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Sound. Acoustics is the study of sound. All sounds are waves produced by vibrating objects - tuning forks, vocal chords, reeds, lips, columns of air, strings, cricket legs Demo – tuning forks - water. Sound. Sound Waves. Sound is a longitudinal wave with compressions and rarefactions.

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Sound
Sound

  • Acoustics is the study of sound.

  • All sounds are waves produced by vibrating objects - tuning forks, vocal chords, reeds, lips, columns of air, strings, cricket legs

  • Demo – tuning forks - water



Sound waves
Sound Waves

  • Sound is a longitudinal wave with compressions and rarefactions.


Sound waves page 272
Sound Wavespage 272


Sound waves1
Sound Waves

  • Notice that the air molecules move in a direction parallel to the direction of the wave.

  • Demo – Slinky on the floor.


Speed of sound
Speed of Sound

  • The speed of propagation


Speed vs temp
Speed vs Temp

  • As temperature increases, the speed of sound increases.

    0.6 m/s per oC


Speed of sound vs speed of light
Speed of Sound vs Speed of Light

Sound = 343 m/s

Light = 300, 000, 000 m/s

Notice that light travels much faster.


Lightning thunder
Lightning & Thunder

  • Light reaches you in an extremely short period of time.

  • Sound reaches you at a much slower rate. It takes about 5 seconds to travel 1 mile.


The delayed sound reaching your ear after the light
The delayed sound reachingyour ear after the light.

  • Other examples include – starters pistol, chopping wood

  • You see the event(light), count the number of seconds until the sound arrives. 5 seconds = 1 mile 10 seconds = 2 miles


Figure 14 36 problem 14 59
Figure 14-36Problem 14-59


Path that sound travels in your ear
Path that sound travels in your ear

Sequence of vibrations from the source

  • To the ear drum

  • To the bones in the middle ear

  • To the oval window in the middle ear

  • To the fluid in the inner ear

  • To the hairs in the cochlea in the inner ear

  • To the nerves which go to the brain on the auditory nerve.


Pitch
Pitch

  • The pitch is determined by the frequency of the sound. Units are Hz or vibrations per sec.

  • Humans hear 20 to 20,000 Hz


Loudness of sound
Loudness of Sound

  • How loud a sound seems is determined by the wave’s amplitude. This is proportional to its energy.

  • We use a decibel scale to measure loudness.


Sound2
Sound

  • Loud noises can damage your hearing. This usually lowers your upper limit. The tiny hairs in the inner ear may fall out.



Reflection of sound
Reflection of Sound

  • Reflection of sound is called an echo.

  • Sound waves reflect off of hard smooth surfaces.

  • Curtains and rugs results in most of the sound being absorbed


Johaan Christian Doppler1803-1853

Doppler effect

A change in frequency (pitch) of sound due to the motion of the source or the receiver


Doppler effect

Doppler: SourceDoppler: Observer

Doppler Effect

Approaching, the frequency is higher because the wavefronts are closer together in time.   Departing, the frequency is lower.


Sound - resonance

  • Sound is produced by vibrating systems.

  • All systems have one or more natural frequencies.

  • A natural frequency is the frequency at which a system tends to vibrate in the absence of any driving or damping force.


Sound - resonance

  • If a system is exposed to a vibration that matches its natural frequency, it will vibrate with an increased amplitude.

  • This results in the amplification (increase in amplitude). of that frequency

  • This phenomenon is called resonance.


Sound - resonance

  • When resonance occurs in systems standing waves are formed.


Sound - resonance

  • In order for standing waves to form in a closed pipe (closed at one end), the length of the pipe L must be an odd multiple of one fourth of the wavelength.

  • The necessary condition is that there is a node at the closed end, and an antinode at the open end.





Sound - resonance

  • In order for standing waves to form in an open pipe (open at both ends), the length of the pipe L must be a whole number multiple of one half of the wavelength.

  • The necessary condition is that there are antinodes at both ends.




Figure 14 29 standing waves in a pipe that is open at both ends
Figure 14-29Standing Waves in a Pipe That Is Open at Both Ends


Sound - resonance

  • Because the speed of sound in air is constant, we can only vary pipe length or frequency to obtain conditions needed for resonance.


Sound - resonance

Example:

A tuning fork with a frequency of 392 Hz is found to cause resonance in an air column spaced by 44.3 cm. the air temperature is 27oC. Find the velocity of sound in air at that temperature.



Terminology – specifically for vibrating air columns.(pipes)

Fundamental frequency – (first harmonic) -

the frequency of the longest standing sound wave that can form in a pipe.

Second harmonic – two times the frequency of the longest standing sound wave that can form in a pipe.

Third harmonic – three times the frequency of the longest standing sound wave that can form in a pipe.


Sound - resonance columns.(pipes)

Beats –

Beats occur when two waves of slightly different frequencies are superimposed. A pulsating variation in loudness is heard.


Sound - resonance columns.(pipes)

Waves on a string –

the necessary condition for standing waves on a string, is that a node exist at either end.


Sound - resonance columns.(pipes)


Sound - resonance columns.(pipes)


Sound - resonance columns.(pipes)


Figure 14 24a harmonics
Figure 14-24a columns.(pipes)Harmonics


Figure 14 24b harmonics
Figure 14-24b columns.(pipes)Harmonics


Figure 14 24c harmonics
Figure 14-24c columns.(pipes)Harmonics


Sound - resonance columns.(pipes)

Example: One of the harmonics on a string 1.3 m long has a frequency of 15.6 Hz. The next higher harmonic has a frequency of 23.4Hz. Find (a) the fundamental frequency, and (b) the speed of the waves on this string.


Sound - resonance columns.(pipes)


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