The
loudspeakers are devices which are designed to reproduce the actual
voice according to the voice frequency signals fed into it. The audible
voice spectrum stretches from around 20Hz to 20 KHz and the interesting
thing is that there is not loudspeaker design which can reproduce all
these frequencies with the same effect. Hence there are different kinds
of loudspeakers like Tweeters, Mid-range Speakers, Sub-Woofers and
Woofers which can reproduce the voice in their own particular frequency
bands only.
In
an audio play back device, the entire audio signals are separated into
different bands and applied to the corresponding type of loudspeaker.
The Tweeters are normally fed with frequencies above 5 kHz, Mid-range
speakers are fed with frequencies in the range of 300 Hz to 5 kHz, and
Sub-Woofers with 300 Hz to 40 Hz and Woofers are fed with frequencies
below 40 Hz.
Since
the musical sound normally falls around the maximum frequency of 5 to 8
KHz, Tweeters are not so common in audio devices. For driving the
Mid-range Speakers a high pass filter of cut-in frequency around 300 Hz
is enough and for Woofers a Low pass filter with cut-off frequency
around 40 Hz will suffice. Bass-beats of the songs appear in the
Sub-Woofer range and a Band-pass filter can be used to separate out
these frequencies from the entire audio signals. This project discusses
the design and implementation of a Multiple Feed Back (MFB) Band pass filter for Sub-Woofer frequencies.
The MFB filters
are very commonly used in circuits due to the fact that they provide
reasonable performance with the simplest circuit. They can be designed
to obtain narrow bandwidth and high gain. They are suitable for the design of Band Pass filters
since the bandwidth, mid-band frequency can be easily adjusted or
varied. These circuits has an amplifier with more than one feedback and
hence the name. The circuit diagram of a MFB band pass circuit using a
single resistor and capacitor feedback is given below;
The
gain of the circuit depends on the value of R1 and R2 and the Q factor
(mid-band frequency divided by bandwidth) and bandwidth depends on the
values of R2 and C. The value of R3 affects only the mid-band frequency
selection and hence it can be used to vary the mid-band frequency
without changing the gain or Q factor.
The equations relating the value of the components to the gain, Q factor, bandwidth and mid-band frequency are given below;
To
start design this circuit select the required value for the mid-band
frequency (Fm), mid-band gain (Am), Bandwidth (B) or Q factor (Q) and
capacitor value (C) and the following equations which are derived from
the above will help to calculate the value of other components;
For
extracting the Sub-Woofer frequencies from the music the band pass
filter here is designed for an ‘Fm’ of 70 Hz, Q of 15 and Am of 100 with
a capacitor value ‘C’ of 0.1uF. The values of R1, R2 and R3 can be
calculated as given below;
Fm = 70 Hz
Q = 15
Am = 100
C = 0.1 uF
Hence for a MFB band pass filter with Fm = 70, Q = 15, Am = 100, one can use the standard value components as given below;
C = 0.1 uF
R1 = 3.3 K
R2 = 680 K
R3 = 1 K
Since
this filter will be used to drive a loudspeaker a buffer circuit needs
to be added at the output of the MBF band pass filter which will drive
the loudspeaker without affecting the filter characteristics. The buffer
can also be made easily with another op-amp. The complete circuit is
given below;
The filters like the above can be considered as a ‘Tuned Amplifier’
which will amplify only a particular frequency range. The R1 and R3
attenuate the input signal and from which the desired frequency is only
amplified out. In this project the circuit amplifies only the Sub-Woofer
frequencies from the music input.
The
audio input is fed from a PC and the filtered audio in this experiment
is observed with a normal headset, since the headset speakers are
designed for the good quality reproduction of both the high frequency
sound and low frequency bass. Play
some music in the PC which has lot of bass like the one used in the
demonstration of this circuit and the output from the circuit will be
the bass-beats only.
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