高等教育：环境工程课程介绍噪声

1、PART 5otherpollutioncontrol亚音速的 超音速的 音调subsonic supersonic pitch straightforward prolonged intriguing rhythmic attribute简明的，明确的 长时间的r 引人兴趣的 有节奏的 特性，属性suretiesfirecracker perception faintest rustic decibel烦人的细微的区别ter m 爆竹感觉，感知微弱的，轻微的沙沙响的分贝Sound and NoiseThat which we hear is called sound. One relatio

2、nship between motion and sound is suggested by the use of the word still, which means both motionless and quiet.We may infer, then, that sound is associated with motion. Motion is a form of energy. Sound must be produced by some source that transmits its energy of motion to us by some means that our

3、 ears can detect.The speed of sound in air under normal conditions on Earth is 334 meters/sec Any object ,such as an airplane，that travels slower than sound is said to be subsonic; one that is faster is supersonic. The sound wave is a succession of compressions and expansions that disturb the normal

4、 density of the medium in which they are propagated、This type of wave is called an elastic wave, The pitch of any given sound is determined by the frequency of the waves that produce it. The energy of a given sound, however, is not determined by the frequency, wavelength, or wave speed Sound power i

5、s related to loudness, but the two are not the same.wUnwanted Sound can be defined as no is e Yet this concept is straightforward enough, but it does not teach us how to predict which sounds will be disliked. After all, a given sound may be pleasant when soft but noise when loud, acceptable for a sh

6、ort time but noise when prolonged ，intriguing when rhythmic but noise when randomly repeated, or reasonable when you make it but noise when someone else make it. Of all the attributes that distinguish between wanted and unwanted sound, the one that we generally consider the most significant is loudn

7、ess.In any event, loudness tends to be annoying, therefore, he louder a sound is, the more likely is to be considered noise. There are yet other subtleties to complicate matters. People often associate noise with power.More difficult to modify, perhaps, is the association of noise with social recogn

8、ition. Such is the case, all too often, with loud speech, a loud motorcycle 9loud music ,especially toy guns and firecrackers.Loudness is not energy, or pressure or frequency or any thing else that can be measured with a physical instrument. Loudness is a sensation, and if you want to know how loud

9、a sound is ,you must get the answer from the person who hears itHowever jt is possible to obtain useful information from instruments by measuring some physical property of sound that is related to the human perception of loudness. The instrument that measures sound is called a decibel meter, and a s

10、cale of such values is called a decibel (abbreviated dB)scale It is set up as follows:(1) The scale starts at zero dB,which represents the softest soundthat is audible to the human ean(2) Each tenfold increase in sound intensity is represented by an additional 10 dB.Thus, a 10-dB sound is 10 times a

11、s intense as the faintest audible sound. The sound level in a quiet library is about 1000 times as intense as the faintest audible sound. Therefore the sound level in the library is 10+10+10 or 30 dB(3) Decibel levels are not directly additive. If one rustling leaf is 10 times the intensity of the f

12、aintest audible sound, two rustling leaves are 20 times as intense, not 100 times as intense, and therefore not 20 decibel louder. It would take 10 rustling leaves to be 10 times as、intense，and only then would the decibel level go up to 20 dBDecibel meters respond to sound pressure, and so does the

13、human ear, but the human ear also responds to the tonal qualities of sound, and particularly to its frequency, so it is important for us to learn about these very interesting attributes.Noise ControlNoise is transmitted from a source to a receiver To control noise, therefore, we can reduce the sourc

14、e ,interrupt the path of transmission, or protect the receiver.The most obvious source reducing the noise production by changing the source in some way It might be possible to modify technological approaches so as to accomplish given objectives more quietly.Thus machinery should be designed so that

15、parts do not needlessly hit or rub against each other. Rotary saws instead of jackhammers could be used to break up street pavement. Ultrasonic pile drivers could replace the noisier steam- powered impact-type pile drivers.Control of noise is a complex and sophisticated technology, and it is most ef

16、fective when it is applied to the original design of the potentially offending source. All too often a device or a machine or an entire industrial facility is designed with a view only to maximize its capacity to carry out its assigned function.If it turns out to be excessively noisy, an acoustical

17、engineer may be called in to sound-proof1it. Under suchcircumstances, the engineer may be forced to accommodate to features of construction that should never have been accepted in the first place. Therefore ,much of his effort may necessarily be applied ,not to the source ,but to the path between so

18、und and receiver.Sound travels through air by compressions and expansions It also travels through other elastic media, including solids such as wood. Such solids vibrate in response to sound and therefore do not effectively interrupt i t s transmission.However, we could use various materials that vi

19、brate very inefficiently ,such as wool or lead 9and absorb the sound energy, converting it to heat. (Very little heat is involved; the sound power of a symphony orchestra will warm up a room about as much as a-watt electric heater.)Sound absorbing media have been developed extensively; they are call

20、ed acoustical materials. We could also build interruption of the sound waves mechanically into more kinds of machinery devices that function in this way are called mufflers.Finally, we may be able to deflect the sound path away from the receiver, as by mechanically directing jet exhaust noise upward instead of down. Such defection is in effect 9an interruption b