T2-11-W4 Sound and Doppler Effect

Sound Waves and the Doppler Effect

Objectives

• Period 1: Describe the characteristics of sound waves

• Period 2: Discuss how sound is detected and perceived, explaining sound’s pitch and loudness.

• Period 3: Describe the Doppler effect

• Period 4: Apply Doppler effect formula to solve problems

Essential Questions

1. What properties does sound share with other waves?

2. How do the physical properties of sound waves relate to our perception of sound?

3. What is the Doppler effect?

4. What are some applications of the Doppler effect?

Vocabulary

• Sound Wave: A wave that transfers sound.

• Pitch: How high or low a sound is perceived.

• Sound Level: A measure of sound intensity.

• Loudness: The perceived volume of sound.

• Decibel (dB): Unit of measurement for sound intensity.

• Doppler Effect: Change in frequency of a wave in relation to an observer moving relative to the wave source.

Characteristics of Sound Waves

• Sound is considered a wave that travels through various mediums (air, liquids, solids).

• Common characteristics include:

  • Frequency: Number of waves passing a point in a second.

  • Period: Time for one complete wave cycle.

  • Wavelength: Distance between successive points of the wave.

  • Wave Speed: Speed at which sound travels in different mediums.

Mechanism of Sound Propagation

• When a source such as a tuning fork vibrates, it compresses and rarefies air particles, creating sound waves that travel through the medium.

• Sound waves cannot travel through a vacuum because they require particles for transmission.

• Sound speed varies based on the medium: faster in solids and liquids than in gases.

Speed of Sound in Various Media

Wave Characteristics

• Amplitude: Maximum height of the wave, affects energy but not frequency.

• Wavelength (λ): Distance between two consecutive crests or troughs.

• Wave Velocity (v): Distance traveled by a wave in one second.

• Frequency (f): Measured in Hertz (Hz), indicating the number of cycles per second.

Doppler Effect

• Describes the change in frequency perceived by an observer due to the movement of the sound source or the observer.

• Frequency (f) received by a detector can be calculated with:

  • When source is moving towards observer: [ f_d = f_s \cdot \left( \frac{v + v_d}{v - v_s} \right) ]

  • When source is moving away from observer: [ f_d = f_s \cdot \left( \frac{v - v_d}{v + v_s} \right) ]

Applications of the Doppler Effect

• Common Uses: Radar and medical imaging (ultrasound), measuring the speed of vehicles, and astronomy to determine the movement of stars.

Human Hearing and Sound Perception

• Human hearing ranges from 20 Hz to 20,000 Hz.

• Sounds less than 20 Hz are termed infrasonic, and sounds above 20,000 Hz are ultrasonic.

• Intensity is connected to amplitude, while loudness relates to the intensity perceived.

Intensity Measurement

• Intensity of sound relates to how we perceive loudness, scaled in decibels (dB).

• Logarithmic scale: every 10 dB increase corresponds to a perceived doubling of loudness.

  • Example:

    • 70 dB is twice as loud as 60 dB.

    • 80 dB is four times louder than 60 dB.

Intensity Levels

Practice Problems

1. Doppler Effect Example: Calculate the frequency perceived by an observer based on the speed of a source emitting a known frequency.

2. Intensity Problem: If 10 babies crying together create a total sound intensity of 79 dB, show the change in intensity level when compared to a single baby.

Conclusion

Understanding sound waves, their characteristics, and the implications of the Doppler effect is crucial in various fields such as physics, audio engineering, and environmental science.