A manufacturer tests a wireless loudspeaker shown in Fig. 1. A battery supplies energy to an electrical circuit containing a coil placed in a magnetic field...
Question 1 Report
A manufacturer tests a wireless loudspeaker shown in Fig. 1. A battery supplies energy to an electrical circuit containing a coil placed in a magnetic field. When the music signal changes, the current in the coil changes. The coil and attached paper cone move rapidly in and out. The moving cone makes nearby air particles vibrate, producing sound waves. A student hears a buzzing sound when the cone is damaged, even though the circuit still works. The manufacturer needs a clear explanation of how electrical energy becomes sound energy before deciding whether to replace the cone.
(a) What part of the loudspeaker vibrates to make the air vibrate? [1]
(b) Label the direction in which the cone moves by drawing a double-headed arrow. [1]
(c) Describe the energy transfers in the loudspeaker. [2]
(d) Which component provides the magnetic field around the coil? [1]
A theatre technician studies Fig. 1 before fitting heavy curtains around a stage. During rehearsals, actors hear a strong echo from the rear wall. The wall is 34 m from the front of the stage. A hand clap is made at the front, and the reflected sound reaches the actor after travelling to the wall and back. Take the speed of sound in air as 340 m/s. The technician checks that the theatre is empty and quiet, so that fan noise does not affect the observation. She needs to calculate whether the delay is long enough to be heard as a separate echo rather than as part of the original sound.
(a) Calculate the distance travelled by the sound from the actor to the wall and back. [1]
(b) Calculate the time taken for the reflected sound to return. [2]
(c) What is the name of this reflected sound? [1]
(d) Give one change to the theatre that would reduce the echo. [1]
Answer Details
Loudspeaker
(a) The cone, also called the diaphragm, vibrates to make the air vibrate. [1]
(b) The arrow must be double-headed and parallel to the cone’s in-and-out motion. [1]
(c) Electrical energy is transferred to kinetic energy of the coil and cone. [1] The moving cone then transfers energy to the air as sound energy. [1]
(d) The magnetic field is provided by a permanent magnet. [1]
Theatre echo
(a) \[34\text{ m}\times2=68\text{ m}\]
The sound travels 68 m to the wall and back. [1]
(b) \[\text{time}=\frac{\text{distance}}{\text{speed}}=\frac{68\text{ m}}{340\text{ m/s}}=0.20\text{ s}\]
The return time is 0.20 s. [2]
(c) The reflected sound is an echo. [1]
(d) Adding curtains, acoustic panels, or other soft material to cover the hard wall would reduce the echo by absorbing sound. [1]
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