Sound lab Digital piano Oscilloscope with speakers Speaker into tuning box for resonance on scope Speaker w/mic and tuning box on scope for beats (203-230) Mic straight into speakers Sounding board Scope freq versus wavelength Pitch versus freq (sin versus square wave) Hydrogen fuel (fracture water) w/ resonance

CH 15 Sound

The nature of Sound Sound is a longitudinal wave A sound wave is created by a vibrating object and is transmitted through a medium

Sound When a diaphragm moves, it compresses the air molecules in front of it. The compression causes the air pressure to rise. On Off

Sound Waves Sound travels in waves known as “Longitudinal” Waves. The picture below shows a Longitudinal wave formed in air molecules due to a sound. The higher density areas are known as compressions. The lower density areas are known as rarefactions. When studying interference in sound waves, treat the compressions as crests and the rarefactions as troughs.

Sound Sound waves move through air by creating variations in air pressure. The air molecules collide, transmitting pressure oscillations. Areas of high pressure are compression Areas of low pressure are call Rarefactions

Distance between compressions is λ Sound travels in all directions Wave front can be created by connecting similar point on the waves Distance between compressions is λ Sound travels in all directions

Note the Energy of the sound is now spread over each “growing” wave front. The area of the wave front is πr2 The Intensity (I) drops by the area of a circle (πr2)

Pressure Amplitude Loudness depends on the pressure amplitude of the wave Sound Intensity I (Watts W/m2) P Power A Area r Radius Area of a sphere

Pressure Amplitude example A bus generates sound intensity of 10μW/m2. A student barely hears the sound from pressure wave of 0.076W. How far away is he?

Pressure Amplitude Sound Intensity I (Watts W) Sound Intensity level is measure in decibels (dB) Io=1x10-12 W/m2 Threshold of human hearing

Intensity Level The threshold for human hearing is 1E-12 W/m2. The average whisper is 1E-10 W/m2. How many dB louder is the whisper?

Intensity Level For every 10x increase in Intensity, the Intensity Level increase 10dB. Event W / m2 dB Threshold of hearing 1x10-12/ 1x10-12 Whisper 1x10-10 / 1x10-12 20 Classroom 1x10-7 / 1x10-12 50 Normal Conversation 1x10-6 / 1x10-12 60 City Street 1x10-5 / 1x10-12 70 Threshold of pain 1x10-0 / 1x10-12 120

Intensity Two different stereo systems can produce a 45dB output and a 25dB output. What is the difference in their intensities?

Practice Problems WS 15.1 1-8 WS 15.2 #’s1-6

Resonance A Phenomenon that occurs when the forced frequency on an object matches the object’s natural frequency, and a dramatic increase in amplitude results.

Tacoma narrows bridge video

Beat Frequency Beats: A variation in amplitude (loudness) caused by the interference of two slightly different frequencies (tones). Ex: A 330Hz and a 332Hz tuning forks are struck simultaneously. What is the beat frequency?

Beat Frequency Example A student has 2 tuning forks. One is 349Hz, the other is an unknown frequency. Together they create a beat of 3Hz. What are the possible frequencies?

Doppler Effect Doppler Effect: the change in wavelength and frequency due to the motion of the source. Police

Doppler Effect Sound waves propagate out from the source in all direction. If the source isn’t moving, the wavelengths are constant Doppler Effect Police Sonic boom from real player library

Doppler Effect Source moving towards you: Source moving away from you: Police

Doppler Effect Example A train is approaching you at 31m/s and blows its whistle of 305hz. What frequency do you hear?

Doppler Effect Example A train is approaching you at 31m/s and blows its whistle of 305hz. b) What frequency does your friend hear if the train has past him and continues moving away?

Expanding Galaxy

Practice Problems Ws 15.2a 1-5 WS 15.3 1-4

Standing Waves Standing Wave: Node: Antinode: Record these vocabulary terms and define their meaning using your text.

Standing Waves Standing Wave: has stationary nodes and antinodes. It is the results of identical waves traveling in opposite direction. Node: The medium is not displaced as the waves pass through Antinode: The displacement caused by interfering waves is largest.

Standing Waves In order for a standing wave to exist, there must be an identical wave traveling in the opposite direction Standing wave demo

Standing Waves Harmonics: Standing wave that consist of more than one pulse 4th Harmonic 1st Harmonic 2nd Harmonic

Standing waves Fundamental Frequency: The lowest frequency that creates a standing wave in a given medium. Harmonics (overtones): frequencies with integer multiples of the fundamental frequency. These frequencies make up the harmonic series.

Harmonic Series in a string Length L 1st Harmonic 2nd Harmonic 3rd Harmonic

Sound End Ch 15