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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


Sound1

Sound


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.


Loudness

Loudness


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


Sound

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

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

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

Sound - resonance

  • When resonance occurs in systems standing waves are formed.


Sound

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

Sound - resonance


Standing waves in a pipe that is open at one end closed pipe

Standing Waves in a Pipe That Is Open at One End(Closed Pipe)


Sound

Sound - resonance


Sound

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.


Sound

Sound - resonance


Sound

Sound - resonance


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

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

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.


Sound

Sound - resonance


Sound

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

Sound - resonance

Beats –

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


Sound

Sound - resonance

Waves on a string –

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


Sound

Sound - resonance


Sound

Sound - resonance


Sound

Sound - resonance


Figure 14 24a harmonics

Figure 14-24aHarmonics


Figure 14 24b harmonics

Figure 14-24bHarmonics


Figure 14 24c harmonics

Figure 14-24cHarmonics


Sound

Sound - resonance

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

Sound - resonance


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