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 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.
I 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.
II 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.
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.
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.