Sound

• Origin – sound originates as a vibration of something (vocal chords, a string, an air column, a reed, a speaker cone)

• Propagation – as a longitudinal or compressional wave

• Perception – relates to physiology

Origin of sound• Vibration frequency – measured in hertz = Hz = cycles

per second.

• human audible range is roughly between 20 Hz and 20 kHz

– The audible range decreases as one ages (especially high frequencies)

– Mosquito ring tones

– http://www.freemosquitoringtones.org/hearing_test/

– http://onlinetonegenerator.com/

– http://www.seventhstring.com/tuningfork/tuningfork.html

• Below 20 Hz is infrasound, above 20 kHz is ultrasound

Propagation of sound• As a longitudinal (compressional) wave

• Speed of wave in air?

– Class lab/demo speed of sound

• Relation between speed of sound and speed of molecules – in a gas, the speed of sound is about the same as the magnitude of the component of the mean molecular velocity in the direction of propagation of sound

• Speed of sound in other materials – essentially square root of ratio of elastic force factor to inertia factor

factorinertia

factorforcev Compare speed of sound

in He, air, water, steel

Sound Waves

• Recall (wave speed) = (frequency) x (wavelength)

• From our measured speed of sound, what is wavelength of sound waves in the audible range?

20 Hz has wavelength of ?

20 kHz has wavelength of ?

• Recall that the energy in a sound wave is proportional to frequency and amplitude squared (as it is in all material waves)

22Afenergywave

Sound Waves

What is oscillating?

Sound Waves

What is oscillating?

Position of particles

Pressure – most commonly thought of as this

Density

Temperature (sort of – recall adiabatic compression and expansion)

Wave phenomena - reflection• Reflection of sound – angle of incidence equals angle of

reflection

World War One acoustic mirror, Kilnsea, East Yorkshire, UK. Concrete mirrors like these were constructed before the invention of radar to detect the sound of approaching enemy aircraft.

Whisper dishes -http://www.youtube.com/watch?v=y26wYeOg_gU

Wave phenomena - reflection

• Definition and explanation

• Conditions for occurrence – change in velocity or density or transmission cross-section

• Viewpoint of boundary conditionshttp://www.acs.psu.edu/drussell/Demos/reflect/reflect.html

• Reverberation http://www.acs.psu.edu/drussell/Demos/RT60/RT60.htm

• Reflection with Doppler effect

Wave phenomena - refraction

• Reflection and refraction animation

http://www.phy.ntnu.edu.tw/java/propagation/propagation.html

• Refraction and curving of sound path http://www.acs.psu.edu/drussell/Demos/refract/refract.html

• Conditions for occurrence of refraction?

Wave phenomena - resonance

• Forced vibration – wiggling something by forcing it to vibrate (speaker diaphragm forced by electromagnetic fields, slinky being wiggled, tuning fork wiggling a table)

• Natural frequency – frequency at which something likes to vibrate (stretched string, drum head, air column)

• Resonance – forced vibration at its natural frequency– Recall standing waves

– http://www.colorado.edu/physics/2000/microwaves/standing_wave2.html

– www.youtube.com/watch?v=HxTZ446tbzE – Try tuning forks on boxes (sympathetic vibrations)

Wave phenomena - interference

Wave phenomena - interference

http://phet.colorado.edu/en/simulation/sound

Wave phenomena - beats

• Do beats demo with tuning forks on sound boxes

Collect data with Logger Pro to show the beats

• http://onlinetonegenerator.com/binauralbeats.html

• Beats are interference in time

• Point out Hewitt’s comb analogy

Sound – physiology and physics (chapter 21 material)

What is the relationship between the physics of a sound wave and our perception of sound?

Pitch relates to ? (see http://onlinetonegenerator.com/)

Loudness relates to ? Run the PhET physics/sound simulation Fourier: Making Waves to show this

Quality (timbre) relates to ? Run the PhET physics/sound simulation Fourier: Making Waves to show this

– Normal conversation

– Leaves rustling

– Threshold of hearing

– Airplane taking off

– Chainsaw

– City traffic

Harmonic content – musical sound

• Run the PhET physics/sound simulation Fourier: Making Waves to show– How sound changes with harmonic content– That pitch remains the same even without the

first harmonic– Various wave forms and relationship to

harmonic content (especially wave packet)

Equal loudness curves for average human hearing

Our hearing is more sensitive to some frequencies than others.

www. lafavre.us/tuning-marimba.htm

• How much power output is there in 10 million people talking loudly?

• What is the difference between a radio wave carrying a song to a radio and the wave carrying the sound from the radio’s speakers to your ears?

• How do we sense sound?

• Is it really possible for an opera singer to shatter glass?

http://www.youtube.com/watch?v=I4jdGf3RzCs

http://www.scientificamerican.com/article.cfm?id=fact-or-fiction-opera-singer-can-shatter-glass