PHYS16 – Lecture 36 & 37 Sound December 5 & 7, 2010

Outline for Sound Sound – Longitudinal Pressure Waves – Sound Velocity and Reflection – Resonance and Standing waves – Intensity and Sound Level – Interference and Beats – Doppler

Resonance and Standing Waves

Guitar Demonstration

Example Question A guitar string with a greater mass density has a/an: A)Greater velocity B)Higher frequency C)Smaller wavelength D)Unchanged wavelength

Example Question So what does the hole do?

Standing Waves with Sound Need two “ends” where sound reflects Reflections constructively and destructively interfere to setup up standing waves

Ruben’s Tube Demo

Standing Waves with Sound Helmholtz Resonator A As A increases, Wavelength decreases and Frequency increases!

Bottle Demo

Intensity and Sound Level

Wave Intensity Intensity is the power per unit area in a wave

Sound Level Sound Level (β) - a relative intensity in decibels (dB) – 0 dB = smallest sound that can be heard – 50 dB = background noise – 70 dB = traffic – 110 dB = rock concert

Example Question If a Rock Concert is 110 dB, what is the intensity level in W/m 2 ? A) 0.1 W/m 2 B) 10 W/m 2 C) W /m 2 D) W/m 2

Example Question A bell is rung in a room and can be assumed to be a point source. If you sit three times the distance your friend is sitting from the bell, what is the decibel drop? A) 0.1 dB B) 1 dB C) 10 dB D) 100 dB

Interference and Beats

Sound Interference Sound waves can interfere destructively or constructively

Dead Zones Places of destructive interference are called Dead Zones For Sound we usually don’t get dead zones because sounds are composed of many different frequencies How does having many frequencies in a sound eliminate dead zones?

Beats Beats occur when two sine waves (two notes) with very similar frequency interfere t t t YYIYYI

Example Question What is the beat frequency between 440 Hz and 442 Hz? A) 441 Hz B) 2 Hz C) -2Hz D) 1 Hz

Doppler Effect

Frequency of sound changes when relative velocity changes – Frequencies on approach sound higher – Frequencies that recede sound lower

Example Question The frequency of an ambulance siren is 1200 Hz. The ambulance is going 60 mph (26.8 m/s). As the ambulance approaches your stopped car, you hear what frequency? f approach = 1300 Hz

Sound + Light = Optical Communication

Main Points - Sound Sound– a longitudinal pressure wave that moves through a medium Velocity – 343 m/s in air Reflection – occurs when velocity changes Resonance and Standing Waves – depends on open/closed ends

Main Points - Sound Intensity and Sound Level Interference and Beats Doppler Effect