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Problems

1. The resistance of a carbon-granule transmitter in the quiescent state is 70 ohms. When it is actuated by sound waves, the change in resistance is 12 per cent above and below this value. If a 3-volt battery and a 100-ohm resistor are connected in series with this transmitter, calculate the current that will flow and the power that will be delivered to the resistor.

2. Assume that the pressure on the diaphragm of the microphone of Fig. 9 is 10 bars and that the microphone is connected through an input transformer to

an amplifier. Calculate the approximate voltage amplification (both as a ratio and in decibels) that the amplifier must have to produce an output voltage of 25 volts.

3. The internal capacitance of a condenser microphone is assumed to be 50 micro-microfarads. Make the calculations required to show that the input resistor of the associated amplifier should be about 100 megohms, and that the amplifier should be close to the microphone with negligible connecting leads,

4. Use Fig. 11 as a basis, and at 500 cycles compute the dbm output at 0.1, 1.0, 10, and 100 bars.

5. Use the data accompanying Fig. 13, and calculate the open-circuit output voltage at 0.1, 1.0, 10, and 100 bars. If the output impedance is assumed to be 50 ohms resistance and 100 feet of microphone cable having a capacitance of 20 micromicrofarads per foot is used between the microphone and a 50-ohm resistance load, calculate the effect of the cable capacitance on the voltage delivered the load at 50, 1000, and 8000 cycles.

6. The output impedance of an amplifier is 15 ohms, and it is to be used to drive four loudspeakers with 15-ohm driving coils. How should the connections be made? If each loudspeaker draws 15 watts and if they are 350 feet from the amplifier, what size wire would you recommend for the line feeding the four speakers?

7. If the voice-coil current of the loudspeaker discussed on page 112 is 1.0 ampere, make an estimate of the power radiated, and of the efficiency. Does this compare with data given on page 132?

8. The diameter of the throat of a horn is 0.75 inch. If it is to transmit up to 7000 cycles, what should be the length of the horn and the diameter of the mouth, assumed to be circular.

9. Use the data for a telephone receiver given in Figs. 26 to 34 and draw to scale a diagram similar to Fig. 35. For the frequency of resonance, calculate the power lost in the direct-current resistance of the windings, the power lost in the magnetic circuit exclusive of the diaphragm, and the power delivered to the diaphragm. If this is assumed completely radiated, calculate the efficiency of the receiver. Compare this with the statement on page 132 and account for differences that may exist. Also compare with the calculations on page 132,

10. What acoustic power would you recommend for a college auditorium that will seat 5000 people? Assuming that you used horn-type speakers, what power-handling capacity should the amplifier have?



Last Update: 2011-05-18