DOPPLER EFFECT

RECAP FROM LAST WEEK…Solving decibel problems requires

knowledge of logarithmic functions.logA - logB = log(A/B)

I=I0

I=

Intensity Level (a.k.a., loudness)

Sound Intensity

EXAMPLE

What is the change in sound intensity needed to double the perceived loudness of a sound?

2 times the loudness means a 10dB increase in sound intensity

So β2-β1=10dB=10log(I2/I0)-10log(I1/I0) solving gives = 10Sound Intensity must increase by a factor of 10 to

get a sound that is perceived to be twice a loud.Means that a 200W stereo is only twice as loud as

a 20W stereo!

THE DOPPLER EFFECT

Why does a siren of a police car sound higher pitched as it is coming toward you, but drops to a lower pitch as it moves away from you?

THE DOPPLER EFFECT

This phenomenon was first identified in 1842 by Austrian physicist Christian Doppler as is called the Doppler effect.

It involves the concepts of velocity (of an object), wavelength and frequency (of sound)

THE DOPPLER EFFECT

When the firetruck is at rest, the sound waves emitted by the truck are symmetrical so a listener in front of the truck will hear the same frequency (same number of condensations per second) as a listener behind the truck.

THE DOPPLER EFFECT

When the firetruck begins to move, the condensations in front of the truck are closer together

The sound waves “bunch up” because the truck gains ground on an already emitted condensation before emitting the next one.

THE DOPPLER EFFECT-MOVING SOURCE

The sound waves in front of the truck have a smaller wavelength (and higher frequency) = higher pitch

The sound waves behind the truck have a larger wavelength (and lower frequency) = lower pitch

Notice the wavelength of the sound perceived by the observer changes, so the observer hears a different frequency

THE DOPPLER EFFECT – MOVING SOURCE

𝑓 0= 𝑓𝑠(1

1−𝑣 𝑠𝑣

)Frequency heard by observer

Frequency emitted by source

Speed of source

Speed of sound

For a source moving TOWARD a stationary observer

is called the Doppler shift

THE DOPPLER EFFECT-MOVING SOURCE

𝑓 0= 𝑓𝑠(1

1+𝑣 𝑠𝑣

)Frequency heard by observer

Frequency emitted by source

Speed of source

Speed of sound

For a source moving AWAY from a stationary observer

is called the Doppler shift

EXAMPLE

A high speed train is traveling at a speed of 44.7 m/s when the engineer sounds the 415 Hz warning horn. The speed of sound is 343 m/s. What are the frequency and wavelength of the sound, as perceived by a person standing at a crossing when the train is (a) approaching and (b) leaving the crossing

EXAMPLE - SOLUTION

= 415(o

λ= = 343/477 = 0.718m= 415(λ= = 343/367 = 0.935m

THE DOPPLER EFFECT – MOVING OBSERVER

If the sound source is stationary, and the observer is moving, there is a Doppler shift.

The observer will intercept more wave condensations per unit time than a stationary observer would.

The observer is detecting a different frequency, fo.

THE DOPPLER EFFECT –MOVING OBSERVER

𝑓 0= 𝑓𝑠(1+𝑣𝑂𝑣

)Frequency heard by observer

Frequency emitted by source

Speed of observer

Speed of sound

For an observer moving TOWARD a stationary source

is called the Doppler shift

THE DOPPLER EFFECT –MOVING OBSERVER

𝑓 0= 𝑓𝑠(1−𝑣𝑂𝑣

)Frequency heard by observer

Frequency emitted by source

Speed of observer

Speed of sound

For an observer moving TOWARD a stationary source

is called the Doppler shift

ASSIGNMENT

P. 504 #63-69 (pick 4)P. 504 #76-80 (pick 4)Due Thursday.

Final exam 6/9 will cover waves and sound (chapters 16 & 17)