Doppler effect for sound

In summary, when a source is moving towards a listener, the wavelength of sound becomes smaller and the number of waves increases. This is because as the source moves, the waves become congested in a smaller distance between the source and the listener. The frequency of the sound also increases because the source is creating more pulses of sound in a shorter amount of time. This phenomenon is not limited to sound waves, but also applies to other types of waves. The concept of relative motion and its effects on the wavelength and frequency of waves is important to understand in order to fully grasp the principles of relativity.
  • #1
hasanhabibul
31
0
we know...when a source is running towards the listener...the wavelenth of sound become small and number of waves inceases...as far as i know ..the logic behind is..when a source runs..then waves become congested in a small distance between source and listener...it is so far so ridiculous to me how waves become congested for the relative motion ...why frequecy i.e. structure of the wave has a headache with motion of both source and the listener... why this happens ...what is the relation of source motion with the frequency when frequency of a source has a constant value?
 
Physics news on Phys.org
  • #2
Can I attempt an answer?

Imagine the source is moving relative to the medium the waves are produced in, and the waves move with a constant velocity through the medium - as is the case with sound. Imagine the source is approaching you, and it is traveling slower than the speed of the waves. At a certain location, it makes the first pulse of sound. It then moves forward behind the advancing sound wave, and a moment later creates the next pulse of sound.

By "pulse of sound" I don't mean a "beep", but since all sounds are waves, sound waves come with crests and troughs. By the "pulse" I mean the point where the source creates the "crest" of each sound wave. For very low frequency sound waves, you can sometimes feel the crests and troughs of sound waves as vibrations in the ground or floor.

Therefore, as the source is moving forwards towards you, the distance between the "crest" of the first sound wave, and the "crest" of the next sound wave will be shorter than if the source were stationary. Therefore the waves will have shorter wavelength. As the waves move with a constant speed, they will therefore arrive at where you are with a higher frequency.

For someone on the far side of the source, who the source is moving away from, the crests between the waves will be further apart than if the source was stationary. Therefore the waves will have a longer wavelength and a lower frequency.

Note that if you are moving through the medium with the same speed and direction as the source, the waves would sound the same as if you both were stationary. Think about it.

I'm waiting for someone to bring in relativity here, which I've ignored. But for normal events we experience in ordinary life here on earth, this is the situation. Mind you, once you've grasped this, you are ready for Einstein's relativity.

If the source is moving faster than the speed of the waves, at a certain point to the side of the source, the crests of each wave will double up and overlap, creating a huge bang or "sonic boom".
 
Last edited:
  • #3


The Doppler effect for sound is a well-known phenomenon that occurs when a sound source is in motion relative to the listener. The key concept to understand is that sound waves are a form of mechanical energy that require a medium, such as air, to travel through. When a sound source is moving towards the listener, the sound waves become compressed, leading to a shorter wavelength and a higher frequency. This is because the source is continuously emitting new waves while moving closer to the listener, causing the waves to overlap and appear more frequent.

The reason behind this phenomenon lies in the basic principles of wave mechanics. When a sound source is stationary, the frequency of the waves it emits remains constant. However, when the source is in motion, the relative motion between the source and the listener causes the wavelength to change, resulting in a perceived change in frequency. This is similar to how a car horn sounds higher pitched when it is approaching and lower pitched when it is moving away.

In essence, the motion of the source affects the frequency of the waves because it alters the distance between consecutive wave crests. This is why the Doppler effect is often described as a "pitch shift" in sound. The closer the source is to the listener, the higher the perceived frequency, and vice versa.

In conclusion, the relationship between source motion and frequency in the Doppler effect for sound is a result of the fundamental principles of wave mechanics. The motion of the source alters the distance between wave crests, leading to a change in frequency perceived by the listener. This phenomenon has been extensively studied and has numerous practical applications, such as in radar technology and medical imaging.
 

1. What is the Doppler effect for sound?

The Doppler effect for sound is the change in frequency or pitch of a sound wave due to the relative motion between the source of the sound and the observer. When the source of the sound is moving towards the observer, the frequency increases and the sound is perceived as higher pitched. When the source is moving away, the frequency decreases and the sound is perceived as lower pitched.

2. What causes the Doppler effect for sound?

The Doppler effect for sound is caused by the compression and expansion of sound waves as they travel through air. When the source of the sound is moving towards the observer, the sound waves are compressed, resulting in a higher frequency. When the source is moving away, the sound waves are expanded, resulting in a lower frequency.

3. How is the Doppler effect for sound used in everyday life?

The Doppler effect for sound is used in various applications, such as in sonar and radar technology. It is also utilized in medical imaging, such as ultrasound, to detect the movement of blood and diagnose heart conditions. In addition, the Doppler effect for sound is used in musical instruments, such as sirens and whistles, to create different pitches and tones.

4. Can the Doppler effect for sound be observed in outer space?

Yes, the Doppler effect for sound can be observed in outer space. However, it is not caused by the movement of a sound source, but rather by the movement of celestial objects. For example, the frequency of radio waves emitted by a star will change as it moves towards or away from Earth, causing a Doppler shift in the detected signal.

5. How is the Doppler effect for sound different from the Doppler effect for light?

The Doppler effect for sound and the Doppler effect for light are similar in that they both involve a perceived change in frequency due to relative motion. However, the Doppler effect for light is caused by the movement of electromagnetic waves, while the Doppler effect for sound is caused by the movement of mechanical waves. Additionally, the speed of light is much faster than the speed of sound, resulting in a greater frequency shift for light compared to sound.

Similar threads

Replies
3
Views
646
Replies
30
Views
560
  • Mechanics
Replies
13
Views
2K
Replies
10
Views
3K
Replies
1
Views
2K
Replies
3
Views
2K
  • Classical Physics
Replies
5
Views
223
  • Mechanics
Replies
5
Views
1K
Replies
1
Views
1K
Replies
8
Views
5K
Back
Top