Thursday, October 18, 2012

ACCOUSTIC


                              ACOUSTICS
                The word acoustics is originally derived from a Greek word meaning to hear. Hence, the acoustics is defined as the science of sound and as such, it discusses the origin, propagation and auditory sensation of sound.
 A sound is produced when part of the atmosphere is compressed suddenly, This compression would have remained stationary at that place only in case  the air was not elastic. But due to elasticity  of air, the particles originally disturbed in turn disturb  the in turn  particles. Ultimately that compression is
 But due t elasticity of air, the particles  originally disturbed in turn  disturb the  neighboring particles, Ultimately the  compression is propagated or spread away  from the source. The sound thus travels in the form of waves and when these waves come near our er-drums, we feel a sensation of hearing.
 Following important facts in connection with the sound are to be noted:
       
         I.            The sounding body which  throws the sound wages is in a state of vibration. If vibrations of the sounding body die out, the sound emitted by the sounding  body will also die out.
        II.             It is absolutely necessary that for a sound to be heard by our ears, the sound  body and ear must be connected by an uninterrupted series  of portions of elastic matter. The physical state of the matter, namely, solid, liquid or  gaseous, is no importance.
       III.             The presence of some matter is required for the transmission of sound. The sound cannot travel in vacuum.
         IV.            The sound waves are longitudinal  waves and hence, each  particle of the medium through which sound wave is proceeding, moves backwards and forwards along a line in the direction in which the sound wave is travelling.
The relation of sound with the human being is so common that  we rarely appreciate its functions in our daily life. It permits us enjoyable experiences like  listening to the musical programmed singing  of birds, etc. It also permits the spoken communication with pour family members and friends. It  also warns  us or alerts us e.g. singing of  telephone, knock at door, etc.
In modern society, many sounds are unpleasant or unwanted and we call them  noise. However, how much a noise annoys depends not  only on its quality but also our altitude to it. The sound need not be loud to develop noise.  For instance, a scratch in a record or a dripping faucet may annoy us much more then a loud thunder. 
 The worst effect of sound is that it can damage and destroy. A sonic  boom may shatter windows and can cause damage to the plaster of walls.  But the most unfortunate case is when the sound damages the delicate organ to receive it, namely, the human  ear.
In the chapter the topic  of acoustics as  related to the construction of building  will be discussed.


                               MEASUREMENT OF SOUND
            Following are the objects or purposes of  measuring the sound :
        I.                                                 It helps in improvising building acoustics and loudspeakers and thus enhances               our enjoyment of music, both in the concert hall and home.
      II.        It indicates when a sound may cause  hearing damage and thus helps in taking corrective measures to be taken, for avoiding the same.
    III.      It permits evaluation of the hearing sensitivity of an individual. 
    IV.       It permits precise and scientific analysis of the annoying sounds and  as such, gives us an objective means of comparing annoying sounds under different conditions.
      V.          It permits the improvement of the quality of our lives .
    VI.          It serves as a powerful diagnostic tool in the noise reduction programmers.
Sound  level meter : A sound  level  meter is an instrument which responds to the  sound  in approximately the same way alas the human  ear and which gives objective reproducible measurements of the sound level.
 The  sound level  is converted to an identical electrical  signal by a high quality microphone and since  the signal is quite small, it must be amplified before it can be read on a meter.
  The quality of the measuring microphone must meet may  high standards. It must have uniform frequency response It must be equally sensitive at all frequencies. The microphone should be equally sensitive to the sounds coming from all angles or in other words, it must possess an unidirectional  characteristic.
      The sound level  meter is a precise instrument and hence provision  is made ot calibrate it for accurate results. It is best done by placing a portable acoustic calibrator  directly over the microphone.  This calibrator is basically a miniature loudspeaker giving a precisely defined soud pressure level to which the sound level meter can be adjusted.

                                    INFLUENCE OF ENVIRONMENT

 The environmental factors which affect the measurement of sound are as follows :
1        Ambient pressure :  The variations in the atmospheric pressure will have as negligible  influence on the microphone sensitivity. For extremely high altitudes and other unusual conditions, the microphone instruction manual should be  referred.
2         Humidity:  The sound level meter and microphone will not be influenced by relative humidity levels up to 9% It is however necessary to shield the instrument  from rain, snow,  etc. ..
3        Magnetic fields :  The influence of  the electrostatic and magnetic filed on  the sound level meter is quite negligible.
4         Temperature :  The  sound level meters are designed to operate accurately over the temperature range of  10oC to + 50C. it is necessary to avoid sudden temperature changes which may cause condensation in the microphone.
5         Vibration : the sound level meters are relatively  insensitive to the vibration. But it is a good practice to provide foam rubber pads or similar material to isolate  them from strong vibrations and shock.
6         Wind : the wind blowing across the microphone disturbs the readings and to minimize  the effect of wind while working  outdoors, a special wind screen  consisting of a ball of porous polyurethane sponge should always be used over the microphone. It  will also give protection ot the microphone from dust, dirt and precipitation. 

              MEASUREMENT REPORT
            It  is necessary to have a good measurement report containing  the following  information :
·        Background noise level :
·         Data on object being measured like machine type load, speed, etc : 
·         Meteorological data and date;
·         Method of calibration;
·         Sketch showing measuring site,  location of microphone and object  being measured;
·         Type and serial number of instrument ;
·         Type of sound signal;
·         Weighing networks and meters responses  used ; etc
With a carefully written report, the future comparisons can be made more accurately  and treated as reliable.
When  the sound waves strike the surfaces of an enclosure in the form of a room with walls, the following three things happen;
·        Some of the sound is reflected back in  the  room.
·         Some  of the sound energy is absorbed by the surfaces and  listeners or persons present in the room.
·         Some of the sound waves are transmitted outside the room through the vibration of walls floors and ceiling .
 The amount of sound reflected or absorbed depends upon the  surfaces while the third category of transmitted sound depends upon the sound insulation properties of the surfaces. 


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