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1 Physics -- Acoustic Phenomena

1.     A source and an observer are at rest on the earth but a strong wind is blowing over them. Is there a Doppler effect?

1.    
A source and an observer
are at rest on the earth but a strong wind is blowing over them. Is there a
Doppler effect?

1 Answers

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When the source and observer are moving in the same direction, the apparent frequency is given by

n’=n (V+W)−V0

        (V+W)−Vs



Where, Vo is the velocity of the observer

Vs is the velocity of the source

W is the velocity of the wind blowing in the same direction.

If the source and observer are at rest i.e.Vo=Vs=0

n’=n

There would be no Doppler shift in frequency because the wind is moving with the same velocity relative to both the observer and source. Notice that the sound medium moving against the direction of sound while both observer and source are stationary gives the same effect as if the source and observer are both moving in the same direction. So there is no Doppler shift in frequency.

More questions on Acoustic Phenomena

A cylindrical wire of resistance 'R' is reformed in a new wire of double length then resistance will be A. 2RB. 4RC. 6RD. 8R

As the length of the wire is changed, it's cross sectional area will also change but volume remains same.

Radius decreases by factor of 2^(1/2).

Resistance=(rho L)/A

Using above relation , new resistance will be 4R

How can we distinguish between the sound of sitar and harmonium without looking at the instruments when th sound is of the same pitch and loudness?

Even though they have same frequency and loudness, the overtones produced by those instruments differ the quality or timber of the sound produced by them. This is why we can distinguish their sound.

This is the same reason why we are able to distinguish voice of different people.

1.     Distinguish between musical sound and noise.

Noise

Music

Sounds which are not pleasant to hear is called Noise

Sounds which are pleasant to hear is called Music

We do not like such sounds

We like such sounds

Example

Sounds produced by musical instruments like guitar, piano

Example

Sounds of Vehicles

Bursting of Crackers

1.     What are the characteristics of musical sound? On what factors do they depend?

The following are the characteristics of a musical sound.

1.Pitch or Frequency

2.Intensity and loudness

3.Quality or Timber.


1.     How are musical sound and noise distinguished from each other?

Noise

Music

Sounds which are not pleasant to hear is called Noise

Sounds which are pleasant to hear is called Music

We do not like such sounds

We like such sounds

Example

Sounds produced by musical instruments like guitar, piano

Example

Sounds of Vehicles

Bursting of Crackers

1.     On what factors does the loudness of a sound depend?

The loudness of sound depends on the amplitude of sound waves. Since the amplitude of a sound wave is equal to the amplitude of vibrations of the source which produces the sound wave, we can also say that: The loudness of sound depends on the amplitude of vibration of the source producing the sound waves.

1.     How do the terms frequency and pitch differ from each other?

  • Frequency is a very well-defined quantity whereas pitch is not a well-defined quantity.
  • Pitch is a property observed only in sound waves. Frequency is a property discussed in all forms of waves including electromagnetic waves and mechanical waves.
  • Frequency is also discussed in oscillations and vibrations.

1.     What are the units in which the intensity of a sound expressed?

The intensity of sound is measured in the units of decibel. The decibel is a unit of measurement used to express the ratio of one value of a physical property to another on a logarithmic scale. One decibel equals 10 times the common logarithm of the power ratio.



1.     What do you mean by ultrasonic?

Sound waves with frequencies higher than the upper audible limit of human hearing are called ultrasound.The limit varies from person to person but is approximately 20,000 Hertz. The physical properties of ultrasound are like the normal audible sound.

1.      Why does a flute give out a musical sound?

The sound from flute comes due to the vibrating columns of air inside it. When air is blown into instruments, it vibrates and then made to resonate to produce sound. In a flute, the air gets vibrated, and the resonator amplifies the vibration and gets modified into sound.

Two notes one produced by a violin and the other by a sitar may have same frequency, yet we can distinguish between them. Explain why?

The quality of a note emitted by a certain instrument depends on the presence or absence of overtones and not on the pitch (frequency) and the loudness (intensity). Thus, sounds from various instruments like violin and guitar may have same pitch and loudness but the quality of the notes emitted by them will be different depending on the number of overtones present in the note.

1.     Two tuning forks vibrate with the same amplitude but the frequency of the first is double the frequency of the sound. Which fork produces more intense sound in air?

If two tuning forks vibrate with the same amplitude but the frequency of the first is double the frequency of the sound, then tuning forks with high frequency will have more intense sound because higher frequency results in higher pitches.

