Today (Finish Chapter 13, Sound)  Temperature and Heat Concepts Tomorrow (Start Chapter 14)  Standing Waves  Beats  Doppler Effect  Example Problems.

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Presentation transcript:

Today (Finish Chapter 13, Sound)  Temperature and Heat Concepts Tomorrow (Start Chapter 14)  Standing Waves  Beats  Doppler Effect  Example Problems

Speed of Sound Sound is a wave, so its speed is related to frequency and wavelength. This is just like what we saw before! Where v = 343 m/s and is the speed of sound in air.

Speed of Sound in a String From before, we saw Where And the speed is related to

Sound: Intensity and Decibels Sound intensity level in decibels What is log? It stands for Logarithmic. It corresponds to a powers of 10. If the sound goes up by 10 decibels, this actually corresponds to the intensity increasing by a factor of 10 (meaning it would be multiplied by 10)!

For a wave that travels in a three-dimensional medium, the intensity is the measure of average power per unit area carried by the wave. What is Intensity? Decreases with the increase of the distance from the source due to spreading out. Point source emitting in all directions no reflection or absorption

Sound: Intensity and Decibels Sound intensity level in decibels

The sound intensity 5 m from a jackhammer is 4.2 x W/m 2. What is the sound intensity level in decibels? (Use reference level of I 0 = 1.00 x W/m 2.) Answer: dB (decibels)

Sound in decibels You are at a rock concert and the sound intensity reaches levels as high as 130 dB. What is the intensity of the music in W/m 2 ? What is the power (Watts) from the speakers if you are 10 meters away?

Standing Sound Waves Open at both ends Standing sound waves (thin pipe open at both ends) Where

Standing sound waves Open at one end Standing sound waves (thin pipe closed at one end ) Where

If two sources have slightly different frequencies, we will get an alternating pattern of constructive and destructive interference, called beats. The beat frequency is the number of times per second that the loudness rises and falls. Beats

Beats example from text A guitarist plucks two strings simultaneously. One string is tuned to a (fundamental) frequency of Hz. It is found that beats are audible, with a beat frequency of 0.5 Hz. What are the possible values for the frequency of the other string?

Doppler Effect Applications: – Doppler Radar – Radar Gun – Echocardiogram: Images Heart with Sound – Motion of Stars/Galaxies – Music (Special effects with voice/sound, i.e. The Beatles)

The Doppler Effect is the change in frequency (pitch) of the sound detected by an observer because the sound source and the observer have different velocities with respect to the medium of sound propagation. The Doppler Effect describes why the pitch of a siren that is approaching you sounds higher than the pitch of a siren that is moving away from you. The Doppler Effect

Moving SOURCE Approaching observer If the source of the sound (the fire truck, for example) is moving towards an observer, the sound waves “bunch up” in front of the source, causing the wavelength observed by a stationary person to shorten. (Smaller wavelengths, bigger frequency.) Receding from observer Behind the source, the sound waves “stretch out”, and the wavelength observed by a stationary person lengthens. (Bigger wavelength, smaller frequency.) The Doppler Effect

Moving SOURCE First “crest” is heard after Second “crest” is emitted after T S First “crest” is emitted In the same time the source travels, It is heard after The Doppler Effect

Moving SOURCE (stationary observer) Approaching observer Moving away from observer The Doppler Effect

A train moving at a speed of 44.7 m/s sounds it’s 415 Hz horn. What is the frequency of the sound heard by a stationary observer standing near the crossing (a) before the train has passed? (b) after the train has passed?

Doppler Example A guitar is tuned to play a note at 440 Hz. If the guitar is in a moving car and the note has a perceived frequency of 410 Hz to a pedestrian on the sidewalk. What is the speed of the car? Assume the observer is stationary on the sidewalk and the car is moving away from the observer.

Moving OBSERVER Approaching source If the observer is moving towards the source, the observer passes more sound waves, causing the number of waves encountered in one second to increase. (Bigger frequency.) Receding from source If the observer is moving away from the source, the observer passes fewer sound waves, causing the number of waves encountered in one second to decrease. (Smaller frequency.) The Doppler Effect

Moving OBSERVER (stationary source) Approaching source Receding from source The Doppler Effect

Speakers A and B emit identical frequencies. A is at rest, speaker B moves toward A at a constant speed. (a) Does a listener at speaker A hear the sound emitted by B at a frequency greater than, less than, or equal to the stationary frequency? (b) Does a listener at speaker B hear the sound of A at a greater than, less than, or equal frequency?

OBSERVER and SOURCE are moving The Doppler Effect