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Beats MCQ - Practice Questions with Answers

Edited By admin | Updated on Sep 25, 2023 25:23 PM | #NEET

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  • Beats is considered one of the most asked concept.

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Three sound waves of equal amplitudes have frequencies (n - 1), n, (n + 1). They superimpose to give beats. The number of beats produced per second will be :

A tuning fork of frequency 512 Hz makes 4 beats per second with the vibrating string of a piano. The beat frequency decreases to 2 beats per second when the tension in the piano string is slightly increased. The frequency of the piano string before increasing the tension was 

Two identical piano wires kept under the same tension T have a fundamental frequency of 600 Hz. The fractional increase in the tension of one of the wires which will lead to the occurrence of 6 beats/s when both the wires oscillate together would be

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The loudspeakers L1 and L2 driven by a common oscillator and amplifier, are set up as shown in the figure. As the frequency of the oscillator increases from zero, the detector at D recorded as a series of maximum and minimum signals. What is the frequency at which the first maximum is observed? (Speed of sound = 330 m/s)

A tuning fork produces 10 beats per second with a sonometer wire of length 95 cm and 100 cm. The frequency of the fork is:

Two tuning forks, A and B produce nodes of frequencies 258 Hz and 262 Hz. An unknown node sounded which produces certain beats. When the same node is sounded with, the beat frequency gets doubled. The unknown frequency is

A closed organ pipe and an open organ pipe of the same length produce 2 beats when they are set into vibrations simultaneously in their fundamental mode. The length of the open pipe is now halved and of the closed organ pipe is doubled. The number of beats produced will be :

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A closed organ pipe and an open organ pipe of the same length produce 2 beats when they are set into vibrations simultaneously in their fundamental mode. The length of the open pipe is now haled and that of the closed organ pipe is doubled. The number of beats produced will be:

R and S are two wires whose fundamental frequencies are 324 \mathrm{hz} and 422 \mathrm{hz} respectively. How many beats in the Third second will be heard by the Fifth harmonic of A and the second harmonic of B? 

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Two-plane harmonic sound waves are expressed by the equation

y_1(x, t)=A \cos (0.7 \pi x-200 \pi t)

y_2(x, t)=A \operatorname{cos}(0.38\pi x-160 \pi t)

How many times does an observer hear maximum intensity in one second?


 

Concepts Covered - 1

Beats

Beats -

When any two sound waves of slightly different frequencies, travelling along the same direction in a medium and superimpose on each other then the intensity of the resultant sound at a particular position rises and falls regularly with time. This phenomenon of regular variation in intensity of sound with time at a particular position is called beats. 

If we struck two tuning forks of slightly different frequencies, one hears a sound of periodically varying amplitude. This phenomenon is called beating.

Beat frequency, equals the difference in frequency between the two sources which we will see below.

Let us consider two sound waves travelling through a medium having equal amplitude with slightly different frequencies f_{1} and  f_{2}. We use equations similar to equation y=A \sin (k x-\omega t) to represent the wave functions for these two waves at a point such that  k x=\pi / 2 :

                                                              \begin{array}{l}{y_{1}=A \sin \left(\frac{\pi}{2}-\omega_{2} t\right)=A \cos \left(2 \pi f_{1} t\right)} \\ \\ {y_{2}=A \sin \left(\frac{\pi}{2}-\omega_{2} t\right)=A \cos \left(2 \pi f_{2} t\right)}\end{array}

By using superposition principle - 

                                                              y=y_{1}+y_{2}=A\left(\cos 2 \pi f_{1}+\cos 2 \pi f_{2} t\right)

We can also write the above equation by using trigonometric identity as - 

                                                   y=\left[2 A \cos 2 \pi\left(\frac{f_{1}-f_{2}}{2}\right) t\right] \cos 2 \pi\left(\frac{f_{1}+f_{2}}{2}\right) t

The graph is like this - 

                                                      

Graphs of the individual waves and the resultant wave are shown in the figure. We can see that the resultant wave has effective frequency equal to average frequency   \frac{f_1+f_2}{2}. From the figure, we can see that this wave is multiplied by the envelope whose equation is given as - 

                                                                    y_{\text {envelope }}=2 A \cos 2 \pi\left(\frac{f_{1}-f_{2}}{2}\right) t

 \begin{array}{l}{\text { A maximum in the amplitude of the resultant sound wave is }} {\text { detected whenever }} \\ \\ {\qquad \cos 2 \pi\left(\frac{f_{1}-f_{2}}{2}\right) t=\pm 1}\end{array}

Hence, there are two maxima in each period of the envelope wave. Because the amplitude varies with frequency as \frac{(f_{1}-f_{2} ) }{ 2} the beat frequency is two times of this value and given by  - 

                                                                                     f_{\mathrm{beat}}=\left|f_{1}-f_{2}\right|

                                                               

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Beats

Physics Part II Textbook for Class XI

Page No. : 382

Line : 29

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