Chapter 15: Sound Barry Latham Physics: Principles & Problems Glencoe Science, 2005
Jan 27, 2016
Chapter 15: SoundBarry Latham
Physics: Principles & ProblemsGlencoe Science, 2005
15.1:Properties and Detection of Sound
Sound Waves- pressure variation transmitted through matter
Speed of Sound- l=v/fPhysics Physlet I.18.2
Detection of Pressure WavesHuman Ear
Tympanic membrane vibrates at the same frequency as the pressure variation
Three tiny bones transfer motion to cochlea
Tiny hairs in cochlea are sensitive to vibration
Nerve cells send impulses to the brain
Perceiving SoundPitch- human perception of frequency
Approximately 20Hz-16,000Hz is the audible rangeLoudness-human perception of amplitude
Sound level- logarithmic scale of pressure variations
Decibel (dB)- unit of loudness 10x higher pressure represents an increase in 20dB Humans perceive an increase of 10dB as being twice as
loud
The Doppler EffectDoppler Shift- change in frequency due to
motion between the sound source and receiverPhysics Physlet I.18.4Motion only depends on the objects getting
closer or further away. It doesn’t matter who does which.
Blue Shift- objects getting closer Source stationary, receiver moving towards source Receiver stationary, source moving towards receiver
Red Shift- objects getting further apart Source stationary, receiver moving away from source Receiver stationary, source moving away from
receiver
Doppler Effectfd=fs(v-vd)/(v-vs)
fs=frequency of source(Hz)fd=detector (your ear?)v=velocity of sound (m/s)vs=velocity of source (m/s)vd=velocity of detector (m/s)
Takes into account direction, so be cautious with positive and negative signs
Physics Physlets E.18.5, P.18.7
15.2 The Physics of MusicSources of Sound
Air rushing from lungs through vocal cords
Lips vibrating in the mouthpiece of a brass instrument
A wooden reed vibrating in a clarinet (woodwind)
Strings vibrating in a guitar, piano, harp, etc.
Resonance in an Air ColumnClosed-pipe resonator- tuning fork
above a hollow tubeThe tube can be adjusted so that it
resonates with the tuning forks frequency
Loudest at the resonance point(s)- amplitudes are added together
Standing Pressure WaveOpen-pipe resonator- similar to closed-pipe, but
not sealed at the other endSound wave reflects off of open end and is
inverted
Closed vs Open Pipe Resonators
Resonance FrequenciesClosed-pipe resonators- the shortest pipe length
that will resonate is l/4=L1
All additional resonances occur with the addition of l/2 to the pipe length
l/4=L1, 3l/4=L2, 5l/4=L3, 7l/4=L4, etc.
Open-pipe resonators- the shortest pipe length that will resonate is l/2=L1
All additional resonances occur with the addition of l/2 to the pipe length
l/2=L1, l=L2, 3l/2=L3, 2l=L4, etc.
Resonant FrequenciesClosed
Pipe
Open Pipe
Resonance on StringsStrings act as open-pipe resonatorsFirst resonance is at l=L1, though, not l/2
l=L1, 3l/2=L2, 2l=L3, etc.
Closed-Pipe HarmonicsFor l1 in a closed-pipe instrument, that frequency is
the fundamental frequency, f1
Each additional odd-numbered resonance point is the next harmonicl3f3, third harmonicl5f5, fifth harmonicl7f7, seventh harmonic
Open-Pipe HarmonicsFor l1 in an open-pipe or stringed instrument, that
frequency is the fundamental frequency, f1
Each additional resonance point is the next harmonicl2f2, second harmonicl3f3, third harmonicl4f4, fourth harmonic
Consonance, Dissonance & BeatsDissonance- an unpleasant set of pitches played at the
same timeConsonance- a pleasant set of pitches played at the
same time (a chord)Both experiences are culturally influencedPhysics Physlets E.18.1, E.18.2
Beats- oscillation of amplitude due to two frequencies being slightly differentfbeat=|fA-fB|Used to tune instruments- when the beats slow down and
disappear, the instrument is in tunePhysics Physets E.18.3