Sound Pitch Loudness Beats Doppler Effect

Physics Ms. Shaver

Sound

  Sound is a longitudinal mechanical wave aka a pressure or compression wave   Compressions(High Pressure) Rarefactions(Low Pressure)

Sound

Tiny hairs inside the inner ear (

cochlea

) translate air pressure into electrical impulses that can be read by the brain

Graphing Sound

Speed of Sound

 Depends on the medium.

 The more elastic the medium the faster sound will travel through it.

 Speed in metals>speed in water>speed in air  Sound can’t travel through vacuum.

Speed of Sound in Air

v = 331 + 0.6 T ( in meters/sec)   T is the temperature in 0 C. In higher humidity, sound will travel faster.   For 15°C, v = 331 + 0.6*15 = 340 m/s Use 340 m/s as an average speed of sound in air.

Faster than the speed of Sound

  Supersonic – motion faster than speed of sound Sonic boom – caused by an object breaking the sound barrier (supersonic planes, bullets)

Plane reaches speed above 770 mph – Explosion of sound waves caused by air crashing behind the plane The air can condense or sweep up vapor from engine – causing this picture

Pitch: the frequency of a sound wave.

 Musical notes have a given pitch.

 The note C has a frequency of 327Hz.

 When two notes differ by a ratio of 2:1 they are one octave apart.

 What would be the next higher C? (Ans:654Hz)

Pitch – frequency of sound wave

   Humans can hear frequencies between 20 Hz and 20,000 Hz Infrasonic frequencies: Below 20 Hz ◦ used by elephants and submarines to communicate over long distances Ultrasonic frequencies ◦ Dog whistles

Loudness = Amplitude

   Loudness is measured in decibels (dB) A +10 dB change we hear as twice as loud A -10 dB change we hear as half as loud

Decibels – Loudness measure

 EXAMPLE: If a sound is 20 dB loud, answer how many dB these would be: 1) 2) 3) A sound twice as loud: A sound half as loud:

30 dB 10 dB

A sound three times as loud:

35 dB

Decibels – Loudness measure

Examples of Sound Intensity Levels

          jet plane taking off air raid siren

threshold of pain

loud rock music

ear damage starts

140 dB 125 dB

120 dB

115 dB

85 dB

busy traffic normal conversation quiet library 70 dB 60 dB 40 dB soft whisper 20 dB

threshold of human hearing 0 dB

Timbre – tone quality

   What makes a particular musical sound different from another, even when they have the same pitch and loudness EX: difference between a guitar and a piano playing the same note at the same loudness Sounds can be described in terms of “coloration,” e.g. bright, dark, warm, harsh, etc.

Timbre – tone quality

  Color of the note is due to the presence of different harmonics. The “oo” has mostly low harmonics, while the “ee” has mostly high harmonics

Harmonics - Review

v = λ f - The velocity of the wave stays the same, even if considering different harmonics….it’s still the same wave!!!

Harmonics - Review

Properties of Sound

 Reflection (Echo)  Refraction  Interference  Diffraction

Echoes

 Echoes are sounds that are reflected back by a hard boundary.

Echoes

    Echolocation – used by many animals like bats to see through dark water or at night Sonar – sound through water Radar – light waves through air Sonograms – sound waves through the human body

Beats

  Interference effect When two waves of close frequencies interact causing alternating constructive and destructive interference

Beats

 The number of beats = difference of two frequencies (absolute value)  EX: f 1 ◦ = 345 Hz; f 2 = 342 Hz Number of beats = 345 – 342 = 3 ◦ There will be three beats per second

http://www.school-for champions.com/science/sound_beat_frequencies.htm#.U2gpwletJW8

Bow (Shock) Waves

When the speed of a moving sound source is greater than the speed of the wave, a pressure ridge builds similar to the wave created by the bow of a ship.

physlet animation

Sonic Boom

 When the pressure ridge of a bow wave of a jet passes over an observer on the ground, the observer experiences a sonic boom.

Doppler Effect  The change in a wave's perceived frequency due to the motion of either the sound source or the observer.

 It is applicable to any type of wave.

 Austrian physicist Christian Doppler (1803-1853).

train sound clip

Simulations

  physlet animation http://www.walter-fendt.de/ph14e/dopplereff.htm

The Doppler Effect http://www.physicsclassroom.com/Class/sound/u11l3b.cfm

The Doppler Effect

 In front of the source the sound waves are compressed (shorter wavelength λ) and this raises the frequency (pitch)  Behind the source the sound waves are stretched (longer wavelength λ) and this drops the frequency (pitch)  Anything moving at the same speed as the source will experience no change in frequency