Question

Which one of the following will remain unchanged when a sound wave travels in air or in water?
(A) Amplitude
(B) Frequency
(C) Wavelength
(D) Speed

Answer 1

Hint: Assuming ideal propagation of the wave we can say that there is no energy loss when the wave moves in any of the mediums. Now energy is independent (exclusively) of the wavelength, speed and amplitude and only dependent on the frequency.

Complete step by step answer:
Energy of a wave is given as \[E = h\nu \], here h is a constant and v is the frequency of the wave.
In an ideal scenario the Energy is not lost to the material while propagating through it.
This means that the energy is constant irrespective of the medium.
 \[ \Rightarrow E = \text{Constant} \Rightarrow \nu = \text{Constant}\]

Hence, it will remain unchanged while passing through various mediums.

Note:
Now one knows that \[E = h\nu = h\dfrac{c}{\lambda }\], hence may get confused that energy changes with only speed of light-c or only the wavelength- \[\lambda \]. But this isn’t true. Basically, the speed and wavelength are inversely proportional.

Additional information: Sound waves travel through any material medium with a speed that depends on the properties of the medium. As sound waves travel through air, the elements of air vibrate to produce changes in density and pressure along the direction of motion of the wave. If the source of the sound waves vibrates sinusoidally, the pressure variations are also sinusoidal.