Warm-Up: Doppler Effect #1 You and a friend are out on the lake, enjoying summer. You are driving along the coast, when you hear music playing from one of the docks. A particular frequency of 320 Hz sounds from the speaker. What is the frequency that you observed if you are moving 16 m/s and are already past the dock? fo = 305 Hz

Going Faster Moving at the Speed of Sound Moving Faster than Sound Moving Source Mach I Supersonic Pressure wave, piled up at nose Shock wave of constructive interference

Sonic Boom When the wave of sound passes you – Sonic Boom! Do you see the plane or hear it first?

Sonic Boom High pressure Low pressure Moisture condenses in low pressure region High pressure Low pressure

Wave Interference SUPERPOSITION ! Wave interaction, described by Superposition –constructive or destructive combination of waves if present at the same place, same time

When Two Waves Meet… If two waves combine... Constructive Interference – Result is a larger amplitude Destructive Interference – Result is a smaller amplitude Waves pass through each other afterwards.

Example 6 The figure above shows two wave pulses that are approaching each other. Which of the following best shows the shape of the resultant pulse when the centers of the pulses, points P and Q coincide?

Example 7 The pulses below are moving towards each other through a medium at one grid square per second. Use the graph below to draw the resulting shape of the two at 1, 2, 3, and 4 seconds later.

Interference by Position If you have the same sound but from two different speakers… Constructive Interference– Waves must be a whole-number wavelength apart Destructive Interference– Waves must be a half wavelength apart

Interference by Position Path difference of ½ λ leads to destructive interference Destructive Constructive

Constructive/Destructive Interference – Applications

Constructive/Destructive Interference – Noise Cancelling Headphones Two Kinds of Noise-Cancelling Headphones Passive - Uses sound-absorbing materials to block sound Active – Uses destructive interference to cancel out outside noise High-density, sound absorbing foam

The ‘Beats’ If two sounds are only slightly off in terms of frequency Produce a periodic rise and fall of amplitude (volume) Throbbing Sound = Beats

The Beats #‘beats’ = how far apart the two frequencies are Ex. Tuning Fork 1: f = 440 Hz Tuning Fork 2: f = *Beat Frequency of 2 Hz?

Example 8 A guitar string produces 4 beats per second when tuned with a 350 Hz tuning fork and 9 beats per second when tuned with a 355 Hz tuning fork. What is the actual frequency of the guitar?