Calculating the True Frequency

Oct 13, 2025 by wbrum.org 104 views

Introduction

When a fire truck is moving away from a stationary person, the sound waves emitted by its siren undergo a phenomenon known as the Doppler effect. This effect causes the frequency of the sound waves to change, resulting in a perceived change in pitch. In this article, we will delve into the world of physics and explore how to calculate the true frequency of the siren.

The Doppler Effect

The Doppler effect is a fundamental concept in physics that describes the change in frequency or pitch of a wave in relation to an observer moving relative to the source of the wave. When the source of the wave is moving away from the observer, the frequency of the wave appears to decrease, resulting in a lower pitch. Conversely, when the source is moving towards the observer, the frequency appears to increase, resulting in a higher pitch.

Calculating the True Frequency

To calculate the true frequency of the siren, we need to use the Doppler effect formula:

f' = f \frac{v}{v + v_s}

where:

$f'$ is the perceived frequency of the siren
$f$ is the true frequency of the siren
$v$ is the speed of sound in air (approximately $343 \, \text{m/s}$ at room temperature and atmospheric pressure)
$v_s$ is the speed of the fire truck

Given that the fire truck is moving away from the stationary person at a speed of $24.5 \, \text{m/s}$ , we can plug in the values to the formula:

f' = 894 \, \text{Hz} \frac{343 \, \text{m/s}}{343 \, \text{m/s} + 24.5 \, \text{m/s}}

Solving for the True Frequency

To solve for the true frequency, we need to rearrange the formula to isolate $f$ :

f = f' \frac{v + v_s}{v}

Plugging in the values, we get:

f = 894 \, \text{Hz} \frac{343 \, \text{m/s} + 24.5 \, \text{m/s}}{343 \, \text{m/s}}

Simplifying the expression, we get:

f = 894 \, \text{Hz} \frac{367.5 \, \text{m/s}}{343 \, \text{m/s}}

f = 894 \, \text{Hz} \times 1.073

f = 958.37 \, \text{Hz}

Conclusion

In conclusion, the true frequency of the siren is approximately $958.37 \, \text{Hz}$ . This is the actual frequency of the siren, unaffected by the Doppler effect. The perceived frequency of $894 \, \text{Hz}$ is a result of the fire truck moving away from the stationary person, causing the frequency to appear lower due to the Doppler effect.

The Importance of the Doppler Effect

The Doppler effect is a fundamental concept in physics that has numerous applications in various fields, including astronomy, medicine, and engineering. Understanding the Doppler effect is crucial in accurately measuring the frequency of sound waves, which is essential in fields such as music, acoustics, and audio engineering.

Real-World Applications

The Doppler effect has numerous real-world applications, including:

Astronomy: The Doppler effect is used to measure the velocity of celestial objects, such as stars and galaxies.
Medicine: The Doppler effect is used in medical imaging techniques, such as Doppler ultrasound, to measure blood flow and detect vascular diseases.
Engineering: The Doppler effect is used in various engineering applications, including radar technology and audio engineering.

Conclusion

Q&A: The Doppler Effect and the Fire Truck's Siren

Q: What is the Doppler effect?

A: The Doppler effect is a fundamental concept in physics that describes the change in frequency or pitch of a wave in relation to an observer moving relative to the source of the wave.

Q: How does the Doppler effect affect the frequency of the siren?

A: When the fire truck is moving away from the stationary person, the frequency of the siren appears to decrease, resulting in a lower pitch. Conversely, when the fire truck is moving towards the stationary person, the frequency appears to increase, resulting in a higher pitch.

Q: What is the formula for calculating the true frequency of the siren?

A: The formula for calculating the true frequency of the siren is:

f' = f \frac{v}{v + v_s}

where:

$f'$ is the perceived frequency of the siren
$f$ is the true frequency of the siren
$v$ is the speed of sound in air (approximately $343 \, \text{m/s}$ at room temperature and atmospheric pressure)
$v_s$ is the speed of the fire truck

Q: How do I calculate the true frequency of the siren?

A: To calculate the true frequency of the siren, you need to plug in the values to the formula and solve for $f$ . In this case, the perceived frequency of the siren is $894 \, \text{Hz}$ , the speed of sound in air is $343 \, \text{m/s}$ , and the speed of the fire truck is $24.5 \, \text{m/s}$ .

Q: What is the true frequency of the siren?

A: The true frequency of the siren is approximately $958.37 \, \text{Hz}$ .

Q: Why is the Doppler effect important?

A: The Doppler effect is a fundamental concept in physics that has numerous applications in various fields, including astronomy, medicine, and engineering. Understanding the Doppler effect is crucial in accurately measuring the frequency of sound waves, which is essential in fields such as music, acoustics, and audio engineering.

Q: What are some real-world applications of the Doppler effect?

A: Some real-world applications of the Doppler effect include:

Astronomy: The Doppler effect is used to measure the velocity of celestial objects, such as stars and galaxies.
Medicine: The Doppler effect is used in medical imaging techniques, such as Doppler ultrasound, to measure blood flow and detect vascular diseases.
Engineering: The Doppler effect is used in various engineering applications, including radar technology and audio engineering.

Q: Can the Doppler effect be used to measure the speed of objects?

A: Yes, the Doppler effect can be used to measure the speed of objects. By measuring the change in frequency of a wave, you can calculate the velocity of the object.

Q: What are some common sources of the Doppler effect?

A: Some common sources of the Doppler effect include:

Sound waves: The Doppler effect is commonly observed in sound waves, such as the siren of a fire truck.
Light waves: The Doppler effect is also observed in light waves, such as the redshift of light from distant galaxies.
Radar waves: The Doppler effect is used in radar technology to measure the velocity of objects.

Conclusion

In conclusion, the Doppler effect is a fundamental concept in physics that describes the change in frequency or pitch of a wave in relation to an observer moving relative to the source of the wave. By understanding the Doppler effect, we can accurately calculate the true frequency of the siren, which is essential in various fields, including astronomy, medicine, and engineering.