Q.18 If the speed of sound is 340 m/s, the length of the shortest closed pipe that resonates at 218 Hz is:

A. 23 cm
B. 17 cm
C. 39 cm
D. 78 cm



Answer :

Certainly! Let's delve into the steps required to find the length of the shortest closed pipe that would resonate at 218 Hz, given the speed of sound as 340 m/s.

### Step-by-Step Solution:

1. Given Data:
- Speed of sound ([tex]\( v \)[/tex]) = 340 m/s
- Frequency ([tex]\( f \)[/tex]) = 218 Hz

2. Identify the formula:
- For a closed pipe, the fundamental frequency (1st harmonic) can be calculated using:
[tex]\[ f = \frac{v}{4L} \][/tex]
where:
- [tex]\( f \)[/tex] is the frequency
- [tex]\( v \)[/tex] is the speed of sound
- [tex]\( L \)[/tex] is the length of the pipe

3. Rearrange the formula to solve for [tex]\( L \)[/tex]:
[tex]\[ L = \frac{v}{4f} \][/tex]

4. Substitute the given values into the equation:
[tex]\[ L = \frac{340 \, \text{m/s}}{4 \times 218 \, \text{Hz}} \][/tex]

5. Calculate the length [tex]\( L \)[/tex]:
[tex]\[ L = \frac{340}{872} \][/tex]
[tex]\[ L = 0.38990825688073394 \, \text{meters} \][/tex]

6. Convert the length from meters to centimeters:
[tex]\[ 1 \, \text{meter} = 100 \, \text{centimeters} \][/tex]
[tex]\[ L = 0.38990825688073394 \times 100 = 38.99082568807339 \, \text{centimeters} \][/tex]

7. Round the length to the nearest centimeter (if necessary):
The length calculated is approximately 39 centimeters.

### Conclusion:
The length of the shortest closed pipe that resonates at a frequency of 218 Hz, given the speed of sound is 340 m/s, is 39 centimeters.

Hence, the correct option is:
C. 39 cm