Questions and Problems

Author: N.H. Crowhurst

1. Compare the action of a moving-iron and moving-coil loudspeaker, showing why the latter have come to be almost universally used.

2. Explain the purpose of the cone, coil, spider, and surround of a moving-coil speaker. Why does a moving-coil speaker have to be so much larger than a moving-coil microphone?

3. Why is some sort of baffle needed for a loudspeaker? Is there any alternative to using a baffle?

4. What is an infinite baffle? Explain why it reduces loudspeaker efficiency if made too small.

5. What two facts are utilized in a bass-reflex baffle?

6. Explain how a reflex baffle reverses the phase of the radiation from the back of the speaker diaphragm. Does this happen at all frequencies?

7. How is the size of the port in a bass reflex enclosure determined?

8. Explain why use of a horn improves the efficiency of a loudspeaker. Why is a simple cone not a good shape for a horn?

9. Show by a simple derivation why the exponential form is the best horn shape. What limits the frequency of a horn loudspeaker (a) at the low-frequency end and (b) at the high-frequency end?

10. Assuming two horns that have the same throat diameter (say, f4 inch), what would be the approximate relationship between their lengths if one works from 100 cycles and the other from 200 cycles?

11. Space can be saved by folding a horn, but what problem does this introduce, and how may it be partially solved?

12. What part of a horn is principally responsible for its directional characteristics in radiation? Give examples and show limitations.

13. Describe briefly (a) the acoustic lens and (b) the acoustic slit, as a means for modifying the radiation properties of a loudspeaker, showing what properties control the frequencies at which their effect becomes active.

14. What are the principal attraction and the main limitation of an electrostatic loudspeaker?

15. What was the principal appeal of the crystal loudspeaker? Why has it become obsolete?