Q10P - Fundamentals Of Physics

Question

Question:  One clue used by your brain to determine the direction of a source of sound is the delay $∆t$ between the arrival of the sound at the ear closer to the source and the arrival at the father ear. Assume that the source is distant so that a wave front from it is approximately planar when it reaches you , and let D represent the separation between your ears.(a) If the source is located at angle  in front of you figure , what is $∆t$ in term of  D and the speed of sound  in air? (b) If you are submerged in water and the speed of sound is directly to your right, what is $∆t$ in terms of V_w and the speed of sound  in water? (c) Based on the time –delay clue, your brain interprets the submerged sound to arrive at an angle $\theta$ from the forward direction. Evaluate$\theta$  for fresh water at 20 ⁰C.

Step-by-Step Solution

Answer

Answer

1. The $∆t$ in term of  and the speed of sound v in air is $\Delta t=\frac{D\sin \theta }{v}$.
2. The  $∆t$in term of  and the speed of sound v  in water is $\Delta t=\frac{D}{v_w}$.
3.  The $\theta$ for fresh water at 20 ⁰c  is $\theta = {13}^0$.

Step 1: Given data

 

• When the wave front reaches the ear, it is planar.
• Time delay between the arrival of the sound for two ear is $∆t$.
• Separation between two ears is D.
• Source is located at an angle θ.

 

Step 2: Determining the concept

By using the distance time relation and trigonometric relation in the given diagram, find time in air and in water. Taking the ratio of  ${\mathit{v}}_{\mathbf{a}\mathbf{i}\mathbf{r}\mathbf{ }\mathbf{ }}\mathit{a}\mathit{n}\mathit{d}\mathbf{ }{\mathit{v}}_{\mathbf{w}\mathbf{a}\mathbf{t}\mathbf{e}\mathbf{r}}$, find angle θ.

The expression for the delay time due to distance is given by,

 $\mathit{\Delta }\mathit{t}\mathbf{=}\frac{\mathbf{d}}{\mathbf{v}}$

Here v is the speed of the sound at 20⁰C , and the value of v is 343 m/s.

Step 3: (a) Determining ∆ t in term of D and the speed of sound v in air

 

Time delay is due to the distance d that the wave front must travel to reach the left ear and then right ear. Now, if  is the velocity of sound in air, then time delay can be calculated as,

$\Delta t=\frac{d}{v}$ ……. (i)

In the figure, in the right angle triangle,

$$\begin{gathered} \sin \theta =\frac{d}{D} \\ d=D \sin \theta \end{gathered}$$

Using these values in equation 1, 

 

$\Delta t=\frac{D\sin \theta }{v}$ …… (ii)

Hence,$\Delta t$  in term of D and the speed of sound v in air is$\Delta t=\frac{D\sin \theta }{v}$ .

Step 4: (b) Determine the ∆ t   in term of D and the speed of sound v in water

Now, as the medium is water, if the velocity of the sound in water is V_w.

And the source is directly towards the right ear, therefore $\theta ={90}^0, \sin 90=1.$

 

Hence, the time delay in water can be calculated as,

$\Delta t=\frac{D}{v_w}$

In the figure, in the right angle triangle,

$$\begin{gathered} \sin \theta =\frac{d}{D} \\ d=D\sin \theta \\ =D\sin 90° \\ =D \end{gathered}$$

Therefore,

$\Delta t=\frac{D}{1482}$

Time delay in water to reach from left ear to right ear is,
$\Delta t=\frac{D}{v_w}$   ……(iii)

Hence,$\Delta t$  in term of D and the speed of sound v  in water is $\Delta t=\frac{D}{v_w}$.

Step 5: (c) Determining the value of θ for fresh water at 20 0 C

 

To find angle  equating equation (ii) and (iii),

$$\begin{gathered} \frac{D\sin \theta }{v}=\frac{D}{v_w} \\ \sin \theta =\frac{v}{v_w} \end{gathered}$$

Substitute 343 m/s for v and 1482 m/s for V_w into the above equation,

$$\begin{gathered} \sin \theta =\frac{343}{1482} \\ =0.231 \\ \theta ={\sin }^{-1}\left(0.231\right) \\ ={13}^0 \end{gathered}$$

Hence, the  $\theta ={13}^0$for fresh water at 20⁰C is $\theta ={13}^0$.