# Important Questions and Answers: Waves in Pipes and Strings

1. The frequency of fundamental note of an open organ pipe double than for closed pipe of same length. Why ?

Ans: We know that the fundamental frequency of an open organ pipe is given by: $$\text{f}_o = \frac{\text{v}}{\text{2l}}$$ Where v is the velocity of sound in air and l is the length of organ pipe.

Also, the fundamental frequency of a closed organ pipe is given by:

$$\text{f}_o = \frac{\text{v}}{\text{4l}}$$

So, the frequency of fundamental note of or an open organ pipe double than for closed pipe of same length.

2. Which organ pipe produces a better quality sound: a closed pipe or open pipe?

Ans: The quality of sound depends upon the number of harmonics or overtones present in it. In an open organ pipe, the frequencies of the waves formed are given by fn=nf1, where f1 is the fundamental frequency and n = 1, 2, 3, 4….

But in case of a closed organ pipe, the frequencies of the waves formed are given by fn = (2n+1)f1, where f1 is the fundamental frequency and n=0, 1, 2, 3….

So, even if the fundamental frequencies are equal, the subsequent frequencies possible in open pipes have fewer intervals whereas the intervals are large in closed pipes. So, a person would feel that sound from opne pipe is better than from closed pipes.

3. Why is loud sound heard at resonance?

Ans: This phenomenon is based on the amplitude of the vibrating substances, one of the factors on which the loudness of sound depends on. At resonance, the forced and free vibrations have same frequency. So, the amplitude of the resultant vibrations will be be very high and large sound is heard.

4. Explain why soldiers are ordered to break steps while crossing a bridge?

Ans: Soldiers are ordered to break steps while crossing the bridge to prevent the collapsing of bridge. When the frequency of soldiers steps and the natural frequency of the bridges becomes equal, the bridge vibrate with maximum amplitude and eventually bridge may collapse.

5. Sometimes, the glasses of the windows of a house are found to be shattered when large sounding bodies like aeroplane pass near the hose. Explain how?

Ans: Every substance has its natural frequency of oscillation when it is set into vibrations, including glass. When the sound produced by anything match this frequency, the amplitude of vibration of glass will be very high, which brings shear to them. When the amount of shearing force exceeds the breaking force, they crack.

Sometimes the explosions can be such that the frequency of their sound matches this natural frequency, causing large scale vibration, at least for some time. This cracks them.

6. One of the nine jewels of Emperor Akbar, widely known as Tansen (the king of Music) , was able to break glass by singing appropriate note. What physical phenomenon could account for this?

Ans: The physical phenomena involved in this trick is resonance. When the natural frequency of the glass coincides with the frequency of the singer’s voice, resonance occurs and when amplitude of the vibration becomes more than the breaking force, the glass cracks.

7. Why is end correction necessary for an organ pipe?

Ans: When an air is set into vibration in an organ pipe, the reflection of sound waves takes place a little above the open end of the pipe. So, the length of the air column which is set into oscillation is not exactly the distance between two ends of the tube. So, to obtain accurate value, end correction is necessary for an organ pipe.

8. What happens when a tuning fork is filled with wax?

Ans: We know that the frequency of a tuning fork refers to the number of times it oscillates or vibrates per second. On  applying the wax, the vibrating ends of the tuning fork become much heavier and as such movement is restricted partially. Therefore, the vibrations per second go down hence there is a gradual lowering of the frequency.

9. Why tuning fork is provided with two prongs?

Ans: Tuning fork with two prongs can oscillate with equal and opposite motion and even oscillation lasts for a longer time. If it is provided with a single prong, hand take away its vibration due to high damping and the oscillation would die away very quickly. So, for the convenient and easy measurement, tuning fork is provided with two prongs.

10. Why does the sound produced by a tuning fork become louder when the stem of the fork is pressed on the top of a table?

Ans: This phenomenon is based on the surface area of the vibrating substances. One of the factors on which the loudness of sound depends on is the surface area of the area available for vibration. When only the tuning fork is used, it causes vibration of air of a very small area, which makes it appear feeble. But when its stem is pressed against a tabletop, the whole of the top vibrates, which causes the vibration of a large area of air. This makes it appear loud.

11. A flute has several holes in it. Why ?

Ans: A flute is an open organ pipe. The length of air column vibrating inside the flute can be changed by putting fingers on the holes. This gives the sound of different frequencies. So, flute is provided with several holes.

12. Explain why guitar is more preferrable than a sarangi to produce quality musical sound?

Ans: Guitar and sarangi both work on the same principles i.e., string vibrates between two fixed points whose sound is magnified by an empty cavity with holes. But the main difference is the number of strings and the difference of thickness of the strings. Guitar has six strings and the tension can be varied by screws. On the other hand, sarangi has three only strings. The thickness and materials of strings of Guitar are totally different whereas in Sarangi, they are same.

The length of guitar string is more, so they can be plucked and held at various points to produce more frequencies. Sarangi strings are short, so there is less variation. So, guitar is more preferable than a sarangi to produce a quality of sound.

13. Why does sonometer hollow from inside?

Ans: Sonometer is a box arrangement whose upper surface can be used to place things which are able to produce sound. This sound is sent below where the air column vibrates to give larger sound and we can hear large sound. This help us for clear reading and accurate measurement. So, sonometer is hollow from inside.

14. Why are musical instruments first warmed up before performing?

Ans: When musical instruments are used for performance, either pipe type or string type or the layers type, they are in motion or are exposed to slightly high temperature of exhaled air or hand or anything else. The frequency of music generated corresponds to that temperature.

When the device is first used after a long period of rest or abstinence, the temperature of the device will be equal to that of the environment, i.e. colder than the performing temperature. So, this will decrease the velocity of sound as well as the frequency produced. Such phenomenon is defined as the device going ‘flat’.

In order to prevent this, they should be warmed up for few minutes, bringing the temperature to the performing level and then brought to use.

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#### One Response to “Important Questions and Answers: Waves in Pipes and Strings”

1. kirti

Q: A pulse is generated at lower end of a hanging rope of uniform density and length L. The speed of the pulse when it reaches the mid point of the rope will be ….?