The echo is Doppler shifted; calculating the frequency of the reflected sound heard by the moving driver gives about 169 Hz, so beats = |169 − 165| ≈ 4 Hz.

At reflection from a denser medium the wave is inverted (crest reflects as trough) producing a 180° phase change, so 'Trough, 180°' is correct.

Reflection from a fixed end inverts the pulse (introduces a phase reversal), so a fixed end produces inversion.

Echoes are produced when sound is reflected from a surface and returns to the listener.

An echo is produced when sound waves reflect from a surface and return to the listener, which is reflection of sound.

When waves pass between media of different densities, both their speed and direction (refraction) generally change; frequency remains the same.

Frequency remains unchanged when a wave passes between media; wavelength and speed change to satisfy v = fλ.

For a short-circuited termination the reflection coefficient is -1, indicating total reflection with a phase inversion.

Critical angle sinθc = vA/vB = V/(2V) = 1/2, so θc = 30°, and total internal reflection occurs for incidence greater than 30°.

Using λ_min (Å) ≈ 12400/ V (V in volts), for 0.1 Å V ≈ 124000 V (124 kV).