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.
