The speed of sound in a certain metal is ${\mathbf{v}}_{\mathbf{m}}$. One end of a long pipe of that metal of length Lis struck a hard blow. A listener at the other end hears two sounds, one from the wave that travels along the pipe’s metal wall and the other from the wave that travels through the air inside the pipe.

(a) Ifrole="math" localid="1661511418960" $\mathbf{V}$ is the speed of sound in air, what is the time interval$\mathbf{\Delta t}$ between the arrivals of the two sounds at the listener’s ear?

(b) If$\mathbf{\Delta t}\mathbf{}\mathbf{=}\mathbf{}\mathbf{1.00}\mathbf{\text{\hspace{0.17em}s}}$and the metal is steel, what is the length$\mathbf{L}$?

1. Speed of sound in metal is$\mathrm{v}$ .
2. Speed of sound in metal is${\mathrm{v}}_{\mathrm{m}}$.
3. Length of tube is$\mathrm{L}$ .

Using the formula of velocity, we can derive the equation for the time interval between the arrivals of the two sounds at the listener’s ear if v is the speed of sound in air. Using this derivation, we can find the length of the pipe.

Formula:

The velocity of the sound wave,

$\mathbf{v}\mathbf{=}\mathbf{d}\mathbf{/}\mathbf{t}$ …(i)

Using equation (i), we can the time taken by the body to be:

$\mathrm{t}=\frac{\mathrm{L}}{\mathrm{v}}$

$\because \mathrm{d}=\mathrm{L}$

Time t for sound to travel in the metal is${\mathrm{t}}_{\mathrm{m}}=\mathrm{L}/{\mathrm{v}}_{\mathrm{m}}$

Time t for sound to travel in air is${\mathrm{t}}_{\mathrm{a}}=\mathrm{L}/\mathrm{v}$

So, the time interval between these sounds can be given as:

$\begin{array}{c}\mathrm{\Delta t}={\mathrm{t}}_{\mathrm{a}}-{\mathrm{t}}_{\mathrm{m}}\\ \mathrm{\Delta t}=\frac{\mathrm{L}}{\mathrm{v}}-\frac{\mathrm{L}}{{\mathrm{v}}_{\mathrm{m}}}\end{array}$

$\mathrm{\Delta t}=\frac{\mathrm{L}({\mathrm{v}}_{\mathrm{m}}-\mathrm{v})}{{\mathrm{v}}_{\mathrm{m}}\mathrm{v}}$ …(a)

Therefore, the time interval between the arrivals of the two sounds at the listener’s ear if $\mathrm{v}$is the speed of sound in air is $\frac{\mathrm{L}({\mathrm{v}}_{\mathrm{m}}-\mathrm{v})}{{\mathrm{v}}_{\mathrm{m}}\mathrm{v}}$.

We have the length of a material using equation (a) as:

$\mathrm{L}=\frac{\mathrm{\Delta t}}{\left(\frac{1}{\mathrm{v}}-\frac{1}{{\mathrm{v}}_{\mathrm{m}}}\right)}$

For metal that is for steel pipe, ${\mathrm{v}}_{\mathrm{m}}=5941\text{\hspace{0.17em}m}/\text{s}$and as $\mathrm{v}=343\text{\hspace{0.17em}m}/\text{s},\mathrm{\Delta t}=1.00\text{\hspace{0.17em}s}$.

Hence, the length value is given as:

$\begin{array}{c}\mathrm{L}=\frac{1.00\text{\hspace{0.17em}s}}{\left(\frac{1}{343\text{\hspace{0.17em}m}/\text{s}}-\frac{1}{5941\text{\hspace{0.17em}m}/\text{s}}\right)}\\ =364\text{\hspace{0.17em}m}\end{array}$

Therefore, the length $L$, if $\mathrm{\Delta t}=1.00\mathrm{s}$and the metal is steel is$364\text{\hspace{0.17em}m}$ .