NDA (II) 2022 General Ability Test · Question 150 of 150

Correct — B, sound waves to electrical signals. A microphone has a thin diaphragm exposed to the air. When a sound wave reaches it, the pressure variations set the diaphragm vibrating, and that mechanical vibration is turned into a varying electrical signal — in the common moving-coil type, by a coil attached to the diaphragm moving in a magnetic field and inducing a voltage that follows the sound. The electrical signal can then be amplified, recorded or transmitted. A loudspeaker performs the reverse conversion, taking an electrical signal and driving a cone to push the air, so the two devices form a matched pair at either end of a sound system.

• (a)electrical signals to sound waves — That is what a loudspeaker or an earphone does, not a microphone. The two are physically similar and the confusion is genuine, but the microphone sits at the input end of a sound system and the loudspeaker at the output end.
• (c)microwaves to sound waves — Microwaves are electromagnetic radiation used in radar, communications and ovens, and nothing in a microphone produces or receives them. The word microphone shares only its Greek prefix with microwave, meaning small, and the resemblance is purely verbal.
• (d)sound waves to microwaves — A microphone produces an electrical signal in a wire, not free electromagnetic radiation. A radio transmitter may later impress that signal on a carrier wave, but that is a separate stage and a separate device.

A device that converts energy from one form to another is called a transducer, and a great deal of elementary physics can be organised by simply listing the input and the output of each one. A microphone is a sound-to-electrical transducer; a loudspeaker is electrical-to-sound; an electric generator is mechanical-to-electrical; a motor is electrical-to-mechanical; a solar cell is light-to-electrical; a light-emitting diode is electrical-to-light. Several of these pairs run the same physics in opposite directions, which is why a moving-coil loudspeaker will work, feebly, as a microphone.

This item is a single-line recall, but the option set is engineered around the prefix micro-, hoping that the word microwave will feel connected to the word microphone. There is no such connection, because a microphone deals in sound and electricity while microwaves belong to the electromagnetic spectrum. The other trap is the reversal in option (a), which describes the loudspeaker. The safe habit for the whole family of such questions is to read each option as an arrow with a source and a destination, then ask which arrow the named device actually draws. In the ear, incidentally, the cochlea is the biological equivalent of a microphone, turning pressure variations into electrical nerve impulses.

• A microphone converts sound energy into an electrical signal; it is a transducer.
• A loudspeaker performs the reverse conversion, turning an electrical signal into sound.
• In a moving-coil microphone the diaphragm's vibration moves a coil in a magnetic field, inducing a voltage by electromagnetic induction.
• Microwaves are electromagnetic radiation and have nothing to do with the working of a microphone.
• An electric generator converts mechanical energy into electrical energy, and a motor does the reverse.

• Reversing the arrow and describing a loudspeaker instead of a microphone.
• Being drawn to microwaves because of the shared prefix.
• Assuming the microphone itself transmits a radio signal, when transmission is a separate stage.

As a one-line device-to-conversion match, often inside a set that also names the loudspeaker, the generator, the motor and the solar cell.

• practice — not a real PYQ

  Which one of the following converts electrical energy into sound energy?

  • (a)Microphone
  • (b)Loudspeaker
  • (c)Solar cell
  • (d)Photodiode

  Answer(b) Loudspeaker — it drives a cone with a varying current so that the cone pushes the air and produces sound.

• practice — not a real PYQ

  An electric generator converts

  • (a)electrical energy into mechanical energy
  • (b)mechanical energy into electrical energy
  • (c)chemical energy into electrical energy
  • (d)light energy into electrical energy

  Answer(b) mechanical energy into electrical energy — by electromagnetic induction, which is the reverse of what an electric motor does.