Question

Which of the following devices does not work on the multiple reflection of sound waves?
A) Stethoscope
B) Hydrophone
C) Soundboard
D) Megaphone

Answer 1

Hint: The phenomenon of Reflection of sound wave is the bouncing back of the sound wave after striking a rigid smooth surface. The wavelength of sound waves is much longer than the light waves.

Complete step by step solution:
Let’s discuss all the instruments in detail
We know, Stethoscope is a device used in medical services to hear sound from the inside of the body. A sound or a vibration from the body reaches the diaphragm of the stethoscope which is placed on the body. The vibration picked by the diaphragm is being protected externally in order to prevent sound loss and thereby channelled through the tube to a specific direction.

When we take a look at the inside of the tube, as a result of wave collision within the wall of the tube, a phenomenon of multiple reflection happens. And this continues till the sound reaches the ear at the other end of the tube. Figure shows what is happening inside the tube of a stethoscope.

Next one is a Hydrophone. We know it is an underwater microphone which is designed to study underwater noise. It works by converting a sound wave into an electrical voltage by detecting changes in pressure.

The speed and distance of sound waves in water is proportional to the pressure changes, and these changes will determine characteristics of the electrical output. The ideal material for making hydrophones is piezoelectric material. They help to generate an electrical potential with the pressure changes.
From the details given about hydrophone, we can find that the property of reflection of sound is not used in the working.

A sound board is a concave board placed behind the speaker in large halls, so that the sound will be audible even at a considerable distance. The speaker is placed at the focus of the concave board.
From the figure given below, we can see how a sound board works with the reflection of sound.

Now we are discussing megaphone.
The shape of a megaphone is like a horn. Speaking tube is a hollow tube– one end is for the source of sound, whereas the other opens to the listeners.
In these devices, the sound waves undergo repeated reflection and the energy of the waves remains confined to the tube. Figure shows how a megaphone works.

The final answer is option (B) Hydrophone.

Note: Sound waves will get reflected from plane and spherical surfaces and obey the laws of reflections. For a better reflection from any surface, it should have dimensions equal or larger than the wavelength of the sound waves falling on it.