1.     Two notes have frequencies 200Hz and 300 Hz respectively, what is the musical interval?

Music interval (T) =1/ (f2- f1)


= 1/(300200)

=1/100

= 0.01sec

1.     How many beats can be heard by sounding together two tuning forks of frequencies 250Hz and 256Hz?

Number of beats (n) =f2- f1

= 256 - 250

= 6 beats /sec

1.     What is decibel?

Smaller unit of the loudness is called decibel. It is one-tenth of bel. It is denoted by db. Loudness of sound in decibel is given by

L (in db) =10 log(I/Io)

1.     In what condition Doppler effect are not applicable?

The Doppler Effect is not applicable in following conditions:

a. If the velocity of sound of the source is greater than that of the sound because the wave gets distorted due to which no change in frequency will be observed.

b. If the velocity of the sound of the observer is greater than that of the sound.

1.     Sound waves can be used to measure blood flow in arteries and veins. Explain?

A Doppler ultrasound is a test that uses high-frequency sound waves to measure the amount of blood flow through your arteries and veins, usually those that supply blood to your arms and legs.

1.     What determines the quality of sound?

Quality of a sound is that characteristic which distinguishes the two sounds of the same loudness and same pitch but emitted by two different instruments.

The quality of sound is determined by the frequencies present in it and their relative loudness.

Even if the sounds of two objects have the same frequency and similar wavelength, they are different.

Example:The sound produced by a guitar and a sitar. Both have the same frequency (i.e. pitch) and loudness (i.e. amplitude), but still one can...

1.     How can bats see in darkness?

To safely navigate and hunt in the dark, bats use echolocation. Echolocation is the use of sound waves and their reflected echoes to identify where objects are in space

1.     Whistle of an approaching train is shriller. Why?

Doppler discovered that if any object giving out sound, is moving toward you, the frequency of the sound increases. That is why as the train gets closer, the sound of the whistle increases, it also became shriller.

1.     Does the velocity of sound in a gas depend on the atomicity of the gas?

Yes, It depends.

Velocity of sound in any gas depends upon density and elasticity of gas.
v=ργP
For monoatomic gasesγ= 5/3 and for diatomic gasesγ=7/5

1.     What is red shift?

If an object emitting light recedes away from an observer, the light reaching the observer will shift towards the red, increasing the wavelength and decreasing the frequency. This is called a redshift.

1.     Why should the difference of the frequencies of two waves should not be greater than 10Hz?

For beats to be audible the difference in frequency of the two waves should not exceed 10Hz.If the beat frequency is less than about 10 Hz, the two waves are very close in frequency and you will hear only one pitch.

If the difference in frequencies of the two waves is more than 10, we shall hear more then 10 beats for second. Due to persistence of hearing, our ear is not able to distinguish between two sounds as separate. If the time internal between them is less than(1/10)th of a second....

1.     What do you mean by intensity and intensity level of sound?

Intensity of the sound is defined as the rate of transfer of sound energy Intensity of sound is defined as the sound energy emitted or received per unit area per second Let I be the intensity of the sound, E be the energy and A be the area and t be the time then the intensity is given by

Intensity (I)= energy passing/(area x time)=E/(A x t)

Intensity level: it is define as the logarithm of the ratio of intensity of the sound to an arbitrarily chosen intensity i.e. L= log10(I/I0)

Where I be the...

State the characteristics of a musical note?

The following are the characteristics of a musical sound.

1.Pitch or Frequency

2.Intensity and loudness

3.Quality or Timber.

1.     What makes the two musical notes of the same pitch but produced from different sources differ from each other?

The reason the same musical note sounds different when played on various instruments is because the harmonic overtones and envelope of each instrument is unique. When a frequency is played, other frequencies, called harmonics, are created. Each instrument has a unique harmonic character. The duration of the notes, or envelope, played on different instruments also varies widely.

1.     What is the Phon unit of?

A unit used to describe the LOUDNESS LEVEL of a given sound or noise is PHON

What is Doppler’s effect of sound in air?

The Doppler effect describes the shift in the frequency of a wave sound when the wave source and/or the receiver is moving.

1.     What do you mean by Threshold of Hearing?

The hearing threshold is the sound level below which a person's ear cannot detect any sound. Any sound below these levels will be inaudible and any sound above the threshold will vary in loudness, dependent on intensity.For adults, 0 dB is the reference level.

1.     How is it that you can recognize a friend from a friend from his voice without seeing him/her?

Due to the quality of sound, one can recognize a friend from his voice without seeing him. The quality of sound emitted by a person depends on the presence of overtones. The quality of sound helps us to distinguish between two sounds. Having the same loudness and pitch. The overtones present in different voices are different whatever they produce the same frequency and the same intensity of sound. In this way, one can recognize a friend from his voice without her/his face.

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