# Frequency of sound received by an observer

• PSN03
In summary, the problem statement does not mention a distance between observer and crossing point. So, using the assumption that the sound travels from the source to the observer in a straight line, the answer is zero.
PSN03
Homework Statement
A source and an observer are situated on two perpendicular tracks as shown in the figure the observer is at rest and source is moving with a speed 50m/s the source emits sound waves of frequency 90Hz which travel in the medium with velocity 200m/s the frequency of sound heard by observer when the source crosses the origin is
Relevant Equations
By dopler's equation we know that
f'=f(v±vo/v±vs)
Where
f'= new frequency
vo=velocity of observer
vs=velocity of source
v=velocity of sound
They are asking us to find the frequency of sound received by observer when the source reaches at origin. According to me when the source reaches origin there would be no component of source's velocity in the direction of observer, hence vs=0
vo=0 since the observer is stationary

Therefore by the formula I should get the answer as
f'=f(v±0/v±0)
=f
Hence no change in frequency should be observed. But my answer is wrong.
Lease tell me where am I going wrong?

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How close is the observer to the crossing point of the tracks ?

PSN03 said:
Hence no change in frequency should be observed. But my answer is wrong.
Lease tell me where am I going wrong?
It takes time for the sound to travel from the source to the observer. At the instant when the source crosses the origin, the observer is hearing the sound that was emitted by the source at an earlier time (when the source was not yet at the origin). Some geometry will come into play. So, you need a good sketch of the situation.

PSN03 and etotheipi
TSny said:
It takes time for the sound to travel from the source to the observer. At the instant when the source crosses the origin, the observer is hearing the sound that was emitted by the source at an earlier time (when the source was not yet at the origin). Some geometry will come into play. So, you need a good sketch of the situation.
Yes...I got it later. Thanks for your help and I feel so stupid for forgetting such a simple point.

No need to feel stupid. My first reaction was like yours. I wonder if the actual problem statement mentions something of a distance between observer and crossing point ?

BvU said:
No need to feel stupid. My first reaction was like yours. I wonder if the actual problem statement mentions something of a distance between observer and crossing point ?
I guess the distance is enough for our answer to come right, though it isn't mentioned.

## What is the definition of frequency of sound received by an observer?

The frequency of sound received by an observer refers to the number of sound waves that pass by a specific point in one second. It is measured in Hertz (Hz) and is directly related to the pitch of the sound.

## How does the frequency of sound affect the human ear?

The human ear is capable of detecting a wide range of frequencies, typically between 20 Hz to 20,000 Hz. Higher frequencies are perceived as higher pitched sounds, while lower frequencies are perceived as lower pitched sounds. The frequency of sound also affects the loudness and quality of the sound.

## What factors can affect the frequency of sound received by an observer?

The frequency of sound can be affected by several factors, including the source of the sound, the distance between the source and the observer, and the medium through which the sound travels. For example, sound travels at different frequencies in air, water, and solids.

## How is the frequency of sound received by an observer measured?

The frequency of sound can be measured using a device called a frequency meter or an oscilloscope. These instruments detect the number of vibrations per second and display it as the frequency in Hz. Alternatively, the frequency can also be calculated by dividing the speed of sound by the wavelength of the sound wave.

## How does the frequency of sound relate to musical notes?

In music, the frequency of sound is directly related to the pitch of a note. Higher frequency sound waves produce higher pitched notes, while lower frequency sound waves produce lower pitched notes. This relationship is what allows us to create and distinguish different musical notes and melodies.

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