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Standing Sound Waves - Practice Questions & MCQ

Edited By admin | Updated on Sep 18, 2023 18:34 AM | #JEE Main

Quick Facts

  • Standing longitudinal wave is considered one of the most asked concept.

  • 36 Questions around this concept.

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A wave y=a\sin \left ( \omega t-kx \right ) on a string meets with another wave producing a node at x=0. Then the equation of the unknown wave is:

Tube A has both ends open while tube B has one end closed, otherwise, they are identical. The ratio of the fundamental frequency of tube A and B

A cylindrical tube, open at both ends, has a fundamental frequency, f, in air. The tube is dipped vertically in water so that half of it is in water. The fundamental frequency of the air column is now

Which of the following is true for longitudinal wave?

Which of the following is true for longitudinal waves?

What is the ratio of frequency for closed organ pipe and open organ pipe having several overtones as 3 & 2 respectively?

(consider all other quantities same for both) 

Concepts Covered - 1

Standing longitudinal wave

Standing waves

When two sets of progressive wave of same type (both longitudinal or both transverse) having the same amplitude and same time period or frequency or wavelength travelling along the same straight line with same speed in opposite directions superimpose, a new set of waves are formed. These are called stationary waves.


Some of the characteristics of standing waves :

(1) In this the disturbance is confined to a particular region between the starting point and reflecting point of the wave.
(2) In this there is no forward motion of the disturbance from one particle to the adjoining particle and so on, beyond this particular region.
(3) The total energy in stationary wave is twice the energy of each of incident and reflected wave. But there is no flow or transference of energy along the stationary wave.
(4) Points in a standing wave, which are permanently at rest. These are called nodes. The distance between two consecutive nodes is λ2

(5) The Points on the standing wave having maximum amplitude is known as antinodes. The distance between two consecutive antinodes is also λ2
(6) All the particles execute simple harmonic motion about their mean position (except those are at nodes) with the same time period.

Note - In standing waves, if the amplitude of component waves are not equal. Resultant amplitude at nodes will not be zero. It will be minimum . Because of this, some energy will pass across nodes and waves will be partially standing.

Let us take an example to understand and derive equation of standing wave - 

Let us take a string and when a string is under tension and set into vibration, transverse harmonic waves propagate along its length. If the length of string is fixed, reflected waves will also exist. These incident and reflected waves will superimpose to produce transverse stationary waves in a string

                                                                    

Incident wave y1=asin2πλ(vt+x)
Reflected wave y2=asin2πλ[(vtx)+π]=asin2πλ(vtx)
Now we can apply principle of superposition on this and get -

y=y1+y2=2acos2πvtλsin2πxλ


Standing Wave in a Closed Organ Pipe -
Organ pipes are the musical instrument which are used for producing musical sound by blowing air into the pipe. In this longitudinal stationary waves are formed due to superimposition of incident and reflected longitudinal waves.

A closed organ pipe is a cylindrical tube having an air column with one end closed. Sound waves are enters from a source vibrating near the open end. An ingoing pressure wave gets reflected from the fixed end. This inverted wave is again reflected at the open end. After two reflections, it moves towards the fixed end and interferes with the new wave sent by the source in that direction. The twice reflected wave has travelled an extra distance of 2l causing a phase advance of 2πλ.2l=4πlλ
Similarly at open ends, the twice reflected wave suffered a phase change of π at the open end.
So the phase difference is δ=4πlλ+π. Also the waves interfere constructively if phase difference is 2nπ

4πlλ+π=2nπl=(2n1)λ4
 

Here n=1,2,3. But if we take n=0,1,2, then the above equation can also be written as -

l=(2n1)λ4


So, the frequency can be written as -

ν=vλ=v(2n1)4l


Equation of standing wave is given by and explained earlier =y=2acos2πtλsin2πxλ
As, general formula for wavelength defined earlier =λ=4L(2n1)
The minimum allowed frequency is obtained by putting n=1

                                                  

(1) First normal mode of vibration : n1=v4L

This is called fundamental frequency. The note so produced is called fundamental note or first harmonic.
(2) Second normal mode of vibration : n2=vλ2=3v4L=3n1

This is called third harmonic or frst overtone.
(3) Third normal mode of vibration : n3=5v4L=5n1

This is called fifth harmonic or second overtone.

Standing Waves in Open Organ Pipes
General formula for wavelength -

λ=2Ln where n=1,2,3


Then the first normal mode of vibration is -

n1=vλ1=v2L
 

This is called fundamental frequency and the node so produced is called fundamental node or first harmonic.

 

(2) Second normal mode of vibration n2=vλ2=vL=2(v2L)=2n1n2=2n1

This is called second harmonic or first overtone.
(3) Third normal mode of vibration n3=vλ3=3v2L,n3=3n1

This is called third harmonic or second overtone.

 

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Standing longitudinal wave

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Standing longitudinal wave

Physics Part II Textbook for Class XI

Page No. : 379

Line : 25

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