Consider the following statements about the microphone and the speaker of a mobile phone: 1. The microphone converts sound to a mechanical signal. 2. The microphone converts sound to an electrical signal. 3. The speaker converts a mechanical signal to sound. 4. The speaker converts an electrical signal to sound. Which of the statements given above are correct?
- A.1 and 3
- B.1 and 4
- C.2 and 3
- D.2 and 4
Correct answer
Explanation
The correct answer is D, 2 and 4. A microphone turns sound into an electrical signal, and a speaker turns an electrical signal back into sound.
Both are transducers, devices that change energy from one form to another. In a microphone, sound waves make a thin diaphragm vibrate; the vibration changes a magnetic field or a capacitance and so produces a varying electrical signal that the phone can process and transmit. A speaker works the other way round: the incoming electrical signal flows through a coil placed in the field of a permanent magnet, the coil and the attached cone vibrate, and the vibrating cone pushes the air to produce sound. The diaphragm's motion is only an intermediate step; the input and output that define each device are sound and electricity.
A is wrong: neither device has a mechanical signal as its final output or input.
B is wrong: statement 1 is incorrect.
C is wrong: statement 3 is incorrect.
Exam tip: microphone = sound to electrical energy; speaker = electrical to sound energy; both rely on electromagnetic induction or its reverse.
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