What kind of waves can survive as standing waves?

$A$ standing wave having $3$ nodes and $2$ antinodes is formed between two atoms having a distance of $1.21 \; \mathring{A}$ between them. The wavelength of the standing wave is .... $\mathring{A}$

The pattern of standing waves formed on a stretched string at two instants of time is shown in the figure. The velocity of the two waves superimposing to form stationary waves is $360 \ m/s$ and their frequencies are $256 \ Hz$.
$(a)$ Calculate the time at which the second curve is plotted.
$(b)$ Mark nodes and antinodes on the curve.
$(c)$ Calculate the distance between $A^{\prime}$ and $C^{\prime}$.

$A$ transverse wave strikes against a rigid wall. What happens to its phase and velocity?

Stationary waves are formed when

$A$ sound source of frequency $170 \, Hz$ is placed near a wall. $A$ man walking from the source towards the wall finds that there is a periodic rise and fall of sound intensity. If the speed of sound in air is $340 \, m/s$,the distance (in metres) separating the two adjacent positions of minimum intensity is