Unit Loudspeaker

    For public address and street announcement purpose we require such type of loudspeaker and a power amplifier which can reproduce the sound waves up to thousand of listeners in a public place. The loudspeaker used for the purposes is called unit loudspeaker. it consists of following two main parts.

1.       Pressure units: Its construction is similar to that or p.m. dynamic loudspeaker. The difference is that it has a large voice coil with a small cone. The impedance of voice coil may be up to 160 ohms whereas the impedance of an ordinary loudspeaker is not more than 20 ohms. It employs metallic cone in place of a paper cone. The power rating of a pressure unit varies between 16 and 20 watts whereas that of an ordinary loudspeaker lies between 3 to 8 watts.

            The pressure units convert the a.f. voltage received from an a. f. amplifier into powerful sound waves. It is known as drive unit.

Horn: if the sound waves reproduced by a pressure unit are allowed to radiate directly in the atmosphere then they would not be able sound waves in a hemisphere then they would not be able sound waves in a hemisphere area only around it. A pressure unit radiates the of aluminum sheet is attached to the pressure unit. The horn converts sound waves of low pressure and high pressure and low velocity in to sound waves of low pressure and high velocity.

            A horn is constructed in such a way that its diameter from its throat to mouth increase in a parabolic or exponential ratio. The total length of an ordinary type of horn is large enough and it does not suit for it to be used on vehicles. Hence, the horns on vehicles are of reflex or folded type. In a reflex horn, the total length of the horn is divided in to two parts and both the parts are fitted in such a way that the total length of the horn is reduced to almost half, whereas its effective length remains the same as of an original type.

COLUMN SPEAKER 

         It is almost impossible to reproduce the entire a.f. range of 20Hz to 20 KHz with a single loudspeaker, even more; the reproduction of the practical a.f. range of 30 Hz to 12 KHz with a single loudspeaker is quite difficult. If the size of the cone is kept between 25 to 35cm then it can reproduce frequencies above 8,000 Hz at the same level. Similarly, if the size of the between 5 to 15  cm then it can reproduce frequencies of the a.f. range of 8000 Hz to 20,000 Hz is reduced to lower level. Hence, the following two alternatives are employed for the faithful and equal level reproduction of the a.f. range of 30 Hz to 12 KHz.  

1.       Coaxial and Triaxial speaker: 

 A coaxial speaker consists of two cones of small and large sizes respectively fitted on the same axis. The smaller speaker is called tweeter and the larger one is called woofer. Tweeter improves high frequency reproduction while woofer improves low frequency reproduction.

               Similarly, a triaxial speaker consists of tweeter for high frequencies, a diaphragm for medium frequencies and woofer for low frequencies.

2.       Multi speaker system  : In this system two of three speakers of different sizes, especially designed to produce maximum output for a low pass filter (a choke of suitable value or a MFD value capacitor) is connected in series with the woofer. Similarly, a high pass filter ( a paper capacitor of suitable value) is connected in series with the tweeter. A box consisting more than one speaker is also known as column speaker.

Microphone mirror

               In sound recording, for adding the echos to the fundamental sound of a singer, polished metallic reflectors are used which are called microphone are also microphone mirrors. Besides, it high impedance microphones are called microphone mirrors.