CLASSIFICATION OF SOUND WAVES

Sound waves can be classified into following three classes:

1.       Audible waves

2.       Infrasonic waves

3.       Ultrasonic waves

Audible waves:

The waves which can be heard by human ears are called audible waves. Their frequency range extends from 20 Hz to 20 KHz but it may vary from person to person in accordance to his age. Children and youth can hear sounds of up to 20 KHz frequency whereas the audible range of old persons is lesser than 20 KHz.

Infrasonic waves: 

Waves having a frequency of less than 20 Hz are called infrasonic waves. These waves cannot be heard by human ears. These waves are produced by the vibration of huge bodies like the earth.

Ultrasonic waves:

Waves having frequency of more than 20 KHz are called ultrasonic waves. These waves also cannot be heard by human ears, but can be heard by bats. The bat can hear waves of frequencies up to 50 to 60 KHz and can produce such waves even.

These waves have proved to be very useful to mankind. These are used for communications and for the determination of depth of sea. Besides it, these are used for increasing agricultural yields, for improving the quality of seeds, for protection against insects and in the field of treatment especially surgery.

ARTICULATION OF SOUND

 Articulation or modification of sound is necessary before broadcasting. For this, the following instructions should be observed:

1.       The broadcasting room should be perfectly sound-proof so that unwanted sounds may not enter in the room.

2.       Microphone, record player, tape recorder and amplifier etc. should have a high efficiently so that all the fundamental and harmonic frequencies remain undistorted.

3.       The frequency range of sound is limited between 30 Hz to 12 KHz since the frequency of all sounds produced by man, sound and musical instruments lies between 30 Hz to 12 KHz. By limiting the frequency range, amplification of high quality and high fidelity is easily possible.

4.       Before sending the sound signals to the transmitter they should be amplified up to a required level.

SOUND

It is a form of energy which is experienced by human ears. It is produced by the vibration of bodies, by man, animals and birds and by natural interruptions. Sound is propagated in the form of waves and some medium is essential for its propagation. The medium may be a solid, liquid or gas but it should have sufficient elasticity.

The velocity of sound in dry air at 0˚C is 332 meters per second and in water it is 1435 meters per second. Similarly the velocity of sound in different medium is different medium is different. The velocity of sound in the air is affected by humidity, temperature, density of dust particles and the speed of air circulation.

FREQUENCY AND WAVELENTH

Frequency: for a wave, the number of cycles completed per second is called its frequency. Its symbol is ƒ and its unit is hertz (Hz).

Wavelength: straight distance travelled by a wave in one cycle is called its wavelength. In other words, the distance between the two consecutive particles vibrating in the same phase is called wavelength. Its symbol is λ (lamda, a Greek letter) and its unit is meter.

Time period: the time taken by one complete set of vibration, i.e., one cycle is called time period. Its symbol is T and its unit is seconds. Therefore

T= 1/ƒ or ƒ=1/T

T= time period, seconds

ƒ =frequency, hertz

Relation: the wavelength decreases by increasing the frequency and conversely the wavelength increases by decreasing the frequency, but their product remains constant and it indicates the velocity of the wave. Hence

Ѵ =ƒ.λ   or   Ѵ = λ/T

Where,        Ѵ = velocity of the wave, meters per second

                     λ = wavelength, meters  

                     ƒ = frequency, hertz

                     T = time period, seconds

Application of Direct Spread Spectrum Signals

 Here we will discuss three types of applications of direct sequence spread spectrum signals: anti jamming signals, low probability intercept and code division multiple access of allow multiple users.

1.      Anti jamming with the help of direct sequence spread spectrum signals

In the frequency band of the interest somebody else transmits the signals intentionally.

Since these signals lie in the frequency band of the transmission, the interfere the required signal. Hence it becomes difficult to detect the required signals. This is called jamming effect.

Such problems normally occur in military applications. If enemy knows the frequency band, then he can send jamming signals intentionally. With the help of spread spectrum method, the transmitted signals are spread over the mid frequency band. Hence these signals appear as noise. Then it becomes difficult for the jammers to send jamming signals. This is called anti jamming. For the anti jamming application three codes are used in conjunction with direct sequence spread spectrum method. These are Golay code (24, 12) expurgated Golay (24, 11) and maximum length shift register code.

2.      Low detectability signal transmission or low probability intercept

For this application, the signal spectral density is intentionally kept small with respect to the channel noise and receiver noise so that the presence of the signal is not detected easily. Consider that the average received signal power at the intended receiver is Pav and the noise power is Nav. Then the signal is transmitted at low power levels such that Pav/Nav << 1. Hence the receivers which are in the vicinity of the intended receiver cannot detect the presence of the signal. The intended receiver recovers the information bearing signal with the help of processing gain and coding gain. Other receivers do not know the pseudo-noise sequence hence they cannot receive the information signal with the help of processing gain and coding gain. This is called as the signal has low probability of being intercepted. It is also called low detect ability signal transmission.

3.      Code division multiple access with direct sequence SS (SSMA)

In this application, many users transmit their signals on the same channel bandwidth. Each transmitter receiver pair has a distinct pseudo-noise (PN) sequence. Thus signals of a particular transmitter are received by its intended receiver only, even if many users are transmitting at the same time. This method is also called spread spectrum multiple access (SSMA). The signals from other users appear as additive interference which is rejected y the spread spectrum decoder. The level of interference depends upon the number of users transmitting at any time. The main advantage of CDMA is that the number of users sharing the same channel can be increased or decreased very easily. Large number of users can transmit on the same channel if their messages are for short periods of time. For this method it is desirable that the PN sequences be mutually orthogonal.

Sensor

A Sensor is a device, which responds to an input quantity by generating a functionally related output usually in the form of an electrical or optical signal.

Thermocouples
 
A thermocouple is a device consisting of two different conductors (usually metal alloys) that produce a voltage proportional to a temperature difference between either ends of the pair of conductors. Thermocouples are a widely used type of temperature sensor for measurement and control and can also be used to convert a heat gradient into electricity. They are inexpensive, interchangeable, are supplied with standard connectors, and can measure a wide range of temperatures. In contrast to most other methods of temperature measurement, thermocouples are self powered and require no external form of excitation. The main limitation with thermocouples is accuracy and system errors of less than one degree Celsius (C) can be difficult to achieve.

Any junction of dissimilar metals will produce an electric potential related to temperature. Thermocouples for practical measurement of temperature are junctions of specific alloys which have a predictable and repeatable relationship between temperature and voltage. Different alloys are used for different temperature ranges. Properties such as resistance to corrosion may also be important when choosing a type of thermocouple. Where the measurement point is far from the measuring instrument, the intermediate connection can be made by extension wires which are less costly than the materials used to make the sensor.

Thermocouples are usually standardized against a reference temperature of 0 degrees Celsius; practical instruments use electronic methods of cold-junction compensation to adjust for varying temperature at the instrument terminals. Electronic instruments can also compensate for the varying characteristics of the thermocouple, and so improve the precision and accuracy of measurements.

Thermocouples are widely used in science and industry; applications include temperature measurement for kilns, gas turbine exhaust, diesel engines, and other industrial processes.