**Interference**

**Beats**

**Sources**

**Doppler**

**Doppler**

**Must use stationary reference frame, because the air is stationary. (this is what the sound moves through)**

**a car travels 25 m/s and a cop travels 35 m/s and emits a frequency of 1200 Hz. What orientations makes the highest and lowest frequencies observed by the car.**

A firetruck has stopped to turn left and is emitting a frequency of 950 Hz. A car approaches the same intersection. If the car hears a frequency of 1000 Hz. How fast is it moving?

While sitting on the side of a road a person hears the frequency of 550 Hz and then 450 Hz, for a police car as it approaches and then recedes away. Assuming the police maintains the same speed, how fast is he going?

34.3 m/s

A bat sitting on a branch emits a frequency of 12 kHz towards a bug, that is moving away at a speed of 10 m/s, what frequency does it hear reflected back towards it? ( note there are two doppler shifts here....)

A electric piano is on top of a parade float, that is moving at a speed of 5 m/s. As it approaches a wall, the sound is reflected off the wall and returns to the piano. The pianist, who has perfect pitch, hears what beat frequency, for the note A 440 Hz?

A guitarist without perfect pitch tunes a string to a tuning harp with a frequency of 285 Hz, and notices he is producing a beat frequency of 3 Hz.

He doesn't know if he should tune 'up or down' so he tunes to another harp with frequency of 290 Hz, and hears a beat frequency of 2 Hz. What is the frequency of the note he is playing?

**Interference**

Geometry (delta r)

Wavelength ( whole or half)

What is the 'n' number

Two speakers emit a frequency of 350 Hz in phase, If they are 6 m apart, and I am on a line that is a perpendicular bisector, 4 m way from the plane of the speakers. Am I hearing constructive or destructive interference?

two main geometries

How far do I walk ( on board) to hear destructive interference? THIS IS A BAD PROBLEM. WHY?

Two speakers a that are 6 m apart and I am on a line that is a perpendicular bisector out 4 m from the plane of the speakers.

If I walk 25 cm 'up' and hear destructive interference, what is the smallest frequency that would produce this result?

What are the next 2 frequencies that would do this?

Two speakers are 5 m apart, and I am 5 m in front of the bottom one. they play a note with a frequency of 275 Hz. What is the "n" at this location?

If I walk to the towards the speak does the "n" increase or decrease?

How far towards do I need to walk to hear constructive interference?

Two speakers are 5 m apart, and I am 5 m in front of one of them. I hear perfectly constructive interference here.

What is the lowest frequency that produces this result?

"I just keep getting the signs mixed up with the Doppler effect equation?"

"Why is there a difference when the observer moves away from a stationary source and when the observer is stationary and the source moves away?"

"Can we go over the last problem from the pre-quiz please?"

"In considering the example problem "a moving siren" the police moved towards and away, what would the frequency be if you moved 25 m/s away from the stationary police car? (answer should look like '1490 Hz') (3 sig fig) "

18 m/s

1434 Hz, 1010 Hz

11320 Hz

13 Hz

288 Hz

Con

WHAT COULD I ASK, (with this geometry)?

572 Hz

1716 Hz, 2861 Hz

**1.66**

**Increase**

**1.24 M**

How far would I have to walk towards to hear constructive again?

**165 Hz.**

**4.05 m**

either 282 or 288

and 288 or 292

1600 Hz

"Harmonics and overtones are still hard for me to grasp. Can we briefly touch on the equations for them again sometime?"