance of particles(質(zhì)點(diǎn)) is in the direction of the wave travel. The displacement of particles of the medium results in alternate regions of high particle density and low particle density. Regions of

9、 high particle density are called compressions. Regions of low particle density are called rarefactions. A CycleRarefactions and compressions both move in the direction of the wave travel. The particles of the medium do not move bodily in the direction of the wave movement. They vibrate about their

10、normal positions. Each complete vibration of a particle is called a cycle ( i.e. from its starting position, to a maximum distance in one direction, back through the starting position, then to a maximum displacement in the opposite direction and back to the starting place). TransmissionAs sound spre

11、ads out in an open-plan office, it meets obstacles: floor, ceiling, partitions (partial-height screens), light fixtures, furniture, etc. These obstacles change the path of the sound. When designing an open-plan office to block sound, designers must consider all of these paths.AttenuationAs sound wav

12、es propagate through any medium, the sound energy diminishes due to spreading, scattering, absorption, and sound transmission loss, which would be introduced in next page.Spreading occurs as the sound expands in spherical waves. The sound level decreases as the distance from the source becomes large

13、r.Scattering occurs as the wave direction changes through diffraction or reflection. Absorption occurs as the sound enters a porous material and gets trapped in the air pockets. The trapped sound energy is converted to other forms of energy.How does sound travel through water?Water is an ideal mediu

14、m for the transmission of sound. The speed of sound in water is 5 times faster than it is in air. Sound travels 334 meters per second (m/s) in air and approximately 1500 m/s in water. This speed can be altered depending on the salinity, temperature and depth of the water. NoiseNoise is defined as unwanted or damaging sound, that is, sound which interferes with what people are trying to do, or sound